CVE-2025-12689: 
vulnerability analysis and mitigation

CVE-2025-12689 is a denial-of-service vulnerability in Mattermost Server affecting the Calls plug-in, caused by improper validation of UTF-8 encoding in WebSocket request fields. Affected versions include 11.0.x ≤ 11.0.4, 10.12.x ≤ 10.12.2, and 10.11.x ≤ 10.11.6. The vulnerability was published on December 17, 2025, and patches were released shortly after. It carries a CVSS v3.1 base score of 6.5 (Medium) (Red Hat CVE, Mattermost Security).

The root cause is classified as CWE-1287 (Improper Validation of Specified Type of Input): Mattermost fails to validate that WebSocket request fields conform to proper UTF-8 encoding before passing them to the Calls plug-in. An authenticated attacker with low privileges can send a malformed WebSocket request containing invalid UTF-8 sequences, triggering a crash in the Calls plug-in. No complex preconditions are required beyond having a valid low-privilege account on the Mattermost instance (Red Hat CVE, GitLab Advisory).

Successful exploitation results in a crash of the Mattermost Calls plug-in, causing a denial of service that disrupts voice and video communication capabilities for all users relying on that plug-in. There is no impact on confidentiality or integrity — only availability is affected. The scope is limited to the Calls plug-in component and does not provide a pathway for lateral movement or data exfiltration (Red Hat CVE, Mattermost Security).

1. Authentication: Log in to a vulnerable Mattermost instance (versions 11.0.x ≤ 11.0.4, 10.12.x ≤ 10.12.2, or 10.11.x ≤ 10.11.6) with any low-privilege account.
2. Establish WebSocket connection: Open a WebSocket connection to the Mattermost server's WebSocket endpoint (typically wss://<host>/api/v4/websocket).
3. Craft malformed request: Construct a WebSocket message targeting the Calls plug-in that includes fields containing invalid or malformed UTF-8 byte sequences (e.g., overlong encodings or invalid continuation bytes).
4. Send malformed request: Transmit the crafted WebSocket message to the server; the Calls plug-in fails to validate the UTF-8 encoding and crashes upon processing the malformed input.
5. Result: The Calls plug-in becomes unavailable, denying voice/video call services to all users until the plug-in is restarted or the server is remediated (Red Hat CVE, GitLab Advisory).

• Network: Unusual or repeated WebSocket connections from a single authenticated user sending malformed or non-standard payloads to the Mattermost WebSocket endpoint (/api/v4/websocket).
• Logs: Mattermost server logs showing unexpected crashes or panic errors in the Calls plug-in process; error messages referencing invalid UTF-8 encoding or malformed WebSocket frames.
• Process: Unexpected termination and restart of the mattermost-plugin-calls process; elevated plug-in crash frequency in a short time window.

Mattermost has released patched versions addressing this vulnerability: 11.0.5, 10.12.3, and 10.11.7. Administrators should upgrade to one of these versions as the primary remediation. As an interim measure, restricting Mattermost access to trusted authenticated users and monitoring for unusual WebSocket request patterns can reduce exposure. No configuration-only workaround is available that fully mitigates the vulnerability without upgrading (Mattermost Security, GitLab Advisory).