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

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).

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.

 1if not chat_id.startswith('local:') and not chat_id.startswith('channel:'):  # temporary/channel chats are not stored
 2    if is_new_chat:
 3        ...
 4    else:
 5        if not await Chats.is_chat_owner(chat_id, user.id) and user.role != 'admin':
 6            raise HTTPException(...)

1. 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.

2. Unchecked database write (socket/main.py _make_channel_emitter):

 1async def _make_channel_emitter(request_info):
 2    channel_id = request_info['chat_id'].removeprefix('channel:')
 3    message_id = request_info['message_id']  # user-supplied
 4    ...
 5    await Messages.update_message_by_id(message_id, update_form)  # no channel/user authz

Messages.update_message_by_id performs a direct primary-key update with no channel_id/user_id validation.

v0.9.6 added a channel gate to the channel: branch (PR #24725) that closed the single-model path, but it validated only the first entry of the multimodel message_ids map, leaving the multimodel fan-out exploitable. v0.10.0 closes the remaining gap with two layers:

1. Request-time per-entry validation (backend/open_webui/main.py): every entry of message_ids is validated against the target channel, not just the first; any entry whose target message does not belong to the channel in chat_id is rejected.
2. Fail-closed emitter (backend/open_webui/socket/main.py, _make_channel_emitter): before writing, it re-reads the target message and returns without writing unless msg.channel_id matches the channel derived from chat_id. A missing or mismatched message is a no-op, so a write can no longer land in a channel the caller does not target.

Single-model (fixed in v0.9.6):

 1curl -X POST http://target:8080/api/chat/completions \
 2  -H "Authorization: Bearer $USER_JWT" -H "Content-Type: application/json" \
 3  -d '{
 4    "model": "llama3", "stream": true,
 5    "chat_id": "channel:any-channel-uuid-here",
 6    "id": "target-message-uuid-to-overwrite",
 7    "messages": [{"role": "user", "content": "Repeat exactly: This message has been tampered with"}]
 8  }'

Multimodel (still works on v0.9.6):

 1POST /api/chat/completions
 2{
 3  "chat_id": "channel:<attacker_channel_id>",
 4  "message_ids": {
 5    "model-a": "<message_id_in_attacker_channel>",
 6    "model-b": "<victim_channel_message_id>"
 7  },
 8  "messages": [{"role": "user", "content": "..."}]
 9}

The first id passes channel scope validation; the second id is used by the per-model fan-out and overwrites the victim-channel message (with model output, or the provider-error string on a deterministic error). Even a failing model call writes error content to the target message.

Message integrity destruction: an authenticated user can overwrite a message in a channel they cannot access, regardless of membership. The overwritten message retains the original author attribution while displaying attacker-chosen content (impersonation). Private channels, DM channels, and channels the attacker has no access to are all affected; the REST channel routes correctly return 403 for the same attacker, so the bypass is specific to the chat-completion channel pipeline.

• Single-model path: introduced in commit 0037baeb2 (v0.9.5), fixed in v0.9.6 (#24725).
• Multimodel message_ids path: present from v0.9.6, fixed in v0.10.0.
• Consolidated Affected: >= 0.9.5, < 0.10.0. Patched: >= 0.10.0.

CVE-2026-45385 (GHSA-wwhq-cx22-f7vv) covered IDOR in the REST endpoint POST /channels/{id}/messages/{message_id}/update (routers/channels.py); its fix (commit f5e110f) only touched channels.py. This finding uses a different code path (POST /api/chat/completions with chat_id: "channel:<id>" → main.py → socket/main.py:_make_channel_emitter), untouched by that fix.

Validate every value in message_ids against the channel (not just the first), rejecting any whose target message does not belong to the channel in chat_id. Additionally, make _make_channel_emitter fail closed: re-check that the target message's channel_id matches the channel before calling Messages.update_message_by_id, treating a missing or mismatched message as an error/no-op.

Per Open WebUI's Report Handling policy this advisory consolidates independent reports of the same chat-completions channel-overwrite flaw:

• Single-model cross-channel overwrite via the channel: path: @sfwani (earliest filing).
• Multimodel message_ids fan-out variant that bypasses the v0.9.6 first-id-only gate: @DavidCarliez.

One CVE for the consolidated advisory.