CVE-2026-5139: Mattermost Server

Medium severity, CVSS 5.4. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler, which allows any authenticated user to overwrite the global default GitLab instance configuration via the {{/gitlab connect <instance-name>}} slash command.. Mattermost Advisory ID: MMSA-2026-00644

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.18 (fixed in 10.11.18); from 11.5.0, before 11.5.6 (fixed in 11.5.6); from 11.6.0, before 11.6.3 (fixed in 11.6.3); from 11.7.0, before 11.7.1 (fixed in 11.7.1)

Published 2026-06-22. Last modified 2026-06-23.