Mattermost versions 9.9.x <= 9.9.1, 9.5.x <= 9.5.7, 9.10.x <= 9.10.0, 9.8.x <= 9.8.2 fail to restrict the input in POST /api/v4/users which allows a user to manipulate the creation date in POST /api/v4/users tricking the admin into believing their account is much older.
CWE-754, CWE-754