CVE-2025-53971: Mattermost Server

Low severity, CVSS 3.8. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 10.5.x <= 10.5.8, 9.11.x <= 9.11.17 fail to properly validate authorization for team scheme role modifications which allows Team Admins to demote Team Members to Guests via the PUT /api/v4/teams/team-id/members/user-id/schemeRoles API endpoint.

Affected products

• Mattermost Mattermost Server: from 9.11.0, before 9.11.18 (fixed in 9.11.18); from 10.5.0, before 10.5.9 (fixed in 10.5.9)

Published 2025-08-21. Last modified 2026-06-17.