source: http://www.securityfocus.com/bid/20483/info

phpList is prone to multiple cross-site scripting vulnerabilities because it fails to properly sanitize user-supplied input. 

An attacker may leverage these issues to have arbitrary script code executed in the browser of an unsuspecting user in the context of the affected site. These may help the attacker steal cookie-based authentication credentials and launch other attacks.

Versions prior to 2.10.3 are vulnerable to this issue.

http://www.example.com/phplist_path/?p=unsubscribe&id=1&unsubscribeemail=%22%3E%3Cscript%3Ealert(document.cookie)%3C/script%3E