diff --git a/README.md b/README.md
index c764516..d374368 100644
--- a/README.md
+++ b/README.md
@@ -30,14 +30,32 @@ export MEALQUEST_PUBLIC_URL=http://localhost:5173
 export MEALQUEST_DATA_DIR="$HOME/.mealquest-dev"
 export MEALQUEST_SHUTDOWN_TIMEOUT=10s
 export MEALQUEST_SECRET_KEY_FILE="$HOME/.mealquest-dev/secret_key"
+# Authentik OAuth2/OpenID provider for MealQuest (redirect URI:
+# $MEALQUEST_PUBLIC_URL/auth/callback). Save its client secret to the file.
+export MEALQUEST_OIDC_ISSUER=https://auth.example.com/application/o/mealquest/
+export MEALQUEST_OIDC_CLIENT_ID=mealquest
+export MEALQUEST_OIDC_CLIENT_SECRET_FILE="$HOME/.mealquest-dev/oidc_client_secret"
+export MEALQUEST_OIDC_ALLOWED_GROUPS=mealquest-admin
+# Optional: sessions end after MEALQUEST_SESSION_IDLE unused (default 720h)
+# and always MEALQUEST_SESSION_MAX after sign-in (default 4320h).
+# export MEALQUEST_SESSION_IDLE=720h
+# export MEALQUEST_SESSION_MAX=4320h
+# Optional: comma-separated IPs/CIDRs of reverse proxies whose
+# X-Forwarded-For/-Proto headers are honoured (production: Traefik). Unset
+# trusts no peer, which is right for local development.
+# export MEALQUEST_TRUSTED_PROXY=10.60.1.33
 make dev
 ```
 
 `make dev` runs the Go server on `:8080` and the Vite dev server on `:5173`
 (which proxies `/api`, `/auth` and `/media` to `:8080`). Open
 http://localhost:5173. Configuration is environment-only and fails fast:
-every variable above is required, and secrets are read only from files named
-by `*_FILE` variables.
+every variable above is required except the commented-out session
+lifetimes (when set they must be positive, with MAX at least IDLE) and
+trusted proxy list (every entry must be an IP or CIDR), and
+secrets are read only from files named by `*_FILE` variables. `MEALQUEST_PUBLIC_URL`, `MEALQUEST_OIDC_ISSUER` and the
+endpoints the issuer advertises must be `https://`; plain `http://` is
+accepted only for `localhost`, `127.0.0.1` and `[::1]`.
 
 ## Common commands
 
diff --git a/cmd/server/boot_test.go b/cmd/server/boot_test.go
index 8ce5eca..157d39f 100644
--- a/cmd/server/boot_test.go
+++ b/cmd/server/boot_test.go
@@ -34,15 +34,24 @@ func useBootMigrations(t *testing.T, m fstest.MapFS) {
 // holds keyFile. Tests that call it must not run in parallel.
 func loadConfig(t *testing.T, keyFile string) config.Config {
 	t.Helper()
-	keyPath := filepath.Join(t.TempDir(), "secret_key")
+	dir := t.TempDir()
+	keyPath := filepath.Join(dir, "secret_key")
 	if err := os.WriteFile(keyPath, []byte(keyFile), 0o600); err != nil {
 		t.Fatal(err)
 	}
+	clientSecretPath := filepath.Join(dir, "oidc_client_secret")
+	if err := os.WriteFile(clientSecretPath, []byte("client-secret\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
 	t.Setenv(config.EnvHTTPAddr, "127.0.0.1:0")
 	t.Setenv(config.EnvPublicURL, "http://localhost")
 	t.Setenv(config.EnvDataDir, t.TempDir())
 	t.Setenv(config.EnvShutdownTimeout, "1s")
 	t.Setenv(config.EnvSecretKeyFile, keyPath)
+	t.Setenv(config.EnvOIDCIssuer, "https://auth.example.com/application/o/mealquest/")
+	t.Setenv(config.EnvOIDCClientID, "mealquest")
+	t.Setenv(config.EnvOIDCClientSecretFile, clientSecretPath)
+	t.Setenv(config.EnvOIDCAllowedGroups, "mealquest-admin")
 	cfg, err := config.Load()
 	if err != nil {
 		t.Fatal(err)
diff --git a/cmd/server/main.go b/cmd/server/main.go
index 79bdbdb..666cce7 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -13,7 +13,9 @@ import (
 	"syscall"
 	"time"
 
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/config"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/forwarded"
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/frontend"
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/secretbox"
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/server"
@@ -31,7 +33,7 @@ func main() {
 		os.Exit(1)
 	}
 
-	logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
+	logger := slog.New(forwarded.LogHandler(slog.NewJSONHandler(os.Stdout, nil)))
 	if err := run(cfg, logger); err != nil {
 		logger.Error("server stopped", "error", err)
 		os.Exit(1)
@@ -56,7 +58,23 @@ func run(cfg config.Config, logger *slog.Logger) error {
 		}
 	}()
 
-	router, err := server.NewRouter(logger, version, frontend.Assets())
+	sessions, err := auth.NewSessionManager(db, cfg.SessionIdle, cfg.SessionMax)
+	if err != nil {
+		return err
+	}
+	authFlow, err := auth.New(ctx, auth.Config{
+		Issuer:        cfg.OIDCIssuer.String(),
+		ClientID:      cfg.OIDCClientID,
+		ClientSecret:  cfg.OIDCClientSecret.Reveal(),
+		PublicURL:     cfg.PublicURL,
+		SecretKey:     []byte(cfg.SecretKey.Reveal()),
+		AllowedGroups: cfg.OIDCAllowedGroups,
+	}, sessions, logger)
+	if err != nil {
+		return err
+	}
+
+	router, err := server.NewRouter(logger, version, cfg.TrustedProxy, authFlow, sessions, frontend.Assets())
 	if err != nil {
 		return err
 	}
@@ -124,6 +142,7 @@ func logListening(logger *slog.Logger, cfg config.Config) {
 	logger.Info("listening",
 		"addr", cfg.HTTPAddr,
 		"public_origin", cfg.PublicOrigin(),
+		"trusted_proxy", cfg.TrustedProxy,
 		"data_dir", cfg.DataDir,
 		"version", version,
 	)
diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index 4beae8c..6f9eaab 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -29,6 +29,15 @@ docker compose project "mealquest" — 10.60.1.14
 One Go service serves the API, the auth flow, media and the PWA. Its SQLite
 database is replicated by Litestream to the NAS.
 
+Traefik terminates TLS. The service honours `X-Forwarded-For` and
+`X-Forwarded-Proto` only from peers listed in `MEALQUEST_TRUSTED_PROXY`
+(comma-separated IPs/CIDRs; production lists `10.60.1.33`); from any other
+peer they are ignored. The client is the rightmost `X-Forwarded-For` address
+that is not a trusted proxy. The resolved client IP is added as `client_ip`
+to every log line written while handling a request. The OIDC redirect URI
+and the Secure cookies are fixed by `MEALQUEST_PUBLIC_URL`, never by request
+headers, so sign-in works whichever peer the request arrives from.
+
 ## Repository layout
 
 | Path | Purpose |
diff --git a/go.mod b/go.mod
index a517797..39a6ce9 100644
--- a/go.mod
+++ b/go.mod
@@ -3,13 +3,16 @@ module code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest
 go 1.25.0
 
 require (
+	github.com/coreos/go-oidc/v3 v3.21.0
 	github.com/go-chi/chi/v5 v5.3.1
 	github.com/oklog/ulid/v2 v2.1.2
+	golang.org/x/oauth2 v0.36.0
 	modernc.org/sqlite v1.59.0
 )
 
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/go-jose/go-jose/v4 v4.1.4 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/mattn/go-isatty v0.0.24 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
diff --git a/go.sum b/go.sum
index 3a07785..ba3f090 100644
--- a/go.sum
+++ b/go.sum
@@ -1,7 +1,11 @@
+github.com/coreos/go-oidc/v3 v3.21.0 h1:wZo4Q9Pum8dYEj0eMUPrqR+kvuGkeUplbLpNCkBqoWM=
+github.com/coreos/go-oidc/v3 v3.21.0/go.mod h1:DYCf24+ncYi+XkIH97GY1+dqoRlbaSI26KVTCI9SrY4=
 github.com/dustin/go-humanize v1.0.1 h1:GzkhY7T5VNhEkwH0PVJgjz+fX1rhBrR7pRT3mDkpeCY=
 github.com/dustin/go-humanize v1.0.1/go.mod h1:Mu1zIs6XwVuF/gI1OepvI0qD18qycQx+mFykh5fBlto=
 github.com/go-chi/chi/v5 v5.3.1 h1:3j4HZLGZQ3JpMCrPJF/Jl3mYJfWLKBfNJ6quurUGCf8=
 github.com/go-chi/chi/v5 v5.3.1/go.mod h1:R+tYY2hNuVUUjxoPtqUdgBqevM9s9njzkTLutVsOCto=
+github.com/go-jose/go-jose/v4 v4.1.4 h1:moDMcTHmvE6Groj34emNPLs/qtYXRVcd6S7NHbHz3kA=
+github.com/go-jose/go-jose/v4 v4.1.4/go.mod h1:x4oUasVrzR7071A4TnHLGSPpNOm2a21K9Kf04k1rs08=
 github.com/google/pprof v0.0.0-20260802141513-ef3492d7dac3 h1:LMLX+LgTNWpfvCBdFebv6EsYotImrt/Ppc5cXIriCSo=
 github.com/google/pprof v0.0.0-20260802141513-ef3492d7dac3/go.mod h1:jl5iWTm0/hd5PjEYEOuwAJ57L/CibdZfrqZ5XA5GrCk=
 github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
@@ -19,6 +23,8 @@ github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
 golang.org/x/mod v0.38.0 h1:MECBjubtXD7yj4HrhIUcywNaGeNVUdfVnxmPajOk4yk=
 golang.org/x/mod v0.38.0/go.mod h1:V6Xz0pq8TQ3dGqVQ1FVHuelZpAL0uNhSkk9ogYP3c40=
+golang.org/x/oauth2 v0.36.0 h1:peZ/1z27fi9hUOFCAZaHyrpWG5lwe0RJEEEeH0ThlIs=
+golang.org/x/oauth2 v0.36.0/go.mod h1:YDBUJMTkDnJS+A4BP4eZBjCqtokkg1hODuPjwiGPO7Q=
 golang.org/x/sync v0.22.0 h1:SZjpbeLmrCk4xhRSZFNZW5gFUeCeFgjekvI/+gfScek=
 golang.org/x/sync v0.22.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
 golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
diff --git a/internal/auth/actor.go b/internal/auth/actor.go
new file mode 100644
index 0000000..c781091
--- /dev/null
+++ b/internal/auth/actor.go
@@ -0,0 +1,45 @@
+package auth
+
+import (
+	"context"
+	"slices"
+)
+
+// Actor is the signed-in household member a request acts for. Handlers
+// record Actor.Subject as updated_by on edits.
+type Actor struct {
+	Subject string
+	Email   string
+	Name    string
+	Groups  []string
+}
+
+// Actor returns the household member s belongs to.
+func (s Session) Actor() Actor {
+	return Actor{Subject: s.Subject, Email: s.Email, Name: s.Name, Groups: slices.Clone(s.Groups)}
+}
+
+type sessionKey struct{}
+
+// WithSession returns a copy of ctx carrying s. The session middleware calls
+// it for every authenticated request.
+func WithSession(ctx context.Context, s Session) context.Context {
+	return context.WithValue(ctx, sessionKey{}, s)
+}
+
+// SessionFrom returns the session WithSession stored in ctx. ok is false when
+// ctx has none, which behind the session middleware means a wiring bug.
+func SessionFrom(ctx context.Context) (s Session, ok bool) {
+	s, ok = ctx.Value(sessionKey{}).(Session)
+	return s, ok
+}
+
+// ActorFrom returns the actor of the session WithSession stored in ctx. ok is
+// false when ctx has none.
+func ActorFrom(ctx context.Context) (actor Actor, ok bool) {
+	s, ok := SessionFrom(ctx)
+	if !ok {
+		return Actor{}, false
+	}
+	return s.Actor(), true
+}
diff --git a/internal/auth/actor_test.go b/internal/auth/actor_test.go
new file mode 100644
index 0000000..fff57cb
--- /dev/null
+++ b/internal/auth/actor_test.go
@@ -0,0 +1,40 @@
+package auth
+
+import (
+	"context"
+	"slices"
+	"testing"
+)
+
+func TestActorContext(t *testing.T) {
+	t.Parallel()
+	if _, ok := ActorFrom(context.Background()); ok {
+		t.Error("ActorFrom found an actor in an empty context")
+	}
+	if _, ok := SessionFrom(context.Background()); ok {
+		t.Error("SessionFrom found a session in an empty context")
+	}
+	s := Session{Subject: "user-123", Email: "cook@example.com", Name: "Home Cook", Groups: []string{"mealquest-admin"}, CSRFToken: "t"}
+	ctx := WithSession(context.Background(), s)
+	gotSession, ok := SessionFrom(ctx)
+	if !ok || gotSession.Subject != s.Subject || gotSession.CSRFToken != s.CSRFToken {
+		t.Errorf("SessionFrom = %+v (ok=%v), want %+v", gotSession, ok, s)
+	}
+	got, ok := ActorFrom(ctx)
+	if !ok || got.Subject != s.Subject || got.Email != s.Email || got.Name != s.Name || !slices.Equal(got.Groups, s.Groups) {
+		t.Errorf("ActorFrom = %+v (ok=%v), want the session's actor", got, ok)
+	}
+}
+
+func TestSessionActor(t *testing.T) {
+	t.Parallel()
+	s := Session{Subject: "user-123", Email: "cook@example.com", Name: "Home Cook", Groups: []string{"mealquest-admin"}, CSRFToken: "t"}
+	a := s.Actor()
+	if a.Subject != s.Subject || a.Email != s.Email || a.Name != s.Name || !slices.Equal(a.Groups, s.Groups) {
+		t.Errorf("Actor = %+v, want the session's identity", a)
+	}
+	a.Groups[0] = "changed"
+	if s.Groups[0] != "mealquest-admin" {
+		t.Error("Actor shares the session's groups slice")
+	}
+}
diff --git a/internal/auth/auth.go b/internal/auth/auth.go
new file mode 100644
index 0000000..d82d33d
--- /dev/null
+++ b/internal/auth/auth.go
@@ -0,0 +1,439 @@
+// Package auth signs the household in with Authentik over OpenID Connect.
+// The service is a confidential client: the authorization code, tokens and
+// client secret stay on the server and never reach JavaScript.
+package auth
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/subtle"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"net/url"
+	"slices"
+	"sync/atomic"
+	"time"
+
+	"github.com/coreos/go-oidc/v3/oidc"
+	"github.com/go-chi/chi/v5"
+	"golang.org/x/oauth2"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/secureurl"
+)
+
+// Routes owned by this package.
+const (
+	LoginPath    = "/auth/login"
+	CallbackPath = "/auth/callback"
+	LogoutPath   = "/auth/logout"
+)
+
+// providerTimeout bounds every request to the identity provider.
+const providerTimeout = 10 * time.Second
+
+// scopes requested at login.
+var scopes = []string{oidc.ScopeOpenID, "profile", "email", "groups", oidc.ScopeOfflineAccess}
+
+// Claims are the verified ID token claims handed to the session layer.
+type Claims struct {
+	Subject           string   `json:"sub"`
+	Email             string   `json:"email"`
+	EmailVerified     bool     `json:"email_verified"`
+	Name              string   `json:"name"`
+	PreferredUsername string   `json:"preferred_username"`
+	Groups            []string `json:"groups"`
+}
+
+// Sessions is the session layer. StartSession is called once per successful
+// login with claims from a verified ID token; it must set the session cookie
+// on w. EndSession is called on logout; it must end the session r carries,
+// if any, and clear the cookie on w. A returned error fails the request with
+// 500.
+type Sessions interface {
+	StartSession(w http.ResponseWriter, r *http.Request, claims Claims) error
+	EndSession(w http.ResponseWriter, r *http.Request) error
+}
+
+// Config is what New needs to reach the identity provider.
+type Config struct {
+	// Issuer is the exact issuer URL; discovery runs against it.
+	Issuer       string
+	ClientID     string
+	ClientSecret string
+	// PublicURL is the service's public URL; its origin prefixes CallbackPath
+	// to form the redirect URI.
+	PublicURL *url.URL
+	// SecretKey is the server key the login cookie's signing key derives from.
+	SecretKey []byte
+	// AllowedGroups are the household's groups; a sign-in is refused unless
+	// the account is in at least one of them.
+	AllowedGroups []string
+}
+
+// Auth serves the login and callback routes.
+type Auth struct {
+	oauth    oauth2.Config
+	verifier *oidc.IDTokenVerifier
+	provider *oidc.Provider
+	client   *http.Client
+	cookie   loginCookie
+	sessions Sessions
+	allowed  []string
+	// endSessionURL is the provider's end_session_endpoint; logout ends
+	// there so the provider's own session ends too.
+	endSessionURL string
+	logger        *slog.Logger
+}
+
+// New discovers the provider at cfg.Issuer and returns the login flow. It
+// fails if discovery fails or the issuer the provider reports differs.
+func New(ctx context.Context, cfg Config, sessions Sessions, logger *slog.Logger) (*Auth, error) {
+	switch {
+	case cfg.Issuer == "", cfg.ClientID == "", cfg.ClientSecret == "":
+		return nil, errors.New("auth: issuer, client ID and client secret are required")
+	case cfg.PublicURL == nil:
+		return nil, errors.New("auth: public URL is required")
+	case len(cfg.SecretKey) == 0:
+		return nil, errors.New("auth: secret key is required")
+	case sessions == nil:
+		return nil, errors.New("auth: session layer is required")
+	case len(cfg.AllowedGroups) == 0:
+		return nil, errors.New("auth: at least one allowed group is required")
+	}
+
+	if err := checkEndpoint("issuer", cfg.Issuer); err != nil {
+		return nil, err
+	}
+
+	client := &http.Client{Timeout: providerTimeout, CheckRedirect: checkRedirect}
+	discovery := *client
+	recorder := &statusRecorder{next: http.DefaultTransport}
+	discovery.Transport = recorder
+	provider, err := oidc.NewProvider(oidc.ClientContext(ctx, &discovery), cfg.Issuer)
+	if err != nil {
+		return nil, discoveryError(err, int(recorder.status.Load()))
+	}
+	endpoint := provider.Endpoint()
+	var discovered struct {
+		JWKSURI       string `json:"jwks_uri"`
+		EndSessionURI string `json:"end_session_endpoint"`
+	}
+	if err := provider.Claims(&discovered); err != nil {
+		return nil, fmt.Errorf("auth: OIDC discovery: %w", err)
+	}
+	// The token endpoint receives the client secret and the key set decides
+	// which tokens verify, so every endpoint used must pass the same rule.
+	for _, e := range []struct{ name, raw string }{
+		{"authorization endpoint", endpoint.AuthURL},
+		{"token endpoint", endpoint.TokenURL},
+		{"JWKS endpoint", discovered.JWKSURI},
+		{"end session endpoint", discovered.EndSessionURI},
+		// Groups come from userinfo when the ID token carries none, and the
+		// access token is sent there.
+		{"userinfo endpoint", provider.UserInfoEndpoint()},
+	} {
+		if err := checkEndpoint(e.name, e.raw); err != nil {
+			return nil, err
+		}
+	}
+	// client_secret_basic, the OIDC default; no auto-detection retries.
+	endpoint.AuthStyle = oauth2.AuthStyleInHeader
+
+	return &Auth{
+		oauth: oauth2.Config{
+			ClientID:     cfg.ClientID,
+			ClientSecret: cfg.ClientSecret,
+			Endpoint:     endpoint,
+			RedirectURL:  cfg.PublicURL.Scheme + "://" + cfg.PublicURL.Host + CallbackPath,
+			Scopes:       scopes,
+		},
+		verifier: provider.Verifier(&oidc.Config{
+			ClientID:             cfg.ClientID,
+			SupportedSigningAlgs: []string{oidc.RS256},
+		}),
+		provider:      provider,
+		client:        client,
+		allowed:       slices.Clone(cfg.AllowedGroups),
+		cookie:        newLoginCookie(cfg.SecretKey),
+		sessions:      sessions,
+		endSessionURL: discovered.EndSessionURI,
+		logger:        logger,
+	}, nil
+}
+
+// statusRecorder records the status of the last response it carries.
+type statusRecorder struct {
+	next   http.RoundTripper
+	status atomic.Int32
+}
+
+func (s *statusRecorder) RoundTrip(req *http.Request) (*http.Response, error) {
+	resp, err := s.next.RoundTrip(req)
+	if err == nil {
+		s.status.Store(int32(resp.StatusCode))
+	}
+	return resp, err
+}
+
+// discoveryError reports a failed discovery. go-oidc puts the provider's
+// response body in its error for a non-200 discovery response, so that
+// error is replaced by the status alone. Transport errors carry no response
+// content and keep their cause.
+func discoveryError(err error, status int) error {
+	var urlErr *url.Error
+	if !errors.As(err, &urlErr) && status != 0 && status != http.StatusOK {
+		return fmt.Errorf("auth: OIDC discovery: provider answered HTTP %d", status)
+	}
+	return fmt.Errorf("auth: OIDC discovery: %w", err)
+}
+
+// checkEndpoint applies secureurl.Check to the provider URL raw, naming the
+// endpoint in the error.
+func checkEndpoint(name, raw string) error {
+	u, err := url.Parse(raw)
+	if err != nil {
+		return fmt.Errorf("auth: OIDC %s %q is not a valid URL", name, raw)
+	}
+	if err := secureurl.Check(u); err != nil {
+		return fmt.Errorf("auth: OIDC %s %q: %w", name, raw, err)
+	}
+	return nil
+}
+
+// maxRedirects matches net/http's default redirect limit.
+const maxRedirects = 10
+
+// checkRedirect is the provider client's redirect policy. On a 307 or 308 to
+// the same host Go resends the body and Authorization header, which carry the
+// code and client secret, so every target must pass secureurl.Check and an
+// https:// request never follows a redirect to plain http://, even loopback.
+func checkRedirect(req *http.Request, via []*http.Request) error {
+	if len(via) >= maxRedirects {
+		return fmt.Errorf("stopped after %d redirects", maxRedirects)
+	}
+	if err := secureurl.Check(req.URL); err != nil {
+		return fmt.Errorf("redirect refused: %w", err)
+	}
+	if via[len(via)-1].URL.Scheme == "https" && req.URL.Scheme != "https" {
+		return errors.New("redirect refused: https:// to plain http:// downgrade")
+	}
+	return nil
+}
+
+// Mount registers GET LoginPath, GET CallbackPath and POST LogoutPath on r.
+func (a *Auth) Mount(r chi.Router) {
+	r.Get(LoginPath, a.login)
+	r.Get(CallbackPath, a.callback)
+	r.Post(LogoutPath, a.logout)
+}
+
+// EndSessionURL is the provider's end_session_endpoint.
+func (a *Auth) EndSessionURL() string { return a.endSessionURL }
+
+// login starts the authorization code flow with PKCE (S256). The optional
+// "return" query parameter must be a same-origin path; anything else fails
+// the login with 400.
+func (a *Auth) login(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Cache-Control", "no-store")
+
+	returnTo, err := returnPath(r.URL.Query().Get("return"))
+	if err != nil {
+		http.Error(w, "invalid return path", http.StatusBadRequest)
+		return
+	}
+	state := loginState{
+		State:    rand.Text(),
+		Nonce:    rand.Text(),
+		Verifier: oauth2.GenerateVerifier(),
+		Return:   returnTo,
+	}
+	if err := a.cookie.set(w, state, time.Now()); err != nil {
+		a.logger.ErrorContext(r.Context(), "set login cookie", "error", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	http.Redirect(w, r, a.oauth.AuthCodeURL(state.State,
+		oidc.Nonce(state.Nonce),
+		oauth2.S256ChallengeOption(state.Verifier),
+	), http.StatusFound)
+}
+
+// callback completes the flow: it checks state against the login cookie,
+// exchanges the code, verifies the ID token and nonce, then starts a session
+// if the account is in an allowed group. Membership is checked on every
+// login, so an account removed from the group is refused at its next one.
+func (a *Auth) callback(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Cache-Control", "no-store")
+	ctx := r.Context()
+	// The login cookie is single-use.
+	a.cookie.clear(w)
+
+	state, err := a.cookie.read(r, time.Now())
+	if err != nil {
+		a.logger.WarnContext(ctx, "login callback without a valid login cookie", "error", err)
+		http.Error(w, "sign-in expired; start again", http.StatusBadRequest)
+		return
+	}
+	q := r.URL.Query()
+	if subtle.ConstantTimeCompare([]byte(q.Get("state")), []byte(state.State)) != 1 {
+		a.logger.WarnContext(ctx, "login callback state mismatch")
+		http.Error(w, "sign-in state mismatch; start again", http.StatusBadRequest)
+		return
+	}
+	if q.Get("error") != "" {
+		// error and error_description are provider-supplied text; only the
+		// fixed category is logged.
+		a.logger.WarnContext(ctx, "identity provider returned an authorization error")
+		http.Error(w, "sign-in was refused", http.StatusUnauthorized)
+		return
+	}
+	code := q.Get("code")
+	if code == "" {
+		http.Error(w, "missing authorization code", http.StatusBadRequest)
+		return
+	}
+
+	claims, err := a.verify(ctx, code, state)
+	if err != nil {
+		a.logger.WarnContext(ctx, "sign-in failed", "error", err)
+		http.Error(w, "sign-in failed", http.StatusUnauthorized)
+		return
+	}
+	if !a.member(claims.Groups) {
+		if claims.Groups == nil {
+			a.logger.WarnContext(ctx, "sign-in refused: no groups claim in the ID token or userinfo", "subject", claims.Subject)
+		} else {
+			a.logger.WarnContext(ctx, "sign-in refused: not in an allowed group", "subject", claims.Subject)
+		}
+		// A session this browser already holds ends too, so a refused
+		// sign-in never leaves it signed in.
+		if err := a.sessions.EndSession(w, r); err != nil {
+			a.logger.ErrorContext(ctx, "end session", "error", err)
+			http.Error(w, "internal error", http.StatusInternalServerError)
+			return
+		}
+		a.refuse(w, r)
+		return
+	}
+	if err := a.sessions.StartSession(w, r, claims); err != nil {
+		a.logger.ErrorContext(ctx, "start session", "error", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	http.Redirect(w, r, state.Return, http.StatusSeeOther)
+}
+
+// logout ends the session, clears the cookie and sends the browser to the
+// provider's end_session_endpoint to end the provider session as well.
+func (a *Auth) logout(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Cache-Control", "no-store")
+	if err := a.sessions.EndSession(w, r); err != nil {
+		a.logger.ErrorContext(r.Context(), "end session", "error", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	http.Redirect(w, r, a.endSessionURL, http.StatusSeeOther)
+}
+
+// verify exchanges code for tokens and returns the verified ID token claims.
+// The verifier enforces RS256, issuer, audience and expiry; nonce is checked
+// here. When the ID token has no groups claim, Groups comes from userinfo;
+// it stays nil when neither has one.
+func (a *Auth) verify(ctx context.Context, code string, state loginState) (Claims, error) {
+	token, err := a.oauth.Exchange(context.WithValue(ctx, oauth2.HTTPClient, a.client), code,
+		oauth2.VerifierOption(state.Verifier))
+	if err != nil {
+		return Claims{}, exchangeError(err)
+	}
+	raw, ok := token.Extra("id_token").(string)
+	if !ok || raw == "" {
+		return Claims{}, errors.New("token response has no id_token")
+	}
+	idToken, err := a.verifier.Verify(ctx, raw)
+	if err != nil {
+		return Claims{}, verifyError(err)
+	}
+	if subtle.ConstantTimeCompare([]byte(idToken.Nonce), []byte(state.Nonce)) != 1 {
+		return Claims{}, errors.New("ID token nonce mismatch")
+	}
+	var claims Claims
+	if err := idToken.Claims(&claims); err != nil {
+		return Claims{}, fmt.Errorf("decode ID token claims: %w", err)
+	}
+	if claims.Subject == "" {
+		return Claims{}, errors.New("ID token has no subject")
+	}
+	if claims.Groups == nil {
+		groups, err := a.userinfoGroups(ctx, token, claims.Subject)
+		if err != nil {
+			return Claims{}, err
+		}
+		claims.Groups = groups
+	}
+	return claims, nil
+}
+
+// userinfoGroups returns the groups claim from the userinfo endpoint, nil if
+// it has none. The userinfo subject must match the ID token's subject.
+func (a *Auth) userinfoGroups(ctx context.Context, token *oauth2.Token, subject string) ([]string, error) {
+	info, err := a.provider.UserInfo(oidc.ClientContext(ctx, a.client), oauth2.StaticTokenSource(token))
+	if err != nil {
+		return nil, userinfoError(err)
+	}
+	if info.Subject != subject {
+		return nil, errors.New("userinfo subject does not match the ID token")
+	}
+	var claims struct {
+		Groups []string `json:"groups"`
+	}
+	if err := info.Claims(&claims); err != nil {
+		return nil, fmt.Errorf("decode userinfo claims: %w", err)
+	}
+	return claims.Groups, nil
+}
+
+// userinfoError reduces a failed userinfo request to a fixed category, like
+// exchangeError: go-oidc's error text can carry the response body.
+func userinfoError(err error) error {
+	var transport *url.Error
+	if errors.As(err, &transport) {
+		return fmt.Errorf("userinfo: request failed: %s: %w", transport.Op, transport.Err)
+	}
+	return errors.New("userinfo: endpoint returned an unusable response")
+}
+
+// verifyError reduces a failed ID token verification to a fixed category.
+// go-oidc's error text can carry the JWKS endpoint's response body or
+// token-supplied values, so only the expiry of an expired token is kept.
+func verifyError(err error) error {
+	var expired *oidc.TokenExpiredError
+	if errors.As(err, &expired) {
+		return fmt.Errorf("verify ID token: token expired at %s", expired.Expiry.UTC().Format(time.RFC3339))
+	}
+	return errors.New("verify ID token: token or provider key set rejected")
+}
+
+// member reports whether groups contains an allowed group.
+func (a *Auth) member(groups []string) bool {
+	return slices.ContainsFunc(groups, func(g string) bool { return slices.Contains(a.allowed, g) })
+}
+
+// exchangeError reduces a failed code exchange to a fixed category and safe
+// metadata. The oauth2 error text can carry the token endpoint's response
+// body or error_description, which may echo credentials, so it is dropped.
+func exchangeError(err error) error {
+	var retrieve *oauth2.RetrieveError
+	var transport *url.Error
+	switch {
+	case errors.As(err, &retrieve):
+		return fmt.Errorf("exchange code: token endpoint rejected the request (HTTP %d)", retrieve.Response.StatusCode)
+	case errors.As(err, &transport):
+		// Request failures never carry the response. The URL is dropped: after
+		// a refused redirect it is the provider-supplied Location.
+		return fmt.Errorf("exchange code: token request failed: %s: %w", transport.Op, transport.Err)
+	default:
+		return errors.New("exchange code: token endpoint returned an unusable response")
+	}
+}
diff --git a/internal/auth/auth_test.go b/internal/auth/auth_test.go
new file mode 100644
index 0000000..407bbb8
--- /dev/null
+++ b/internal/auth/auth_test.go
@@ -0,0 +1,1075 @@
+package auth
+
+import (
+	"bytes"
+	"context"
+	"crypto"
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"maps"
+	"math/big"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"slices"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+)
+
+const (
+	testClientID     = "mealquest"
+	testClientSecret = "client-s3cret"
+	testKID          = "test-key"
+)
+
+var testPublicURL = &url.URL{Scheme: "https", Host: "meals.example.com"}
+
+// testAllowedGroups are the household's groups in tests.
+var testAllowedGroups = []string{"mealquest-admin", "mealquest-family"}
+
+// fakeProvider is an OIDC provider serving discovery, JWKS and a token
+// endpoint. Each issued code maps to the ID token the token endpoint returns.
+type fakeProvider struct {
+	t   *testing.T
+	srv *httptest.Server
+	key *rsa.PrivateKey
+
+	mu    sync.Mutex
+	codes map[string]issuedCode
+	// discoveryOverrides replace fields of the discovery document.
+	discoveryOverrides map[string]any
+	// tokenReply, when set, is returned by the token endpoint instead of a
+	// token.
+	tokenReply *tokenReply
+	// userinfo is the userinfo endpoint's response; userinfoHits counts
+	// requests to it.
+	userinfo     map[string]any
+	userinfoHits int
+	// userinfoReply, when set, is returned by the userinfo endpoint instead.
+	userinfoReply *tokenReply
+	// jwksReply, when set, is returned by the JWKS endpoint instead.
+	jwksReply *tokenReply
+}
+
+type tokenReply struct {
+	status int
+	body   string
+}
+
+type issuedCode struct {
+	challenge string
+	idToken   string
+}
+
+func newFakeProvider(t *testing.T) *fakeProvider {
+	t.Helper()
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		t.Fatal(err)
+	}
+	p := &fakeProvider{t: t, key: key, codes: map[string]issuedCode{}}
+	mux := http.NewServeMux()
+	mux.HandleFunc("GET /.well-known/openid-configuration", p.discovery)
+	mux.HandleFunc("GET /jwks", p.jwks)
+	mux.HandleFunc("POST /token", p.token)
+	mux.HandleFunc("GET /userinfo", p.userinfoEndpoint)
+	p.srv = httptest.NewServer(mux)
+	t.Cleanup(p.srv.Close)
+	return p
+}
+
+func (p *fakeProvider) issuer() string { return p.srv.URL }
+
+func (p *fakeProvider) discovery(w http.ResponseWriter, _ *http.Request) {
+	doc := map[string]any{
+		"issuer":                                p.issuer(),
+		"authorization_endpoint":                p.issuer() + "/authorize",
+		"token_endpoint":                        p.issuer() + "/token",
+		"jwks_uri":                              p.issuer() + "/jwks",
+		"userinfo_endpoint":                     p.issuer() + "/userinfo",
+		"end_session_endpoint":                  p.issuer() + "/end-session/",
+		"response_types_supported":              []string{"code"},
+		"subject_types_supported":               []string{"public"},
+		"id_token_signing_alg_values_supported": []string{"RS256", "HS256"},
+	}
+	p.mu.Lock()
+	maps.Copy(doc, p.discoveryOverrides)
+	p.mu.Unlock()
+	writeJSON(w, doc)
+}
+
+func (p *fakeProvider) jwks(w http.ResponseWriter, _ *http.Request) {
+	p.mu.Lock()
+	reply := p.jwksReply
+	p.mu.Unlock()
+	if reply != nil {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(reply.status)
+		io.WriteString(w, reply.body)
+		return
+	}
+	writeJSON(w, map[string]any{"keys": []map[string]string{{
+		"kty": "RSA",
+		"kid": testKID,
+		"use": "sig",
+		"alg": "RS256",
+		"n":   b64(p.key.N.Bytes()),
+		"e":   b64(big.NewInt(int64(p.key.E)).Bytes()),
+	}}})
+}
+
+// token checks client authentication, the code and the PKCE verifier before
+// returning the code's ID token.
+func (p *fakeProvider) token(w http.ResponseWriter, r *http.Request) {
+	p.mu.Lock()
+	reply := p.tokenReply
+	p.mu.Unlock()
+	if reply != nil {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(reply.status)
+		io.WriteString(w, reply.body)
+		return
+	}
+	id, secret, ok := r.BasicAuth()
+	if !ok || id != testClientID || secret != testClientSecret {
+		http.Error(w, `{"error":"invalid_client"}`, http.StatusUnauthorized)
+		return
+	}
+	if err := r.ParseForm(); err != nil {
+		http.Error(w, `{"error":"invalid_request"}`, http.StatusBadRequest)
+		return
+	}
+	p.mu.Lock()
+	issued, ok := p.codes[r.PostForm.Get("code")]
+	delete(p.codes, r.PostForm.Get("code"))
+	p.mu.Unlock()
+	if r.PostForm.Get("grant_type") != "authorization_code" || !ok ||
+		challengeS256(r.PostForm.Get("code_verifier")) != issued.challenge {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusBadRequest)
+		io.WriteString(w, `{"error":"invalid_grant"}`)
+		return
+	}
+	writeJSON(w, map[string]any{
+		"access_token": "access-token",
+		"token_type":   "Bearer",
+		"expires_in":   3600,
+		"id_token":     issued.idToken,
+	})
+}
+
+// userinfoEndpoint checks the access token and returns p.userinfo, which
+// defaults to the test user without a groups claim.
+func (p *fakeProvider) userinfoEndpoint(w http.ResponseWriter, r *http.Request) {
+	if r.Header.Get("Authorization") != "Bearer access-token" {
+		http.Error(w, `{"error":"invalid_token"}`, http.StatusUnauthorized)
+		return
+	}
+	p.mu.Lock()
+	p.userinfoHits++
+	info, reply := p.userinfo, p.userinfoReply
+	p.mu.Unlock()
+	if reply != nil {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(reply.status)
+		io.WriteString(w, reply.body)
+		return
+	}
+	if info == nil {
+		info = map[string]any{"sub": "user-123", "email": "cook@example.com"}
+	}
+	writeJSON(w, info)
+}
+
+func (p *fakeProvider) setUserinfo(info map[string]any) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	p.userinfo = info
+}
+
+func (p *fakeProvider) userinfoRequests() int {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	return p.userinfoHits
+}
+
+// authorize stands in for the user signing in at the provider: it records a
+// code bound to the PKCE challenge and returns it.
+func (p *fakeProvider) authorize(challenge, idToken string) string {
+	code := rand.Text()
+	p.mu.Lock()
+	p.codes[code] = issuedCode{challenge: challenge, idToken: idToken}
+	p.mu.Unlock()
+	return code
+}
+
+// claims returns valid ID token claims for nonce.
+func (p *fakeProvider) claims(nonce string) map[string]any {
+	now := time.Now()
+	return map[string]any{
+		"iss":                p.issuer(),
+		"sub":                "user-123",
+		"aud":                testClientID,
+		"iat":                now.Unix(),
+		"exp":                now.Add(5 * time.Minute).Unix(),
+		"nonce":              nonce,
+		"email":              "cook@example.com",
+		"email_verified":     true,
+		"name":               "Home Cook",
+		"preferred_username": "cook",
+		"groups":             []string{"mealquest-admin"},
+	}
+}
+
+func (p *fakeProvider) signRS256(claims map[string]any) string {
+	return signRS256(p.t, p.key, claims)
+}
+
+func signRS256(t *testing.T, key *rsa.PrivateKey, claims map[string]any) string {
+	t.Helper()
+	signing := jwtSigningInput(t, "RS256", claims)
+	digest := sha256.Sum256([]byte(signing))
+	sig, err := rsa.SignPKCS1v15(rand.Reader, key, crypto.SHA256, digest[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	return signing + "." + b64(sig)
+}
+
+// signHS256 signs with the client secret, as in an algorithm-confusion attack.
+func (p *fakeProvider) signHS256(claims map[string]any) string {
+	signing := jwtSigningInput(p.t, "HS256", claims)
+	mac := hmac.New(sha256.New, []byte(testClientSecret))
+	mac.Write([]byte(signing))
+	return signing + "." + b64(mac.Sum(nil))
+}
+
+func jwtSigningInput(t *testing.T, alg string, claims map[string]any) string {
+	t.Helper()
+	header, err := json.Marshal(map[string]string{"alg": alg, "typ": "JWT", "kid": testKID})
+	if err != nil {
+		t.Fatal(err)
+	}
+	body, err := json.Marshal(claims)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b64(header) + "." + b64(body)
+}
+
+func challengeS256(verifier string) string {
+	sum := sha256.Sum256([]byte(verifier))
+	return b64(sum[:])
+}
+
+func b64(b []byte) string { return base64.RawURLEncoding.EncodeToString(b) }
+
+func writeJSON(w http.ResponseWriter, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(v)
+}
+
+// memorySessions is an in-memory Sessions that records the claims it was
+// handed and how many sessions were ended.
+type memorySessions struct {
+	mu     sync.Mutex
+	claims []Claims
+	ended  int
+	err    error
+}
+
+func (m *memorySessions) StartSession(w http.ResponseWriter, _ *http.Request, claims Claims) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.err != nil {
+		return m.err
+	}
+	m.claims = append(m.claims, claims)
+	http.SetCookie(w, &http.Cookie{Name: "test_session", Value: claims.Subject})
+	return nil
+}
+
+func (m *memorySessions) EndSession(http.ResponseWriter, *http.Request) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.err != nil {
+		return m.err
+	}
+	m.ended++
+	return nil
+}
+
+func (m *memorySessions) started() []Claims {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return slices.Clone(m.claims)
+}
+
+type harness struct {
+	auth     *Auth
+	provider *fakeProvider
+	sessions *memorySessions
+	router   http.Handler
+	logs     *syncBuffer
+}
+
+// syncBuffer is a bytes.Buffer safe for concurrent log writes.
+type syncBuffer struct {
+	mu  sync.Mutex
+	buf bytes.Buffer
+}
+
+func (b *syncBuffer) Write(p []byte) (int, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.Write(p)
+}
+
+func (b *syncBuffer) String() string {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.String()
+}
+
+func newHarness(t *testing.T) *harness {
+	t.Helper()
+	p := newFakeProvider(t)
+	sessions := &memorySessions{}
+	logs := &syncBuffer{}
+	a, err := New(context.Background(), Config{
+		Issuer:        p.issuer(),
+		ClientID:      testClientID,
+		ClientSecret:  testClientSecret,
+		PublicURL:     testPublicURL,
+		SecretKey:     []byte("server-secret-key"),
+		AllowedGroups: testAllowedGroups,
+	}, sessions, slog.New(slog.NewTextHandler(logs, nil)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := chi.NewRouter()
+	a.Mount(r)
+	return &harness{auth: a, provider: p, sessions: sessions, router: r, logs: logs}
+}
+
+func (h *harness) get(target string, cookies ...*http.Cookie) *http.Response {
+	req := httptest.NewRequest(http.MethodGet, target, nil)
+	for _, c := range cookies {
+		req.AddCookie(c)
+	}
+	rec := httptest.NewRecorder()
+	h.router.ServeHTTP(rec, req)
+	return rec.Result()
+}
+
+// login is a started login: the authorize URL's query and the login cookie.
+type login struct {
+	query  url.Values
+	cookie *http.Cookie
+}
+
+func (h *harness) login(t *testing.T, target string) login {
+	t.Helper()
+	resp := h.get(target)
+	if resp.StatusCode != http.StatusFound {
+		t.Fatalf("GET %s status = %d, want 302", target, resp.StatusCode)
+	}
+	loc, err := url.Parse(resp.Header.Get("Location"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := loc.Scheme+"://"+loc.Host+loc.Path, h.provider.issuer()+"/authorize"; got != want {
+		t.Fatalf("redirect to %s, want %s", got, want)
+	}
+	var cookie *http.Cookie
+	for _, c := range resp.Cookies() {
+		if c.Name == loginCookieName {
+			cookie = c
+		}
+	}
+	if cookie == nil {
+		t.Fatalf("login set no %s cookie", loginCookieName)
+	}
+	return login{query: loc.Query(), cookie: cookie}
+}
+
+// callback completes l at the provider with idToken and calls the callback.
+func (h *harness) callback(l login, state, idToken string) *http.Response {
+	code := h.provider.authorize(l.query.Get("code_challenge"), idToken)
+	q := url.Values{"code": {code}, "state": {state}}
+	return h.get(CallbackPath+"?"+q.Encode(), l.cookie)
+}
+
+func TestLoginRedirect(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	l := h.login(t, LoginPath+"?return=/plan")
+
+	want := map[string]string{
+		"client_id":             testClientID,
+		"response_type":         "code",
+		"scope":                 "openid profile email groups offline_access",
+		"redirect_uri":          "https://meals.example.com/auth/callback",
+		"code_challenge_method": "S256",
+	}
+	for k, v := range want {
+		if got := l.query.Get(k); got != v {
+			t.Errorf("authorize %s = %q, want %q", k, got, v)
+		}
+	}
+	for _, k := range []string{"state", "nonce", "code_challenge"} {
+		if l.query.Get(k) == "" {
+			t.Errorf("authorize URL has no %s", k)
+		}
+	}
+
+	c := l.cookie
+	if !c.HttpOnly || !c.Secure || c.SameSite != http.SameSiteLaxMode || c.Path != "/" {
+		t.Errorf("login cookie HttpOnly=%v Secure=%v SameSite=%v Path=%q, want HttpOnly, Secure, Lax, /",
+			c.HttpOnly, c.Secure, c.SameSite, c.Path)
+	}
+	if c.MaxAge <= 0 || c.MaxAge > 600 {
+		t.Errorf("login cookie MaxAge = %d, want 1-600 seconds", c.MaxAge)
+	}
+}
+
+func TestLoginStateIsFreshEachTime(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	a, b := h.login(t, LoginPath), h.login(t, LoginPath)
+	for _, k := range []string{"state", "nonce", "code_challenge"} {
+		if a.query.Get(k) == b.query.Get(k) {
+			t.Errorf("%s repeated across logins", k)
+		}
+	}
+}
+
+func TestCallbackSuccess(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	l := h.login(t, LoginPath+"?return="+url.QueryEscape("/plan?week=2026-W40"))
+
+	resp := h.callback(l, l.query.Get("state"), h.provider.signRS256(h.provider.claims(l.query.Get("nonce"))))
+
+	if resp.StatusCode != http.StatusSeeOther {
+		t.Fatalf("callback status = %d, want 303", resp.StatusCode)
+	}
+	if got := resp.Header.Get("Location"); got != "/plan?week=2026-W40" {
+		t.Errorf("callback redirect = %q, want the return path", got)
+	}
+	started := h.sessions.started()
+	want := Claims{
+		Subject:           "user-123",
+		Email:             "cook@example.com",
+		EmailVerified:     true,
+		Name:              "Home Cook",
+		PreferredUsername: "cook",
+		Groups:            []string{"mealquest-admin"},
+	}
+	if len(started) != 1 || started[0].Subject != want.Subject || started[0].Email != want.Email ||
+		started[0].Name != want.Name || started[0].PreferredUsername != want.PreferredUsername ||
+		!started[0].EmailVerified || !slices.Equal(started[0].Groups, want.Groups) {
+		t.Errorf("session started with %+v, want [%+v]", started, want)
+	}
+	assertLoginCookieCleared(t, resp)
+}
+
+func TestCallbackDefaultReturnPath(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	l := h.login(t, LoginPath)
+	resp := h.callback(l, l.query.Get("state"), h.provider.signRS256(h.provider.claims(l.query.Get("nonce"))))
+	if resp.StatusCode != http.StatusSeeOther || resp.Header.Get("Location") != "/" {
+		t.Errorf("callback = %d to %q, want 303 to /", resp.StatusCode, resp.Header.Get("Location"))
+	}
+}
+
+func TestCallbackRejected(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		// state returns the state sent to the callback.
+		state func(l login) string
+		// token returns the ID token the provider issues.
+		token      func(p *fakeProvider, l login) string
+		wantStatus int
+	}{
+		{
+			name:       "mismatched state",
+			state:      func(login) string { return "attacker-state" },
+			wantStatus: http.StatusBadRequest,
+		},
+		{
+			name: "wrong nonce",
+			token: func(p *fakeProvider, _ login) string {
+				return p.signRS256(p.claims("other-nonce"))
+			},
+			wantStatus: http.StatusUnauthorized,
+		},
+		{
+			name: "missing nonce",
+			token: func(p *fakeProvider, _ login) string {
+				c := p.claims("")
+				delete(c, "nonce")
+				return p.signRS256(c)
+			},
+			wantStatus: http.StatusUnauthorized,
+		},
+		{
+			name: "HS256-signed token",
+			token: func(p *fakeProvider, l login) string {
+				return p.signHS256(p.claims(l.query.Get("nonce")))
+			},
+			wantStatus: http.StatusUnauthorized,
+		},
+		{
+			name: "expired token",
+			token: func(p *fakeProvider, l login) string {
+				c := p.claims(l.query.Get("nonce"))
+				c["iat"] = time.Now().Add(-2 * time.Hour).Unix()
+				c["exp"] = time.Now().Add(-time.Hour).Unix()
+				return p.signRS256(c)
+			},
+			wantStatus: http.StatusUnauthorized,
+		},
+		{
+			name: "wrong issuer",
+			token: func(p *fakeProvider, l login) string {
+				c := p.claims(l.query.Get("nonce"))
+				c["iss"] = "https://evil.example.com"
+				return p.signRS256(c)
+			},
+			wantStatus: http.StatusUnauthorized,
+		},
+		{
+			name: "wrong audience",
+			token: func(p *fakeProvider, l login) string {
+				c := p.claims(l.query.Get("nonce"))
+				c["aud"] = "another-client"
+				return p.signRS256(c)
+			},
+			wantStatus: http.StatusUnauthorized,
+		},
+		{
+			name: "signed by an unknown key",
+			token: func(p *fakeProvider, l login) string {
+				key, err := rsa.GenerateKey(rand.Reader, 2048)
+				if err != nil {
+					p.t.Fatal(err)
+				}
+				return signRS256(p.t, key, p.claims(l.query.Get("nonce")))
+			},
+			wantStatus: http.StatusUnauthorized,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			h := newHarness(t)
+			l := h.login(t, LoginPath)
+			state := l.query.Get("state")
+			if tc.state != nil {
+				state = tc.state(l)
+			}
+			token := h.provider.signRS256(h.provider.claims(l.query.Get("nonce")))
+			if tc.token != nil {
+				token = tc.token(h.provider, l)
+			}
+
+			resp := h.callback(l, state, token)
+
+			if resp.StatusCode != tc.wantStatus {
+				t.Errorf("callback status = %d, want %d", resp.StatusCode, tc.wantStatus)
+			}
+			if n := len(h.sessions.started()); n != 0 {
+				t.Errorf("%d sessions started, want none", n)
+			}
+			assertLoginCookieCleared(t, resp)
+		})
+	}
+}
+
+func TestCallbackWithoutLoginCookie(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	l := h.login(t, LoginPath)
+	l.cookie = &http.Cookie{Name: "unrelated", Value: "x"}
+	resp := h.callback(l, l.query.Get("state"), h.provider.signRS256(h.provider.claims(l.query.Get("nonce"))))
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("callback status = %d, want 400", resp.StatusCode)
+	}
+	if n := len(h.sessions.started()); n != 0 {
+		t.Errorf("%d sessions started, want none", n)
+	}
+}
+
+func TestCallbackTamperedLoginCookie(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	l := h.login(t, LoginPath)
+	payload, sig, _ := strings.Cut(l.cookie.Value, ".")
+	data, err := base64.RawURLEncoding.DecodeString(payload)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var s loginState
+	if err := json.Unmarshal(data, &s); err != nil {
+		t.Fatal(err)
+	}
+	s.Return = "/somewhere-else"
+	data, err = json.Marshal(s)
+	if err != nil {
+		t.Fatal(err)
+	}
+	l.cookie = &http.Cookie{Name: loginCookieName, Value: b64(data) + "." + sig}
+
+	resp := h.callback(l, l.query.Get("state"), h.provider.signRS256(h.provider.claims(l.query.Get("nonce"))))
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("callback status = %d, want 400", resp.StatusCode)
+	}
+}
+
+// The provider's error and error_description are untrusted text and never
+// reach the logs; only the fixed category does.
+func TestCallbackProviderError(t *testing.T) {
+	t.Parallel()
+	const (
+		errorMarker       = "provider-error-marker"
+		descriptionMarker = "description-marker-" + testClientSecret
+	)
+	h := newHarness(t)
+	l := h.login(t, LoginPath)
+	q := url.Values{
+		"error":             {errorMarker},
+		"error_description": {descriptionMarker},
+		"state":             {l.query.Get("state")},
+	}
+	resp := h.get(CallbackPath+"?"+q.Encode(), l.cookie)
+	if resp.StatusCode != http.StatusUnauthorized {
+		t.Errorf("callback status = %d, want 401", resp.StatusCode)
+	}
+	assertLoginCookieCleared(t, resp)
+	if n := len(h.sessions.started()); n != 0 {
+		t.Errorf("started %d sessions, want 0", n)
+	}
+	logs := h.logs.String()
+	for _, marker := range []string{errorMarker, descriptionMarker} {
+		if strings.Contains(logs, marker) {
+			t.Errorf("logs contain provider text %q: %s", marker, logs)
+		}
+	}
+	if !strings.Contains(logs, "identity provider returned an authorization error") {
+		t.Errorf("logs = %q, want the authorization error category", logs)
+	}
+}
+
+func TestCallbackSessionError(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	h.sessions.err = errors.New("database down")
+	l := h.login(t, LoginPath)
+	resp := h.callback(l, l.query.Get("state"), h.provider.signRS256(h.provider.claims(l.query.Get("nonce"))))
+	if resp.StatusCode != http.StatusInternalServerError {
+		t.Errorf("callback status = %d, want 500", resp.StatusCode)
+	}
+}
+
+// A failed code exchange is logged by category and status only: the token
+// endpoint's body and error_description may echo credentials.
+func TestCallbackExchangeFailureDoesNotLogProviderResponse(t *testing.T) {
+	t.Parallel()
+	const leaked = "leaked-" + testClientSecret
+	tests := []struct {
+		name     string
+		reply    tokenReply
+		wantLogs string
+	}{
+		{
+			name:     "error response",
+			reply:    tokenReply{http.StatusBadRequest, `{"error":"invalid_grant","error_description":"` + leaked + `"}`},
+			wantLogs: "HTTP 400",
+		},
+		{
+			name:     "unparseable error response",
+			reply:    tokenReply{http.StatusInternalServerError, leaked},
+			wantLogs: "HTTP 500",
+		},
+		{
+			name:     "success without access token",
+			reply:    tokenReply{http.StatusOK, `{"id_token":"` + leaked + `"}`},
+			wantLogs: "unusable response",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			h := newHarness(t)
+			h.provider.mu.Lock()
+			h.provider.tokenReply = &tc.reply
+			h.provider.mu.Unlock()
+			l := h.login(t, LoginPath)
+
+			resp := h.callback(l, l.query.Get("state"), "unused")
+
+			if resp.StatusCode != http.StatusUnauthorized {
+				t.Errorf("callback status = %d, want 401", resp.StatusCode)
+			}
+			logs := h.logs.String()
+			if strings.Contains(logs, leaked) {
+				t.Errorf("logs contain the provider response: %s", logs)
+			}
+			if !strings.Contains(logs, tc.wantLogs) {
+				t.Errorf("logs = %q, want %q", logs, tc.wantLogs)
+			}
+		})
+	}
+}
+
+func TestCallbackKeySetFailureDoesNotLogProviderResponse(t *testing.T) {
+	t.Parallel()
+	const sentinel = "sentinel-jwks-body-5c1e"
+	tests := []struct {
+		name  string
+		reply tokenReply
+	}{
+		{"error response", tokenReply{http.StatusInternalServerError, sentinel}},
+		{"malformed success", tokenReply{http.StatusOK, `{"keys":"` + sentinel + `"`}},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			h := newHarness(t)
+			h.provider.mu.Lock()
+			h.provider.jwksReply = &tc.reply
+			h.provider.mu.Unlock()
+
+			// The returned error, checked through verify directly.
+			l := h.login(t, LoginPath)
+			req := httptest.NewRequest(http.MethodGet, CallbackPath, nil)
+			req.AddCookie(l.cookie)
+			state, err := h.auth.cookie.read(req, time.Now())
+			if err != nil {
+				t.Fatal(err)
+			}
+			code := h.provider.authorize(l.query.Get("code_challenge"), h.provider.signRS256(h.provider.claims(l.query.Get("nonce"))))
+			_, err = h.auth.verify(context.Background(), code, state)
+			if err == nil {
+				t.Fatal("verify succeeded with an unusable key set")
+			}
+			if strings.Contains(err.Error(), sentinel) {
+				t.Errorf("verify error contains the JWKS response: %v", err)
+			}
+
+			// The logs and response, through the callback.
+			l = h.login(t, LoginPath)
+			resp := h.callback(l, l.query.Get("state"), h.provider.signRS256(h.provider.claims(l.query.Get("nonce"))))
+
+			if resp.StatusCode != http.StatusUnauthorized {
+				t.Errorf("callback status = %d, want 401", resp.StatusCode)
+			}
+			body, _ := io.ReadAll(resp.Body)
+			if strings.Contains(string(body), sentinel) {
+				t.Errorf("response contains the JWKS response: %s", body)
+			}
+			logs := h.logs.String()
+			if strings.Contains(logs, sentinel) {
+				t.Errorf("logs contain the JWKS response: %s", logs)
+			}
+			if want := "token or provider key set rejected"; !strings.Contains(logs, want) {
+				t.Errorf("logs = %q, want %q", logs, want)
+			}
+			if len(h.sessions.started()) != 0 {
+				t.Error("a session started")
+			}
+		})
+	}
+}
+
+func TestLoginCookieExpires(t *testing.T) {
+	t.Parallel()
+	c := newLoginCookie([]byte("server-secret-key"))
+	now := time.Now()
+	rec := httptest.NewRecorder()
+	if err := c.set(rec, loginState{State: "s", Nonce: "n", Verifier: "v", Return: "/"}, now); err != nil {
+		t.Fatal(err)
+	}
+	req := httptest.NewRequest(http.MethodGet, CallbackPath, nil)
+	req.AddCookie(rec.Result().Cookies()[0])
+
+	if _, err := c.read(req, now.Add(loginCookieTTL-time.Second)); err != nil {
+		t.Errorf("read before expiry: %v", err)
+	}
+	if _, err := c.read(req, now.Add(loginCookieTTL)); err == nil {
+		t.Error("read at expiry succeeded")
+	}
+}
+
+func TestLoginRejectsOffOriginReturn(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	for _, ret := range []string{
+		"https://evil.example.com/",
+		"http://meals.example.com/plan",
+		"//evil.example.com/",
+		"/\\evil.example.com",
+		"\\\\evil.example.com",
+		"javascript:alert(1)",
+		"plan",
+		"/plan\r\nSet-Cookie: x=y",
+	} {
+		resp := h.get(LoginPath + "?return=" + url.QueryEscape(ret))
+		if resp.StatusCode != http.StatusBadRequest {
+			t.Errorf("return=%q: status = %d, want 400", ret, resp.StatusCode)
+		}
+		if loc := resp.Header.Get("Location"); loc != "" {
+			t.Errorf("return=%q: redirected to %q", ret, loc)
+		}
+		if len(resp.Cookies()) != 0 {
+			t.Errorf("return=%q: set cookies %v", ret, resp.Cookies())
+		}
+	}
+}
+
+func TestNewFailsWhenDiscoveryFails(t *testing.T) {
+	t.Parallel()
+	srv := httptest.NewServer(http.NotFoundHandler())
+	t.Cleanup(srv.Close)
+	_, err := New(context.Background(), Config{
+		Issuer:        srv.URL,
+		ClientID:      testClientID,
+		ClientSecret:  testClientSecret,
+		PublicURL:     testPublicURL,
+		SecretKey:     []byte("k"),
+		AllowedGroups: testAllowedGroups,
+	}, &memorySessions{}, slog.New(slog.DiscardHandler))
+	if err == nil {
+		t.Fatal("New succeeded without a discovery document")
+	}
+}
+
+func TestNewDiscoveryErrorOmitsProviderBody(t *testing.T) {
+	t.Parallel()
+	const sentinel = "sentinel-discovery-body-7f3a"
+	for _, status := range []int{http.StatusNotFound, http.StatusInternalServerError} {
+		t.Run(http.StatusText(status), func(t *testing.T) {
+			t.Parallel()
+			srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				w.WriteHeader(status)
+				io.WriteString(w, sentinel)
+			}))
+			t.Cleanup(srv.Close)
+			logs := &syncBuffer{}
+			_, err := New(context.Background(), Config{
+				Issuer:        srv.URL,
+				ClientID:      testClientID,
+				ClientSecret:  testClientSecret,
+				PublicURL:     testPublicURL,
+				SecretKey:     []byte("k"),
+				AllowedGroups: testAllowedGroups,
+			}, &memorySessions{}, slog.New(slog.NewTextHandler(logs, nil)))
+			if err == nil {
+				t.Fatal("New succeeded without a discovery document")
+			}
+			// cmd/server logs the returned error at startup.
+			slog.New(slog.NewTextHandler(logs, nil)).Error("startup failed", "err", err)
+			if strings.Contains(logs.String(), sentinel) {
+				t.Errorf("logs contain the provider response body: %s", logs)
+			}
+			if want := fmt.Sprintf("auth: OIDC discovery: provider answered HTTP %d", status); err.Error() != want {
+				t.Errorf("error = %q, want %q", err, want)
+			}
+		})
+	}
+}
+
+func TestNewFailsOnIssuerMismatch(t *testing.T) {
+	t.Parallel()
+	p := newFakeProvider(t)
+	// Discovery trims the trailing slash, so this fetches the provider's
+	// document, which reports the issuer without it.
+	_, err := New(context.Background(), Config{
+		Issuer:        p.issuer() + "/",
+		ClientID:      testClientID,
+		ClientSecret:  testClientSecret,
+		PublicURL:     testPublicURL,
+		SecretKey:     []byte("k"),
+		AllowedGroups: testAllowedGroups,
+	}, &memorySessions{}, slog.New(slog.DiscardHandler))
+	if err == nil {
+		t.Fatal("New accepted a provider reporting a different issuer")
+	}
+}
+
+func assertLoginCookieCleared(t *testing.T, resp *http.Response) {
+	t.Helper()
+	for _, c := range resp.Cookies() {
+		if c.Name == loginCookieName && c.MaxAge < 0 {
+			return
+		}
+	}
+	t.Errorf("callback did not clear the %s cookie", loginCookieName)
+}
+
+func TestNewRejectsRemoteHTTPIssuer(t *testing.T) {
+	t.Parallel()
+	for _, issuer := range []string{"http://auth.example.com/application/o/mealquest/", "http://192.0.2.10"} {
+		_, err := New(context.Background(), Config{
+			Issuer:        issuer,
+			ClientID:      testClientID,
+			ClientSecret:  testClientSecret,
+			PublicURL:     testPublicURL,
+			SecretKey:     []byte("k"),
+			AllowedGroups: testAllowedGroups,
+		}, &memorySessions{}, slog.New(slog.DiscardHandler))
+		if err == nil || !strings.Contains(err.Error(), "issuer") {
+			t.Errorf("New(issuer %q) error = %v, want issuer rejected", issuer, err)
+		}
+	}
+}
+
+func TestNewRejectsRemoteHTTPEndpoint(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		field, name string
+	}{
+		{"authorization_endpoint", "authorization endpoint"},
+		{"token_endpoint", "token endpoint"},
+		{"jwks_uri", "JWKS endpoint"},
+		{"end_session_endpoint", "end session endpoint"},
+		{"userinfo_endpoint", "userinfo endpoint"},
+	}
+	for _, tc := range tests {
+		t.Run(tc.field, func(t *testing.T) {
+			t.Parallel()
+			p := newFakeProvider(t)
+			p.discoveryOverrides = map[string]any{tc.field: "http://auth.example.com/" + tc.field}
+			_, err := New(context.Background(), Config{
+				Issuer:        p.issuer(),
+				ClientID:      testClientID,
+				ClientSecret:  testClientSecret,
+				PublicURL:     testPublicURL,
+				SecretKey:     []byte("k"),
+				AllowedGroups: testAllowedGroups,
+			}, &memorySessions{}, slog.New(slog.DiscardHandler))
+			if err == nil || !strings.Contains(err.Error(), tc.name) {
+				t.Errorf("New error = %v, want %s rejected", err, tc.name)
+			}
+		})
+	}
+}
+
+// A token endpoint on https:// that redirects the code exchange to plain
+// http:// must not receive a follow-up request: Go would resend the client
+// secret in the Authorization header. The plaintext target is loopback, so
+// only the downgrade rule stops it.
+func TestCallbackRefusesTokenRedirectToHTTP(t *testing.T) {
+	t.Parallel()
+	for _, status := range []int{http.StatusTemporaryRedirect, http.StatusPermanentRedirect} {
+		t.Run(http.StatusText(status), func(t *testing.T) {
+			t.Parallel()
+			var plaintextHits atomic.Int32
+			plaintext := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				plaintextHits.Add(1)
+				http.Error(w, "unreachable", http.StatusTeapot)
+			}))
+			t.Cleanup(plaintext.Close)
+			var tlsHits atomic.Int32
+			tlsToken := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				tlsHits.Add(1)
+				http.Redirect(w, r, plaintext.URL+"/token", status)
+			}))
+			t.Cleanup(tlsToken.Close)
+
+			p := newFakeProvider(t)
+			p.discoveryOverrides = map[string]any{"token_endpoint": tlsToken.URL + "/token"}
+			sessions := &memorySessions{}
+			logs := &syncBuffer{}
+			a, err := New(context.Background(), Config{
+				Issuer:        p.issuer(),
+				ClientID:      testClientID,
+				ClientSecret:  testClientSecret,
+				PublicURL:     testPublicURL,
+				SecretKey:     []byte("server-secret-key"),
+				AllowedGroups: testAllowedGroups,
+			}, sessions, slog.New(slog.NewTextHandler(logs, nil)))
+			if err != nil {
+				t.Fatal(err)
+			}
+			// Trust the test server's certificate; the redirect policy stays.
+			a.client.Transport = tlsToken.Client().Transport
+			r := chi.NewRouter()
+			a.Mount(r)
+			h := &harness{provider: p, sessions: sessions, router: r, logs: logs}
+			l := h.login(t, LoginPath)
+
+			resp := h.callback(l, l.query.Get("state"), "unused")
+
+			if resp.StatusCode != http.StatusUnauthorized {
+				t.Errorf("callback status = %d, want 401", resp.StatusCode)
+			}
+			if n := tlsHits.Load(); n != 1 {
+				t.Errorf("https token endpoint hit %d times, want 1", n)
+			}
+			if n := plaintextHits.Load(); n != 0 {
+				t.Errorf("plaintext endpoint hit %d times, want 0", n)
+			}
+			if n := len(sessions.started()); n != 0 {
+				t.Errorf("started %d sessions, want 0", n)
+			}
+			if got := logs.String(); !strings.Contains(got, "downgrade") {
+				t.Errorf("logs = %q, want the downgrade refusal", got)
+			}
+		})
+	}
+}
+
+func TestCheckRedirect(t *testing.T) {
+	t.Parallel()
+	request := func(raw string) *http.Request {
+		req, err := http.NewRequest(http.MethodPost, raw, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return req
+	}
+	tests := []struct {
+		from, to string
+		ok       bool
+	}{
+		{"https://auth.example.com/token", "https://auth2.example.com/token", true},
+		{"http://127.0.0.1:9000/token", "http://localhost:9001/token", true},
+		{"http://127.0.0.1:9000/token", "https://auth.example.com/token", true},
+		{"https://auth.example.com/token", "http://auth.example.com/token", false},
+		{"https://127.0.0.1:9000/token", "http://127.0.0.1:9001/token", false},
+		{"http://127.0.0.1:9000/token", "http://auth.example.com/token", false},
+		{"https://auth.example.com/token", "ftp://auth.example.com/token", false},
+	}
+	for _, tc := range tests {
+		err := checkRedirect(request(tc.to), []*http.Request{request(tc.from)})
+		if (err == nil) != tc.ok {
+			t.Errorf("redirect %s -> %s: error = %v, want ok=%v", tc.from, tc.to, err, tc.ok)
+		}
+	}
+	via := make([]*http.Request, maxRedirects)
+	for i := range via {
+		via[i] = request("https://auth.example.com/token")
+	}
+	if err := checkRedirect(request("https://auth.example.com/token"), via); err == nil {
+		t.Errorf("redirect after %d hops: error = nil, want limit", maxRedirects)
+	}
+}
diff --git a/internal/auth/cookie.go b/internal/auth/cookie.go
new file mode 100644
index 0000000..4be2d7d
--- /dev/null
+++ b/internal/auth/cookie.go
@@ -0,0 +1,124 @@
+package auth
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+)
+
+const (
+	loginCookieName = "__Host-mealquest_login"
+	loginCookieTTL  = 10 * time.Minute
+	// loginCookieKeyLabel separates the login cookie's signing key from other
+	// keys derived from the same server secret.
+	loginCookieKeyLabel = "mealquest auth login cookie v1"
+)
+
+// loginState travels from login to callback in the signed login cookie.
+type loginState struct {
+	State    string `json:"s"`
+	Nonce    string `json:"n"`
+	Verifier string `json:"v"`
+	Return   string `json:"r"`
+	Expires  int64  `json:"e"` // Unix seconds
+}
+
+// loginCookie signs and verifies the login cookie with HMAC-SHA256.
+type loginCookie struct {
+	key []byte
+}
+
+func newLoginCookie(secretKey []byte) loginCookie {
+	mac := hmac.New(sha256.New, secretKey)
+	mac.Write([]byte(loginCookieKeyLabel))
+	return loginCookie{key: mac.Sum(nil)}
+}
+
+func (c loginCookie) sign(payload string) string {
+	mac := hmac.New(sha256.New, c.key)
+	mac.Write([]byte(payload))
+	return base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
+}
+
+func (c loginCookie) set(w http.ResponseWriter, s loginState, now time.Time) error {
+	s.Expires = now.Add(loginCookieTTL).Unix()
+	data, err := json.Marshal(s)
+	if err != nil {
+		return err
+	}
+	payload := base64.RawURLEncoding.EncodeToString(data)
+	http.SetCookie(w, &http.Cookie{
+		Name:     loginCookieName,
+		Value:    payload + "." + c.sign(payload),
+		Path:     "/",
+		MaxAge:   int(loginCookieTTL / time.Second),
+		Secure:   true,
+		HttpOnly: true,
+		// Lax so the cookie rides the provider's top-level redirect back.
+		SameSite: http.SameSiteLaxMode,
+	})
+	return nil
+}
+
+func (c loginCookie) clear(w http.ResponseWriter) {
+	http.SetCookie(w, &http.Cookie{
+		Name:     loginCookieName,
+		Path:     "/",
+		MaxAge:   -1,
+		Secure:   true,
+		HttpOnly: true,
+		SameSite: http.SameSiteLaxMode,
+	})
+}
+
+func (c loginCookie) read(r *http.Request, now time.Time) (loginState, error) {
+	cookie, err := r.Cookie(loginCookieName)
+	if err != nil {
+		return loginState{}, errors.New("login cookie missing")
+	}
+	payload, sig, ok := strings.Cut(cookie.Value, ".")
+	if !ok || !hmac.Equal([]byte(sig), []byte(c.sign(payload))) {
+		return loginState{}, errors.New("login cookie signature invalid")
+	}
+	data, err := base64.RawURLEncoding.DecodeString(payload)
+	if err != nil {
+		return loginState{}, fmt.Errorf("login cookie encoding: %w", err)
+	}
+	var s loginState
+	if err := json.Unmarshal(data, &s); err != nil {
+		return loginState{}, fmt.Errorf("login cookie payload: %w", err)
+	}
+	if now.Unix() >= s.Expires {
+		return loginState{}, errors.New("login cookie expired")
+	}
+	return s, nil
+}
+
+// returnPath validates the optional post-login return path. Empty means "/".
+// Only same-origin absolute paths are allowed: no scheme, host, protocol-
+// relative "//" prefix, backslashes or control characters.
+func returnPath(raw string) (string, error) {
+	if raw == "" {
+		return "/", nil
+	}
+	if !strings.HasPrefix(raw, "/") || strings.HasPrefix(raw, "//") || strings.ContainsRune(raw, '\\') {
+		return "", errors.New("return path must be a same-origin path")
+	}
+	for _, c := range raw {
+		if c < 0x20 || c == 0x7f {
+			return "", errors.New("return path contains a control character")
+		}
+	}
+	u, err := url.Parse(raw)
+	if err != nil || u.Scheme != "" || u.Host != "" || u.User != nil {
+		return "", errors.New("return path must be a same-origin path")
+	}
+	return raw, nil
+}
diff --git a/internal/auth/gate_test.go b/internal/auth/gate_test.go
new file mode 100644
index 0000000..0f18362
--- /dev/null
+++ b/internal/auth/gate_test.go
@@ -0,0 +1,264 @@
+package auth
+
+import (
+	"context"
+	"errors"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/url"
+	"slices"
+	"strings"
+	"testing"
+)
+
+// signInWith completes a login whose ID token has claims edited by edit.
+func (h *harness) signInWith(t *testing.T, edit func(claims map[string]any)) *http.Response {
+	t.Helper()
+	l := h.login(t, LoginPath)
+	c := h.provider.claims(l.query.Get("nonce"))
+	edit(c)
+	return h.callback(l, l.query.Get("state"), h.provider.signRS256(c))
+}
+
+// assertRefused checks resp is the refused-sign-in page and starts no
+// session.
+func assertRefused(t *testing.T, resp *http.Response) {
+	t.Helper()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("callback status = %d, want 403", resp.StatusCode)
+	}
+	if ct := resp.Header.Get("Content-Type"); ct != "text/html; charset=utf-8" {
+		t.Errorf("Content-Type = %q, want text/html; charset=utf-8", ct)
+	}
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, want := range []string{
+		"This kitchen is for the household only",
+		`<form method="post" action="/auth/logout">`,
+		"Sign out",
+	} {
+		if !strings.Contains(string(body), want) {
+			t.Errorf("refused page lacks %q:\n%s", want, body)
+		}
+	}
+	for _, c := range resp.Cookies() {
+		// Clearing a session cookie (negative Max-Age) is expected.
+		if (c.Name == SessionCookieName || c.Name == "test_session") && c.MaxAge >= 0 {
+			t.Errorf("refused sign-in set session cookie %s", c.Name)
+		}
+	}
+	assertLoginCookieCleared(t, resp)
+}
+
+func TestGroupGateMember(t *testing.T) {
+	t.Parallel()
+	// Any one allowed group is enough, alongside groups that are not allowed.
+	h := newHarness(t)
+	resp := h.signInWith(t, func(c map[string]any) { c["groups"] = []string{"guests", "mealquest-family"} })
+	if resp.StatusCode != http.StatusSeeOther {
+		t.Fatalf("callback status = %d, want 303", resp.StatusCode)
+	}
+	if started := h.sessions.started(); len(started) != 1 || !slices.Equal(started[0].Groups, []string{"guests", "mealquest-family"}) {
+		t.Errorf("sessions started = %+v, want one with the ID token groups", started)
+	}
+	if n := h.provider.userinfoRequests(); n != 0 {
+		t.Errorf("userinfo requested %d times, want 0 when the ID token has groups", n)
+	}
+}
+
+func TestGroupGateRefused(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name     string
+		groups   any // ID token groups claim; nil removes it
+		userinfo map[string]any
+	}{
+		{name: "non-member", groups: []string{"guests", "MEALQUEST-ADMIN"}},
+		{name: "empty groups claim", groups: []string{}},
+		{name: "groups claim missing from ID token and userinfo", groups: nil},
+		{
+			name:     "non-member in userinfo",
+			groups:   nil,
+			userinfo: map[string]any{"sub": "user-123", "groups": []string{"guests"}},
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			h := newHarness(t)
+			if tc.userinfo != nil {
+				h.provider.setUserinfo(tc.userinfo)
+			}
+			resp := h.signInWith(t, func(c map[string]any) {
+				if tc.groups == nil {
+					delete(c, "groups")
+				} else {
+					c["groups"] = tc.groups
+				}
+			})
+			assertRefused(t, resp)
+			if started := h.sessions.started(); len(started) != 0 {
+				t.Errorf("sessions started = %+v, want none", started)
+			}
+			if !strings.Contains(h.logs.String(), "sign-in refused") {
+				t.Errorf("refusal not logged:\n%s", h.logs.String())
+			}
+		})
+	}
+}
+
+func TestGroupGateUsesUserinfoWhenIDTokenHasNoGroups(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	h.provider.setUserinfo(map[string]any{"sub": "user-123", "groups": []string{"mealquest-admin"}})
+	resp := h.signInWith(t, func(c map[string]any) { delete(c, "groups") })
+	if resp.StatusCode != http.StatusSeeOther {
+		t.Fatalf("callback status = %d, want 303", resp.StatusCode)
+	}
+	if started := h.sessions.started(); len(started) != 1 || !slices.Equal(started[0].Groups, []string{"mealquest-admin"}) {
+		t.Errorf("sessions started = %+v, want one with the userinfo groups", started)
+	}
+	if n := h.provider.userinfoRequests(); n != 1 {
+		t.Errorf("userinfo requested %d times, want 1", n)
+	}
+}
+
+func TestGroupGateUserinfoFailures(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name     string
+		userinfo map[string]any
+	}{
+		{"subject mismatch", map[string]any{"sub": "someone-else", "groups": []string{"mealquest-admin"}}},
+		{"malformed groups", map[string]any{"sub": "user-123", "groups": "mealquest-admin"}},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			h := newHarness(t)
+			h.provider.setUserinfo(tc.userinfo)
+			resp := h.signInWith(t, func(c map[string]any) { delete(c, "groups") })
+			if resp.StatusCode != http.StatusUnauthorized {
+				t.Errorf("callback status = %d, want 401", resp.StatusCode)
+			}
+			if started := h.sessions.started(); len(started) != 0 {
+				t.Errorf("sessions started = %+v, want none", started)
+			}
+		})
+	}
+}
+
+// A failed userinfo request fails the sign-in without logging the
+// endpoint's response, which may echo credentials.
+func TestGroupGateUserinfoErrorNotLogged(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	h.provider.mu.Lock()
+	h.provider.userinfoReply = &tokenReply{status: http.StatusInternalServerError, body: `{"error":"leaked-s3cret"}`}
+	h.provider.mu.Unlock()
+	resp := h.signInWith(t, func(c map[string]any) { delete(c, "groups") })
+	if resp.StatusCode != http.StatusUnauthorized {
+		t.Errorf("callback status = %d, want 401", resp.StatusCode)
+	}
+	if started := h.sessions.started(); len(started) != 0 {
+		t.Errorf("sessions started = %+v, want none", started)
+	}
+	logs := h.logs.String()
+	if !strings.Contains(logs, "userinfo") || strings.Contains(logs, "leaked-s3cret") {
+		t.Errorf("logs = %s, want the userinfo failure without the response body", logs)
+	}
+}
+
+// Membership is checked at every login: after removal from the group, the
+// next login is refused and starts no session.
+func TestGroupGateRemovedMemberRefusedAtNextLogin(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	h.signIn(t)
+	if n := len(h.rows(t)); n != 1 {
+		t.Fatalf("sessions after first login = %d, want 1", n)
+	}
+
+	resp := h.signInWith(t, func(c map[string]any) { c["groups"] = []string{"former-household"} })
+	assertRefused(t, resp)
+	if n := len(h.rows(t)); n != 1 {
+		t.Errorf("sessions after refused login = %d, want still 1", n)
+	}
+}
+
+// A refused sign-in from a browser that still holds a session ends that
+// session and clears its cookie.
+func TestGroupGateRefusalEndsExistingSession(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	_, cookie := h.signIn(t)
+
+	l := h.login(t, LoginPath)
+	c := h.provider.claims(l.query.Get("nonce"))
+	c["groups"] = []string{"former-household"}
+	code := h.provider.authorize(l.query.Get("code_challenge"), h.provider.signRS256(c))
+	q := url.Values{"code": {code}, "state": {l.query.Get("state")}}
+	resp := h.get(CallbackPath+"?"+q.Encode(), l.cookie, cookie)
+
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("callback status = %d, want 403", resp.StatusCode)
+	}
+	if n := len(h.rows(t)); n != 0 {
+		t.Errorf("sessions after refused login = %d, want 0", n)
+	}
+	if _, err := h.use(cookie); !errors.Is(err, ErrNoSession) {
+		t.Errorf("old session lookup error = %v, want ErrNoSession", err)
+	}
+	cleared := false
+	for _, c := range resp.Cookies() {
+		cleared = cleared || (c.Name == SessionCookieName && c.MaxAge < 0)
+	}
+	if !cleared {
+		t.Error("refused sign-in did not clear the session cookie")
+	}
+}
+
+func TestGroupGateRefusalEndSessionError(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	h.sessions.err = errors.New("database down")
+	resp := h.signInWith(t, func(c map[string]any) { c["groups"] = []string{"guests"} })
+	if resp.StatusCode != http.StatusInternalServerError {
+		t.Errorf("callback status = %d, want 500", resp.StatusCode)
+	}
+}
+
+func TestNewRequiresAllowedGroups(t *testing.T) {
+	t.Parallel()
+	p := newFakeProvider(t)
+	_, err := New(context.Background(), Config{
+		Issuer:       p.issuer(),
+		ClientID:     testClientID,
+		ClientSecret: testClientSecret,
+		PublicURL:    testPublicURL,
+		SecretKey:    []byte("k"),
+	}, &memorySessions{}, slog.New(slog.DiscardHandler))
+	if err == nil || !strings.Contains(err.Error(), "allowed group") {
+		t.Errorf("New error = %v, want missing allowed groups named", err)
+	}
+}
+
+func TestNewRequiresUserinfoEndpoint(t *testing.T) {
+	t.Parallel()
+	p := newFakeProvider(t)
+	p.discoveryOverrides = map[string]any{"userinfo_endpoint": nil}
+	_, err := New(context.Background(), Config{
+		Issuer:        p.issuer(),
+		ClientID:      testClientID,
+		ClientSecret:  testClientSecret,
+		PublicURL:     testPublicURL,
+		SecretKey:     []byte("k"),
+		AllowedGroups: testAllowedGroups,
+	}, &memorySessions{}, slog.New(slog.DiscardHandler))
+	if err == nil || !strings.Contains(err.Error(), "userinfo endpoint") {
+		t.Errorf("New error = %v, want the missing userinfo endpoint named", err)
+	}
+}
diff --git a/internal/auth/refused.go b/internal/auth/refused.go
new file mode 100644
index 0000000..399e705
--- /dev/null
+++ b/internal/auth/refused.go
@@ -0,0 +1,37 @@
+package auth
+
+import (
+	"io"
+	"net/http"
+)
+
+// refusedPage is shown when a sign-in succeeds at the provider but the
+// account is not in the household's group. Signing out ends the provider
+// session too, so another account can sign in.
+const refusedPage = `<!doctype html>
+<html lang="en">
+<head>
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<title>MealQuest</title>
+</head>
+<body>
+<main>
+<h1>This kitchen is for the household only</h1>
+<p>The account you signed in with is not part of our kitchen.</p>
+<form method="post" action="` + LogoutPath + `">
+<button type="submit">Sign out</button>
+</form>
+</main>
+</body>
+</html>
+`
+
+// refuse writes the refused-sign-in page with 403.
+func (a *Auth) refuse(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	w.WriteHeader(http.StatusForbidden)
+	if _, err := io.WriteString(w, refusedPage); err != nil {
+		a.logger.WarnContext(r.Context(), "write refused sign-in page", "error", err)
+	}
+}
diff --git a/internal/auth/session.go b/internal/auth/session.go
new file mode 100644
index 0000000..89d061e
--- /dev/null
+++ b/internal/auth/session.go
@@ -0,0 +1,238 @@
+package auth
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"database/sql"
+	"encoding/base64"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"time"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/store"
+)
+
+// SessionCookieName is the session cookie. The __Host- prefix makes the
+// browser insist on Secure, Path=/ and no Domain.
+const SessionCookieName = "__Host-mealquest_session"
+
+// sessionIDBytes is the size of the random session ID: 256 bits.
+const sessionIDBytes = 32
+
+// ErrNoSession means the request carries no live session: the cookie is
+// missing or malformed, or its session is unknown, ended or expired.
+var ErrNoSession = errors.New("auth: no valid session")
+
+// Session is a signed-in household member's server-side session.
+type Session struct {
+	Subject   string
+	Email     string
+	Name      string
+	Groups    []string
+	CSRFToken string
+	CreatedAt time.Time
+	// LastSeenAt is the last time the session was used.
+	LastSeenAt time.Time
+	// ExpiresAt is when the session ends unless used again first.
+	ExpiresAt time.Time
+}
+
+// SessionManager keeps sessions in the sessions table. The cookie holds only
+// the random session ID; the table holds only its SHA-256 hash. A session
+// ends after idle without use and always max after it was created.
+type SessionManager struct {
+	db   *store.Store
+	idle time.Duration
+	max  time.Duration
+	// now is the clock; tests replace it.
+	now func() time.Time
+}
+
+// NewSessionManager returns a SessionManager over db. idle and max must be
+// positive with max >= idle.
+func NewSessionManager(db *store.Store, idle, max time.Duration) (*SessionManager, error) {
+	switch {
+	case db == nil:
+		return nil, errors.New("auth: session store is required")
+	case idle <= 0 || max <= 0:
+		return nil, errors.New("auth: session idle and max lifetimes must be positive")
+	case max < idle:
+		return nil, errors.New("auth: session max lifetime must be at least the idle lifetime")
+	}
+	return &SessionManager{db: db, idle: idle, max: max, now: time.Now}, nil
+}
+
+// StartSession creates a session for claims and sets the session cookie.
+func (m *SessionManager) StartSession(w http.ResponseWriter, r *http.Request, claims Claims) error {
+	if claims.Subject == "" {
+		return errors.New("auth: session needs a subject")
+	}
+	id, csrf := randomToken(), randomToken()
+	groups := claims.Groups
+	if groups == nil {
+		groups = []string{}
+	}
+	groupsJSON, err := json.Marshal(groups)
+	if err != nil {
+		return fmt.Errorf("auth: encode session groups: %w", err)
+	}
+
+	now := m.timestamp()
+	if _, err := m.db.Writer.ExecContext(r.Context(), `
+		INSERT INTO sessions (id_hash, subject, email, name, groups, csrf_token, created_at, last_seen_at, expires_at)
+		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+		hashSessionID(id), claims.Subject, claims.Email, claims.Name, string(groupsJSON), csrf,
+		now, now, m.expiry(now.Time, now.Time),
+	); err != nil {
+		return fmt.Errorf("auth: create session: %w", err)
+	}
+
+	// The cookie outlives idle gaps so a returning device presents its ID;
+	// the server decides whether the session is still live. Max-Age is whole
+	// seconds, rounded up so the cookie never ends before the absolute limit.
+	http.SetCookie(w, sessionCookie(id, cookieMaxAge(m.max)))
+	return nil
+}
+
+// cookieMaxAge is lifetime in whole seconds, rounded up. It rounds the
+// quotient rather than adding to lifetime, which would overflow near the
+// largest time.Duration.
+func cookieMaxAge(lifetime time.Duration) int {
+	seconds := lifetime / time.Second
+	if lifetime%time.Second != 0 {
+		seconds++
+	}
+	return int(seconds)
+}
+
+// SessionFor returns the live session r carries and slides its last use to
+// now. It returns ErrNoSession when there is none; an expired session is
+// deleted.
+func (m *SessionManager) SessionFor(r *http.Request) (Session, error) {
+	idHash, err := sessionIDHash(r)
+	if err != nil {
+		return Session{}, err
+	}
+	now := m.timestamp()
+
+	var s Session
+	live := false
+	err = m.db.WithTx(r.Context(), func(tx *sql.Tx) error {
+		var groupsJSON string
+		var created, lastSeen, expires store.Timestamp
+		err := tx.QueryRowContext(r.Context(), `
+			SELECT subject, email, name, groups, csrf_token, created_at, last_seen_at, expires_at
+			FROM sessions WHERE id_hash = ?`, idHash,
+		).Scan(&s.Subject, &s.Email, &s.Name, &groupsJSON, &s.CSRFToken, &created, &lastSeen, &expires)
+		if errors.Is(err, sql.ErrNoRows) {
+			return nil
+		}
+		if err != nil {
+			return fmt.Errorf("auth: read session: %w", err)
+		}
+
+		if !now.Before(expires.Time) || now.Sub(lastSeen.Time) >= m.idle || now.Sub(created.Time) >= m.max {
+			if _, err := tx.ExecContext(r.Context(), `DELETE FROM sessions WHERE id_hash = ?`, idHash); err != nil {
+				return fmt.Errorf("auth: delete expired session: %w", err)
+			}
+			return nil
+		}
+
+		if err := json.Unmarshal([]byte(groupsJSON), &s.Groups); err != nil {
+			return fmt.Errorf("auth: decode session groups: %w", err)
+		}
+		expiresAt := m.expiry(created.Time, now.Time)
+		if _, err := tx.ExecContext(r.Context(), `
+			UPDATE sessions SET last_seen_at = ?, expires_at = ? WHERE id_hash = ?`,
+			now, expiresAt, idHash,
+		); err != nil {
+			return fmt.Errorf("auth: slide session: %w", err)
+		}
+		s.CreatedAt, s.LastSeenAt, s.ExpiresAt = created.Time, now.Time, expiresAt.Time
+		live = true
+		return nil
+	})
+	if err != nil {
+		return Session{}, err
+	}
+	if !live {
+		return Session{}, ErrNoSession
+	}
+	return s, nil
+}
+
+// EndSession deletes the session r carries, if any, and clears the cookie.
+func (m *SessionManager) EndSession(w http.ResponseWriter, r *http.Request) error {
+	http.SetCookie(w, sessionCookie("", -1))
+	idHash, err := sessionIDHash(r)
+	if errors.Is(err, ErrNoSession) {
+		return nil
+	}
+	if err != nil {
+		return err
+	}
+	if _, err := m.db.Writer.ExecContext(r.Context(), `DELETE FROM sessions WHERE id_hash = ?`, idHash); err != nil {
+		return fmt.Errorf("auth: delete session: %w", err)
+	}
+	return nil
+}
+
+// timestamp is the clock's current instant at stored precision.
+func (m *SessionManager) timestamp() store.Timestamp {
+	return store.Timestamp{Time: m.now().UTC().Truncate(time.Millisecond)}
+}
+
+// expiry is when a session created at created and last used at lastSeen
+// ends: idle after lastSeen, but never later than max after created.
+func (m *SessionManager) expiry(created, lastSeen time.Time) store.Timestamp {
+	idle, absolute := lastSeen.Add(m.idle), created.Add(m.max)
+	if absolute.Before(idle) {
+		return store.Timestamp{Time: absolute}
+	}
+	return store.Timestamp{Time: idle}
+}
+
+// sessionCookie is the session cookie carrying value, with maxAge seconds;
+// a negative maxAge clears it.
+func sessionCookie(value string, maxAge int) *http.Cookie {
+	return &http.Cookie{
+		Name:     SessionCookieName,
+		Value:    value,
+		Path:     "/",
+		MaxAge:   maxAge,
+		Secure:   true,
+		HttpOnly: true,
+		SameSite: http.SameSiteLaxMode,
+	}
+}
+
+// sessionIDHash returns the stored hash of r's session ID, or ErrNoSession
+// when the cookie is missing or is not a well-formed ID.
+func sessionIDHash(r *http.Request) (string, error) {
+	c, err := r.Cookie(SessionCookieName)
+	if err != nil {
+		return "", ErrNoSession
+	}
+	raw, err := base64.RawURLEncoding.DecodeString(c.Value)
+	if err != nil || len(raw) != sessionIDBytes {
+		return "", ErrNoSession
+	}
+	return hashSessionID(c.Value), nil
+}
+
+// hashSessionID is the SHA-256 hex digest of the cookie value.
+func hashSessionID(id string) string {
+	sum := sha256.Sum256([]byte(id))
+	return hex.EncodeToString(sum[:])
+}
+
+// randomToken returns 256 random bits, base64url-encoded without padding.
+// crypto/rand.Read never returns an error; it crashes the program instead.
+func randomToken() string {
+	b := make([]byte, sessionIDBytes)
+	rand.Read(b)
+	return base64.RawURLEncoding.EncodeToString(b)
+}
diff --git a/internal/auth/session_test.go b/internal/auth/session_test.go
new file mode 100644
index 0000000..3f86f02
--- /dev/null
+++ b/internal/auth/session_test.go
@@ -0,0 +1,523 @@
+package auth
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/store"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/migrations"
+)
+
+const (
+	testIdle = 30 * 24 * time.Hour
+	testMax  = 180 * 24 * time.Hour
+)
+
+// testEpoch is the fake clock's starting instant.
+var testEpoch = time.Date(2026, 10, 3, 18, 30, 0, 0, time.UTC)
+
+// sessionHarness runs the real login flow against a fake provider with a
+// SessionManager over a migrated database and a fake clock.
+type sessionHarness struct {
+	*harness
+	db       *store.Store
+	sessions *SessionManager
+	auth     *Auth
+	clock    time.Time
+}
+
+func newSessionHarness(t *testing.T) *sessionHarness {
+	t.Helper()
+	ctx := context.Background()
+	db, err := store.Open(ctx, t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if err := db.Close(); err != nil {
+			t.Error(err)
+		}
+	})
+	if err := store.Migrate(ctx, db.Writer, migrations.FS); err != nil {
+		t.Fatal(err)
+	}
+	sessions, err := NewSessionManager(db, testIdle, testMax)
+	if err != nil {
+		t.Fatal(err)
+	}
+	h := &sessionHarness{db: db, sessions: sessions, clock: testEpoch}
+	sessions.now = func() time.Time { return h.clock }
+
+	p := newFakeProvider(t)
+	logs := &syncBuffer{}
+	a, err := New(ctx, Config{
+		Issuer:        p.issuer(),
+		ClientID:      testClientID,
+		ClientSecret:  testClientSecret,
+		PublicURL:     testPublicURL,
+		SecretKey:     []byte("server-secret-key"),
+		AllowedGroups: testAllowedGroups,
+	}, sessions, slog.New(slog.NewTextHandler(logs, nil)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := chi.NewRouter()
+	a.Mount(r)
+	h.harness = &harness{provider: p, router: r, logs: logs}
+	h.auth = a
+	return h
+}
+
+func (h *sessionHarness) advance(d time.Duration) { h.clock = h.clock.Add(d) }
+
+// signIn completes a login and returns the callback response and the
+// session cookie it set.
+func (h *sessionHarness) signIn(t *testing.T) (*http.Response, *http.Cookie) {
+	t.Helper()
+	l := h.login(t, LoginPath)
+	resp := h.callback(l, l.query.Get("state"), h.provider.signRS256(h.provider.claims(l.query.Get("nonce"))))
+	if resp.StatusCode != http.StatusSeeOther {
+		t.Fatalf("callback status = %d, want 303", resp.StatusCode)
+	}
+	for _, c := range resp.Cookies() {
+		if c.Name == SessionCookieName {
+			return resp, c
+		}
+	}
+	t.Fatalf("callback set no %s cookie", SessionCookieName)
+	return nil, nil
+}
+
+// use looks up the session cookie c names.
+func (h *sessionHarness) use(c *http.Cookie) (Session, error) {
+	req := httptest.NewRequest(http.MethodGet, "/api/anything", nil)
+	if c != nil {
+		req.AddCookie(c)
+	}
+	return h.sessions.SessionFor(req)
+}
+
+type sessionRow struct {
+	idHash, subject, email, name, groups, csrf string
+	created, lastSeen, expires                 store.Timestamp
+}
+
+func (h *sessionHarness) rows(t *testing.T) []sessionRow {
+	t.Helper()
+	rs, err := h.db.Reader.Query(`SELECT id_hash, subject, email, name, groups, csrf_token,
+		created_at, last_seen_at, expires_at FROM sessions`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rs.Close()
+	var out []sessionRow
+	for rs.Next() {
+		var r sessionRow
+		if err := rs.Scan(&r.idHash, &r.subject, &r.email, &r.name, &r.groups, &r.csrf,
+			&r.created, &r.lastSeen, &r.expires); err != nil {
+			t.Fatal(err)
+		}
+		out = append(out, r)
+	}
+	if err := rs.Err(); err != nil {
+		t.Fatal(err)
+	}
+	return out
+}
+
+// sessionSetCookie returns the raw Set-Cookie header for the session cookie.
+func sessionSetCookie(t *testing.T, resp *http.Response) string {
+	t.Helper()
+	for _, v := range resp.Header.Values("Set-Cookie") {
+		if strings.HasPrefix(v, SessionCookieName+"=") {
+			return v
+		}
+	}
+	t.Fatalf("no Set-Cookie for %s in %q", SessionCookieName, resp.Header.Values("Set-Cookie"))
+	return ""
+}
+
+// assertSessionCookieAttributes checks the attributes the __Host- prefix
+// and D-004 require, on the raw header so nothing is lost in parsing.
+func assertSessionCookieAttributes(t *testing.T, raw string) {
+	t.Helper()
+	attrs := map[string]bool{}
+	for _, a := range strings.Split(raw, ";")[1:] {
+		name, _, _ := strings.Cut(strings.TrimSpace(a), "=")
+		attrs[strings.ToLower(name)] = true
+	}
+	for _, want := range []string{"Path=/", "HttpOnly", "Secure", "SameSite=Lax"} {
+		if !slices.Contains(strings.Split(raw, "; "), want) {
+			t.Errorf("session Set-Cookie %q lacks %s", raw, want)
+		}
+	}
+	if attrs["domain"] {
+		t.Errorf("session Set-Cookie %q has a Domain", raw)
+	}
+}
+
+func sha256Hex(s string) string {
+	sum := sha256.Sum256([]byte(s))
+	return hex.EncodeToString(sum[:])
+}
+
+func TestCallbackStartsSession(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	resp, cookie := h.signIn(t)
+
+	assertSessionCookieAttributes(t, sessionSetCookie(t, resp))
+	if cookie.MaxAge != int(testMax/time.Second) {
+		t.Errorf("session cookie MaxAge = %d, want %d", cookie.MaxAge, int(testMax/time.Second))
+	}
+
+	rows := h.rows(t)
+	if len(rows) != 1 {
+		t.Fatalf("sessions has %d rows, want 1", len(rows))
+	}
+	r := rows[0]
+	if len(cookie.Value) != 43 {
+		t.Errorf("session ID %q is %d chars, want 43 (256 bits, base64url)", cookie.Value, len(cookie.Value))
+	}
+	if r.idHash != sha256Hex(cookie.Value) {
+		t.Errorf("id_hash = %q, want the SHA-256 of the cookie value", r.idHash)
+	}
+	if r.subject != "user-123" || r.email != "cook@example.com" || r.name != "Home Cook" ||
+		r.groups != `["mealquest-admin"]` {
+		t.Errorf("session row = %+v, want the ID token claims", r)
+	}
+	if len(r.csrf) != 43 || r.csrf == cookie.Value {
+		t.Errorf("csrf_token = %q, want a separate 256-bit token", r.csrf)
+	}
+	if !r.created.Equal(testEpoch) || !r.lastSeen.Equal(testEpoch) || !r.expires.Equal(testEpoch.Add(testIdle)) {
+		t.Errorf("created, last_seen, expires = %v, %v, %v, want %v, %v, %v",
+			r.created, r.lastSeen, r.expires, testEpoch, testEpoch, testEpoch.Add(testIdle))
+	}
+
+	s, err := h.use(cookie)
+	if err != nil {
+		t.Fatalf("SessionFor = %v, want the new session", err)
+	}
+	if s.Subject != "user-123" || s.CSRFToken != r.csrf || !slices.Equal(s.Groups, []string{"mealquest-admin"}) {
+		t.Errorf("SessionFor = %+v", s)
+	}
+}
+
+func TestStartSessionStoresOnlyTheHash(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	_, cookie := h.signIn(t)
+	var n int
+	err := h.db.Reader.QueryRow(`SELECT count(*) FROM sessions WHERE
+		id_hash = ?1 OR subject = ?1 OR email = ?1 OR name = ?1 OR groups = ?1 OR csrf_token = ?1`,
+		cookie.Value).Scan(&n)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 0 {
+		t.Error("the raw session ID is stored in the sessions table")
+	}
+}
+
+func TestSessionLastSeenSlides(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	_, cookie := h.signIn(t)
+
+	// Used every 20 days, the session outlives several idle periods.
+	for range 4 {
+		h.advance(20 * 24 * time.Hour)
+		s, err := h.use(cookie)
+		if err != nil {
+			t.Fatalf("at %v SessionFor = %v, want the session", h.clock, err)
+		}
+		if !s.LastSeenAt.Equal(h.clock) || !s.ExpiresAt.Equal(h.clock.Add(testIdle)) {
+			t.Errorf("at %v session last seen %v, expires %v, want %v, %v",
+				h.clock, s.LastSeenAt, s.ExpiresAt, h.clock, h.clock.Add(testIdle))
+		}
+		r := h.rows(t)[0]
+		if !r.lastSeen.Equal(h.clock) || !r.expires.Equal(h.clock.Add(testIdle)) || !r.created.Equal(testEpoch) {
+			t.Errorf("at %v row created %v, last seen %v, expires %v", h.clock, r.created, r.lastSeen, r.expires)
+		}
+	}
+}
+
+func TestSessionIdleExpiry(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	_, cookie := h.signIn(t)
+
+	h.advance(testIdle - time.Millisecond)
+	if _, err := h.use(cookie); err != nil {
+		t.Fatalf("just inside the idle limit SessionFor = %v, want the session", err)
+	}
+	h.advance(testIdle)
+	if _, err := h.use(cookie); !errors.Is(err, ErrNoSession) {
+		t.Fatalf("after idle SessionFor = %v, want ErrNoSession", err)
+	}
+	if n := len(h.rows(t)); n != 0 {
+		t.Errorf("expired session left %d rows, want 0", n)
+	}
+	// Turning the clock back cannot revive it.
+	h.clock = testEpoch
+	if _, err := h.use(cookie); !errors.Is(err, ErrNoSession) {
+		t.Errorf("after deletion SessionFor = %v, want ErrNoSession", err)
+	}
+}
+
+func TestSessionAbsoluteExpiry(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	_, cookie := h.signIn(t)
+
+	for h.clock.Before(testEpoch.Add(testMax - 20*24*time.Hour)) {
+		h.advance(20 * 24 * time.Hour)
+		if _, err := h.use(cookie); err != nil {
+			t.Fatalf("at %v SessionFor = %v, want the session", h.clock, err)
+		}
+	}
+	// Last used 20 days before the absolute limit: the idle window would run
+	// past it, so expiry is capped there.
+	if r := h.rows(t)[0]; !r.expires.Equal(testEpoch.Add(testMax)) {
+		t.Errorf("expires_at = %v, want the absolute limit %v", r.expires, testEpoch.Add(testMax))
+	}
+
+	h.clock = testEpoch.Add(testMax - time.Millisecond)
+	if _, err := h.use(cookie); err != nil {
+		t.Fatalf("just inside the absolute limit SessionFor = %v, want the session", err)
+	}
+	h.clock = testEpoch.Add(testMax)
+	if _, err := h.use(cookie); !errors.Is(err, ErrNoSession) {
+		t.Fatalf("at the absolute limit, used 1ms ago, SessionFor = %v, want ErrNoSession", err)
+	}
+	if n := len(h.rows(t)); n != 0 {
+		t.Errorf("expired session left %d rows, want 0", n)
+	}
+}
+
+// A max with fractional seconds rounds the cookie Max-Age up, so the browser
+// keeps the cookie until the server-side absolute limit.
+func TestSessionCookieOutlivesFractionalMax(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	h.sessions.idle, h.sessions.max = 1500*time.Millisecond, 2500*time.Millisecond
+	_, cookie := h.signIn(t)
+	if cookie.MaxAge != 3 {
+		t.Errorf("session cookie MaxAge = %d, want 3", cookie.MaxAge)
+	}
+	if r := h.rows(t)[0]; !r.expires.Equal(testEpoch.Add(1500 * time.Millisecond)) {
+		t.Errorf("expires_at = %v, want %v", r.expires, testEpoch.Add(1500*time.Millisecond))
+	}
+
+	h.advance(time.Second)
+	if _, err := h.use(cookie); err != nil {
+		t.Fatalf("at 1s SessionFor = %v, want the session", err)
+	}
+	h.clock = testEpoch.Add(2500*time.Millisecond - time.Millisecond)
+	if _, err := h.use(cookie); err != nil {
+		t.Fatalf("just inside the absolute limit SessionFor = %v, want the session", err)
+	}
+	h.clock = testEpoch.Add(2500 * time.Millisecond)
+	if _, err := h.use(cookie); !errors.Is(err, ErrNoSession) {
+		t.Errorf("at the absolute limit SessionFor = %v, want ErrNoSession", err)
+	}
+}
+
+// A lifetime near the largest accepted duration still yields a positive,
+// rounded-up cookie Max-Age rather than overflowing.
+func TestSessionCookieMaxAgeNearLargestMax(t *testing.T) {
+	t.Parallel()
+	largest, err := time.ParseDuration("2562047h47m16.854s")
+	if err != nil {
+		t.Fatal(err)
+	}
+	h := newSessionHarness(t)
+	h.sessions.max = largest
+	_, cookie := h.signIn(t)
+	if want := 9223372037; cookie.MaxAge != want {
+		t.Errorf("session cookie MaxAge = %d, want %d", cookie.MaxAge, want)
+	}
+}
+
+// A lifetime shortened by configuration applies to existing sessions even
+// though their stored expires_at is later.
+func TestSessionLifetimeChecksUseCurrentConfig(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	_, cookie := h.signIn(t)
+	h.sessions.idle = 24 * time.Hour
+	h.advance(24 * time.Hour)
+	if _, err := h.use(cookie); !errors.Is(err, ErrNoSession) {
+		t.Errorf("past the shortened idle SessionFor = %v, want ErrNoSession", err)
+	}
+
+	_, cookie = h.signIn(t)
+	h.sessions.idle, h.sessions.max = 24*time.Hour, 48*time.Hour
+	for range 3 {
+		h.advance(12 * time.Hour)
+		if _, err := h.use(cookie); err != nil {
+			t.Fatalf("SessionFor = %v, want the session", err)
+		}
+	}
+	h.advance(12 * time.Hour)
+	if _, err := h.use(cookie); !errors.Is(err, ErrNoSession) {
+		t.Errorf("past the shortened max SessionFor = %v, want ErrNoSession", err)
+	}
+
+	// Used recently and with expires_at still ahead, but older than a max
+	// shortened after that use: only the absolute check ends it.
+	h.sessions.idle, h.sessions.max = testIdle, testMax
+	_, cookie = h.signIn(t)
+	h.advance(10 * 24 * time.Hour)
+	if _, err := h.use(cookie); err != nil {
+		t.Fatalf("SessionFor = %v, want the session", err)
+	}
+	h.sessions.max = 10 * 24 * time.Hour
+	h.advance(time.Hour)
+	if _, err := h.use(cookie); !errors.Is(err, ErrNoSession) {
+		t.Errorf("past a max shortened after the last use SessionFor = %v, want ErrNoSession", err)
+	}
+}
+
+func TestSessionForRejectsBadCookies(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	h.signIn(t)
+	for name, c := range map[string]*http.Cookie{
+		"missing":      nil,
+		"empty":        {Name: SessionCookieName, Value: ""},
+		"not base64":   {Name: SessionCookieName, Value: strings.Repeat("!", 43)},
+		"short":        {Name: SessionCookieName, Value: "AAAA"},
+		"unknown ID":   {Name: SessionCookieName, Value: randomToken()},
+		"other cookie": {Name: "mealquest_session", Value: randomToken()},
+	} {
+		if _, err := h.use(c); !errors.Is(err, ErrNoSession) {
+			t.Errorf("%s: SessionFor = %v, want ErrNoSession", name, err)
+		}
+	}
+	if n := len(h.rows(t)); n != 1 {
+		t.Errorf("sessions has %d rows, want the signed-in session only", n)
+	}
+}
+
+func (h *sessionHarness) logout(cookies ...*http.Cookie) *http.Response {
+	req := httptest.NewRequest(http.MethodPost, LogoutPath, nil)
+	for _, c := range cookies {
+		req.AddCookie(c)
+	}
+	rec := httptest.NewRecorder()
+	h.router.ServeHTTP(rec, req)
+	return rec.Result()
+}
+
+func TestLogout(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	_, cookie := h.signIn(t)
+	_, other := h.signIn(t)
+
+	resp := h.logout(cookie)
+	if resp.StatusCode != http.StatusSeeOther {
+		t.Fatalf("logout status = %d, want 303", resp.StatusCode)
+	}
+	want := h.provider.issuer() + "/end-session/"
+	if got := resp.Header.Get("Location"); got != want || h.auth.EndSessionURL() != want {
+		t.Errorf("logout redirect = %q, EndSessionURL = %q, want %q", got, h.auth.EndSessionURL(), want)
+	}
+	raw := sessionSetCookie(t, resp)
+	assertSessionCookieAttributes(t, raw)
+	if !strings.HasPrefix(raw, SessionCookieName+"=;") || !strings.Contains(raw, "Max-Age=0") {
+		t.Errorf("logout Set-Cookie = %q, want the cookie cleared", raw)
+	}
+
+	rows := h.rows(t)
+	if len(rows) != 1 || rows[0].idHash != sha256Hex(other.Value) {
+		t.Errorf("after logout sessions = %+v, want only the other session", rows)
+	}
+	if _, err := h.use(cookie); !errors.Is(err, ErrNoSession) {
+		t.Errorf("logged-out cookie: SessionFor = %v, want ErrNoSession", err)
+	}
+	if _, err := h.use(other); err != nil {
+		t.Errorf("other session: SessionFor = %v, want it untouched", err)
+	}
+}
+
+func TestLogoutWithoutSession(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	resp := h.logout()
+	if resp.StatusCode != http.StatusSeeOther || resp.Header.Get("Location") != h.provider.issuer()+"/end-session/" {
+		t.Errorf("logout = %d to %q, want 303 to the end session endpoint", resp.StatusCode, resp.Header.Get("Location"))
+	}
+	assertSessionCookieAttributes(t, sessionSetCookie(t, resp))
+}
+
+func TestLogoutIsPostOnly(t *testing.T) {
+	t.Parallel()
+	h := newSessionHarness(t)
+	_, cookie := h.signIn(t)
+	if resp := h.get(LogoutPath, cookie); resp.StatusCode != http.StatusMethodNotAllowed {
+		t.Errorf("GET %s status = %d, want 405", LogoutPath, resp.StatusCode)
+	}
+	if _, err := h.use(cookie); err != nil {
+		t.Errorf("after GET %s SessionFor = %v, want the session", LogoutPath, err)
+	}
+}
+
+func TestLogoutSessionError(t *testing.T) {
+	t.Parallel()
+	h := newHarness(t)
+	h.sessions.err = errors.New("database down")
+	rec := httptest.NewRecorder()
+	h.router.ServeHTTP(rec, httptest.NewRequest(http.MethodPost, LogoutPath, nil))
+	if rec.Code != http.StatusInternalServerError {
+		t.Errorf("logout status = %d, want 500", rec.Code)
+	}
+}
+
+func TestNewRequiresEndSessionEndpoint(t *testing.T) {
+	t.Parallel()
+	p := newFakeProvider(t)
+	p.discoveryOverrides = map[string]any{"end_session_endpoint": nil}
+	_, err := New(context.Background(), Config{
+		Issuer:        p.issuer(),
+		ClientID:      testClientID,
+		ClientSecret:  testClientSecret,
+		PublicURL:     testPublicURL,
+		SecretKey:     []byte("k"),
+		AllowedGroups: testAllowedGroups,
+	}, &memorySessions{}, slog.New(slog.DiscardHandler))
+	if err == nil || !strings.Contains(err.Error(), "end session endpoint") {
+		t.Errorf("New error = %v, want the missing end session endpoint named", err)
+	}
+}
+
+func TestNewSessionManagerValidates(t *testing.T) {
+	t.Parallel()
+	db := &store.Store{}
+	for _, tc := range []struct {
+		name      string
+		db        *store.Store
+		idle, max time.Duration
+	}{
+		{"nil store", nil, time.Hour, time.Hour},
+		{"zero idle", db, 0, time.Hour},
+		{"negative max", db, time.Hour, -time.Hour},
+		{"max below idle", db, 2 * time.Hour, time.Hour},
+	} {
+		if _, err := NewSessionManager(tc.db, tc.idle, tc.max); err == nil {
+			t.Errorf("%s: NewSessionManager succeeded", tc.name)
+		}
+	}
+}
diff --git a/internal/config/config.go b/internal/config/config.go
index 5673595..d3ab5e2 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -11,6 +11,9 @@ import (
 	"strconv"
 	"strings"
 	"time"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/forwarded"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/secureurl"
 )
 
 // Environment variable names.
@@ -20,6 +23,23 @@ const (
 	EnvDataDir         = "MEALQUEST_DATA_DIR"
 	EnvShutdownTimeout = "MEALQUEST_SHUTDOWN_TIMEOUT"
 	EnvSecretKeyFile   = "MEALQUEST_SECRET_KEY_FILE"
+
+	EnvOIDCIssuer           = "MEALQUEST_OIDC_ISSUER"
+	EnvOIDCClientID         = "MEALQUEST_OIDC_CLIENT_ID"
+	EnvOIDCClientSecretFile = "MEALQUEST_OIDC_CLIENT_SECRET_FILE"
+	EnvOIDCAllowedGroups    = "MEALQUEST_OIDC_ALLOWED_GROUPS"
+
+	EnvSessionIdle = "MEALQUEST_SESSION_IDLE"
+	EnvSessionMax  = "MEALQUEST_SESSION_MAX"
+
+	EnvTrustedProxy = "MEALQUEST_TRUSTED_PROXY"
+)
+
+// Session lifetime defaults (D-004): long enough that the kitchen tablet
+// stays signed in.
+const (
+	DefaultSessionIdle = 720 * time.Hour  // 30 days
+	DefaultSessionMax  = 4320 * time.Hour // 180 days
 )
 
 // Config is the validated service configuration.
@@ -29,6 +49,22 @@ type Config struct {
 	DataDir         string
 	ShutdownTimeout time.Duration
 	SecretKey       Secret
+
+	// OIDCIssuer is the Authentik issuer URL used for discovery.
+	OIDCIssuer       *url.URL
+	OIDCClientID     string
+	OIDCClientSecret Secret
+	// OIDCAllowedGroups lists the groups whose members may sign in.
+	OIDCAllowedGroups []string
+
+	// SessionIdle ends a session left unused this long.
+	SessionIdle time.Duration
+	// SessionMax ends every session this long after sign-in, used or not.
+	SessionMax time.Duration
+
+	// TrustedProxy lists the reverse proxies whose X-Forwarded-For and
+	// X-Forwarded-Proto headers are honoured. Empty trusts no peer.
+	TrustedProxy forwarded.Trusted
 }
 
 // PublicOrigin returns the scheme and host of PublicURL, the only parts of it
@@ -47,10 +83,23 @@ func load(lookup func(string) (string, bool)) (Config, error) {
 	r := reader{lookup: lookup}
 	cfg := Config{
 		HTTPAddr:        r.listenAddr(EnvHTTPAddr),
-		PublicURL:       r.httpURL(EnvPublicURL),
+		PublicURL:       r.secureURL(EnvPublicURL),
 		DataDir:         r.dir(EnvDataDir),
 		ShutdownTimeout: r.duration(EnvShutdownTimeout),
 		SecretKey:       r.secretFile(EnvSecretKeyFile),
+
+		OIDCIssuer:        r.secureURL(EnvOIDCIssuer),
+		OIDCClientID:      r.str(EnvOIDCClientID),
+		OIDCClientSecret:  r.secretFile(EnvOIDCClientSecretFile),
+		OIDCAllowedGroups: r.list(EnvOIDCAllowedGroups),
+
+		SessionIdle: r.sessionDuration(EnvSessionIdle, DefaultSessionIdle),
+		SessionMax:  r.sessionDuration(EnvSessionMax, DefaultSessionMax),
+
+		TrustedProxy: r.trustedProxy(EnvTrustedProxy),
+	}
+	if cfg.SessionIdle > 0 && cfg.SessionMax > 0 && cfg.SessionMax < cfg.SessionIdle {
+		r.fail(EnvSessionMax, "must be at least %s (%s)", EnvSessionIdle, cfg.SessionIdle)
 	}
 	if len(r.problems) > 0 {
 		return Config{}, &Error{Problems: r.problems}
@@ -114,6 +163,41 @@ func (r *reader) required(name string) (string, bool) {
 	return v, true
 }
 
+func (r *reader) str(name string) string {
+	v, _ := r.required(name)
+	return v
+}
+
+// list parses a comma-separated list. Every entry must be non-empty after
+// trimming spaces.
+func (r *reader) list(name string) []string {
+	v, ok := r.required(name)
+	if !ok {
+		return nil
+	}
+	items := strings.Split(v, ",")
+	for i, item := range items {
+		items[i] = strings.TrimSpace(item)
+		if items[i] == "" {
+			r.fail(name, "empty entry in comma-separated list %q", v)
+			return nil
+		}
+	}
+	return items
+}
+
+// trustedProxy parses an optional comma-separated list of IP addresses and
+// CIDR prefixes with forwarded.ParseTrusted. Unset or blank trusts no peer.
+func (r *reader) trustedProxy(name string) forwarded.Trusted {
+	v, _ := r.lookup(name)
+	t, err := forwarded.ParseTrusted(v)
+	if err != nil {
+		r.fail(name, "%v", err)
+		return nil
+	}
+	return t
+}
+
 func (r *reader) listenAddr(name string) string {
 	v, ok := r.required(name)
 	if !ok {
@@ -155,15 +239,21 @@ func validHostname(h string) bool {
 	return true
 }
 
-func (r *reader) httpURL(name string) *url.URL {
+// secureURL reads an absolute URL that must pass secureurl.Check: https://,
+// or http:// to a loopback host only.
+func (r *reader) secureURL(name string) *url.URL {
 	v, ok := r.required(name)
 	if !ok {
 		return nil
 	}
 	// The value is never echoed: it may carry credentials.
 	u, err := url.Parse(v)
-	if err != nil || (u.Scheme != "http" && u.Scheme != "https") || u.Hostname() == "" {
-		r.fail(name, "invalid URL (want absolute http:// or https:// URL)")
+	if err != nil {
+		r.fail(name, "invalid URL (want absolute https:// URL)")
+		return nil
+	}
+	if err := secureurl.Check(u); err != nil {
+		r.fail(name, "invalid URL: %v", err)
 		return nil
 	}
 	if u.User != nil {
@@ -195,6 +285,32 @@ func (r *reader) duration(name string) time.Duration {
 	if !ok {
 		return 0
 	}
+	return r.parseDuration(name, v)
+}
+
+// optionalDuration returns def when name is unset. A set value, even an empty
+// one, must be a valid positive duration.
+func (r *reader) optionalDuration(name string, def time.Duration) time.Duration {
+	v, ok := r.lookup(name)
+	if !ok {
+		return def
+	}
+	return r.parseDuration(name, v)
+}
+
+// sessionDuration is optionalDuration restricted to whole milliseconds:
+// session expiry is stored as a store.Timestamp, which keeps millisecond
+// precision, so a finer duration could not be honoured.
+func (r *reader) sessionDuration(name string, def time.Duration) time.Duration {
+	d := r.optionalDuration(name, def)
+	if d%time.Millisecond != 0 {
+		r.fail(name, "duration must be a whole number of milliseconds, got %s", d)
+		return 0
+	}
+	return d
+}
+
+func (r *reader) parseDuration(name, v string) time.Duration {
 	d, err := time.ParseDuration(v)
 	if err != nil {
 		r.fail(name, "invalid duration %q (want e.g. 10s, 1m)", v)
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index 01a184f..8816bf7 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -3,13 +3,17 @@ package config
 import (
 	"errors"
 	"fmt"
+	"net/netip"
 	"net/url"
 	"os"
 	"path/filepath"
+	"reflect"
 	"slices"
 	"strings"
 	"testing"
 	"time"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/forwarded"
 )
 
 func TestLoad(t *testing.T) {
@@ -21,6 +25,10 @@ func TestLoad(t *testing.T) {
 		t.Fatal(err)
 	}
 	unreadablePath := filepath.Join(dir, "missing_secret")
+	clientSecretPath := filepath.Join(dir, "oidc_client_secret")
+	if err := os.WriteFile(clientSecretPath, []byte("client-s3cret\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
 	emptyPath := filepath.Join(dir, "empty_secret")
 	if err := os.WriteFile(emptyPath, []byte("\n"), 0o600); err != nil {
 		t.Fatal(err)
@@ -33,6 +41,11 @@ func TestLoad(t *testing.T) {
 			EnvDataDir:         dir,
 			EnvShutdownTimeout: "15s",
 			EnvSecretKeyFile:   secretPath,
+
+			EnvOIDCIssuer:           "https://auth.example.com/application/o/mealquest/",
+			EnvOIDCClientID:         "mealquest",
+			EnvOIDCClientSecretFile: clientSecretPath,
+			EnvOIDCAllowedGroups:    "mealquest-admin",
 		}
 	}
 	with := func(changes map[string]string, unset ...string) map[string]string {
@@ -70,6 +83,7 @@ func TestLoad(t *testing.T) {
 			env:  map[string]string{},
 			badVars: []string{
 				EnvHTTPAddr, EnvPublicURL, EnvDataDir, EnvShutdownTimeout, EnvSecretKeyFile,
+				EnvOIDCIssuer, EnvOIDCClientID, EnvOIDCClientSecretFile, EnvOIDCAllowedGroups,
 			},
 		},
 		{
@@ -139,6 +153,24 @@ func TestLoad(t *testing.T) {
 			env:     with(map[string]string{EnvPublicURL: "http://:8080"}),
 			badVars: []string{EnvPublicURL},
 		},
+		{
+			name:    "plain HTTP public URL on a remote host",
+			env:     with(map[string]string{EnvPublicURL: "http://meals.example.com"}),
+			badVars: []string{EnvPublicURL},
+		},
+		{
+			name:    "plain HTTP public URL on a non-loopback IP",
+			env:     with(map[string]string{EnvPublicURL: "http://192.168.1.20:8080"}),
+			badVars: []string{EnvPublicURL},
+		},
+		{
+			name: "plain HTTP public URL on 127.0.0.1",
+			env:  with(map[string]string{EnvPublicURL: "http://127.0.0.1:8080"}),
+		},
+		{
+			name: "plain HTTP public URL on [::1]",
+			env:  with(map[string]string{EnvPublicURL: "http://[::1]:8080"}),
+		},
 		{
 			name:    "relative public URL",
 			env:     with(map[string]string{EnvPublicURL: "meals.example.com"}),
@@ -169,6 +201,143 @@ func TestLoad(t *testing.T) {
 			env:     with(map[string]string{"MEALQUEST_SECRET_KEY": "inline"}),
 			badVars: []string{"MEALQUEST_SECRET_KEY"},
 		},
+		{
+			name:    "inline OIDC client secret rejected",
+			env:     with(map[string]string{"MEALQUEST_OIDC_CLIENT_SECRET": "inline"}),
+			badVars: []string{"MEALQUEST_OIDC_CLIENT_SECRET"},
+		},
+		{
+			name:    "unreadable OIDC client secret file",
+			env:     with(map[string]string{EnvOIDCClientSecretFile: unreadablePath}),
+			badVars: []string{EnvOIDCClientSecretFile},
+		},
+		{
+			name:    "relative OIDC issuer",
+			env:     with(map[string]string{EnvOIDCIssuer: "auth.example.com"}),
+			badVars: []string{EnvOIDCIssuer},
+		},
+		{
+			name:    "non-http OIDC issuer",
+			env:     with(map[string]string{EnvOIDCIssuer: "ftp://auth.example.com"}),
+			badVars: []string{EnvOIDCIssuer},
+		},
+		{
+			name:    "plain HTTP OIDC issuer on a remote host",
+			env:     with(map[string]string{EnvOIDCIssuer: "http://auth.example.com/application/o/mealquest/"}),
+			badVars: []string{EnvOIDCIssuer},
+		},
+		{
+			name: "plain HTTP OIDC issuer on localhost",
+			env:  with(map[string]string{EnvOIDCIssuer: "http://localhost:9000/application/o/mealquest/"}),
+		},
+		{
+			name: "several allowed groups",
+			env:  with(map[string]string{EnvOIDCAllowedGroups: "mealquest-admin, family"}),
+		},
+		{
+			name:    "empty allowed group entry",
+			env:     with(map[string]string{EnvOIDCAllowedGroups: "mealquest-admin,,family"}),
+			badVars: []string{EnvOIDCAllowedGroups},
+		},
+		{
+			name:    "trailing comma in allowed groups",
+			env:     with(map[string]string{EnvOIDCAllowedGroups: "mealquest-admin,"}),
+			badVars: []string{EnvOIDCAllowedGroups},
+		},
+		{
+			name:    "blank OIDC client ID",
+			env:     with(map[string]string{EnvOIDCClientID: "  "}),
+			badVars: []string{EnvOIDCClientID},
+		},
+		{
+			name: "session lifetimes set",
+			env:  with(map[string]string{EnvSessionIdle: "24h", EnvSessionMax: "48h"}),
+		},
+		{
+			name: "session max equal to idle",
+			env:  with(map[string]string{EnvSessionIdle: "24h", EnvSessionMax: "24h"}),
+		},
+		{
+			name:    "invalid session idle",
+			env:     with(map[string]string{EnvSessionIdle: "a month"}),
+			badVars: []string{EnvSessionIdle},
+		},
+		{
+			name:    "empty session idle",
+			env:     with(map[string]string{EnvSessionIdle: ""}),
+			badVars: []string{EnvSessionIdle},
+		},
+		{
+			name:    "non-positive session max",
+			env:     with(map[string]string{EnvSessionMax: "-1h"}),
+			badVars: []string{EnvSessionMax},
+		},
+		{
+			name: "whole-millisecond session lifetimes",
+			env:  with(map[string]string{EnvSessionIdle: "1ms", EnvSessionMax: "2ms"}),
+		},
+		{
+			name:    "sub-millisecond session idle",
+			env:     with(map[string]string{EnvSessionIdle: "1500us", EnvSessionMax: "2ms"}),
+			badVars: []string{EnvSessionIdle},
+		},
+		{
+			name:    "sub-millisecond session max",
+			env:     with(map[string]string{EnvSessionIdle: "1ms", EnvSessionMax: "1ms500us"}),
+			badVars: []string{EnvSessionMax},
+		},
+		{
+			name:    "session max below idle",
+			env:     with(map[string]string{EnvSessionIdle: "48h", EnvSessionMax: "24h"}),
+			badVars: []string{EnvSessionMax},
+		},
+		{
+			name:    "session max below default idle",
+			env:     with(map[string]string{EnvSessionMax: "719h"}),
+			badVars: []string{EnvSessionMax},
+		},
+		{
+			name: "trusted proxy IP",
+			env:  with(map[string]string{EnvTrustedProxy: "10.60.1.33"}),
+		},
+		{
+			name: "trusted proxy IPs and CIDRs",
+			env:  with(map[string]string{EnvTrustedProxy: "10.60.1.33, 172.16.0.0/12,2001:db8::/32,::1"}),
+		},
+		{
+			name: "blank trusted proxy trusts no peer",
+			env:  with(map[string]string{EnvTrustedProxy: "  "}),
+		},
+		{
+			name:    "trusted proxy not an IP",
+			env:     with(map[string]string{EnvTrustedProxy: "traefik"}),
+			badVars: []string{EnvTrustedProxy},
+		},
+		{
+			name:    "trusted proxy invalid CIDR",
+			env:     with(map[string]string{EnvTrustedProxy: "10.60.1.0/33"}),
+			badVars: []string{EnvTrustedProxy},
+		},
+		{
+			name:    "trusted proxy CIDR with host bits",
+			env:     with(map[string]string{EnvTrustedProxy: "10.60.1.33/24"}),
+			badVars: []string{EnvTrustedProxy},
+		},
+		{
+			name:    "trusted proxy empty entry",
+			env:     with(map[string]string{EnvTrustedProxy: "10.60.1.33,,10.60.1.34"}),
+			badVars: []string{EnvTrustedProxy},
+		},
+		{
+			name:    "trusted proxy trusting everyone",
+			env:     with(map[string]string{EnvTrustedProxy: "0.0.0.0/0"}),
+			badVars: []string{EnvTrustedProxy},
+		},
+		{
+			name:    "trusted proxy with port",
+			env:     with(map[string]string{EnvTrustedProxy: "10.60.1.33:443"}),
+			badVars: []string{EnvTrustedProxy},
+		},
 		{
 			name: "several invalid values are all named",
 			env: with(map[string]string{
@@ -211,7 +380,7 @@ func TestLoad(t *testing.T) {
 					t.Errorf("error message %q does not name %s", err.Error(), v)
 				}
 			}
-			if cfg != (Config{}) {
+			if !reflect.ValueOf(cfg).IsZero() {
 				t.Errorf("load() returned non-zero Config alongside error: %+v", cfg)
 			}
 		})
@@ -226,12 +395,21 @@ func TestLoadValues(t *testing.T) {
 	if err := os.WriteFile(secretPath, []byte("s3cret-value\r\n"), 0o600); err != nil {
 		t.Fatal(err)
 	}
+	clientSecretPath := filepath.Join(dir, "oidc_client_secret")
+	if err := os.WriteFile(clientSecretPath, []byte("client-s3cret\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
 	env := map[string]string{
 		EnvHTTPAddr:        "127.0.0.1:9000",
 		EnvPublicURL:       "http://localhost:9000",
 		EnvDataDir:         dir,
 		EnvShutdownTimeout: "1m30s",
 		EnvSecretKeyFile:   secretPath,
+
+		EnvOIDCIssuer:           "https://auth.example.com/application/o/mealquest/",
+		EnvOIDCClientID:         "mealquest",
+		EnvOIDCClientSecretFile: clientSecretPath,
+		EnvOIDCAllowedGroups:    " mealquest-admin ,family",
 	}
 	cfg, err := load(func(k string) (string, bool) { v, ok := env[k]; return v, ok })
 	if err != nil {
@@ -253,6 +431,40 @@ func TestLoadValues(t *testing.T) {
 	if cfg.SecretKey.Reveal() != "s3cret-value" {
 		t.Errorf("SecretKey.Reveal() = %q, want trailing newline trimmed", cfg.SecretKey.Reveal())
 	}
+	if got, want := cfg.OIDCIssuer.String(), "https://auth.example.com/application/o/mealquest/"; got != want {
+		t.Errorf("OIDCIssuer = %q, want %q", got, want)
+	}
+	if cfg.OIDCClientID != "mealquest" {
+		t.Errorf("OIDCClientID = %q", cfg.OIDCClientID)
+	}
+	if cfg.OIDCClientSecret.Reveal() != "client-s3cret" {
+		t.Errorf("OIDCClientSecret.Reveal() = %q", cfg.OIDCClientSecret.Reveal())
+	}
+	if want := []string{"mealquest-admin", "family"}; !slices.Equal(cfg.OIDCAllowedGroups, want) {
+		t.Errorf("OIDCAllowedGroups = %q, want %q", cfg.OIDCAllowedGroups, want)
+	}
+	if cfg.SessionIdle != 720*time.Hour || cfg.SessionMax != 4320*time.Hour {
+		t.Errorf("SessionIdle, SessionMax = %v, %v, want defaults 720h, 4320h", cfg.SessionIdle, cfg.SessionMax)
+	}
+
+	if cfg.TrustedProxy != nil {
+		t.Errorf("TrustedProxy = %v, want none when unset", cfg.TrustedProxy)
+	}
+
+	env[EnvSessionIdle] = "12h"
+	env[EnvSessionMax] = "36h"
+	env[EnvTrustedProxy] = "10.60.1.33, 172.16.0.0/12"
+	cfg, err = load(func(k string) (string, bool) { v, ok := env[k]; return v, ok })
+	if err != nil {
+		t.Fatalf("load() error = %v", err)
+	}
+	if cfg.SessionIdle != 12*time.Hour || cfg.SessionMax != 36*time.Hour {
+		t.Errorf("SessionIdle, SessionMax = %v, %v, want 12h, 36h", cfg.SessionIdle, cfg.SessionMax)
+	}
+	want := forwarded.Trusted{netip.MustParsePrefix("10.60.1.33/32"), netip.MustParsePrefix("172.16.0.0/12")}
+	if !slices.Equal(cfg.TrustedProxy, want) {
+		t.Errorf("TrustedProxy = %v, want %v", cfg.TrustedProxy, want)
+	}
 }
 
 func TestSecretRedacted(t *testing.T) {
@@ -260,7 +472,7 @@ func TestSecretRedacted(t *testing.T) {
 
 	s := Secret{value: "s3cret-value"}
 	for _, format := range []string{"%v", "%s", "%+v", "%#v"} {
-		cfg := Config{SecretKey: s}
+		cfg := Config{SecretKey: s, OIDCClientSecret: s}
 		for _, out := range []string{fmt.Sprintf(format, s), fmt.Sprintf(format, cfg)} {
 			if strings.Contains(out, "s3cret-value") {
 				t.Errorf("format %s leaked secret: %s", format, out)
diff --git a/internal/forwarded/forwarded.go b/internal/forwarded/forwarded.go
new file mode 100644
index 0000000..8ac429e
--- /dev/null
+++ b/internal/forwarded/forwarded.go
@@ -0,0 +1,221 @@
+// Package forwarded resolves where a request really came from. The service
+// runs behind a reverse proxy (Traefik), so the TCP peer is usually the proxy;
+// the X-Forwarded-For and X-Forwarded-Proto headers it adds are believed only
+// when the peer is a trusted proxy. From anyone else they are ignored, so a
+// client cannot spoof its address or scheme.
+package forwarded
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"net/netip"
+	"strings"
+)
+
+// Header names honoured from trusted proxies.
+const (
+	HeaderFor   = "X-Forwarded-For"
+	HeaderProto = "X-Forwarded-Proto"
+)
+
+// ErrInvalidHeader marks a malformed forwarded header from a trusted peer.
+var ErrInvalidHeader = errors.New("invalid forwarded header")
+
+// Trusted is the set of proxy addresses whose forwarded headers are honoured.
+// The zero value trusts no peer.
+type Trusted []netip.Prefix
+
+// ParseTrusted parses a comma-separated list of IP addresses and CIDR
+// prefixes. A blank list trusts no peer. Each entry must be a plain address
+// or a prefix with no host bits set; zones, IPv4-mapped IPv6 forms and
+// zero-length prefixes, which would trust every peer, are rejected.
+func ParseTrusted(s string) (Trusted, error) {
+	if strings.TrimSpace(s) == "" {
+		return nil, nil
+	}
+	var t Trusted
+	for _, entry := range strings.Split(s, ",") {
+		entry = strings.TrimSpace(entry)
+		if entry == "" {
+			return nil, fmt.Errorf("empty entry in comma-separated list %q", s)
+		}
+		p, err := parseEntry(entry)
+		if err != nil {
+			return nil, err
+		}
+		t = append(t, p)
+	}
+	return t, nil
+}
+
+func parseEntry(entry string) (netip.Prefix, error) {
+	var p netip.Prefix
+	if strings.Contains(entry, "/") {
+		var err error
+		p, err = netip.ParsePrefix(entry)
+		if err != nil {
+			return netip.Prefix{}, fmt.Errorf("invalid CIDR %q", entry)
+		}
+		if p != p.Masked() {
+			return netip.Prefix{}, fmt.Errorf("CIDR %q has host bits set (did you mean %s?)", entry, p.Masked())
+		}
+	} else {
+		a, err := netip.ParseAddr(entry)
+		if err != nil {
+			return netip.Prefix{}, fmt.Errorf("invalid IP address %q", entry)
+		}
+		if a.Zone() != "" {
+			return netip.Prefix{}, fmt.Errorf("IP address %q must not have a zone", entry)
+		}
+		p = netip.PrefixFrom(a, a.BitLen())
+	}
+	switch {
+	case p.Addr().Is4In6():
+		return netip.Prefix{}, fmt.Errorf("%q is IPv4-mapped IPv6; write the IPv4 form", entry)
+	case p.Bits() == 0:
+		return netip.Prefix{}, fmt.Errorf("%q would trust every peer", entry)
+	}
+	return p, nil
+}
+
+// Contains reports whether a is a trusted proxy address. IPv4-mapped IPv6
+// addresses, as a dual-stack listener reports IPv4 peers, match their IPv4
+// form.
+func (t Trusted) Contains(a netip.Addr) bool {
+	a = normalize(a)
+	for _, p := range t {
+		if p.Contains(a) {
+			return true
+		}
+	}
+	return false
+}
+
+func normalize(a netip.Addr) netip.Addr { return a.Unmap().WithZone("") }
+
+// Origin is where a request came from.
+type Origin struct {
+	// ClientIP is the client's address.
+	ClientIP netip.Addr
+	// Scheme is "https" or "http": the scheme the client used.
+	Scheme string
+}
+
+// Resolve works out r's Origin. When the TCP peer (r.RemoteAddr) is not in
+// trusted, both forwarded headers are ignored: the client is the peer and the
+// scheme is that of the connection.
+//
+// When the peer is trusted:
+//   - X-Forwarded-For entries (across all its header lines, in order) are
+//     walked right to left, skipping trusted proxy addresses; the first
+//     untrusted address is the client. Entries left of it are client-supplied
+//     and not examined. If every entry is trusted the leftmost is the client;
+//     with no X-Forwarded-For the peer is the client.
+//   - X-Forwarded-Proto, if present, must be a single "http" or "https" and
+//     is the scheme; without it the connection's scheme is used.
+//
+// A malformed header from a trusted peer is an error wrapping
+// ErrInvalidHeader; any other error means RemoteAddr is not an IP and port.
+func Resolve(r *http.Request, trusted Trusted) (Origin, error) {
+	peer, err := netip.ParseAddrPort(r.RemoteAddr)
+	if err != nil {
+		return Origin{}, fmt.Errorf("peer address %q is not an IP and port", r.RemoteAddr)
+	}
+	o := Origin{ClientIP: normalize(peer.Addr()), Scheme: "http"}
+	if r.TLS != nil {
+		o.Scheme = "https"
+	}
+	if !trusted.Contains(o.ClientIP) {
+		return o, nil
+	}
+	if values := r.Header.Values(HeaderFor); len(values) > 0 {
+		o.ClientIP, err = clientFromChain(strings.Split(strings.Join(values, ","), ","), trusted)
+		if err != nil {
+			return Origin{}, err
+		}
+	}
+	switch values := r.Header.Values(HeaderProto); len(values) {
+	case 0:
+	case 1:
+		switch scheme := strings.ToLower(strings.TrimSpace(values[0])); scheme {
+		case "http", "https":
+			o.Scheme = scheme
+		default:
+			return Origin{}, fmt.Errorf("%w: %s %q is not http or https", ErrInvalidHeader, HeaderProto, values[0])
+		}
+	default:
+		return Origin{}, fmt.Errorf("%w: %d %s headers, want one", ErrInvalidHeader, len(values), HeaderProto)
+	}
+	return o, nil
+}
+
+// clientFromChain returns the rightmost untrusted address in chain, or the
+// leftmost address when all are trusted.
+func clientFromChain(chain []string, trusted Trusted) (netip.Addr, error) {
+	var client netip.Addr
+	for i := len(chain) - 1; i >= 0; i-- {
+		a, err := netip.ParseAddr(strings.TrimSpace(chain[i]))
+		if err != nil {
+			return netip.Addr{}, fmt.Errorf("%w: %s entry %q is not an IP address", ErrInvalidHeader, HeaderFor, strings.TrimSpace(chain[i]))
+		}
+		client = normalize(a)
+		if !trusted.Contains(client) {
+			break
+		}
+	}
+	return client, nil
+}
+
+type contextKey struct{}
+
+// NewContext returns ctx carrying o.
+func NewContext(ctx context.Context, o Origin) context.Context {
+	return context.WithValue(ctx, contextKey{}, o)
+}
+
+// FromContext returns the Origin ctx carries; ok is false when the request
+// was not resolved.
+func FromContext(ctx context.Context) (o Origin, ok bool) {
+	o, ok = ctx.Value(contextKey{}).(Origin)
+	return o, ok
+}
+
+// ClientIP returns the resolved client address ctx carries.
+func ClientIP(ctx context.Context) (netip.Addr, bool) {
+	o, ok := FromContext(ctx)
+	return o.ClientIP, ok
+}
+
+// Scheme returns the resolved request scheme ctx carries.
+func Scheme(ctx context.Context) (string, bool) {
+	o, ok := FromContext(ctx)
+	return o.Scheme, ok
+}
+
+// LogAttr is the log attribute key for the resolved client address.
+const LogAttr = "client_ip"
+
+// LogHandler wraps h so every record logged with a resolved request's
+// context carries the client address as LogAttr.
+func LogHandler(h slog.Handler) slog.Handler { return logHandler{h} }
+
+type logHandler struct{ slog.Handler }
+
+func (h logHandler) Handle(ctx context.Context, rec slog.Record) error {
+	if ip, ok := ClientIP(ctx); ok {
+		rec = rec.Clone()
+		rec.AddAttrs(slog.String(LogAttr, ip.String()))
+	}
+	return h.Handler.Handle(ctx, rec)
+}
+
+func (h logHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	return logHandler{h.Handler.WithAttrs(attrs)}
+}
+
+func (h logHandler) WithGroup(name string) slog.Handler {
+	return logHandler{h.Handler.WithGroup(name)}
+}
diff --git a/internal/forwarded/forwarded_test.go b/internal/forwarded/forwarded_test.go
new file mode 100644
index 0000000..298e9ab
--- /dev/null
+++ b/internal/forwarded/forwarded_test.go
@@ -0,0 +1,260 @@
+package forwarded
+
+import (
+	"bytes"
+	"context"
+	"crypto/tls"
+	"errors"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"slices"
+	"strings"
+	"testing"
+)
+
+func TestParseTrusted(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		in      string
+		want    []string
+		wantErr string
+	}{
+		{in: "", want: nil},
+		{in: "   ", want: nil},
+		{in: "10.60.1.33", want: []string{"10.60.1.33/32"}},
+		{in: "10.60.1.33, 172.16.0.0/12", want: []string{"10.60.1.33/32", "172.16.0.0/12"}},
+		{in: "2001:db8::1,2001:db8::/32", want: []string{"2001:db8::1/128", "2001:db8::/32"}},
+		{in: "traefik", wantErr: "invalid IP address"},
+		{in: "10.60.1.33:443", wantErr: "invalid IP address"},
+		{in: "10.60.1.0/33", wantErr: "invalid CIDR"},
+		{in: "10.60.1.33/24", wantErr: "did you mean 10.60.1.0/24"},
+		{in: "10.60.1.33,,10.60.1.34", wantErr: "empty entry"},
+		{in: "10.60.1.33,", wantErr: "empty entry"},
+		{in: "fe80::1%eth0", wantErr: "zone"},
+		{in: "::ffff:10.60.1.33", wantErr: "IPv4-mapped"},
+		{in: "0.0.0.0/0", wantErr: "every peer"},
+		{in: "::/0", wantErr: "every peer"},
+	}
+	for _, tt := range tests {
+		got, err := ParseTrusted(tt.in)
+		if tt.wantErr != "" {
+			if err == nil || !strings.Contains(err.Error(), tt.wantErr) {
+				t.Errorf("ParseTrusted(%q) error = %v, want one containing %q", tt.in, err, tt.wantErr)
+			}
+			continue
+		}
+		if err != nil {
+			t.Errorf("ParseTrusted(%q) error = %v", tt.in, err)
+			continue
+		}
+		var gotStr []string
+		for _, p := range got {
+			gotStr = append(gotStr, p.String())
+		}
+		if !slices.Equal(gotStr, tt.want) {
+			t.Errorf("ParseTrusted(%q) = %v, want %v", tt.in, gotStr, tt.want)
+		}
+	}
+}
+
+func TestTrustedContainsMappedIPv4(t *testing.T) {
+	t.Parallel()
+	trusted := mustTrusted(t, "10.60.1.33")
+	if !trusted.Contains(netip.MustParseAddr("::ffff:10.60.1.33")) {
+		t.Error("IPv4-mapped form of a trusted address is not trusted")
+	}
+	if Trusted(nil).Contains(netip.MustParseAddr("10.60.1.33")) {
+		t.Error("an empty set trusts a peer")
+	}
+}
+
+func mustTrusted(t *testing.T, s string) Trusted {
+	t.Helper()
+	tr, err := ParseTrusted(s)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return tr
+}
+
+func TestResolve(t *testing.T) {
+	t.Parallel()
+	// 10.60.1.33 is Traefik; 10.60.2.0/24 is an inner proxy tier.
+	trusted := mustTrusted(t, "10.60.1.33,10.60.2.0/24")
+	tests := []struct {
+		name       string
+		peer       string
+		tls        bool
+		xff        []string
+		xfp        []string
+		wantIP     string
+		wantScheme string
+	}{
+		{
+			name: "untrusted peer, no headers",
+			peer: "198.51.100.9:5000", wantIP: "198.51.100.9", wantScheme: "http",
+		},
+		{
+			name: "untrusted peer over TLS",
+			peer: "198.51.100.9:5000", tls: true, wantIP: "198.51.100.9", wantScheme: "https",
+		},
+		{
+			name: "untrusted peer spoofing both headers is ignored",
+			peer: "198.51.100.9:5000", xff: []string{"203.0.113.7"}, xfp: []string{"https"},
+			wantIP: "198.51.100.9", wantScheme: "http",
+		},
+		{
+			name: "untrusted peer's malformed headers are ignored, not rejected",
+			peer: "198.51.100.9:5000", xff: []string{"not-an-ip"}, xfp: []string{"gopher"},
+			wantIP: "198.51.100.9", wantScheme: "http",
+		},
+		{
+			name: "trusted peer: client from X-Forwarded-For, scheme from X-Forwarded-Proto",
+			peer: "10.60.1.33:41000", xff: []string{"203.0.113.7"}, xfp: []string{"https"},
+			wantIP: "203.0.113.7", wantScheme: "https",
+		},
+		{
+			name: "trusted peer seen as IPv4-mapped IPv6",
+			peer: "[::ffff:10.60.1.33]:41000", xff: []string{"203.0.113.7"}, xfp: []string{"https"},
+			wantIP: "203.0.113.7", wantScheme: "https",
+		},
+		{
+			name: "trusted peer without headers is itself the client",
+			peer: "10.60.1.33:41000", wantIP: "10.60.1.33", wantScheme: "http",
+		},
+		{
+			name: "rightmost untrusted address wins over a client-supplied prefix",
+			peer: "10.60.1.33:41000", xff: []string{"1.2.3.4, 203.0.113.7"}, xfp: []string{"https"},
+			wantIP: "203.0.113.7", wantScheme: "https",
+		},
+		{
+			name: "trusted proxies in the chain are skipped",
+			peer: "10.60.1.33:41000", xff: []string{"1.2.3.4, 203.0.113.7, 10.60.2.5"}, xfp: []string{"https"},
+			wantIP: "203.0.113.7", wantScheme: "https",
+		},
+		{
+			name: "several header lines form one chain",
+			peer: "10.60.1.33:41000", xff: []string{"1.2.3.4", "203.0.113.7, 10.60.2.5"},
+			wantIP: "203.0.113.7", wantScheme: "http",
+		},
+		{
+			name: "garbage left of the client is not examined",
+			peer: "10.60.1.33:41000", xff: []string{"spoofed, 203.0.113.7"},
+			wantIP: "203.0.113.7", wantScheme: "http",
+		},
+		{
+			name: "all trusted: leftmost is the client",
+			peer: "10.60.1.33:41000", xff: []string{"10.60.2.5, 10.60.2.6"},
+			wantIP: "10.60.2.5", wantScheme: "http",
+		},
+		{
+			name: "IPv6 client",
+			peer: "10.60.1.33:41000", xff: []string{"2001:db8::7"}, xfp: []string{"HTTPS"},
+			wantIP: "2001:db8::7", wantScheme: "https",
+		},
+		{
+			name: "trusted peer may downgrade to http",
+			peer: "10.60.1.33:41000", tls: true, xfp: []string{"http"},
+			wantIP: "10.60.1.33", wantScheme: "http",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			r := request(tt.peer, tt.xff, tt.xfp)
+			if tt.tls {
+				r.TLS = &tls.ConnectionState{}
+			}
+			o, err := Resolve(r, trusted)
+			if err != nil {
+				t.Fatalf("Resolve error = %v", err)
+			}
+			if o.ClientIP.String() != tt.wantIP || o.Scheme != tt.wantScheme {
+				t.Errorf("Resolve = %s %s, want %s %s", o.ClientIP, o.Scheme, tt.wantIP, tt.wantScheme)
+			}
+		})
+	}
+}
+
+func TestResolveRejectsMalformedHeadersFromTrustedPeer(t *testing.T) {
+	t.Parallel()
+	trusted := mustTrusted(t, "10.60.1.33")
+	for _, tt := range []struct {
+		name     string
+		xff, xfp []string
+	}{
+		{"X-Forwarded-For not an IP", []string{"not-an-ip"}, nil},
+		{"X-Forwarded-For with a port", []string{"203.0.113.7:5000"}, nil},
+		{"X-Forwarded-For empty entry", []string{"203.0.113.7, "}, nil},
+		{"X-Forwarded-For empty", []string{""}, nil},
+		{"X-Forwarded-Proto unknown scheme", nil, []string{"ftp"}},
+		{"X-Forwarded-Proto list", nil, []string{"https, http"}},
+		{"X-Forwarded-Proto repeated", nil, []string{"https", "https"}},
+	} {
+		_, err := Resolve(request("10.60.1.33:41000", tt.xff, tt.xfp), trusted)
+		if !errors.Is(err, ErrInvalidHeader) {
+			t.Errorf("%s: Resolve error = %v, want ErrInvalidHeader", tt.name, err)
+		}
+	}
+}
+
+func TestResolveBadPeerAddress(t *testing.T) {
+	t.Parallel()
+	r := request("@", nil, nil)
+	_, err := Resolve(r, nil)
+	if err == nil || errors.Is(err, ErrInvalidHeader) {
+		t.Errorf("Resolve error = %v, want a peer address error", err)
+	}
+}
+
+func request(peer string, xff, xfp []string) *http.Request {
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+	r.RemoteAddr = peer
+	for _, v := range xff {
+		r.Header.Add(HeaderFor, v)
+	}
+	for _, v := range xfp {
+		r.Header.Add(HeaderProto, v)
+	}
+	return r
+}
+
+func TestContextHelpers(t *testing.T) {
+	t.Parallel()
+	if _, ok := ClientIP(context.Background()); ok {
+		t.Error("ClientIP ok on an unresolved context")
+	}
+	if _, ok := Scheme(context.Background()); ok {
+		t.Error("Scheme ok on an unresolved context")
+	}
+	ctx := NewContext(context.Background(), Origin{ClientIP: netip.MustParseAddr("203.0.113.7"), Scheme: "https"})
+	if ip, ok := ClientIP(ctx); !ok || ip.String() != "203.0.113.7" {
+		t.Errorf("ClientIP = %s %v, want 203.0.113.7 true", ip, ok)
+	}
+	if s, ok := Scheme(ctx); !ok || s != "https" {
+		t.Errorf("Scheme = %q %v, want https true", s, ok)
+	}
+}
+
+func TestLogHandler(t *testing.T) {
+	t.Parallel()
+	var buf bytes.Buffer
+	logger := slog.New(LogHandler(slog.NewTextHandler(&buf, nil))).With("component", "test")
+	ctx := NewContext(context.Background(), Origin{ClientIP: netip.MustParseAddr("203.0.113.7"), Scheme: "https"})
+
+	logger.InfoContext(ctx, "resolved")
+	logger.InfoContext(context.Background(), "unresolved")
+
+	lines := strings.Split(strings.TrimSpace(buf.String()), "\n")
+	if len(lines) != 2 {
+		t.Fatalf("logged %d lines, want 2: %q", len(lines), buf.String())
+	}
+	if !strings.Contains(lines[0], "client_ip=203.0.113.7") || !strings.Contains(lines[0], "component=test") {
+		t.Errorf("resolved line = %q, want client_ip and the With attribute", lines[0])
+	}
+	if strings.Contains(lines[1], "client_ip") {
+		t.Errorf("unresolved line = %q, want no client_ip", lines[1])
+	}
+}
diff --git a/internal/secureurl/secureurl.go b/internal/secureurl/secureurl.go
new file mode 100644
index 0000000..09f3129
--- /dev/null
+++ b/internal/secureurl/secureurl.go
@@ -0,0 +1,32 @@
+// Package secureurl holds the one transport rule for URLs that carry
+// credentials or session cookies: HTTPS, or plain HTTP to a loopback host for
+// local development and tests.
+package secureurl
+
+import (
+	"errors"
+	"fmt"
+	"net/url"
+	"strings"
+)
+
+// loopbackHosts are the only hosts plain HTTP is allowed for, compared
+// case-insensitively. url.URL's Hostname strips the brackets from "[::1]".
+var loopbackHosts = map[string]bool{"localhost": true, "127.0.0.1": true, "::1": true}
+
+// Check reports an error unless u is an absolute https:// URL, or an http://
+// URL whose host is localhost, 127.0.0.1 or ::1.
+func Check(u *url.URL) error {
+	switch {
+	case u.Hostname() == "":
+		return errors.New("URL has no host")
+	case u.Scheme == "https":
+		return nil
+	case u.Scheme == "http" && loopbackHosts[strings.ToLower(u.Hostname())]:
+		return nil
+	case u.Scheme == "http":
+		return errors.New("plain http:// is allowed only for localhost, 127.0.0.1 or [::1]; use https://")
+	default:
+		return fmt.Errorf("unsupported scheme %q (want https://)", u.Scheme)
+	}
+}
diff --git a/internal/secureurl/secureurl_test.go b/internal/secureurl/secureurl_test.go
new file mode 100644
index 0000000..bebc707
--- /dev/null
+++ b/internal/secureurl/secureurl_test.go
@@ -0,0 +1,40 @@
+package secureurl
+
+import (
+	"net/url"
+	"testing"
+)
+
+func TestCheck(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		raw string
+		ok  bool
+	}{
+		{"https://meals.example.com", true},
+		{"https://auth.example.com/application/o/mealquest/", true},
+		{"https://192.0.2.10:8443", true},
+		{"http://localhost:8080", true},
+		{"http://LOCALHOST:8080", true},
+		{"http://127.0.0.1:8080/path", true},
+		{"http://[::1]:8080", true},
+		{"http://[::1]", true},
+		{"http://meals.example.com", false},
+		{"http://192.0.2.10", false},
+		{"http://127.0.0.2", false},
+		{"http://localhost.example.com", false},
+		{"http://0.0.0.0:8080", false},
+		{"ftp://localhost", false},
+		{"https://", false},
+		{"meals.example.com", false},
+	}
+	for _, tc := range tests {
+		u, err := url.Parse(tc.raw)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := Check(u); (err == nil) != tc.ok {
+			t.Errorf("Check(%q) error = %v, want ok=%v", tc.raw, err, tc.ok)
+		}
+	}
+}
diff --git a/internal/server/csrf.go b/internal/server/csrf.go
new file mode 100644
index 0000000..65c21a3
--- /dev/null
+++ b/internal/server/csrf.go
@@ -0,0 +1,42 @@
+package server
+
+import (
+	"crypto/subtle"
+	"log/slog"
+	"net/http"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
+)
+
+// CSRFHeader carries the session's CSRF token, which the client reads from
+// GET /api/v1/me.
+const CSRFHeader = "X-MealQuest-CSRF"
+
+// requireCSRF passes GET requests to next; every other
+// request must carry CSRFHeader equal to its session's CSRF token or get 403
+// csrf. It runs behind requireSession, which puts the session in the context.
+func requireCSRF(logger *slog.Logger) func(http.Handler) http.Handler {
+	forbidden := apiErrorHandler(logger, http.StatusForbidden, "csrf", "missing or invalid CSRF token")
+	internal := apiErrorHandler(logger, http.StatusInternalServerError, "internal", "internal error")
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			switch r.Method {
+			case http.MethodGet:
+				next.ServeHTTP(w, r)
+				return
+			}
+			s, ok := auth.SessionFrom(r.Context())
+			if !ok || s.CSRFToken == "" {
+				// An empty token would match a missing header.
+				logger.ErrorContext(r.Context(), "csrf check without a session token", "session", ok)
+				internal(w, r)
+				return
+			}
+			if subtle.ConstantTimeCompare([]byte(r.Header.Get(CSRFHeader)), []byte(s.CSRFToken)) != 1 {
+				forbidden(w, r)
+				return
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}
diff --git a/internal/server/csrf_test.go b/internal/server/csrf_test.go
new file mode 100644
index 0000000..7e1dd54
--- /dev/null
+++ b/internal/server/csrf_test.go
@@ -0,0 +1,166 @@
+package server
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
+)
+
+const (
+	csrfJSON             = `{"error":{"code":"csrf","message":"missing or invalid CSRF token","retryable":false}}`
+	methodNotAllowedJSON = `{"error":{"code":"method_not_allowed","message":"method not allowed","retryable":false}}`
+)
+
+func TestCSRFThroughRouter(t *testing.T) {
+	t.Parallel()
+
+	router := newTestRouter(t, "dev")
+	for _, tc := range []struct {
+		name, method, path string
+		header             []string // nil sends no CSRF header
+		wantStatus         int
+		wantBody           string
+	}{
+		{"POST without header", http.MethodPost, "/api/v1/me", nil, http.StatusForbidden, csrfJSON},
+		{"POST with wrong header", http.MethodPost, "/api/v1/me", []string{"wrong-token"}, http.StatusForbidden, csrfJSON},
+		{"POST with empty header", http.MethodPost, "/api/v1/me", []string{""}, http.StatusForbidden, csrfJSON},
+		{"POST with token prefix", http.MethodPost, "/api/v1/me", []string{"csrf-token"}, http.StatusForbidden, csrfJSON},
+		{"PATCH without header", http.MethodPatch, "/api/v1/settings/household", nil, http.StatusForbidden, csrfJSON},
+		{"DELETE with wrong header", http.MethodDelete, "/api/v1/me", []string{"wrong-token"}, http.StatusForbidden, csrfJSON},
+		// The right token passes the check and reaches the router, which has
+		// no POST /api/v1/me.
+		{"POST with right header", http.MethodPost, "/api/v1/me", []string{testSession.CSRFToken}, http.StatusMethodNotAllowed, methodNotAllowedJSON},
+		{"PATCH with right header", http.MethodPatch, "/api/v1/settings/household", []string{testSession.CSRFToken}, http.StatusNotFound, notFoundJSON},
+		{"GET without header", http.MethodGet, "/api/v1/me", nil, http.StatusOK, ""},
+		// Only GET is exempt; HEAD and OPTIONS need the header too.
+		{"HEAD without header", http.MethodHead, "/api/v1/me", nil, http.StatusForbidden, csrfJSON},
+		{"OPTIONS without header", http.MethodOptions, "/api/v1/me", nil, http.StatusForbidden, csrfJSON},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			req := httptest.NewRequest(tc.method, tc.path, nil)
+			req.AddCookie(validSessionCookie)
+			if tc.header != nil {
+				req.Header.Set(CSRFHeader, tc.header[0])
+			}
+			rec := httptest.NewRecorder()
+			router.ServeHTTP(rec, req)
+
+			if rec.Code != tc.wantStatus {
+				t.Errorf("status = %d, want %d (body %s)", rec.Code, tc.wantStatus, rec.Body.String())
+			}
+			if tc.wantBody != "" && rec.Body.String() != tc.wantBody {
+				t.Errorf("body = %s, want %s", rec.Body.String(), tc.wantBody)
+			}
+		})
+	}
+}
+
+func TestCSRFUnauthenticatedGets401(t *testing.T) {
+	t.Parallel()
+
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/me", nil)
+	rec := httptest.NewRecorder()
+	newTestRouter(t, "dev").ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusUnauthorized || rec.Body.String() != unauthenticatedJSON {
+		t.Errorf("response = %d %s, want 401 %s", rec.Code, rec.Body.String(), unauthenticatedJSON)
+	}
+}
+
+func TestRequireCSRF(t *testing.T) {
+	t.Parallel()
+
+	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
+	for _, tc := range []struct {
+		name       string
+		method     string
+		session    *auth.Session
+		header     string
+		wantStatus int
+	}{
+		{"right header reaches handler", http.MethodPost, &testSession, testSession.CSRFToken, http.StatusNoContent},
+		{"wrong header", http.MethodPut, &testSession, "nope", http.StatusForbidden},
+		{"GET needs no header", http.MethodGet, &testSession, "", http.StatusNoContent},
+		{"HEAD needs the header", http.MethodHead, &testSession, "", http.StatusForbidden},
+		{"OPTIONS needs the header", http.MethodOptions, &testSession, "", http.StatusForbidden},
+		{"no session in context", http.MethodPost, nil, "", http.StatusInternalServerError},
+		{"session without token", http.MethodPost, &auth.Session{Subject: "user-123"}, "", http.StatusInternalServerError},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			called := false
+			h := requireCSRF(logger)(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				called = true
+				w.WriteHeader(http.StatusNoContent)
+			}))
+			ctx := context.Background()
+			if tc.session != nil {
+				ctx = auth.WithSession(ctx, *tc.session)
+			}
+			req := httptest.NewRequestWithContext(ctx, tc.method, "/api/v1/anything", nil)
+			if tc.header != "" {
+				req.Header.Set(CSRFHeader, tc.header)
+			}
+			rec := httptest.NewRecorder()
+			h.ServeHTTP(rec, req)
+
+			if rec.Code != tc.wantStatus {
+				t.Errorf("status = %d, want %d", rec.Code, tc.wantStatus)
+			}
+			if called != (tc.wantStatus == http.StatusNoContent) {
+				t.Errorf("handler called = %v with status %d", called, rec.Code)
+			}
+		})
+	}
+}
+
+func TestMe(t *testing.T) {
+	t.Parallel()
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/me", nil)
+	req.AddCookie(validSessionCookie)
+	rec := httptest.NewRecorder()
+	newTestRouter(t, "dev").ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200 (body %s)", rec.Code, rec.Body.String())
+	}
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Errorf("Content-Type = %q, want application/json", ct)
+	}
+	if cc := rec.Header().Get("Cache-Control"); cc != "no-store" {
+		t.Errorf("Cache-Control = %q, want no-store", cc)
+	}
+	var got map[string]any
+	if err := json.Unmarshal(rec.Body.Bytes(), &got); err != nil {
+		t.Fatalf("decode %s: %v", rec.Body.String(), err)
+	}
+	want := map[string]any{
+		"name":      "Home Cook",
+		"email":     "cook@example.com",
+		"household": map[string]any{"label": "Our kitchen"},
+		"csrfToken": "csrf-token-123",
+	}
+	gotJSON, _ := json.Marshal(got)
+	wantJSON, _ := json.Marshal(want)
+	if string(gotJSON) != string(wantJSON) {
+		t.Errorf("body = %s, want %s", gotJSON, wantJSON)
+	}
+}
+
+func TestMeNeedsSession(t *testing.T) {
+	t.Parallel()
+
+	rec := httptest.NewRecorder()
+	newTestRouter(t, "dev").ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/api/v1/me", nil))
+	if rec.Code != http.StatusUnauthorized || rec.Body.String() != unauthenticatedJSON {
+		t.Errorf("response = %d %s, want 401 %s", rec.Code, rec.Body.String(), unauthenticatedJSON)
+	}
+}
diff --git a/internal/server/login_test.go b/internal/server/login_test.go
new file mode 100644
index 0000000..d928de7
--- /dev/null
+++ b/internal/server/login_test.go
@@ -0,0 +1,140 @@
+package server
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+	"testing/fstest"
+	"time"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/forwarded"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/store"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/migrations"
+)
+
+// newDiscoveryProvider serves the OIDC discovery document auth.New needs;
+// login only redirects to its authorization endpoint.
+func newDiscoveryProvider(t *testing.T) *httptest.Server {
+	t.Helper()
+	var srv *httptest.Server
+	srv = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/.well-known/openid-configuration" {
+			http.NotFound(w, r)
+			return
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(map[string]any{
+			"issuer":                                srv.URL,
+			"authorization_endpoint":                srv.URL + "/authorize",
+			"token_endpoint":                        srv.URL + "/token",
+			"jwks_uri":                              srv.URL + "/jwks",
+			"userinfo_endpoint":                     srv.URL + "/userinfo",
+			"end_session_endpoint":                  srv.URL + "/end-session/",
+			"response_types_supported":              []string{"code"},
+			"subject_types_supported":               []string{"public"},
+			"id_token_signing_alg_values_supported": []string{"RS256"},
+		})
+	}))
+	t.Cleanup(srv.Close)
+	return srv
+}
+
+// newLoginRouter wires the production router as cmd/server does: the real
+// auth flow and session manager, trusting Traefik only.
+func newLoginRouter(t *testing.T, issuer string) http.Handler {
+	t.Helper()
+	ctx := context.Background()
+	db, err := store.Open(ctx, t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if err := db.Close(); err != nil {
+			t.Error(err)
+		}
+	})
+	if err := store.Migrate(ctx, db.Writer, migrations.FS); err != nil {
+		t.Fatal(err)
+	}
+	sessions, err := auth.NewSessionManager(db, 720*time.Hour, 4320*time.Hour)
+	if err != nil {
+		t.Fatal(err)
+	}
+	logger := slog.New(forwarded.LogHandler(slog.NewTextHandler(io.Discard, nil)))
+	flow, err := auth.New(ctx, auth.Config{
+		Issuer:        issuer,
+		ClientID:      "mealquest",
+		ClientSecret:  "client-s3cret",
+		PublicURL:     &url.URL{Scheme: "https", Host: "meals.example.com"},
+		SecretKey:     []byte("server-secret-key"),
+		AllowedGroups: []string{"mealquest-admin"},
+	}, sessions, logger)
+	if err != nil {
+		t.Fatal(err)
+	}
+	trusted, err := forwarded.ParseTrusted(traefik)
+	if err != nil {
+		t.Fatal(err)
+	}
+	r, err := NewRouter(logger, "dev", trusted, flow, sessions,
+		fstest.MapFS{"index.html": {Data: []byte(fixtureIndex)}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return r
+}
+
+// TestLoginThroughRouter starts a login through the production router. The
+// redirect URI and the Secure login cookie come from MEALQUEST_PUBLIC_URL
+// whatever the direct peer: forwarded headers from an untrusted peer are
+// ignored but never fail sign-in.
+func TestLoginThroughRouter(t *testing.T) {
+	t.Parallel()
+	provider := newDiscoveryProvider(t)
+	router := newLoginRouter(t, provider.URL)
+	// Traefik reaches the service over plain http on its internal address;
+	// the host headers must not leak into the redirect URI.
+	headers := map[string]string{
+		forwarded.HeaderFor:   "203.0.113.7",
+		forwarded.HeaderProto: "https",
+		"X-Forwarded-Host":    "evil.example.com",
+	}
+	tests := []struct {
+		name string
+		peer string
+	}{
+		{"trusted proxy", traefik + ":41000"},
+		{"untrusted peer", "172.17.0.1:5000"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			rec := httptest.NewRecorder()
+			router.ServeHTTP(rec, originRequest("http://mealquest:8080"+auth.LoginPath, tt.peer, headers))
+			resp := rec.Result()
+			if resp.StatusCode != http.StatusFound {
+				t.Fatalf("login status = %d, want 302", resp.StatusCode)
+			}
+			loc, err := url.Parse(resp.Header.Get("Location"))
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got, want := loc.Scheme+"://"+loc.Host+loc.Path, provider.URL+"/authorize"; got != want {
+				t.Errorf("redirect to %s, want %s", got, want)
+			}
+			if got, want := loc.Query().Get("redirect_uri"), "https://meals.example.com/auth/callback"; got != want {
+				t.Errorf("redirect_uri = %q, want %q from the public URL", got, want)
+			}
+			cookies := resp.Cookies()
+			if len(cookies) != 1 || cookies[0].Name != "__Host-mealquest_login" || !cookies[0].Secure || !cookies[0].HttpOnly {
+				t.Errorf("login cookies = %v, want one Secure, HttpOnly __Host-mealquest_login", cookies)
+			}
+		})
+	}
+}
diff --git a/internal/server/me.go b/internal/server/me.go
new file mode 100644
index 0000000..e9a8a26
--- /dev/null
+++ b/internal/server/me.go
@@ -0,0 +1,52 @@
+package server
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
+)
+
+// householdLabel is the single shared household identity.
+const householdLabel = "Our kitchen"
+
+type meResponse struct {
+	Name      string        `json:"name"`
+	Email     string        `json:"email"`
+	Household householdInfo `json:"household"`
+	CSRFToken string        `json:"csrfToken"`
+}
+
+type householdInfo struct {
+	Label string `json:"label"`
+}
+
+// me answers GET /api/v1/me with the signed-in member, the household and the
+// session's CSRF token for CSRFHeader.
+func me(logger *slog.Logger) http.HandlerFunc {
+	internal := apiErrorHandler(logger, http.StatusInternalServerError, "internal", "internal error")
+	return func(w http.ResponseWriter, r *http.Request) {
+		s, ok := auth.SessionFrom(r.Context())
+		if !ok {
+			logger.ErrorContext(r.Context(), "GET /api/v1/me without a session")
+			internal(w, r)
+			return
+		}
+		body, err := json.Marshal(meResponse{
+			Name:      s.Name,
+			Email:     s.Email,
+			Household: householdInfo{Label: householdLabel},
+			CSRFToken: s.CSRFToken,
+		})
+		if err != nil {
+			panic(err) // string fields cannot fail to marshal
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.Header().Set("Cache-Control", "no-store")
+		w.WriteHeader(http.StatusOK)
+		if _, err := w.Write(body); err != nil {
+			logger.WarnContext(r.Context(), "write me response", "error", err)
+		}
+	}
+}
diff --git a/internal/server/origin.go b/internal/server/origin.go
new file mode 100644
index 0000000..9f711ad
--- /dev/null
+++ b/internal/server/origin.go
@@ -0,0 +1,34 @@
+package server
+
+import (
+	"errors"
+	"log/slog"
+	"net/http"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/forwarded"
+)
+
+// resolveOrigin resolves every request's client address and scheme with
+// forwarded.Resolve and puts them in the context (read them with
+// forwarded.ClientIP and forwarded.Scheme). Forwarded headers are honoured
+// only from a peer in trusted. A malformed forwarded header from a trusted
+// peer gets 400 bad_request.
+func resolveOrigin(logger *slog.Logger, trusted forwarded.Trusted) func(http.Handler) http.Handler {
+	badRequest := apiErrorHandler(logger, http.StatusBadRequest, "bad_request", "invalid forwarded headers")
+	internal := apiErrorHandler(logger, http.StatusInternalServerError, "internal", "internal error")
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			o, err := forwarded.Resolve(r, trusted)
+			switch {
+			case errors.Is(err, forwarded.ErrInvalidHeader):
+				logger.WarnContext(r.Context(), "rejected forwarded headers", "peer", r.RemoteAddr, "error", err)
+				badRequest(w, r)
+			case err != nil:
+				logger.ErrorContext(r.Context(), "resolve request origin", "error", err)
+				internal(w, r)
+			default:
+				next.ServeHTTP(w, r.WithContext(forwarded.NewContext(r.Context(), o)))
+			}
+		})
+	}
+}
diff --git a/internal/server/origin_test.go b/internal/server/origin_test.go
new file mode 100644
index 0000000..1df8212
--- /dev/null
+++ b/internal/server/origin_test.go
@@ -0,0 +1,145 @@
+package server
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"testing/fstest"
+
+	"github.com/go-chi/chi/v5"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/forwarded"
+)
+
+// traefik is the production proxy's address.
+const traefik = "10.60.1.33"
+
+// probeAuth mounts GET /auth/probe, which reports the request's resolved
+// origin, in place of the real auth flow.
+type probeAuth struct{}
+
+func (probeAuth) Mount(r chi.Router) {
+	r.Get("/auth/probe", func(w http.ResponseWriter, r *http.Request) {
+		ip, ipOK := forwarded.ClientIP(r.Context())
+		scheme, schemeOK := forwarded.Scheme(r.Context())
+		if !ipOK || !schemeOK {
+			http.Error(w, "origin not resolved", http.StatusInternalServerError)
+			return
+		}
+		fmt.Fprintf(w, "%s %s", ip, scheme)
+	})
+}
+
+// newOriginRouter builds the production router trusting Traefik, logging
+// through forwarded.LogHandler as cmd/server does.
+func newOriginRouter(t *testing.T, logs io.Writer) http.Handler {
+	t.Helper()
+	trusted, err := forwarded.ParseTrusted(traefik)
+	if err != nil {
+		t.Fatal(err)
+	}
+	logger := slog.New(forwarded.LogHandler(slog.NewJSONHandler(logs, nil)))
+	r, err := NewRouter(logger, "dev", trusted, probeAuth{}, stubSessions{},
+		fstest.MapFS{"index.html": {Data: []byte(fixtureIndex)}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return r
+}
+
+func originRequest(target, peer string, headers map[string]string) *http.Request {
+	req := httptest.NewRequest(http.MethodGet, target, nil)
+	req.RemoteAddr = peer
+	for k, v := range headers {
+		req.Header.Set(k, v)
+	}
+	return req
+}
+
+func TestRouterResolvesOrigin(t *testing.T) {
+	t.Parallel()
+	spoofed := map[string]string{forwarded.HeaderFor: "203.0.113.7", forwarded.HeaderProto: "https"}
+	tests := []struct {
+		name string
+		peer string
+		want string
+	}{
+		{"trusted proxy's forwarded headers are honoured", traefik + ":41000", "203.0.113.7 https"},
+		{"untrusted peer spoofing them is ignored", "198.51.100.9:5000", "198.51.100.9 http"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			rec := httptest.NewRecorder()
+			newOriginRouter(t, io.Discard).ServeHTTP(rec, originRequest("/auth/probe", tt.peer, spoofed))
+			if rec.Code != http.StatusOK || rec.Body.String() != tt.want {
+				t.Errorf("probe = %d %q, want 200 %q", rec.Code, rec.Body.String(), tt.want)
+			}
+		})
+	}
+}
+
+func TestRouterLogsResolvedClientIP(t *testing.T) {
+	t.Parallel()
+	spoofed := map[string]string{forwarded.HeaderFor: "203.0.113.7", forwarded.HeaderProto: "https"}
+	tests := []struct {
+		name string
+		peer string
+		want string
+	}{
+		{"trusted proxy", traefik + ":41000", "203.0.113.7"},
+		{"untrusted peer spoofing", "198.51.100.9:5000", "198.51.100.9"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			var logs bytes.Buffer
+			// A failed session lookup is logged with the request's context.
+			req := originRequest("/api/v1/me", tt.peer, spoofed)
+			req.AddCookie(brokenSessionCookie)
+			rec := httptest.NewRecorder()
+			newOriginRouter(t, &logs).ServeHTTP(rec, req)
+			if rec.Code != http.StatusInternalServerError {
+				t.Fatalf("status = %d, want 500", rec.Code)
+			}
+			var line map[string]any
+			if err := json.Unmarshal(logs.Bytes(), &line); err != nil {
+				t.Fatalf("log %q: %v", logs.String(), err)
+			}
+			if line["msg"] != "look up session" || line[forwarded.LogAttr] != tt.want {
+				t.Errorf("log = %v, want the session error with %s=%s", line, forwarded.LogAttr, tt.want)
+			}
+		})
+	}
+}
+
+func TestRouterRejectsMalformedForwardedHeaders(t *testing.T) {
+	t.Parallel()
+	const badRequestJSON = `{"error":{"code":"bad_request","message":"invalid forwarded headers","retryable":false}}`
+	for _, headers := range []map[string]string{
+		{forwarded.HeaderFor: "not-an-ip"},
+		{forwarded.HeaderProto: "ftp"},
+	} {
+		var logs bytes.Buffer
+		rec := httptest.NewRecorder()
+		newOriginRouter(t, &logs).ServeHTTP(rec, originRequest("/health", traefik+":41000", headers))
+		if rec.Code != http.StatusBadRequest || rec.Body.String() != badRequestJSON {
+			t.Errorf("%v: response = %d %s, want 400 %s", headers, rec.Code, rec.Body.String(), badRequestJSON)
+		}
+		if !bytes.Contains(logs.Bytes(), []byte("rejected forwarded headers")) {
+			t.Errorf("%v: logs = %q, want the rejection logged", headers, logs.String())
+		}
+
+		// The same headers from an untrusted peer are not examined at all.
+		rec = httptest.NewRecorder()
+		newOriginRouter(t, io.Discard).ServeHTTP(rec, originRequest("/health", "198.51.100.9:5000", headers))
+		if rec.Code != http.StatusOK {
+			t.Errorf("%v from an untrusted peer: status = %d, want 200", headers, rec.Code)
+		}
+	}
+}
diff --git a/internal/server/server.go b/internal/server/server.go
index 929c846..46520c2 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -9,6 +9,7 @@ import (
 
 	"github.com/go-chi/chi/v5"
 
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/forwarded"
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/frontend"
 )
 
@@ -16,9 +17,18 @@ import (
 // SPA's index.html.
 var reservedPrefixes = []string{"/api", "/auth", "/media", "/health"}
 
-// NewRouter returns the HTTP handler for the service. version is reported by
-// GET /health; assets is the web build served for every other GET.
-func NewRouter(logger *slog.Logger, version string, assets fs.FS) (http.Handler, error) {
+// RouteMounter registers its own routes on the router.
+type RouteMounter interface {
+	Mount(r chi.Router)
+}
+
+// NewRouter returns the HTTP handler for the service. Every request first
+// has its client address and scheme resolved, honouring forwarded headers
+// only from a peer in trusted (see resolveOrigin). version is reported by
+// GET /health; auth mounts the /auth routes; sessions guards /api/v1 and
+// /media; assets is the web build served for every other GET. /health, /auth
+// and the web build are public. Requests to /api/v1 other than GET also need the session's CSRF token in CSRFHeader.
+func NewRouter(logger *slog.Logger, version string, trusted forwarded.Trusted, auth RouteMounter, sessions SessionLookup, assets fs.FS) (http.Handler, error) {
 	notFound := apiErrorHandler(logger, http.StatusNotFound, "not_found", "not found")
 	methodNotAllowed := apiErrorHandler(logger, http.StatusMethodNotAllowed, "method_not_allowed", "method not allowed")
 
@@ -28,16 +38,31 @@ func NewRouter(logger *slog.Logger, version string, assets fs.FS) (http.Handler,
 	}
 
 	r := chi.NewRouter()
+	r.Use(resolveOrigin(logger, trusted))
 	r.MethodNotAllowed(methodNotAllowed)
 	r.NotFound(notFound)
 
 	r.Get("/health", health(logger, version))
+	auth.Mount(r)
+	// The session middleware wraps the whole /api/v1 and /media subrouters
+	// rather than being added with Use: chi skips a subrouter's middleware
+	// when it has no routes, which would leave it unguarded.
+	signedIn := requireSession(logger, sessions)
+	csrf := requireCSRF(logger)
+	apiV1, media := chi.NewRouter(), chi.NewRouter()
+	for _, sub := range []*chi.Mux{apiV1, media} {
+		sub.MethodNotAllowed(methodNotAllowed)
+		sub.NotFound(notFound)
+	}
+	apiV1.Get("/me", me(logger))
 	r.Route("/api", func(r chi.Router) {
 		r.MethodNotAllowed(methodNotAllowed)
 		r.NotFound(notFound)
+		r.Mount("/v1", signedIn(csrf(apiV1)))
 	})
-	// Catch-all for the PWA. /auth and /media have no routes yet; the frontend
-	// handler passes them to notFound rather than the SPA fallback.
+	r.Mount("/media", signedIn(media))
+	// Catch-all for the PWA. Unknown /auth paths reach the frontend handler,
+	// which passes them to notFound rather than the SPA fallback.
 	r.Method(http.MethodGet, "/*", web)
 	r.Method(http.MethodHead, "/*", web)
 	return r, nil
diff --git a/internal/server/server_test.go b/internal/server/server_test.go
index f1adc7e..7419658 100644
--- a/internal/server/server_test.go
+++ b/internal/server/server_test.go
@@ -10,6 +10,8 @@ import (
 	"strings"
 	"testing"
 	"testing/fstest"
+
+	"github.com/go-chi/chi/v5"
 )
 
 const (
@@ -25,13 +27,39 @@ func newTestRouter(t *testing.T, version string) http.Handler {
 		"index.html":             {Data: []byte(fixtureIndex)},
 		"assets/index-AbC123.js": {Data: []byte(fixtureAsset)},
 	}
-	r, err := NewRouter(logger, version, assets)
+	r, err := NewRouter(logger, version, nil, stubAuth{}, stubSessions{}, assets)
 	if err != nil {
 		t.Fatal(err)
 	}
 	return r
 }
 
+// stubAuth mounts a fixed GET /auth/login in place of the real auth flow.
+type stubAuth struct{}
+
+func (stubAuth) Mount(r chi.Router) {
+	r.Get("/auth/login", func(w http.ResponseWriter, _ *http.Request) {
+		io.WriteString(w, "stub login")
+	})
+}
+
+func TestAuthRoutesMounted(t *testing.T) {
+	t.Parallel()
+
+	router := newTestRouter(t, "dev")
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/auth/login", nil))
+	if rec.Code != http.StatusOK || rec.Body.String() != "stub login" {
+		t.Errorf("GET /auth/login = %d %q, want the mounted handler", rec.Code, rec.Body.String())
+	}
+
+	rec = httptest.NewRecorder()
+	router.ServeHTTP(rec, httptest.NewRequest(http.MethodPost, "/auth/login", nil))
+	if rec.Code != http.StatusMethodNotAllowed {
+		t.Errorf("POST /auth/login status = %d, want 405", rec.Code)
+	}
+}
+
 func TestHealth(t *testing.T) {
 	t.Parallel()
 
@@ -98,8 +126,10 @@ func TestReservedPathsNotFound(t *testing.T) {
 	}{
 		{http.MethodGet, "/api"},
 		{http.MethodGet, "/api/unknown"},
+		{http.MethodGet, "/api/v1/unknown"},
+		{http.MethodPost, "/api/v1/unknown"},
 		{http.MethodPost, "/api/unknown"},
-		{http.MethodGet, "/auth/callback"},
+		{http.MethodGet, "/auth/unknown"},
 		{http.MethodGet, "/media/photo.jpg"},
 		{http.MethodGet, "/health/deep"},
 		{http.MethodGet, "/auth/../quests/42"},
@@ -109,8 +139,13 @@ func TestReservedPathsNotFound(t *testing.T) {
 		t.Run(tt.method+" "+tt.path, func(t *testing.T) {
 			t.Parallel()
 
+			// Signed in with the CSRF token, so /api/v1 and /media reach
+			// their routers.
+			req := httptest.NewRequest(tt.method, tt.path, nil)
+			req.AddCookie(validSessionCookie)
+			req.Header.Set(CSRFHeader, testSession.CSRFToken)
 			rec := httptest.NewRecorder()
-			router.ServeHTTP(rec, httptest.NewRequest(tt.method, tt.path, nil))
+			router.ServeHTTP(rec, req)
 
 			if rec.Code != http.StatusNotFound {
 				t.Errorf("status = %d, want 404", rec.Code)
diff --git a/internal/server/session.go b/internal/server/session.go
new file mode 100644
index 0000000..e718389
--- /dev/null
+++ b/internal/server/session.go
@@ -0,0 +1,37 @@
+package server
+
+import (
+	"errors"
+	"log/slog"
+	"net/http"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
+)
+
+// SessionLookup finds the live session a request carries; it returns
+// auth.ErrNoSession when there is none. *auth.SessionManager implements it.
+type SessionLookup interface {
+	SessionFor(r *http.Request) (auth.Session, error)
+}
+
+// requireSession is the RequireSession middleware. It passes requests with a
+// live session to next with the session in the context (read it with
+// auth.SessionFrom or auth.ActorFrom); others get 401 unauthenticated.
+func requireSession(logger *slog.Logger, sessions SessionLookup) func(http.Handler) http.Handler {
+	unauthenticated := apiErrorHandler(logger, http.StatusUnauthorized, "unauthenticated", "sign in required")
+	internal := apiErrorHandler(logger, http.StatusInternalServerError, "internal", "internal error")
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			s, err := sessions.SessionFor(r)
+			switch {
+			case errors.Is(err, auth.ErrNoSession):
+				unauthenticated(w, r)
+			case err != nil:
+				logger.ErrorContext(r.Context(), "look up session", "error", err)
+				internal(w, r)
+			default:
+				next.ServeHTTP(w, r.WithContext(auth.WithSession(r.Context(), s)))
+			}
+		})
+	}
+}
diff --git a/internal/server/session_test.go b/internal/server/session_test.go
new file mode 100644
index 0000000..d221b4c
--- /dev/null
+++ b/internal/server/session_test.go
@@ -0,0 +1,152 @@
+package server
+
+import (
+	"errors"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"testing"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/auth"
+)
+
+const (
+	unauthenticatedJSON = `{"error":{"code":"unauthenticated","message":"sign in required","retryable":false}}`
+	internalJSON        = `{"error":{"code":"internal","message":"internal error","retryable":false}}`
+)
+
+var (
+	validSessionCookie  = &http.Cookie{Name: auth.SessionCookieName, Value: "valid"}
+	brokenSessionCookie = &http.Cookie{Name: auth.SessionCookieName, Value: "broken"}
+	testSession         = auth.Session{
+		Subject:   "user-123",
+		Email:     "cook@example.com",
+		Name:      "Home Cook",
+		Groups:    []string{"mealquest-admin"},
+		CSRFToken: "csrf-token-123",
+	}
+)
+
+// stubSessions knows one valid session cookie; the broken cookie fails the
+// lookup as a database error would.
+type stubSessions struct{}
+
+func (stubSessions) SessionFor(r *http.Request) (auth.Session, error) {
+	c, err := r.Cookie(auth.SessionCookieName)
+	switch {
+	case err != nil:
+		return auth.Session{}, auth.ErrNoSession
+	case c.Value == validSessionCookie.Value:
+		return testSession, nil
+	case c.Value == brokenSessionCookie.Value:
+		return auth.Session{}, errors.New("database down")
+	default:
+		return auth.Session{}, auth.ErrNoSession
+	}
+}
+
+func TestProtectedPathsRequireSession(t *testing.T) {
+	t.Parallel()
+
+	router := newTestRouter(t, "dev")
+	for _, tc := range []struct {
+		method, path string
+		cookie       *http.Cookie
+	}{
+		{http.MethodGet, "/api/v1/settings/household", nil},
+		{http.MethodPatch, "/api/v1/settings/household", nil},
+		{http.MethodGet, "/api/v1", nil},
+		{http.MethodGet, "/media/photo.jpg", nil},
+		{http.MethodGet, "/api/v1/settings/household", &http.Cookie{Name: auth.SessionCookieName, Value: "unknown"}},
+	} {
+		t.Run(tc.method+" "+tc.path, func(t *testing.T) {
+			t.Parallel()
+			req := httptest.NewRequest(tc.method, tc.path, nil)
+			if tc.cookie != nil {
+				req.AddCookie(tc.cookie)
+			}
+			rec := httptest.NewRecorder()
+			router.ServeHTTP(rec, req)
+
+			if rec.Code != http.StatusUnauthorized {
+				t.Errorf("status = %d, want 401", rec.Code)
+			}
+			if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+				t.Errorf("Content-Type = %q, want application/json", ct)
+			}
+			if got := rec.Body.String(); got != unauthenticatedJSON {
+				t.Errorf("body = %s, want %s", got, unauthenticatedJSON)
+			}
+		})
+	}
+}
+
+func TestPublicPathsNeedNoSession(t *testing.T) {
+	t.Parallel()
+
+	router := newTestRouter(t, "dev")
+	for _, path := range []string{"/health", "/auth/login", "/", "/assets/index-AbC123.js", "/quests/42"} {
+		t.Run(path, func(t *testing.T) {
+			t.Parallel()
+			rec := httptest.NewRecorder()
+			router.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, path, nil))
+			if rec.Code != http.StatusOK {
+				t.Errorf("GET %s without a cookie = %d, want 200", path, rec.Code)
+			}
+		})
+	}
+}
+
+func TestSessionLookupFailure(t *testing.T) {
+	t.Parallel()
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/settings/household", nil)
+	req.AddCookie(brokenSessionCookie)
+	rec := httptest.NewRecorder()
+	newTestRouter(t, "dev").ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusInternalServerError || rec.Body.String() != internalJSON {
+		t.Errorf("response = %d %s, want 500 %s", rec.Code, rec.Body.String(), internalJSON)
+	}
+}
+
+func TestRequireSessionAttachesActor(t *testing.T) {
+	t.Parallel()
+
+	var got auth.Actor
+	var ok bool
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		got, ok = auth.ActorFrom(r.Context())
+		w.WriteHeader(http.StatusNoContent)
+	})
+	h := requireSession(slog.New(slog.NewTextHandler(io.Discard, nil)), stubSessions{})(next)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/settings/household", nil)
+	req.AddCookie(validSessionCookie)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusNoContent {
+		t.Fatalf("status = %d, want the handler's 204", rec.Code)
+	}
+	if !ok || got.Subject != "user-123" || got.Email != "cook@example.com" || got.Name != "Home Cook" ||
+		!slices.Equal(got.Groups, []string{"mealquest-admin"}) {
+		t.Errorf("actor = %+v (ok=%v), want the session's household member", got, ok)
+	}
+}
+
+func TestRequireSessionStopsUnauthenticated(t *testing.T) {
+	t.Parallel()
+
+	called := false
+	next := http.HandlerFunc(func(http.ResponseWriter, *http.Request) { called = true })
+	h := requireSession(slog.New(slog.NewTextHandler(io.Discard, nil)), stubSessions{})(next)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/api/v1/settings/household", nil))
+
+	if called || rec.Code != http.StatusUnauthorized {
+		t.Errorf("handler called = %v, status = %d; want not called and 401", called, rec.Code)
+	}
+}