From Secunia (http://secunia.com/advisories/22405/)
Description:
MustLive has reported a vulnerability in phplist, which can be exploited by malicious people to conduct cross-site scripting attacks.
Input passed to the "unsubscribeemail" parameter in index.php is not properly sanitised before being returned to the user. This can be exploited to execute arbitrary HTML and script code in a user's browser session in context of an affected site.
The vulnerability is reported in all versions prior to 2.10.3.
Solution:
Update to version 2.10.3.