WP-PHPList plugin is prone to a cross-site scripting vulnerability. Application fails to properly clean up user-supplied input. An attacker may execute arbitrary script code in the browser of an user in the context of the affected site. In this way the attacker can steal cookie-based authentication credentials. There other attacks are also possible.
Update the plugin.
Type XSS (Cross Site Scripting) OWASP Top 10 A3: Cross Site Scripting (XSS)