CVE-2026-6046: Mattermost Server

Medium severity, CVSS 5.3. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to validate that a username returned during bot registration belongs to a bot account, which allows an unprivileged attacker to intercept private messages sent by plugins via direct message channels by pre-registering a user account with a predictable plugin bot username.. Mattermost Advisory ID: MMSA-2026-00649

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.17 (fixed in 10.11.17); from 11.5.0, before 11.5.5 (fixed in 11.5.5); from 11.6.0, before 11.6.2 (fixed in 11.6.2)

Published 2026-06-12. Last modified 2026-06-18.