CVE-2026-4055: Mattermost Server

Medium severity, CVSS 4.3. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 11.5.x <= 11.5.1 fail to validate team-level run_create permission against the target team when creating a playbook run which allows an authenticated team member to create runs in teams where they lack permission via specifying a different team ID in the run creation API request. Mattermost Advisory ID: MMSA-2026-00629

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.17 (fixed in 10.11.17); from 11.5.0, before 11.5.5 (fixed in 11.5.5); from 11.6.0, before 11.6.2 (fixed in 11.6.2)

Published 2026-05-21. Last modified 2026-07-23.