CVE-2026-6062: Mattermost Server

Medium severity, CVSS 6.4. EPSS: 0.2% 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 validate channel ownership of an existing subscription before applying edits which allows an authenticated attacker to hijack subscriptions from channels they have no access to via a crafted PUT request to the subscription edit endpoint.. Mattermost Advisory ID: MMSA-2026-00650

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.