Open WebUI: Private channel messages can be disclosed through cross-channel thread parent_id binding

A normal authenticated user can read the content of a message in a private channel they do not belong to. GET /api/v1/channels/{id}/messages/{message_id}/thread authorizes the caller against the URL channel, but the underlying thread lookup loads the thread parent by id and returns it without verifying the parent belongs to that channel. By requesting a thread in a channel they can access while supplying a victim channel's message id as the thread root, the attacker receives the victim message — content, channel id, and author.

• backend/open_webui/models/messages.py — get_messages_by_parent_id()
• backend/open_webui/routers/channels.py — get_channel_thread_messages() (read), new_message_handler() (parent/reply binding on write)

get_messages_by_parent_id(channel_id, parent_id) filters the thread replies by channel_id, but loads the thread parent by id alone and appends it without requiring parent.channel_id == channel_id:

 1message = await db.get(Message, parent_id)     # loaded by id only — no channel binding
 2if not message:
 3    return []
 4# replies are filtered by channel_id ...
 5if len(all_messages) < limit:
 6    all_messages.append(message)               # parent appended unconditionally

get_channel_thread_messages() authorizes only the URL channel, then calls get_messages_by_parent_id(id, message_id) with the caller-supplied message_id. The reply insert path (new_message_handler → insert_new_message) also stored a caller-supplied parent_id without binding it to the channel.

A non-member can disclose the content (plus channel id and author metadata) of a private-channel message whose id they know or obtain. Direct reads of the victim channel/message/thread return 403; the disclosure is via the thread parent of a channel the attacker can access. Read-only, one message per known id.

(reporter) Validated on v0.9.6: GET /channels/{attacker_channel}/messages/{victim_message_id}/thread returned the victim's private message — content, victim channel id, and author — although direct reads of the victim channel returned 403.

Bind the thread parent to the requested channel: get_messages_by_parent_id returns [] unless the parent exists and parent.channel_id == channel_id. Defence-in-depth on the write path: new_message_handler rejects a supplied parent_id/reply_to_id whose message does not belong to the URL channel.

• Affected: < 0.10.0 (last affected release 0.9.6)
• Patched: v0.10.0 (PR #25766). get_messages_by_parent_id binds the thread parent to the requested channel (returns [] unless parent.channel_id == channel_id), and new_message_handler rejects a caller-supplied parent_id/reply_to_id whose message does not belong to the channel.