diff --git a/cmd/server/main.go b/cmd/server/main.go
index 89f6b9d..229a69a 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -57,17 +57,18 @@ func run(cfg config.Config, logger *slog.Logger) error {
 		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()),
+		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, authFlow, frontend.Assets())
+	router, err := server.NewRouter(logger, version, authFlow, sessions, frontend.Assets())
 	if err != nil {
 		return err
 	}
diff --git a/internal/auth/actor.go b/internal/auth/actor.go
new file mode 100644
index 0000000..ac65b84
--- /dev/null
+++ b/internal/auth/actor.go
@@ -0,0 +1,35 @@
+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 actorKey struct{}
+
+// WithActor returns a copy of ctx carrying actor. The session middleware
+// calls it for every authenticated request.
+func WithActor(ctx context.Context, actor Actor) context.Context {
+	return context.WithValue(ctx, actorKey{}, actor)
+}
+
+// ActorFrom returns the actor WithActor stored in ctx. ok is false when ctx
+// has none, which behind the session middleware means a wiring bug.
+func ActorFrom(ctx context.Context) (actor Actor, ok bool) {
+	actor, ok = ctx.Value(actorKey{}).(Actor)
+	return actor, ok
+}
diff --git a/internal/auth/actor_test.go b/internal/auth/actor_test.go
new file mode 100644
index 0000000..c2f239e
--- /dev/null
+++ b/internal/auth/actor_test.go
@@ -0,0 +1,32 @@
+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")
+	}
+	want := Actor{Subject: "user-123", Email: "cook@example.com", Name: "Home Cook", Groups: []string{"mealquest-admin"}}
+	got, ok := ActorFrom(WithActor(context.Background(), want))
+	if !ok || got.Subject != want.Subject || got.Email != want.Email || got.Name != want.Name || !slices.Equal(got.Groups, want.Groups) {
+		t.Errorf("ActorFrom = %+v (ok=%v), want %+v", got, ok, want)
+	}
+}
+
+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
index 629ed19..b1cf2c9 100644
--- a/internal/auth/auth.go
+++ b/internal/auth/auth.go
@@ -12,6 +12,7 @@ import (
 	"log/slog"
 	"net/http"
 	"net/url"
+	"slices"
 	"time"
 
 	"github.com/coreos/go-oidc/v3/oidc"
@@ -65,15 +66,20 @@ type Config struct {
 	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
@@ -92,6 +98,8 @@ func New(ctx context.Context, cfg Config, sessions Sessions, logger *slog.Logger
 		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 {
@@ -118,6 +126,9 @@ func New(ctx context.Context, cfg Config, sessions Sessions, logger *slog.Logger
 		{"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
@@ -138,7 +149,9 @@ func New(ctx context.Context, cfg Config, sessions Sessions, logger *slog.Logger
 			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,
@@ -218,7 +231,9 @@ func (a *Auth) login(w http.ResponseWriter, r *http.Request) {
 }
 
 // callback completes the flow: it checks state against the login cookie,
-// exchanges the code, verifies the ID token and nonce, then starts a session.
+// 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()
@@ -256,6 +271,15 @@ func (a *Auth) callback(w http.ResponseWriter, r *http.Request) {
 		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.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)
@@ -278,7 +302,8 @@ func (a *Auth) logout(w http.ResponseWriter, r *http.Request) {
 
 // verify exchanges code for tokens and returns the verified ID token claims.
 // The verifier enforces RS256, issuer, audience and expiry; nonce is checked
-// here.
+// 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))
@@ -303,9 +328,50 @@ func (a *Auth) verify(ctx context.Context, code string, state loginState) (Claim
 	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")
+}
+
+// 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.
diff --git a/internal/auth/auth_test.go b/internal/auth/auth_test.go
index a9d83e6..4ecf409 100644
--- a/internal/auth/auth_test.go
+++ b/internal/auth/auth_test.go
@@ -36,6 +36,9 @@ const (
 
 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 {
@@ -50,6 +53,12 @@ type fakeProvider struct {
 	// 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
 }
 
 type tokenReply struct {
@@ -73,6 +82,7 @@ func newFakeProvider(t *testing.T) *fakeProvider {
 	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
@@ -86,6 +96,7 @@ func (p *fakeProvider) discovery(w http.ResponseWriter, _ *http.Request) {
 		"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"},
@@ -148,6 +159,41 @@ func (p *fakeProvider) token(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// 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 {
@@ -291,11 +337,12 @@ func newHarness(t *testing.T) *harness {
 	sessions := &memorySessions{}
 	logs := &syncBuffer{}
 	a, err := New(context.Background(), Config{
-		Issuer:       p.issuer(),
-		ClientID:     testClientID,
-		ClientSecret: testClientSecret,
-		PublicURL:    testPublicURL,
-		SecretKey:    []byte("server-secret-key"),
+		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)
@@ -730,11 +777,12 @@ func TestNewFailsWhenDiscoveryFails(t *testing.T) {
 	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"),
+		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")
@@ -747,11 +795,12 @@ func TestNewFailsOnIssuerMismatch(t *testing.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"),
+		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")
@@ -772,11 +821,12 @@ 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"),
+			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)
@@ -793,6 +843,7 @@ func TestNewRejectsRemoteHTTPEndpoint(t *testing.T) {
 		{"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) {
@@ -800,11 +851,12 @@ func TestNewRejectsRemoteHTTPEndpoint(t *testing.T) {
 			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"),
+				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)
@@ -840,11 +892,12 @@ func TestCallbackRefusesTokenRedirectToHTTP(t *testing.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"),
+				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)
diff --git a/internal/auth/gate_test.go b/internal/auth/gate_test.go
new file mode 100644
index 0000000..fffa832
--- /dev/null
+++ b/internal/auth/gate_test.go
@@ -0,0 +1,218 @@
+package auth
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"net/http"
+	"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 sets 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() {
+		if c.Name == SessionCookieName || c.Name == "test_session" {
+			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)
+	}
+}
+
+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_test.go b/internal/auth/session_test.go
index 20e03fe..3f86f02 100644
--- a/internal/auth/session_test.go
+++ b/internal/auth/session_test.go
@@ -62,11 +62,12 @@ func newSessionHarness(t *testing.T) *sessionHarness {
 	p := newFakeProvider(t)
 	logs := &syncBuffer{}
 	a, err := New(ctx, Config{
-		Issuer:       p.issuer(),
-		ClientID:     testClientID,
-		ClientSecret: testClientSecret,
-		PublicURL:    testPublicURL,
-		SecretKey:    []byte("server-secret-key"),
+		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)
@@ -490,11 +491,12 @@ func TestNewRequiresEndSessionEndpoint(t *testing.T) {
 	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"),
+		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)
diff --git a/internal/server/server.go b/internal/server/server.go
index 45a8471..4ae887b 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -22,9 +22,10 @@ type RouteMounter interface {
 }
 
 // NewRouter returns the HTTP handler for the service. version is reported by
-// GET /health; auth mounts the /auth routes; assets is the web build served
-// for every other GET.
-func NewRouter(logger *slog.Logger, version string, auth RouteMounter, assets fs.FS) (http.Handler, error) {
+// 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.
+func NewRouter(logger *slog.Logger, version string, 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")
 
@@ -39,13 +40,23 @@ func NewRouter(logger *slog.Logger, version string, auth RouteMounter, assets fs
 
 	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)
+	apiV1, media := chi.NewRouter(), chi.NewRouter()
+	for _, sub := range []*chi.Mux{apiV1, media} {
+		sub.MethodNotAllowed(methodNotAllowed)
+		sub.NotFound(notFound)
+	}
 	r.Route("/api", func(r chi.Router) {
 		r.MethodNotAllowed(methodNotAllowed)
 		r.NotFound(notFound)
+		r.Mount("/v1", signedIn(apiV1))
 	})
-	// Catch-all for the PWA. Unknown /auth paths and /media (no routes yet)
-	// reach the frontend handler, which 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 21d63e8..2b2cee1 100644
--- a/internal/server/server_test.go
+++ b/internal/server/server_test.go
@@ -27,7 +27,7 @@ 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, stubAuth{}, assets)
+	r, err := NewRouter(logger, version, stubAuth{}, stubSessions{}, assets)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -126,6 +126,8 @@ 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/unknown"},
 		{http.MethodGet, "/media/photo.jpg"},
@@ -137,8 +139,11 @@ func TestReservedPathsNotFound(t *testing.T) {
 		t.Run(tt.method+" "+tt.path, func(t *testing.T) {
 			t.Parallel()
 
+			// Signed in, so /api/v1 and /media reach their routers.
+			req := httptest.NewRequest(tt.method, tt.path, nil)
+			req.AddCookie(validSessionCookie)
 			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..ade1f51
--- /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's actor in the context (read it with
+// 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.WithActor(r.Context(), s.Actor())))
+			}
+		})
+	}
+}
diff --git a/internal/server/session_test.go b/internal/server/session_test.go
new file mode 100644
index 0000000..a8b4ac0
--- /dev/null
+++ b/internal/server/session_test.go
@@ -0,0 +1,151 @@
+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"},
+	}
+)
+
+// 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)
+	}
+}