GHSA-x2ff-v5v8-m75m Open WebUI: Cross-channel message overwrite via chat completion API (single-model and multimodel message_ids)
SummaryAny 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:**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.**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.**Unchecked database write** (`socket/main.py` `_make_channel_emitter`):`Messages.update_message_by_id` performs a direct primary-key update with no `channel_id`/`user_id` validation.Fix (shipped in v0.10.0)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:**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.**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.PoCSingle-model (fixed in v0.9.6):Multimodel (still works on v0.9.6):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.Impact**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.Affected versionsSingle-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`.Distinction from existing CVEsCVE-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.Suggested fixValidate **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.ConsolidationPer 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.
CVSS v3.1 score: 7.1