Monitorix 3.13.0 allows remote attackers to bypass Basic Authentication in a default installation (i.e., an installation without a hosts_deny option). This issue occurred because a new access-control feature was introduced without considering that some exiting installations became unsafe, upon an update to 3.13.0, unless the new feature was immediately configured.
https://www.monitorix.org/news.html?n=20210127
https://github.com/mikaku/Monitorix/issues/309
https://github.com/mikaku/Monitorix/compare/v3.13.0...v3.13.1
https://github.com/mikaku/Monitorix/commit/d6816e20da1a98bcdc6372d9c36a093df5238f4a