Open WebUI versions 0.11.0 through 0.11.3 contain an improper authorization flaw in the chat‑fork endpoint. The handler stores the newly forked chat in the source chat’s folder without evaluating whether the requester has write permission for that folder. This allows a user who previously had write access and was later downgraded to read‑only to submit a POST request to /api/v1/chats/{id}/fork, thereby creating attacker‑controlled chat content that the folder owner and other members can view.

The vulnerability affects the open‑webui product from the open‑webui vendor. All releases starting at 0.11.0 and prior to 0.11.4 are susceptible. Installing or upgrading to Open WebUI 0.11.4 or later removes the flaw.

The CVSS score of 5.3 indicates a moderate threat. EPSS information is not available, and the vulnerability is not listed in CISA KEV. The likely attack vector is an internal or remote user with read‑only membership who previously had write access; by sending a crafted POST request to the fork endpoint, the attacker can inject unauthorized content visible to other folder members, potentially exposing sensitive information or corrupting data integrity. The lack of an official workaround means the recommended mitigation is to apply the patch or upgrade to a fixed version.