Open WebUI Broken Authorization Allows Task Cancellation

Open WebUI versions 0.8.12 and earlier suffer from a broken object-level authorization vulnerability that allows authenticated, low-privilege users to enumerate and stop tasks belonging to other users. This vulnerability, identified as CVE-2026-45399 and a prior disclosure as CVE-2025-63681, enables an attacker to disrupt system-wide chat usage by repeatedly canceling active tasks. The vulnerability stems from insufficient authorization checks on the /api/tasks and /api/tasks/stop/{task_id} endpoints, which operate on a global task namespace. A fix was implemented in version 0.9.0 of Open WebUI. This issue highlights the risk of relying on weak authorization schemes, where simply being an authenticated user grants access to sensitive system functions.

Attack Chain

1. An attacker obtains a valid user account on the Open WebUI instance.
2. The attacker authenticates to the Open WebUI instance using their credentials.
3. The attacker sends a GET request to /api/tasks to enumerate all active task IDs on the system.
4. The server returns a list of task IDs, including those belonging to other users.
5. The attacker selects a task ID belonging to another user.
6. The attacker sends a POST request to /api/tasks/stop/{task_id}, replacing {task_id} with the target task ID.
7. The server, lacking proper authorization checks, attempts to stop the specified task.
8. The targeted user’s task is interrupted, causing disruption to their ongoing activity.

Impact

This vulnerability impacts all users in a multi-user Open WebUI deployment, particularly those running background tasks such as chat generation. A single low-privilege user can effectively cause a denial-of-service by continuously canceling tasks, making the chat functionality unusable for other users. This affects integrity and availability, allowing unauthorized interruption of legitimate operations.

Recommendation

• Upgrade to Open WebUI version 0.9.0 or later to remediate CVE-2026-45399 and CVE-2025-63681, which addresses the broken object-level authorization.
• Deploy the Sigma rule “Detect Open WebUI Task Enumeration” to identify potential exploitation attempts using the /api/tasks endpoint.
• Deploy the Sigma rule “Detect Open WebUI Task Cancellation” to detect unauthorized task cancellation attempts via the /api/tasks/stop/{task_id} endpoint.