Update the WordPress WP Maintenance plugin to the latest available version (at least 6.0.8).
bugb hunter discovered and reported this Cross Site Scripting (XSS) vulnerability in WordPress WP Maintenance 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 6.0.8.