CVE-2026-8821: Mattermost Server

High severity, CVSS 7.1. 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, 10.11.x <= 10.11.22 fail to validate channel member-management permission during playbook run creation, allowing an authenticated channel member to add an arbitrary user to a restricted channel via the run owner field.. Mattermost Advisory ID: MMSA-2026-00677

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.23 (fixed in 10.11.23); 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.