Eclipse Mosquitto 1.5.x before 1.5.5 allows ACL bypass: if the option per_listener_settings was set to true, and the default listener was in use, and the default listener specified an acl_file, then the acl file was being ignored.
https://github.com/eclipse/mosquitto/issues/1073
https://github.com/eclipse/mosquitto/commit/9097577b49b7fdcf45d30975976dd93808ccc0c4
https://github.com/eclipse/mosquitto/blob/master/ChangeLog.txt