CVE-2026-16047: Mattermost Server

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

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate that users have read access to a channel before linking a board to it, which allows an authenticated attacker to discover the membership of private channels on the same team via creating, patching, importing, or bulk-creating boards with an arbitrary channelId. Mattermost Advisory ID: MMSA-2026-00674

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.22 (fixed in 10.11.22); 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.