It was reported that, in certain situations, cached data in Django could be served to a different session, or to a user with no session at all. The original report from upstream:

""
In certain situations, Django may allow caches to store private data
related to a particular session and then serve that data to requests
with a different session, or no session at all. This can both lead to
information disclosure, and can be a vector for cache poisoning.

When using Django sessions, Django will set a ``Vary: Cookie`` header
to ensure caches do not serve cached data to requests from other
sessions. However, older versions of Internet Explorer (most likely
only Internet Explorer 6, and Internet Explorer 7 if run on Windows XP
or Windows Server 2003) are unable to handle the ``Vary`` header in
combination with many content types. Therefore, Django would remove
the header if the request was made by Internet Explorer.

To remedy this, the special behavior for these older Internet
Explorer versions has been removed, and the ``Vary`` header is no
longer stripped from the response. In addition, modifications to the
``Cache-Control`` header for all Internet Explorer requests with a
``Content-Disposition`` header, have also been removed as they were
found to have similar issues.
""

This issue affects Django versions 1.4, 1.5, 1.6, and 1.7. It has been fixed in versions 1.4.13, 1.5.8, 1.6.5, and 1.7 beta 4.

Acknowledgements:

Red Hat would like to thank the upstream Django project for reporting this issue. Upstream acknowledges Michael Nelson, Natalia Bidart, and James Westby as the original reporters.