CVE-2026-4635: Mattermost Server

Medium severity, CVSS 5.3. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to archive the channel before removing persistent notifications which allows authenticated user to crash the server via timing the creation of persistent notification message between the server deleting existing persistent notifications and archiving the channel.. Mattermost Advisory ID: MMSA-2026-00637

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.15 (fixed in 10.11.15); from 11.4.0, before 11.4.5 (fixed in 11.4.5); from 11.5.0, before 11.5.4 (fixed in 11.5.4); from 11.6.0, before 11.6.1 (fixed in 11.6.1)

Published 2026-05-22. Last modified 2026-07-23.