CVE-2026-6517: Mattermost Desktop

High severity, CVSS 7.7. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost Desktop App versions <=6.1 5.5.13.0 fail to restrict the allow list of domains to which NTLM credentials were forwarded to in the Mattermost Desktop App which allows any user on a server without the image proxy enabled to intercept other users credentials via embedding an image that routes to an external web server. Mattermost Advisory ID: MMSA-2026-00651

Affected products

• Mattermost Mattermost Desktop: up to and including 5.13.0; from 6.1.0, up to and including 6.1.5

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