Mattermost versions 10.6.x <= 10.6.1, 10.5.x <= 10.5.2, 10.4.x <= 10.4.4, 9.11.x <= 9.11.11 fail to check the correct permissions which allows authenticated users who only have permission to invite non-guest users to a team to add guest users to that team via the API to add a single user to a team.

Mattermost versions 10.6.x <= 10.6.1, 10.5.x <= 10.5.2, 10.4.x <= 10.4 ...

Mattermost Fails to Validate Team Invite Permissions