Executive Summary

CVE-2026-42276 is an Insecure Direct Object Reference (IDOR) vulnerability in the Onyx AI platform's /chat/stop-chat-session endpoint.

This endpoint allows any authenticated user to stop another user's active chat session by providing the session UUID, without verifying that the session belongs to the caller.

Because the endpoint checks only authentication but not ownership, an attacker who knows a chat session UUID can interrupt another user's ongoing large language model (LLM) generation mid-stream.

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Impact Analysis

This vulnerability can allow an attacker to terminate other users' active chat sessions unexpectedly.

As a result, users may experience interruptions in receiving responses from the AI platform, leading to loss of partial output or incomplete interactions.

The attack requires low complexity and only authenticated privileges, but no user interaction, making it relatively easy for attackers with access to exploit.

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Detection Guidance

This vulnerability can be detected by monitoring requests to the POST /chat/stop-chat-session/{chat_session_id} endpoint where an authenticated user attempts to stop chat sessions using session UUIDs that do not belong to them.

To detect potential exploitation, you can log and analyze API calls to this endpoint, checking for cases where the session UUID does not match the authenticated user's sessions.

Example commands to detect suspicious activity might include:

• Using web server logs or API gateway logs, filter POST requests to /chat/stop-chat-session and correlate the authenticated user with the session UUID being stopped.
• If using Linux command line tools, you might run: `grep 'POST /chat/stop-chat-session' /var/log/onyx/access.log | awk '{print $user, $session_uuid}'` to extract user and session UUID pairs for analysis.
• Use network monitoring tools to capture and inspect HTTP POST requests to the vulnerable endpoint, looking for mismatches between the authenticated user and the session UUID targeted.

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Mitigation Strategies

The immediate mitigation step is to upgrade the Onyx application to a patched version where this vulnerability is fixed.

• Upgrade to Onyx version 3.0.9, 3.1.6, or 3.2.6 or later, as these versions include the patch that verifies session ownership before allowing a chat session to be stopped.
• Restrict access to the /chat/stop-chat-session endpoint to only trusted users or roles until the upgrade can be applied.
• Monitor logs for suspicious activity involving the stopping of chat sessions by users other than the session owners.

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Compliance Impact

The vulnerability allows any authenticated user to terminate another user's active chat session without proper authorization checks. This could lead to denial of service for legitimate users and potential loss of partial output data during LLM generation.

While the CVE description and resources do not explicitly mention compliance with standards such as GDPR or HIPAA, the authorization bypass could impact data integrity and availability, which are important aspects of these regulations.

Specifically, interruption of user sessions without consent might violate principles of user control and data protection under such regulations, potentially leading to compliance risks.

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