The MITRE CVE dictionary describes this issue as:
webmail.php in SquirrelMail 1.4.0 to 1.4.5 allows remote attackers to inject arbitrary web pages into the right frame via a URL in the right_frame parameter. NOTE: this has been called a cross-site scripting (XSS) issue, but it is different than what is normally identified as XSS.
Find out more about CVE-2006-0188 from the
MITRE CVE dictionary dictionary and