CVE-2026-26233: Mattermost Server

Medium severity, CVSS 6.5. EPSS: 0.5% chance of exploitation in the next 30 days.

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to rate limit login requests which allows unauthenticated remote attackers to cause denial of service (server crash and restart) via HTTP/2 single packet attack with 100+ parallel login requests.. Mattermost Advisory ID: MMSA-2025-00566

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.12 (fixed in 10.11.12); from 11.2.0, before 11.2.4 (fixed in 11.2.4); from 11.3.0, before 11.3.2 (fixed in 11.3.2); from 11.4.0, before 11.4.1 (fixed in 11.4.1)

Published 2026-03-25. Last modified 2026-06-17.