The form library in Django 1.3.x before 1.3.6, 1.4.x before 1.4.4, and 1.5 before release candidate 2 allows remote attackers to bypass intended resource limits for formsets and cause a denial of service (memory consumption) or trigger server errors via a modified max_num parameter.
https://www.djangoproject.com/weblog/2013/feb/19/security/
http://www.debian.org/security/2013/dsa-2634