CVE-2026-59215: Open WebUI: Private channel messages can be disclosed through cross-channel thread parent_id binding

Summary

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/{messageid}/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.

Affected component

- backend/openwebui/models/messages.py — getmessagesbyparentid() - backend/openwebui/routers/channels.py — getchannelthreadmessages() (read), newmessagehandler() (parent/reply binding on write)

Root cause

getmessagesbyparentid(channelid, parentid) filters the thread replies by channelid, but loads the thread parent by id alone and appends it without requiring parent.channelid == channelid:

python message = await db.get(Message, parentid) # loaded by id only — no channel binding if not message: return [] replies are filtered by channelid ... if len(allmessages) < limit: allmessages.append(message) # parent appended unconditionally

getchannelthreadmessages() authorizes only the URL channel, then calls getmessagesbyparentid(id, messageid) with the caller-supplied messageid. The reply insert path (newmessagehandler → insertnewmessage) also stored a caller-supplied parentid without binding it to the channel.

Impact

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.

Proof of Concept

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

Fix

Bind the thread parent to the requested channel: getmessagesbyparentid returns [] unless the parent exists and parent.channelid == channelid. Defence-in-depth on the write path: newmessagehandler rejects a supplied parentid/replytoid whose message does not belong to the URL channel.

Affected / Patched

- Affected: < 0.10.0 (last affected release 0.9.6) - Patched: v0.10.0 (PR #25766). getmessagesbyparentid binds the thread parent to the requested channel (returns [] unless parent.channelid == channelid), and newmessagehandler rejects a caller-supplied parentid/replytoid whose message does not belong to the channel.