CVE-2026-26230: Mattermost Server

Low severity, CVSS 3.8. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 10.11.x <= 10.11.10 fail to properly validate permission requirements in the team member roles API endpoint which allows team administrators to demote members to guest role. Mattermost Advisory ID: MMSA-2025-00531

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.11 (fixed in 10.11.11)

Published 2026-03-16. Last modified 2026-06-17.