GHSA-gh7p-78x6-jw6m: Open WebUI: /api/v1/channels/{id}/members exposes full user model including sensitive credentials

Summary The channel members endpoint serializes and returns full user models for channel participants, including settings objects. A normal user in a DM can retrieve admin-only sensitive configuration such as webhook URLs and tool server key material (settings.ui.toolServers[].key), which is not available via standard user info APIs. Details The endpoint GET /api/v1/channels/{id}/members returns the full serialized user model for every member in the channel. In both the DM and non-DM code paths, the handler constructs the response with [UserModelResponse(**user.model_dump(), is_active=...)] and returns it as the users list. Because UserModel (models/users.py) includes a settings object (UserSettings) and arbitrary UI configuration (settings.ui), the endpoint exposes other users' sensitive configuration to any channel participant. Practically, a regular user who participates in a DM or group can call /api/v1/channels/{id}/members and receive other members' settings, including admin-only details such as webhook notification URLs and tool server configuration, including credential fields like settings.ui.toolServers[].key. These values are not returned by the normal user profile endpoints (e.g., /api/v1/users/{user_id}/info). PoC 1. Start a local Open WebUI instance 2. Log in as admin and in the Admin Panel, go to Settings -> General and check *Channels (Beta)*, then press Save. 3. Create a low-privilege user in the Users tab 4. Click on the admin's profile bottom left, Settings and Integrations. Then click the + after *Manage Tool Servers* to add some tool server with a secret Bearer token (eg. KEY) 5. Log in as the attacker with the low-privilege account and create a new Direct Message channel with the admin user: <img width="690" height="383" alt="image" src="https://github.com/user-attachments/assets/70208661-a0db-4457-9984-119056ca3daf" /> 7. After creating, open DevTools with F12 and go to the Network tab. Then in the DM UI click on the *Users* icon top r