CVE-2026-82920: Mattermost Server

Medium severity, CVSS 5.5. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7 fail to enforce authorization boundaries on the access control policy update endpoint which allows a channel or team administrator to detach a system-assigned ABAC parent policy via a crafted PUT /api/v4/access_control_policies request with an empty imports list.. Mattermost Advisory ID: MMSA-2026-00724

Affected products

• Mattermost Mattermost Server: from 11.7.0, before 11.7.8 (fixed in 11.7.8); from 11.8.0, before 11.8.5 (fixed in 11.8.5); from 11.9.0, before 11.9.1 (fixed in 11.9.1)

Published 2026-09-14. Last modified 2026-10-07.