CVE-2025-20621: Mattermost Server

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

Mattermost versions 10.2.x <= 10.2.0, 9.11.x <= 9.11.5, 10.0.x <= 10.0.3, 10.1.x <= 10.1.3 fail to properly handle posts with attachments containing fields that cannot be cast to a String, which allows an attacker to cause the webapp to crash via creating and sending such a post to a channel.

Affected products

• Mattermost Mattermost Server: from 9.11.0, before 9.11.6 (fixed in 9.11.6); from 10.0.0, before 10.0.4 (fixed in 10.0.4); from 10.1.0, before 10.1.4 (fixed in 10.1.4); version 10.2.0 only

Published 2025-01-16. Last modified 2026-06-17.