Drupal 6.x before 6.29 and 7.x before 7.24 uses the PHP mt_rand function to generate random numbers, which uses predictable seeds and allows remote attackers to predict security strings and bypass intended restrictions via a brute force attack.
https://drupal.org/SA-CORE-2013-003
http://www.openwall.com/lists/oss-security/2013/11/22/4
http://www.debian.org/security/2013/dsa-2828