Open WebUI is an extensible, feature-rich, and user-friendly self-hosted AI platform. From 0.9.5 before 0.10.0, 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 issue is fixed in version 0.10.0.

Affected: PyPI: open-webui

## 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.

```python 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…

Affected: PyPI: open-webui

## 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.

```python 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…