Open WebUI: Any authenticated user can hang the server via a cyclic chat message history

## Summary Chat histories are stored as an unvalidated JSON object. The walk that reconstructs a chat's message chain detected repeats using each message's own `id` field while moving through the history by map key, so a message that simply omitted `id` was never recorded as visited. A history whose messages referenced each other in a parent cycle therefore made the walk run forever. Any account with the default `user` role could store such a chat and trigger the walk, blocking the server for everyone. ## Preconditions One account with the default `user` role. No administrator rights, no additional permissions, no configuration change and no non-default setting: creating a chat is available to every user out of the box. The attack runs entirely against the attacker's own chat, so no knowledge of any other user's data is needed. Instances where every account is trusted are affected in the sense that the fault is reachable, but require a user acting deliberately. ## Impact The walk is synchronous and runs on the asyncio event loop, so while it spins, every request from every user is blocked, including unauthenticated `/health` and administrator endpoints. External health checks and orchestrator liveness probes fail alongside the UI. The list it appends to grows without bound, so a memory-capped deployment ends in an out-of-memory kill rather than a hang. The work is not cancelled when the client disconnects, so one fire-and-forget request is enough and the attacker can disconnect immediately. The malformed chat stays in the database, so restarting the process does not clear the condition: the next request that walks that chat hangs the new process, and recovery requires deleting the stored chat. No data is disclosed, altered or deleted. ## Fix Fixed in 0.11.1 by https://github.com/open-webui/open-webui/commit/5c79ccc9e5c9efc2bc024d8f0b9757652ece929a. The walk now records the map key it is currently positioned at instead of the message's self-reported `id`, so it terminates after at most one step per stored message whatever the message contents are. Upgrading fully resolves the issue, including for chats stored while the deployment was on an affected version, which become harmless once the walk terminates. ## Root cause Affected component: the message-chain reconstruction helper in `backend/open_webui/utils/misc.py`, reached from every path that rebuilds a chat's history, including chat completion, per-chat statistics, context compaction, subagents and timers. Affected setup: all builds from 0.5.0 up to and including 0.11.0. The loop's visited set was keyed on the message body's `id` field while the loop itself advanced by looking the parent up as a key in the history map, so the two used different notions of identity. That `id` field is part of the stored chat object and is fully attacker-controlled, and the guard explicitly skipped recording it when absent, which left the exit condition unreachable for any message that omitted it. The write path does not validate the structure of a chat's history, so a history containing id-less messages in a parent cycle was persisted exactly as submitted. ## Proof of concept As a default-role user on a running instance, store one chat of two messages that reference each other as parents, with the `id` field omitted: ```json {"chat":{"title":"poc","history":{"currentId":"A","messages":{ "A":{"parentId":"B","role":"user","content":"a","childrenIds":[]}, "B":{"parentId":"A","role":"assistant","content":"b","childrenIds":[]}}}}} ``` `POST /api/v1/chats/new` stores it verbatim. A single subsequent `GET /api/v1/chats/stats/usage` as the same user then never returns. Sampled during the hang, the worker consumed one full CPU core and reached 1.86 GB resident within 60 seconds, still growing. Unauthenticated `GET /health` and administrator API calls both time out for as long as the process lives, and continue to do so after the attacker's connection closes. Restarting the server restores s

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-835

Also known as

CVE-2026-88002, PYSEC-2026-3879

• github.com/open-webui/open-webui/security/advisories/GHSA-jqhh-cjmq-vmv6
• github.com/open-webui/open-webui/pull/28034
• github.com/open-webui/open-webui/commit/5c79ccc9e5c9efc2bc024d8f0b9757652ece929a
• github.com/open-webui/open-webui
• github.com/open-webui/open-webui/releases/tag/v0.11.1