diff --git a/app/api/admin/auth/route.ts b/app/api/admin/auth/route.ts
index 2dd882ab..1720059b 100644
Binary files a/app/api/admin/auth/route.ts and b/app/api/admin/auth/route.ts differ
diff --git a/app/api/auth/stalwart-context/route.ts b/app/api/auth/stalwart-context/route.ts
index 24426454..d4f06d51 100644
--- a/app/api/auth/stalwart-context/route.ts
+++ b/app/api/auth/stalwart-context/route.ts
@@ -1,6 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { logger } from '@/lib/logger';
-import { JmapAuthVerificationError, normalizeJmapServerUrl, validateProxyAuthHeader, verifyJmapAuth } from '@/lib/auth/verify-jmap-auth';
+import { JmapAuthVerificationError, assertBasicAuthMatchesUsername, normalizeJmapServerUrl, validateProxyAuthHeader, verifyJmapAuth } from '@/lib/auth/verify-jmap-auth';
 import { setStalwartAuthContext } from '@/lib/stalwart/auth-context';
 import { configManager } from '@/lib/admin/config-manager';
 import { isPublicHttpUrl } from '@/lib/security/url-guard';
@@ -57,15 +57,22 @@ export async function POST(request: NextRequest) {
     }
 
     const slot = getSlot(request, bodySlot);
-    // Trusted (admin-configured) URLs skip the upstream re-fetch: the caller
-    // just authenticated to JMAP with these credentials, and the cookie we
-    // write here is only ever consumed for requests on behalf of this same
-    // user - a bogus auth header would just yield 401s downstream, not
-    // privilege escalation. For untrusted custom endpoints we still verify
-    // upstream as before.
-    const normalizedServerUrl = upstreamTrusted
-      ? (validateProxyAuthHeader(authHeader), normalizeJmapServerUrl(upstreamUrl))
-      : await verifyJmapAuth(upstreamUrl, authHeader, { trusted: false });
+    // Trusted (admin-configured) URLs skip the upstream re-fetch, but we
+    // still bind the cookie's `username` to the credential when we can verify
+    // locally. Without this, a caller can POST username="admin@host" +
+    // authHeader=<their own Basic creds>, and downstream consumers that read
+    // the cookie-derived username (audit logs, login tracker) accept the
+    // spoof. Bearer tokens are opaque so only the format check runs;
+    // authorization sinks must key off the credential itself, not the
+    // cookie's username claim (see admin/auth's authHeader-hashed cache key).
+    let normalizedServerUrl: string;
+    if (upstreamTrusted) {
+      validateProxyAuthHeader(authHeader);
+      assertBasicAuthMatchesUsername(authHeader, username);
+      normalizedServerUrl = normalizeJmapServerUrl(upstreamUrl);
+    } else {
+      normalizedServerUrl = await verifyJmapAuth(upstreamUrl, authHeader, { trusted: false });
+    }
 
     await setStalwartAuthContext(slot, {
       serverUrl: normalizedServerUrl,
diff --git a/lib/auth/verify-jmap-auth.ts b/lib/auth/verify-jmap-auth.ts
index 1d2d1616..06a55708 100644
--- a/lib/auth/verify-jmap-auth.ts
+++ b/lib/auth/verify-jmap-auth.ts
@@ -40,6 +40,31 @@ export function validateProxyAuthHeader(authHeader: string): void {
   }
 }
 
+/**
+ * For a `Basic` Authorization header, assert that the user portion of the
+ * credentials matches `claimedUsername`. Prevents callers of routes that
+ * accept independent `username` + `authHeader` fields from binding a cookie
+ * to one identity while authenticating as another. No-op for Bearer.
+ */
+export function assertBasicAuthMatchesUsername(authHeader: string, claimedUsername: string): void {
+  const match = /^Basic\s+(\S+)$/i.exec(authHeader);
+  if (!match) return;
+  let decoded: string;
+  try {
+    decoded = Buffer.from(match[1], 'base64').toString('utf8');
+  } catch {
+    throw new JmapAuthVerificationError('Invalid Authorization header', 400);
+  }
+  const colon = decoded.indexOf(':');
+  if (colon < 0) {
+    throw new JmapAuthVerificationError('Invalid Authorization header', 400);
+  }
+  const credUser = decoded.slice(0, colon);
+  if (credUser !== claimedUsername) {
+    throw new JmapAuthVerificationError('Username does not match credentials', 400);
+  }
+}
+
 export async function verifyJmapAuth(
   serverUrl: string,
   authHeader: string,