Monstra CMS 3.0.4 allows remote attackers to execute arbitrary PHP code via a mixed-case file extension, as demonstrated by the 123.PhP filename, because plugins\box\filesmanager\filesmanager.admin.php mishandles the forbidden_types variable.
https://github.com/AlwaysHereFight/monstra_cms-3.0.4--getshell/blob/master/README.md