CVE-2025-12419: Mattermost Server

Critical severity, CVSS 9.9. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 10.12.x <= 10.12.1, 10.11.x <= 10.11.4, 10.5.x <= 10.5.12, 11.0.x <= 11.0.3 fail to properly validate OAuth state tokens during OpenID Connect authentication which allows an authenticated attacker with team creation privileges to take over a user account via manipulation of authentication data during the OAuth completion flow. This requires email verification to be disabled (default: disabled), OAuth/OpenID Connect to be enabled, and the attacker to control two users in the SSO system with one of them never having logged into Mattermost.

Affected products

• Mattermost Mattermost Server: from 10.5.0, before 10.5.13 (fixed in 10.5.13); from 10.11.0, before 10.11.5 (fixed in 10.11.5); from 10.12.0, before 10.12.2 (fixed in 10.12.2); from 11.0.0, before 11.0.4 (fixed in 11.0.4)

Published 2025-11-27. Last modified 2026-06-17.