The vulnerability is an Insecure Direct Object Reference (CWE‑639) in the /chat/stop-chat-session endpoint. It allows any authenticated user to stop any other user's chat session by providing a chat session UUID. The primary impact is abrupt interruption of LLM generation, causing loss of context and possible incomplete responses, which can degrade user experience and disrupt business processes. No elevation of privileges or data exfiltration is possible; the flaw only affects session control.

Affected systems are the Onyx open‑source AI platform, specifically versions 3.0.0 through 3.0.8, 3.1.0 through 3.1.5, and 3.2.0 through 3.2.5. The vendor is onyx-dot-app, product Onyx. The issue was first discovered before these release dates and has been fixed in 3.0.9, 3.1.6, and 3.2.6.

The CVSS score of 4.3 indicates moderate severity. EPSS is not available, and the vulnerability is not listed in the CISA KEV catalog. Attack requires that an attacker be authenticated to the platform and know a particular chat session UUID; the endpoint performs no ownership check. Exploitation is straightforward with internal or compromised credentials and can be executed over any trusted communication channel. Because the affected code runs in a server environment, the attacker can disrupt multiple users by repeatedly submitting stop-session requests.