Update the WordPress Frontend File Manager plugin to the latest available version (at least 21.3).
Raad Haddad (Cloudyrion GmbH) discovered and reported this Arbitrary File Upload vulnerability in WordPress Frontend File Manager Plugin. This could allow a malicious actor to upload any type of file to your website. This can include backdoors which are then executed to gain further access to your website. This vulnerability has been fixed in version 21.3.