Validate API paths against the page origin, not a placeholder

The guard resolved paths against a fixed placeholder origin, so
//same-origin.invalid/api/v1/me passed and fetch sent the CSRF token to
that host. apiFetch now resolves against location.origin, refuses
anything off that origin or outside /api/v1/, and fetches the validated
path-absolute URL.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/api/client.test.ts

@@ -37,6 +37,10 @@ describe('apiFetch', () => {
   it.each([
     'https://evil.example/api/v1/me',
     '//evil.example/api/v1/me',
+    // The placeholder origin an earlier guard compared against.
+    '//same-origin.invalid/api/v1/me',
+    'https://same-origin.invalid/api/v1/me',
+    '/\\evil.example/api/v1/me',
     '/auth/logout',
     '/api/v1/../auth/logout',
     '/api/v1/%2e%2e/auth/logout',

web/src/api/client.ts

@@ -8,16 +8,17 @@ export const ME_PATH = '/api/v1/me'
 // API_PREFIX is the only path apiFetch sends to.
 const API_PREFIX = '/api/v1/'
 
-// requireApiPath refuses any path that would not resolve to a same-origin URL
-// under /api/v1/, so the CSRF token is never sent anywhere else. Resolving
-// against a placeholder origin catches absolute and protocol-relative URLs
-// and dot segments that climb out of /api/v1/.
-function requireApiPath(method: string, path: string) {
-  const base = 'https://same-origin.invalid'
-  const url = new URL(path, base)
-  if (!path.startsWith('/') || url.origin !== base || !url.pathname.startsWith(API_PREFIX)) {
+// apiURL resolves path against the page origin and returns it as the
+// path-absolute URL to fetch. It refuses any path that would not resolve to a
+// same-origin URL under /api/v1/, so the CSRF token is never sent anywhere
+// else: absolute and protocol-relative URLs on another origin, and dot
+// segments that climb out of /api/v1/.
+function apiURL(origin: string, method: string, path: string): string {
+  const url = new URL(path, origin)
+  if (!path.startsWith('/') || url.origin !== origin || !url.pathname.startsWith(API_PREFIX)) {
     throw new Error(`${method} ${path}: not a same-origin ${API_PREFIX} path`)
   }
+  return url.pathname + url.search
 }
 
 // Me is the GET /api/v1/me response: the signed-in member, the household and
@@ -30,7 +31,8 @@ export type Me = {
 }
 
 // ApiLocation is the part of window.location the client reads and navigates.
-export type ApiLocation = Pick<Location, 'pathname' | 'search' | 'assign'>
+// origin is the page origin every request must stay on.
+export type ApiLocation = Pick<Location, 'origin' | 'pathname' | 'search' | 'assign'>
 
 export type ApiRequest = {
   method?: 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE'
@@ -87,15 +89,15 @@ export function createApiClient(location: ApiLocation): ApiClient {
 
   async function apiFetch<T>(path: string, request: ApiRequest = {}): Promise<T> {
     const method = request.method ?? 'GET'
-    requireApiPath(method, path)
-    let response = await send(method, path, request.body)
+    const url = apiURL(location.origin, method, path)
+    let response = await send(method, url, request.body)
     if (response.status === 403) {
       const error = await parseForbidden(method, path, response)
       // Only a non-GET carries the token, so only it can be refused for it.
       if (error.code !== 'csrf' || method === 'GET') throw error
       // The session's token changed: refresh it once and retry once.
       await refreshMe()
-      response = await send(method, path, request.body)
+      response = await send(method, url, request.body)
       if (response.status === 403) throw await parseForbidden(method, path, response)
     }
     return decode<T>(method, path, response)

web/src/test/fetchStub.ts

@@ -42,7 +42,8 @@ export function request(fetchMock: Mock<typeof fetch>, n: number) {
   }
 }
 
-// fakeLocation stands in for window.location, recording navigations.
+// fakeLocation stands in for window.location on https://mealquest.test,
+// recording navigations.
 export function fakeLocation(pathname = '/', search = ''): ApiLocation & { assign: Mock<(url: string | URL) => void> } {
-  return { pathname, search, assign: vi.fn<(url: string | URL) => void>() }
+  return { origin: 'https://mealquest.test', pathname, search, assign: vi.fn<(url: string | URL) => void>() }
 }