fix: enforce OAUTH_ALLOWED_DOMAINS on token exchange endpoint (#23639)

The OAuth token exchange endpoint skipped the domain allowlist check that the normal OAuth callback enforces. An attacker with a valid OAuth token from a non-allowed domain (e.g. gmail.com) could bypass the admin's domain restriction policy entirely.

Adds the same domain validation check used in the OAuth callback, denying access when the email domain is not in the allowed list.

backend/open_webui/routers/auths.py

@@ -54,6 +54,7 @@ from open_webui.config import (
     OAUTH_PROVIDERS,
     OAUTH_MERGE_ACCOUNTS_BY_EMAIL,
 )
+from open_webui.utils.oauth import auth_manager_config
 from pydantic import BaseModel
 
 from open_webui.utils.misc import parse_duration, validate_email_format
@@ -1295,6 +1296,17 @@ async def token_exchange(
         )
     email = email.lower()
 
+    # Enforce domain allowlist — same check as the normal OAuth callback
+    if (
+        '*' not in auth_manager_config.OAUTH_ALLOWED_DOMAINS
+        and email.split('@')[-1] not in auth_manager_config.OAUTH_ALLOWED_DOMAINS
+    ):
+        log.warning(f'Token exchange denied: email domain not in allowed domains list')
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail=ERROR_MESSAGES.ACCESS_PROHIBITED,
+        )
+
     # Try to find the user by OAuth sub
     user = await Users.get_user_by_oauth_sub(provider, sub, db=db)