(frontend) alternative auth without relying on sameSite cookie

src/frontend/src/App.tsx

@@ -12,6 +12,7 @@ import { routes } from './routes'
 import './i18n/init'
 import { queryClient } from '@/api/queryClient'
 import { AppInitialization } from '@/components/AppInitialization'
+import { TransitCodeGate } from '@/features/auth/components/TransitCodeGate'
 import { useIsSdkContext } from '@/features/sdk/hooks/useIsSdkContext'
 import { useApplyA11yFonts } from '@/hooks/useApplyA11yFonts'
 
@@ -24,23 +25,29 @@ function App() {
 
   return (
     <QueryClientProvider client={queryClient}>
-      {!isSDKContext && <AppInitialization />}
-      <Suspense fallback={null}>
-        <I18nProvider locale={i18n.language}>
-          <Layout>
-            <Switch>
-              {Object.entries(routes).map(([, route], i) => (
-                <Route key={i} path={route.path} component={route.Component} />
-              ))}
-              <Route component={NotFoundScreen} />
-            </Switch>
-          </Layout>
-          <ReactQueryDevtools
-            initialIsOpen={false}
-            buttonPosition="bottom-left"
-          />
-        </I18nProvider>
-      </Suspense>
+      <TransitCodeGate>
+        {!isSDKContext && <AppInitialization />}
+        <Suspense fallback={null}>
+          <I18nProvider locale={i18n.language}>
+            <Layout>
+              <Switch>
+                {Object.entries(routes).map(([, route], i) => (
+                  <Route
+                    key={i}
+                    path={route.path}
+                    component={route.Component}
+                  />
+                ))}
+                <Route component={NotFoundScreen} />
+              </Switch>
+            </Layout>
+            <ReactQueryDevtools
+              initialIsOpen={false}
+              buttonPosition="bottom-left"
+            />
+          </I18nProvider>
+        </Suspense>
+      </TransitCodeGate>
     </QueryClientProvider>
   )
 }

src/frontend/src/api/fetchApi.ts

@@ -1,17 +1,23 @@
 import { ApiError } from './ApiError'
 import { apiUrl } from './apiUrl'
+import { getAccessToken } from '@/stores/accessToken'
 
 export const fetchApi = async <T = Record<string, unknown>>(
   url: string,
   options?: RequestInit
 ): Promise<T> => {
   const csrfToken = getCsrfToken()
+  // Embedded (iframe) mode: the user access token obtained through the
+  // transit code exchange authenticates requests in place of the session
+  // cookie, which is blocked in third-party contexts.
+  const accessToken = getAccessToken()
   const response = await fetch(apiUrl(url), {
     credentials: 'include',
     ...options,
     headers: {
       'Content-Type': 'application/json',
       ...(!!csrfToken && { 'X-CSRFToken': csrfToken }),
+      ...(!!accessToken && { Authorization: `Bearer ${accessToken}` }),
       ...options?.headers,
     },
   })

src/frontend/src/features/auth/api/exchangeAccessToken.ts

@@ -0,0 +1,64 @@
+import { fetchApi } from '@/api/fetchApi'
+import { setAccessToken } from '@/stores/accessToken'
+import { consumeTransitCodeFromFragment } from '../utils/transitCode'
+
+type ApiAccessToken = {
+  access_token: string
+  token_type: string
+  expires_in: number
+  scope: string
+}
+
+/**
+ * Exchange a single-use transit code for a user access token.
+ *
+ * The endpoint is unauthenticated: the code itself is the credential.
+ */
+export const exchangeAccessToken = (code: string): Promise<ApiAccessToken> => {
+  return fetchApi<ApiAccessToken>('/users/exchange-access-token/', {
+    method: 'POST',
+    body: JSON.stringify({ code }),
+  })
+}
+
+const runInitialization = async (): Promise<void> => {
+  const code = consumeTransitCodeFromFragment()
+
+  if (!code) {
+    return
+  }
+
+  try {
+    const { access_token } = await exchangeAccessToken(code)
+    setAccessToken(access_token)
+  } catch (error) {
+    console.warn('Transit code exchange failed:', error)
+  }
+}
+
+let initialization: Promise<void> | null = null
+
+/**
+ * Bootstrap the embedded (iframe) authentication, if applicable.
+ *
+ * When, and only when, a transit code is present in the URL fragment,
+ * exchange it for a user access token and keep it in the in-memory
+ * accessToken store: fetchApi then sends it as a Bearer header on every
+ * api call, authenticating the user exactly like a session cookie would.
+ *
+ * Must complete before anything fires an authenticated query, which the
+ * TransitCodeGate component guarantees by gating the app tree on it.
+ *
+ * Memoized: the fragment is consumed and the code exchanged exactly once,
+ * however many times this is called (StrictMode double-invoked effects,
+ * among others). Subsequent calls await the same promise.
+ *
+ * A failed exchange (expired or already used code) is not fatal: the app
+ * starts unauthenticated, falling back to the regular session flow.
+ */
+export const initializeAccessTokenFromFragment = (): Promise<void> => {
+  if (!initialization) {
+    initialization = runInitialization()
+  }
+  return initialization
+}

src/frontend/src/features/auth/api/fetchUser.ts

@@ -2,6 +2,7 @@ import { ApiError } from '@/api/ApiError'
 import { fetchApi } from '@/api/fetchApi'
 import { type ApiUser } from './ApiUser'
 import { attemptSilentLogin, canAttemptSilentLogin } from '../utils/silentLogin'
+import { getAccessToken } from '@/stores/accessToken'
 
 /**
  * fetch the logged-in user from the api.
@@ -25,7 +26,13 @@ export const fetchUser = (
         if (error instanceof ApiError && error.statusCode === 401) {
           // make sure to not resolve the promise while trying to silent login
           // so that consumers of fetchUser don't think the work already ended
-          if (opts.attemptSilent && canAttemptSilentLogin()) {
+          // Never attempt a silent login in embedded (token) mode: an OIDC
+          // redirect inside the iframe would break the embed.
+          if (
+            opts.attemptSilent &&
+            !getAccessToken() &&
+            canAttemptSilentLogin()
+          ) {
             attemptSilentLogin(30)
           } else {
             resolve(false)

src/frontend/src/features/auth/components/TransitCodeGate.tsx

@@ -0,0 +1,67 @@
+import { useEffect, useState } from 'react'
+import { LoadingScreen } from '@/components/LoadingScreen'
+import { useHash } from '@/hooks/useHash'
+import { initializeAccessTokenFromFragment } from '../api/exchangeAccessToken'
+import { hasTransitCodeInFragment } from '../utils/transitCode'
+
+/**
+ * Gates the app tree on the embedded (iframe) authentication bootstrap.
+ *
+ * Without a transit code in the URL fragment — the overwhelmingly common
+ * case — the component early returns children synchronously: no state,
+ * no effect, no extra render, no loading screen.
+ *
+ * When a transit code is present, children are not mounted until it has
+ * been exchanged for a user access token, so that every authenticated
+ * query already carries the Authorization header. A loading screen is
+ * displayed in the meantime, as UserAware does.
+ */
+export const TransitCodeGate = ({
+  children,
+}: {
+  children: React.ReactNode
+}) => {
+  const hash = useHash()
+
+  // Latch the decision on the initial hash: the bootstrap scrubs the
+  // fragment as soon as it starts, and the gate must not flip back to the
+  // fast path while the exchange is still in flight.
+  const [needsExchange] = useState(() => hasTransitCodeInFragment(hash))
+
+  if (!needsExchange) {
+    return children
+  }
+
+  return <TransitCodeExchange>{children}</TransitCodeExchange>
+}
+
+/**
+ * Only ever mounted when a transit code is present: runs the memoized
+ * bootstrap (safe against StrictMode double-invoked effects) and holds
+ * children back until it settles.
+ */
+const TransitCodeExchange = ({ children }: { children: React.ReactNode }) => {
+  const [isReady, setIsReady] = useState(false)
+
+  useEffect(() => {
+    let isMounted = true
+    initializeAccessTokenFromFragment().finally(() => {
+      console.log('$$ transit code exchange finished')
+      if (isMounted) {
+        console.log('$$ setIsReady')
+        setIsReady(true)
+      }
+    })
+    return () => {
+      isMounted = false
+    }
+  }, [])
+
+  console.log('$$ isReady', isReady)
+
+  return isReady ? (
+    children
+  ) : (
+    <LoadingScreen header={false} footer={false} delay={1000} />
+  )
+}

src/frontend/src/features/auth/utils/transitCode.ts

@@ -0,0 +1,46 @@
+const TRANSIT_CODE_FRAGMENT_PARAM = 'transit_code'
+
+/**
+ * Whether a URL fragment carries a transit code. Pure check, does not
+ * consume anything.
+ */
+export const hasTransitCodeInFragment = (hash: string): boolean => {
+  if (!hash) {
+    return false
+  }
+  return new URLSearchParams(hash.replace(/^#/, '')).has(
+    TRANSIT_CODE_FRAGMENT_PARAM
+  )
+}
+
+/**
+ * Extract the transit code from the URL fragment, if any.
+ *
+ * The fragment is scrubbed from the address bar immediately, before any
+ * network call, so the code never lingers in the browser history. Any
+ * other fragment content is preserved.
+ */
+export const consumeTransitCodeFromFragment = (): string | null => {
+  if (typeof window === 'undefined' || !window.location.hash) {
+    return null
+  }
+
+  const params = new URLSearchParams(window.location.hash.substring(1))
+  const code = params.get(TRANSIT_CODE_FRAGMENT_PARAM)
+
+  if (!code) {
+    return null
+  }
+
+  params.delete(TRANSIT_CODE_FRAGMENT_PARAM)
+  const remaining = params.toString()
+  window.history.replaceState(
+    null,
+    '',
+    window.location.pathname +
+      window.location.search +
+      (remaining ? `#${remaining}` : '')
+  )
+
+  return code
+}

src/frontend/src/hooks/useHash.ts

@@ -0,0 +1,10 @@
+import { useLocationProperty } from 'wouter/use-browser-location'
+
+const hashSelector = () =>
+  typeof window !== 'undefined' ? window.location.hash : ''
+
+/**
+ * Reactive window.location.hash, subscribed to wouter's navigation
+ * events (the same low-level primitive wouter builds useSearch upon).
+ */
+export const useHash = (): string => useLocationProperty(hashSelector, () => '')

src/frontend/src/stores/accessToken.ts

@@ -0,0 +1,32 @@
+import { proxy } from 'valtio'
+
+type State = {
+  accessToken: string | null
+}
+
+/**
+ * User access token for the embedded (iframe) mode.
+ *
+ * When Meet is rendered inside an iframe, third-party session cookies are
+ * blocked: the host application passes a single-use transit code in the
+ * URL fragment, exchanged at startup for a user access token (see
+ * features/auth/api/exchangeAccessToken) that authenticates every api
+ * call exactly like a session cookie would.
+ *
+ * The token deliberately lives in this in-memory store only: unlike other
+ * stores, it is never persisted (no subscribe/localStorage) and never
+ * appears in a URL. It is lost on reload, in which case the host page is
+ * expected to mint a fresh transit code.
+ *
+ * A non-null token also tells the app it is running in embedded mode:
+ * components can react to it with useSnapshot(accessTokenStore).
+ */
+export const accessTokenStore = proxy<State>({
+  accessToken: null,
+})
+
+export const setAccessToken = (accessToken: string | null) => {
+  accessTokenStore.accessToken = accessToken
+}
+
+export const getAccessToken = () => accessTokenStore.accessToken