Executive brief
Open WebUI is a web-based chat interface that allows users to create and store conversations. An authenticated user can craft a malicious chat with cyclic message references and omitted ID fields, causing the server's message-history reconstruction logic to enter an infinite loop. This blocks the entire server—all users lose access to the web interface, health checks fail, and orchestrator monitoring stops working—until an administrator manually deletes the malicious chat from the database or the server is restarted.
Technical details
The vulnerability is a classic infinite-loop denial of service caused by a mismatch between loop traversal and visited-set tracking in `backend/open_webui/utils/misc.py`. The code walks a chat's message chain by following `parentId` keys in a history map, but tracks visited messages using their `id` field. When a message omits the `id` field (which the API does not validate), the visited guard fails to record it, allowing cycles to persist indefinitely. The walk is synchronous and runs on the asyncio event loop, so it blocks every concurrent request—including unauthenticated `/health` endpoints and administrator APIs. The malformed chat persists in the database across restarts, so the hang resumes on the next server restart. Any default-role user can trigger the attack via a single POST to create a chat followed by a GET to trigger history reconstruction. No authentication bypass, data exfiltration, or modification occurs. Fixed in version 0.11.1 by recording the traversal position (map key) instead of the message's self-reported `id` field, guaranteeing termination after at most one step per stored message.
Affected products
- Open WebUI Open WebUI 0.5.0 to 0.11.0 inclusive
Timeline
- 2026-08-29: disclosed
- 2026-09-09: advisory: Published to GitHub Advisory Database
- 2026: patched: Fixed in version 0.11.1 by commit 5c79ccc9e5c9efc2bc024d8f0b9757652ece929a