functions/page_header.php in SquirrelMail 1.4.21 and earlier does not prevent page rendering inside a frame in a third-party HTML document, which makes it easier for remote attackers to conduct clickjacking attacks via a crafted web site.
https://exchange.xforce.ibmcloud.com/vulnerabilities/68512
https://bugzilla.redhat.com/show_bug.cgi?id=720693
http://www.squirrelmail.org/security/issue/2011-07-12
http://www.mandriva.com/security/advisories?name=MDVSA-2011:123
http://www.debian.org/security/2011/dsa-2291
http://support.apple.com/kb/HT5130
http://rhn.redhat.com/errata/RHSA-2012-0103.html
http://lists.apple.com/archives/security-announce/2012/Feb/msg00000.html