Input validation vulnerability in Email Newsletter 20.13.6

The Email Newsletter plugin for WordPress is vulnerable to a type of cyberattack called Reflected Cross-Site Scripting. This type of attack can be used to inject malicious code into webpages, which can then be executed by a user who has been tricked into performing an action like clicking on a link. Versions of the Email Newsletter plugin for WordPress up to and including 20.13.6 are vulnerable to this attack due to a lack of safeguards for preventing malicious input and not properly encoding the output.