Mattermost Team Join Permission Vulnerability via Manipulated Invite Token

A vulnerability exists in Mattermost versions 10.11.x through 10.11.1, 10.10.x through 10.10.2, and 10.5.x through 10.5.10. These versions fail to properly verify a user's permission to join a team using the original invite token. This flaw allows any user to join any team on a Mattermost server, bypassing existing restrictions, by manipulating the RelayState.

Impact

Exploitation of this vulnerability allows unauthorized users to join teams on the Mattermost server, disregarding any access restrictions that may be in place.

Remediation

Users can upgrade to Mattermost version 11.0.0 or 10.12.1, depending on their current version, to address this vulnerability.