The zend_hash_init function in PHP 5 before 5.2.1 and PHP 4 before 4.4.5 when running on a 64-bit platform allows context-dependent attackers to cause a denial of service (infinite loop) by unserializing certain integer expressions which only cause 32-bit arguments to be used after the check for a negative value as demonstrated by an \a:2147483649:{\ argument.