diff --git a/frontend/src/hooks.server.ts b/frontend/src/hooks.server.ts
index 2c155dce..5fe167c1 100644
--- a/frontend/src/hooks.server.ts
+++ b/frontend/src/hooks.server.ts
@@ -58,21 +58,40 @@ const handleParaglide: Handle = ({ event, resolve }) =>
 	});
 
 const userGroup: Handle = async ({ event, resolve }) => {
-	const auth = event.cookies.get('auth_token');
+	// One-off cleanup of the legacy Basic-credentials cookie from before the Spring Session migration (#523).
+	if (event.cookies.get('auth_token')) {
+		event.cookies.delete('auth_token', { path: '/' });
+	}
 
-	if (auth) {
-		try {
-			const apiUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
-			const response = await fetch(`${apiUrl}/api/users/me`, {
-				headers: { Authorization: auth }
-			});
-			if (response.ok) {
-				const user = await response.json();
-				event.locals.user = user;
+	const sessionId = event.cookies.get('fa_session');
+	if (!sessionId) {
+		return resolve(event);
+	}
+
+	try {
+		const apiUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+		const response = await fetch(`${apiUrl}/api/users/me`, {
+			headers: { Cookie: `fa_session=${sessionId}` }
+		});
+
+		if (response.ok) {
+			event.locals.user = await response.json();
+		} else if (response.status === 401) {
+			// Backend rejected the session (expired or invalidated). Drop the stale
+			// cookie and surface the reason on the login page. PUBLIC_PATHS check
+			// avoids a redirect loop if the user is already on /login.
+			event.cookies.delete('fa_session', { path: '/' });
+			const isPublic = PUBLIC_PATHS.some((p) => event.url.pathname.startsWith(p));
+			if (!isPublic) {
+				throw redirect(302, '/login?reason=expired');
 			}
-		} catch (error) {
-			console.error('Error fetching user in hook:', error);
 		}
+	} catch (error) {
+		// Don't swallow SvelteKit redirects — they're thrown as objects with a `status` field.
+		if (error instanceof Object && 'status' in error && 'location' in error) {
+			throw error;
+		}
+		console.error('Error fetching user in hook:', error);
 	}
 
 	return resolve(event);
@@ -83,14 +102,11 @@ export const handleFetch: HandleFetch = async ({ event, request, fetch }) => {
 	const isApi = request.url.startsWith(apiUrl) || request.url.includes('/api/');
 
 	if (isApi) {
-		// If the request already carries an explicit Authorization header (e.g. the
-		// login action sends Basic auth), pass it through unchanged.
-		if (request.headers.has('Authorization')) {
-			return fetch(request);
-		}
-
-		// Password reset endpoints are public — no auth header needed.
+		// Auth endpoints that establish/check their own credentials manage cookies themselves;
+		// don't double-inject a stale fa_session.
 		const PUBLIC_API_PATHS = [
+			'/api/auth/login',
+			'/api/auth/logout',
 			'/api/auth/forgot-password',
 			'/api/auth/reset-password',
 			'/api/auth/invite/',
@@ -100,24 +116,20 @@ export const handleFetch: HandleFetch = async ({ event, request, fetch }) => {
 			return fetch(request);
 		}
 
-		const token = event.cookies.get('auth_token');
-
-		if (!token) {
+		const sessionId = event.cookies.get('fa_session');
+		if (!sessionId) {
 			return new Response('Unauthorized', { status: 401 });
 		}
 
-		// Clone the request first to preserve the body
-		const clonedRequest = request.clone();
-
-		// Create new request with Authorization header and preserved body
-		const modifiedRequest = new Request(clonedRequest, {
+		// Clone first so the body stream is preserved on the new Request.
+		const cloned = request.clone();
+		const modified = new Request(cloned, {
 			headers: {
-				...Object.fromEntries(clonedRequest.headers),
-				Authorization: token
+				...Object.fromEntries(cloned.headers),
+				Cookie: `fa_session=${sessionId}`
 			}
 		});
-
-		return fetch(modifiedRequest);
+		return fetch(modified);
 	}
 
 	return fetch(request);