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.
{ "cpes": [ "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "cpe:2.3:a:mattermost:mattermost_server:9.9.0:*:*:*:*:*:*:*" ], "severity": "Medium" }