Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6, 9.7.x <= 9.7.5, 9.8.x <= 9.8.1 fail to disallow users to set their own remote username, when shared channels were enabled, which allows a user on a remote to set their remote username prop to an arbitrary string, which would be then synced to the local server as long as the user hadn't been synced before.

Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6, 9.7.x <= 9.7.5, 9. ...

Mattermost allows a user on a remote to set their remote username prop to an arbitrary string