Update the WordPress Ninja Forms File Uploads Extension premium plugin to the latest available version (at least 3.3.13).
Nuno Correia (Blaze Security) discovered and reported this Cross Site Scripting (XSS) vulnerability in WordPress Ninja Forms File Uploads Extension Plugin. This could allow a malicious actor to inject malicious scripts, such as redirects, advertisements, and other HTML payloads into your website which will be executed when guests visit your site. This vulnerability has been fixed in version 3.3.13.