Open WebUI: Low-privilege authenticated users can enumerate and stop global background tasks, causing system-wide chat disruption

Summary

Any authenticated user with low privileges can enumerate active background tasks across the system and stop tasks belonging to other users via the GET /api/tasks and POST /api/tasks/stop/{task_id} methods. This allows a casual user to disrupt system-wide chat usage by continuously canceling other users' active tasks. This is a real authorization vulnerability affecting integrity and usability in multi-user deployments.

Details

Open WebUI exposes GET /api/tasks and POST /api/tasks/stop/{task_id} to any verified user. These endpoints operate on a global task namespace and accept raw task_id values without checking whether the task belongs to the current caller.

As a result, a normal authenticated user can enumerate active global task IDs and stop tasks belonging to other users.

Root cause:

1. Route authorization is too weak.

In backend/open_webui/main.py, both endpoints only require get_verified_user:

@app.post('/api/tasks/stop/{task_id}')
async def stop_task_endpoint(request: Request, task_id: str, user=Depends(get_verified_user)):
    result = await stop_task(request.app.state.redis, task_id)

@app.get('/api/tasks')
async def list_tasks_endpoint(request: Request, user=Depends(get_verified_user)):
    return {'tasks': await list_tasks(request.app.state.redis)}

get_verified_user accepts both user and admin roles in backend/open_webui/utils/auth.py.

2. The helper operates on a global namespace.

In backend/open_webui/tasks.py, task listing is global:

async def list_tasks(redis):
    if redis:
        return await redis_list_tasks(redis)
    return list(tasks.keys())

In backend/open_webui/tasks.py, task stopping is by raw task_id:

async def stop_task(redis, task_id: str):
    if redis:
        item_id = await redis.hget(REDIS_TASKS_KEY, task_id)
        await redis_send_command(redis, {'action': 'stop', 'task_id': task_id})
        await redis_cleanup_task(redis, task_id, item_id or None)

There is no owner check, no user_id check, and no mapping from task_id back to the current caller before stop or cleanup.

This also appears unintended because the codebase already has a scoped route, GET /api/tasks/chat/{chat_id}, which checks whether the chat belongs to the current user before returning task IDs.

Relevant code references:
- backend/open_webui/main.py:1975
- backend/open_webui/main.py:1984
- backend/open_webui/main.py:1989
- backend/open_webui/tasks.py:127
- backend/open_webui/tasks.py:145
- backend/open_webui/utils/auth.py:415

Suggested remediation:
- Store task ownership metadata such as user_id and chat_id, then enforce owner-only access for non-admin users
- Suggested implementation locations:
- backend/open_webui/main.py: add authentication checks for /api/tasks and /api/tasks/stop/{task_id}
- backend/open_webui/tasks.py: add support for storing/querying task ownership metadata such as user_id and chat_id, and support owner-scoped listing/stopping

PoC

Preconditions:

• Default main branch deployment
• Authentication enabled
• Two normal user accounts, or any multi-user deployment where the attacker has one authenticated non-admin account
• At least one task actively running for another user

This does not require any weakened security settings.

PoC objective:

1. Show that a non-admin user can see global active task IDs that are not their own
2. Show that the same user can stop another user's active task

Reproduction steps:

Step 1. Victim starts a long-running task

Using the UI, User A starts a long response generation or another background task and leaves it running.

Expected security model:
User B should not be able to see or control User A's task.

Step 2. Attacker enumerates global task IDs

Using User B's authenticated token:

curl -i -H "Authorization: Bearer <USER_B_TOKEN>" http://<open-webui-host>/api/tasks

Expected result:

• only User B's own task IDs should be returned, or
• the endpoint should be admin-only

Actual result:
the response returns the global active task list.

Example response shape:

{"tasks":["<task-id-a>","<task-id-b>"]}

This exposes task IDs belonging to other users.

Step 3. Attacker stops a foreign task

Pick a task ID that belongs to User A and send:

curl -i -X POST -H "Authorization: Bearer <USER_B_TOKEN>" http://<open-webui-host>/api/tasks/stop/<FOREIGN_TASK_ID>

Expected result:

• 403 Forbidden, or
• 404 Not Found for non-owned tasks, or
• admin-only access

Actual result:
the server accepts the request and attempts to stop the foreign task.

Example response shape:

{"status":true,"message":"Task <FOREIGN_TASK_ID> stopped."}

Step 4. Observe boundary violation

User A's running task is interrupted or disappears from the active task set even though User B does not own it.

What actions become possible that should not be possible:

• enumerate globally active task IDs across users
• cancel another user's in-progress generation or background work
• repeat this for every returned task ID, causing broad cross-user disruption

Copy-paste PoC summary:

1. Enumerate all active tasks as a normal non-admin user

curl -s -H "Authorization: Bearer <USER_B_TOKEN>" http://<open-webui-host>/api/tasks

2. Stop a task that does not belong to that user

curl -s -X POST -H "Authorization: Bearer <USER_B_TOKEN>" http://<open-webui-host>/api/tasks/stop/<FOREIGN_TASK_ID>

Impact

Type of vulnerability:
broken object-level authorization affecting a global runtime control-plane endpoint.

Who is impacted:

• all users in a multi-user Open WebUI deployment
• any user currently running a background task, especially chat generation tasks
• administrators indirectly, because normal users can disrupt system-wide usage without admin privileges

Direct impact:

• cross-user task ID disclosure
• cross-user task cancellation

Practical impact:

• interruption of long-running chat responses
• interruption of background indexing or ingestion tasks associated with shared runtime jobs
• one ordinary authenticated low-privilege user can continuously poll /api/tasks and immediately cancel every newly created active task
• with a simple loop or script, this becomes a practical persistent denial-of-service against chat usage for all users on the instance
• in a multi-user deployment, normal users may be unable to complete any chat generation while the attacker continues polling and cancelling tasks

Why severity is meaningful:

• privileges required: low, only an authenticated non-admin account
• scope: cross-user
• impact class: integrity and availability
• exploitation complexity: low once logged in

This is not full account takeover or privilege escalation, but it enables platform-wide operational disruption from a low-privilege account. In practice, sustained exploitation can make chat functionality effectively unusable for other users on the system.

Resolution

Fixed in commit e7ff4768f (#23454, "Add ownership checks to global task endpoints"), first released in v0.9.0 (Apr 2026).

The fix takes a simpler approach than per-task ownership tracking, which would have required a schema change to attribute every task to a user_id:

• GET /api/tasks and POST /api/tasks/stop/{task_id} are restricted to admin-only via Depends(get_admin_user). Cross-user enumeration and termination are no longer reachable from a non-admin account.
• A new scoped POST /api/tasks/chat/{chat_id}/stop endpoint covers the legitimate non-admin use case (a user stopping their own in-progress generation), reusing the same chat-ownership check the existing GET /api/tasks/chat/{chat_id} already enforces.

CVE-2025-63681 was a prior disclosure of the same authorization gap against v0.6.33; the fix in v0.9.0 also resolves that.

Users on >= 0.9.0 are not affected.