Update the WordPress WP HTML Mail plugin to the latest available version (at least 3.1).
Chloe Chamberland (Wordfence) discovered and reported this Cross Site Scripting (XSS) vulnerability in WordPress WP HTML Mail 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 3.1.