The _sanitize_globals function in CodeIgniter 1.5.3 before 20070628 allows remote attackers to unset arbitrary global variables with unspecified impact, as demonstrated by a _SERVER cookie.
http://www.securityfocus.com/archive/1/473190/100/0/threaded
http://securityreason.com/securityalert/2877
http://secunia.com/advisories/25991
http://lists.grok.org.uk/pipermail/full-disclosure/2007-July/064500.html