CVE-2026-6673: Mattermost Server

Medium severity, CVSS 6.4. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret and disrupt the Jira integration via POST to /ac/installed during the pending-install window.. Mattermost Advisory ID: MMSA-2026-00654

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.18 (fixed in 10.11.18); from 11.5.0, before 11.5.6 (fixed in 11.5.6); from 11.6.0, before 11.6.3 (fixed in 11.6.3); from 11.7.0, before 11.7.1 (fixed in 11.7.1)

Published 2026-06-22. Last modified 2026-06-23.