PHP 4 before 4.4.5, and PHP 5 before 5.2.1, when register_globals is enabled, allows context-dependent attackers to execute arbitrary code via deserialization of session data, which overwrites arbitrary global variables, as demonstrated by calling session_decode on a string beginning with "_SESSION|s:39:".
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A11034
https://exchange.xforce.ibmcloud.com/vulnerabilities/33658
http://www.vupen.com/english/advisories/2007/2374
http://www.vupen.com/english/advisories/2007/1991
http://www.securityfocus.com/bid/23120
http://www.php-security.org/MOPB/MOPB-31-2007.html
http://security.gentoo.org/glsa/glsa-200705-19.xml
http://secunia.com/advisories/25850