fix: never follow redirects on OAuth token and revocation requests

The code exchange, token refresh and revocation POSTs carry the
authorization code, refresh token and client secret, and used a plain
`fetch` that follows redirects. The endpoints come from a validated
discovery document, but a redirect from them (or from an endpoint that
changed after discovery) re-sent the body to a host nobody validated.
These requests now fail on any redirect. A failed refresh still keeps
the refresh token, as for any other outage.

app/api/auth/token/route.ts

@@ -52,11 +52,14 @@ async function revokeRefreshToken(token: string, serverId: string | null, metada
   }, serverId, { fallbackClientId: DEFAULT_CLIENT_ID });
 
   try {
+    // Never follow a redirect with the refresh token and client secret in
+    // the body: the endpoint was validated, its redirect target was not.
     const revocationResponse = await fetch(metadata.revocation_endpoint, {
       method: 'POST',
       headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
       body: params.toString(),
       signal: AbortSignal.timeout(REVOCATION_TIMEOUT_MS),
+      redirect: 'error',
     });
     if (!revocationResponse.ok) {
       logger.warn('Token revocation returned error', { status: revocationResponse.status });
@@ -205,6 +208,8 @@ export async function PUT(request: NextRequest) {
       method: 'POST',
       headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
       body: params.toString(),
+      // See revokeRefreshToken: the body holds the refresh token.
+      redirect: 'error',
     });
 
     if (!tokenResponse.ok) {

lib/__tests__/oauth-token-route.test.ts

@@ -145,6 +145,28 @@ describe('oauth token route - access token cache (#552)', () => {
     expect(fetchMock).toHaveBeenCalledTimes(1);
   });
 
+  it('never follows a redirect with the refresh token in the body', async () => {
+    cookieStore.set('jmap_rt', 'refresh-token');
+    fetchMock.mockResolvedValue({
+      ok: true,
+      json: async () => ({ access_token: 'fresh-token', expires_in: 1800 }),
+    });
+
+    await callPut();
+
+    expect(fetchMock.mock.calls[0][1]).toMatchObject({ method: 'POST', redirect: 'error' });
+  });
+
+  it('keeps the refresh token when the token endpoint redirects', async () => {
+    cookieStore.set('jmap_rt', 'refresh-token');
+    fetchMock.mockRejectedValue(new TypeError('fetch failed: unexpected redirect'));
+
+    const { status } = await callPut();
+
+    expect(status).toBe(500);
+    expect(cookieStore.get('jmap_rt')).toBeTruthy();
+  });
+
   it('ignores an expired cached token', async () => {
     cookieStore.set('jmap_rt', 'refresh-token');
     seedCachedToken('long-gone', -600);

lib/oauth/token-exchange.ts

@@ -174,10 +174,14 @@ export async function exchangeCodeForTokens(
     code_verifier: codeVerifier,
   }, serverId);
 
+  // The body carries the code, the PKCE verifier and the client secret. The
+  // endpoint was validated; a redirect would re-send all of it to a host
+  // that was not, so none is followed.
   const tokenResponse = await fetch(tokenEndpoint, {
     method: 'POST',
     headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
     body: params.toString(),
+    redirect: 'error',
   });
 
   if (!tokenResponse.ok) {