Mattermost versions 10.5.x <= 10.5.1, 10.4.x <= 10.4.3, 9.11.x <= 9.11.9 fail to invalidate the cache when a user account is converted to a bot which allows an attacker to login to the bot exactly one time via normal credentials.
Update Mattermost to versions 10.6.0, 10.5.2, 10.4.4, 9.11.10 or higher.