Executive brief
Open WebUI is a user-friendly interface for AI chat that supports collaborative channels where team members can post and edit messages. A vulnerability allows any channel member to overwrite another member's message through the chat completions endpoint, causing the modified text to appear as if written by the original author. This enables fraud, reputation damage, and data integrity loss without leaving a clear audit trail.
Technical details
The vulnerability is an authorization bypass (CWE-639, CWE-862) in the channel message editing flow. The chat completions endpoint (`POST /api/chat/completions`) checks whether the caller has write access to the channel and whether the target message belongs to that channel, but fails to verify that the caller authored the message being edited. In contrast, the dedicated channel message update route (`POST /api/v1/channels/<channel_id>/messages/<message_id>/update`) correctly enforces authorship checks and rejects cross-member edits with a 403 response. An authenticated channel member with write access can exploit this inconsistency by submitting a message ID belonging to another user to the completions endpoint, causing the target message text to be replaced via AI completion while retaining the original author's identity in the stored record. The vulnerability requires channels to be enabled (disabled by default in `ENABLE_CHANNELS`), attacker membership with write permissions to the target channel, and knowledge of the target message ID (which is visible to all channel members). Fixed in version 0.11.1 via commit 7d392bedc, which adds author identity verification to the channel completions branch matching the dedicated edit route logic.
Affected products
- Open WebUI open-webui >= 0.9.5, < 0.11.1
Timeline
- 2026-09-10: disclosed
- 2026-09-10: patched: Fixed in version 0.11.1
- 2026-08-31: advisory: Published to GitHub Advisory Database