CVE-2026-15754: Mattermost Server

Medium severity, CVSS 4.2. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 11.7.x <= 11.7.6, 11.8.x <= 11.8.3 The access control policy unassign endpoint fails to re-validate that each target channel still belongs to the requesting admin's team, which allows an authenticated team administrator to remove ABAC (attribute-based access control) policy assignments from channels outside their team via the policy unassign API after a channel has been moved to another team.. Mattermost Advisory ID: MMSA-2026-00718

Affected products

• Mattermost Mattermost Server: from 11.7.0, before 11.7.7 (fixed in 11.7.7); from 11.8.0, before 11.8.4 (fixed in 11.8.4)

Published 2026-08-17. Last modified 2026-08-18.