CVE-2026-10080: Mattermost Server

Medium severity, CVSS 6.5. EPSS: 0.4% chance of exploitation in the next 30 days.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fails to validate WebSocket command field types which allows an authenticated user to crash the plugin process and deny service to all Boards users via a custom_focalboard_SUBSCRIBE_TEAM message with a non-string teamId.. Mattermost Advisory ID: MMSA-2026-00687

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.22 (fixed in 10.11.22); from 11.7.0, before 11.7.7 (fixed in 11.7.7); from 11.8.0, before 11.8.4 (fixed in 11.8.4)

Published 2026-08-17. Last modified 2026-08-18.