CVE-2025-12689: Mattermost Server

Medium severity, CVSS 6.5. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 11.0.x <= 11.0.4, 10.12.x <= 10.12.2, 10.11.x <= 10.11.6 fail to check WebSocket request field for proper UTF-8 format, which allows attacker to crash Calls plug-in via sending malformed request.

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.7 (fixed in 10.11.7); from 10.12.0, before 10.12.3 (fixed in 10.12.3); from 11.0.0, before 11.0.5 (fixed in 11.0.5)

Published 2025-12-17. Last modified 2026-06-17.