Update the plugin.
An unknown person discovered and reported this Cross Site Scripting (XSS) vulnerability in WordPress YITH Maintenance Mode 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 1.2.0.
Multiple Authenticated Stored CrossSite Scripting (XSS) vulnerabilities
Authenticated Stored CrossSite Scripting (XSS) vulnerability