Mattermost Group Member ID Enumeration Vulnerability for Authenticated Guests

A vulnerability exists in Mattermost versions 11.2.x through 11.2.2, 10.11.x through 10.11.10, 11.4.x through 11.4.0, and 11.3.x through 11.3.1. These versions fail to properly restrict view permissions when accessing group member IDs, allowing authenticated guest users to enumerate user IDs beyond their permitted visibility range via the group retrieval endpoint.

Impact

Exploitation of this vulnerability could lead to unauthorized enumeration of user IDs, potentially allowing guest users to gain insights into group member compositions that should remain private.

Remediation

Users can upgrade to Mattermost versions 11.5.0 or 11.6.0 to address this vulnerability.