Update the WordPress Admin CSS MU plugin to the latest available version (at least 2.7).
Dave Jong (Patchstack) discovered and reported this Server Side Request Forgery (SSRF) vulnerability in WordPress Admin CSS MU Plugin. This could allow a malicious actor to cause a website to execute website requests to an arbitrary domain of the attacker. This could allow a malicious actor to find sensitive information of other services running on the system. This vulnerability has been fixed in version 2.7.