James Bennett of Django reports:

Django's form library includes tools for generating formsets -- multiple instances of a single form, to be submitted simultaneously (e.g., for mass editing/updating of similar objects). Formsets allow a parameter, max_num, specifying the maximum number of individual forms which may be displayed. A hidden input in the formset then specifies the number of forms being submitted.

Django accepts the submitted hidden value, and attempts to instantiate that many form objects. Sufficiently large values will result in extreme memory consumption; values exceeding sys.maxint will, in this case, result in an HTTP 500 response (due to an uncaught OverflowError from the over-large value). In the former case, the result is an effective denial-of-service attack. In the latter, an attacker can cause arbitrary server errors.

To resolve this, Django will now supply a (suitably large) default value for the maximum number of forms, which will be used when max_num is not supplied. Application developers can still manually specify max_num to suit the needs of their applications, but absent this the default value -- 1000 -- will ensure that attackers cannot cause overflow errors or excessive memory consumption.

External reference:
https://www.djangoproject.com/weblog/2013/feb/19/security/