The flaw lies in the Socket.IO event handler for ydoc:document:update; it verifies membership in a document’s room but does not confirm that the sender has write rights. As a result, users with read‑only permissions who join the room can emit update events that alter the Yjs document state, leading to unintended changes in collaborative documents. This represents an authorization bypass that compromises data integrity.

Open WebUI version 0.9.0 and earlier are affected. The product is hosted by the vendor open‑webui:open‑webui. No narrower version range is specified beyond the pre‑0.9.0 release.

The CVSS score of 5.4 places the vulnerability in the moderate severity range. EPSS is not available, and the flaw is not yet listed in CISA's KEV catalog. An attacker who can act as a read‑only user and join the Socket.IO room can issue update events, thereby modifying collaborative documents in real time. The attack vector is likely network‑based, requiring connectivity to the WebSocket endpoint. No additional elevating capabilities are needed; having read‑only access to the document room is sufficient to exploit the bug.