PHP 4.x and 5.x before 5.2.1, when running on multi-threaded systems, does not ensure thread safety for libc crypt function calls using protection schemes such as a mutex, which creates race conditions that allow remote attackers to overwrite internal program memory and gain system access.
https://exchange.xforce.ibmcloud.com/vulnerabilities/34601
http://www.securityfocus.com/bid/24109
http://secunia.com/advisories/25434
http://blog.php-security.org/archives/82-Suhosin-0.9.20-and-crypt-Thread-Safety-Vulnerability.html