CVE-2026-21388: Mattermost Server

Medium severity, CVSS 6.5. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost Plugins versions <=2.3.1 fail to limit the request body size on the {{/lifecycle}} webhook endpoint which allows an authenticated attacker to cause memory exhaustion and denial of service via sending an oversized JSON payload. Mattermost Advisory ID: MMSA-2026-00610

Affected products

• Mattermost Mattermost Server: up to and including 2.3.1

Published 2026-04-09. Last modified 2026-06-17.