The password hashing API in Drupal 7.x before 7.34 and the Secure Password Hashes (aka phpass) module 6.x-2.x before 6.x-2.1 for Drupal allows remote attackers to cause a denial of service (CPU and memory consumption) via a crafted request.
https://www.drupal.org/node/2378375
https://www.drupal.org/node/2378367
https://www.drupal.org/SA-CORE-2014-006
http://www.openwall.com/lists/oss-security/2014/11/21/1
http://www.openwall.com/lists/oss-security/2014/11/20/3
http://www.openwall.com/lists/oss-security/2014/11/20/21
http://www.debian.org/security/2014/dsa-3075