The Image module in Drupal 7.x before 7.19, when a private file system is used, does not properly restrict access to derivative images, which allows remote attackers to read derivative images of otherwise restricted images via unspecified vectors.
https://drupal.org/SA-CORE-2013-001
http://secunia.com/advisories/51717
http://seclists.org/oss-sec/2013/q1/211