CVE-2026-27659: Mattermost Server

Medium severity, CVSS 4.6. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to properly validate CSRF tokens in the /api/v4/access_control_policies/{policy_id}/activate endpoint, which allows an attacker to trick an admin into changing access control policy active status via a crafted request.. Mattermost Advisory ID: MMSA-2026-00578

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.11 (fixed in 10.11.11); from 11.2.0, before 11.2.3 (fixed in 11.2.3); from 11.3.0, before 11.3.2 (fixed in 11.3.2); from 11.4.0, before 11.4.1 (fixed in 11.4.1)

Published 2026-03-25. Last modified 2026-06-17.