Open WebUI: Cross-channel message overwrite via chat completion API (single-model and multimodel message_ids)

Summary

Any authenticated user can overwrite the content of a message in a channel they do not belong to (including private and DM channels) by sending a chat completion request with a channel:-prefixed chat_id and a target message_id. The channel: path routes pipeline output through _make_channel_emitter, which writes to the Messages table using the caller-supplied message_id without binding it to the channel.

This advisory consolidates two filings of the same flaw: the original single-model form, and a multimodel message_ids variant that survives the partial fix shipped in v0.9.6 (see "Fix status" below).

Details (as introduced in v0.9.5)

When a user submits a chat completion request with a chat_id starting with channel:, three authorization gaps combined in v0.9.5:

1. Ownership check skipped (main.py): the channel: prefix caused the entire ownership/membership verification block to be skipped, with no channel membership/write check replacing it.

if not chat_id.startswith('local:') and not chat_id.startswith('channel:'):  # temporary/channel chats are not stored
    if is_new_chat:
        ...
    else:
        if not await Chats.is_chat_owner(chat_id, user.id) and user.role != 'admin':
            raise HTTPException(...)

2. Message ID from user input: id (and each value of the multimodel message_ids map) comes directly from the request body and is passed as message_id to the channel emitter.

3. Unchecked database write (socket/main.py ):