SquirrelMail 1.4.6 and earlier, with register_globals enabled, allows remote attackers to hijack cookies in src/redirect.php via unknown vectors. NOTE: while "cookie theft" is frequently associated with XSS, the vendor disclosure is too vague to be certain of this.
https://exchange.xforce.ibmcloud.com/vulnerabilities/27632
http://www.vupen.com/english/advisories/2006/2708