Open WebUI’s POST /api/v1/channels/{channel_id}/messages/{message_id}/update endpoint incorrectly allowed users with only read permission to modify messages that they did not own in standard channels. This is an instance of Improper Authorization (CWE‑862). The flaw occurs because when access_control is set to None, the authorization check defaults to true for read access, enabling unauthorized message updates. An attacker who can reach the endpoint and has read rights can therefore alter another user’s message content, compromising data integrity and potentially disrupting trust in the platform.

The vulnerability affects versions of Open WebUI prior to 0.8.6. Administrators running the open-webui platform should verify that the deployed instance is 0.8.6 or later; earlier releases are susceptible to unauthorized message updates via the POST /api/v1/channels/{channel_id}/messages/{message_id}/update endpoint.

The CVSS score of 6.5 classifies this as a moderate severity flaw. Exploitation requires that the attacker be able to reach the vulnerable endpoint and possess read permission, which is common in local or compromised environments. The EPSS score is not available, and the vulnerability is not currently listed in the CISA KEV catalog, indicating that known exploits are not widespread. Still, any user who obtains even read access can manipulate messages, so the risk persists until remediation is applied.