CVE-2026-3590: Mattermost Server

Medium severity, CVSS 6.5. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests.. Mattermost Advisory ID: MMSA-2026-00624

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.13 (fixed in 10.11.13); from 11.3.0, before 11.3.3 (fixed in 11.3.3); from 11.4.0, before 11.4.3 (fixed in 11.4.3); from 11.5.0, before 11.5.1 (fixed in 11.5.1)

Published 2026-04-15. Last modified 2026-06-17.