CVE-2026-20796: Mattermost Server

Low severity, CVSS 3.1. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 10.11.x <= 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /common_teams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.10 (fixed in 10.11.10)

Published 2026-02-13. Last modified 2026-06-17.