diff --git a/app/api/auth/session/route.ts b/app/api/auth/session/route.ts
index a21f526f..5711df57 100644
--- a/app/api/auth/session/route.ts
+++ b/app/api/auth/session/route.ts
@@ -4,7 +4,12 @@ import { logger } from '@/lib/logger';
 import { encryptSession, decryptSession } from '@/lib/auth/crypto';
 import { SESSION_COOKIE_MAX_AGE, sessionCookieName } from '@/lib/auth/session-cookie';
 import { getCookieOptions } from '@/lib/oauth/cookie-config';
-import { JmapAuthVerificationError, verifyJmapAuth } from '@/lib/auth/verify-jmap-auth';
+import {
+  JmapAuthVerificationError,
+  normalizeJmapServerUrl,
+  validateProxyAuthHeader,
+  verifyJmapAuth,
+} from '@/lib/auth/verify-jmap-auth';
 import {
   clearStalwartAuthContextInStore,
   setStalwartAuthContextInStore,
@@ -74,7 +79,13 @@ export async function POST(request: NextRequest) {
     const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot < MAX_ACCOUNT_SLOTS ? bodySlot : getSlot(request);
     const cookieName = sessionCookieName(slot);
     const authHeader = `Basic ${Buffer.from(`${username}:${password}`).toString('base64')}`;
-    const normalizedServerUrl = await verifyJmapAuth(upstreamUrl, authHeader, { trusted: upstreamTrusted });
+    // Trusted (admin-configured) URLs skip the upstream re-fetch: the cookie
+    // we write here is only ever consumed for requests on behalf of this same
+    // user, so bogus credentials would just yield 401s downstream rather than
+    // privilege escalation. Untrusted custom endpoints still verify upstream.
+    const normalizedServerUrl = upstreamTrusted
+      ? (validateProxyAuthHeader(authHeader), normalizeJmapServerUrl(upstreamUrl))
+      : await verifyJmapAuth(upstreamUrl, authHeader, { trusted: false });
     const token = encryptSession(normalizedServerUrl, username, password);
     const cookieStore = await cookies();
     cookieStore.set(cookieName, token, COOKIE_OPTIONS);