James Bennett of Django reports:

Issue: Host header poisoning

Several previous Django security releases have attempted to address persistent issues with the HTTP Host header. Django contains code -- and some functionality shipped with Django itself makes use of that code -- for constructing a fully-qualified URL based on the incoming HTTP request. Depending on configuration, this makes use of the Host header, and so an attacker who can cause a Django application to respond to arbitrary Host headers can cause Django to generate, and display to end users, URLs on arbitrary domains.

Previous iterations of this issue (see CVE-2011-4139 and CVE-2012-4520) have focused on tightening Django's parsing of Host headers, to eliminate various means by which attackers -- using techniques such as username/password pairs in their submitted Host header -- could exploit this.

Ultimately, however, Django alone cannot ensure that an attacker is unable to submit, and cause Django to accept, arbitrary Host headers. Properly securing this in a deployed Django instance additionally requires configuration of the web server, and both the configuration and the achievable level of security vary with the server being used.

In light of this, the get_host() method of django.http.HttpRequest will now validate the Host header against a new setting, ALLOWED_HOSTS. This setting is a list of strings (by default, an empty list) corresponding to acceptable values for the header, with some support for wildcards.

Code which does not use get_host(), or Django configurations which can determine the current hostname without recourse to HTTP headers (i.e., when Django's sites framework is enabled) will not be affected by this change.

Since this is hardening/tightening of a previous issue, it does not have a new CVE number.

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