Update the WordPress Ninja Forms File Uploads Extension premium plugin to the latest available version (at least 3.0.23).
Jasper Weijts (Onvio) discovered and reported this Arbitrary File Upload vulnerability in WordPress Ninja Forms File Uploads Extension 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 3.0.23.
Unauthenticated Arbitrary File Upload vulnerability
Reflected CrossSite Scripting (XSS) vulnerability