Open WebUI: Any authenticated user can cancel another user's chat generation via the chat delete endpoint

Summary

DELETE /api/v1/chats/{id} cancelled a chat's in-flight tasks before it checked whether the caller was allowed to delete that chat. Any authenticated user who knew another user's chat id could therefore abort that user's running model response, title generation or tag generation. The deletion itself was still refused, so the only missing control was on the cancellation side effect.

Preconditions

Default configuration, no special deployment shape. The attacker needs a normal account with the default user role and nothing else: the chat.delete permission is not required, and revoking it does not prevent the cancellation. The attacker also needs the victim's chat id, which is returned by the read-only shared-chat endpoint when a chat or a folder has been shared with them - otherwise enumerating the chat id requires guessing the chat id or brute forcing it, and the victim must have a generation running at that moment.

Impact

A user can repeatedly interrupt another user's generations without any write access to the target chat. Nothing is deleted, modified or disclosed, and the victim can simply regenerate, so the effect is limited to availability of in-flight responses. Because the attacker only needs a chat id, the interruption can be scripted and repeated for as long as the id stays valid.

Fix

Fixed in https://github.com/open-webui/open-webui/pull/27006, released in 0.11.0. The handler now resolves and authorizes the chat first and only cancels tasks and deletes once the caller is an admin or a permitted owner; an unauthorized caller gets 401 or 404 with no cancellation.

Root cause

Affected component: delete_chat_by_id in backend/open_webui/routers/chats.py, serving DELETE /api/v1/chats/{id}. Affected setup: every build from 0.9.6 up to and including 0.10.2.