CVE-2026-28735: Mattermost Server

Medium severity, CVSS 5.4. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate the OAuth token scope on the callback which allows an authenticated Mattermost user to gain access to private repositories via modifying the scope parameter in the GitHub authorization URL.. Mattermost Advisory ID: MMSA-2026-00628

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.15 (fixed in 10.11.15); from 11.4.0, before 11.4.5 (fixed in 11.4.5); from 11.5.0, before 11.5.4 (fixed in 11.5.4); from 11.6.0, before 11.6.1 (fixed in 11.6.1)

Published 2026-05-22. Last modified 2026-07-23.