inc/ticket.class.php in GLPI 0.83.9 and earlier allows remote attackers to unserialize arbitrary PHP objects via the _predefined_fields parameter to front/ticket.form.php.
https://forge.indepnet.net/projects/glpi/repository/revisions/21169/diff
https://euvd.enisa.europa.eu/vulnerability/EUVD-2013-2178
http://www.securityfocus.com/bid/60823
http://www.exploit-db.com/exploits/26530
http://seclists.org/oss-sec/2013/q2/645