The following flaw was found in Django:

In previous versions of Django, the session backends created a new empty record in the session storage anytime ``request.session`` was accessed and there was a session key provided in the request cookies that didn't already have a session record. This could allow an attacker to easily create many new session records simply by sending repeated requests with unknown session keys, potentially filling up the session store or causing other users' session records to be evicted.

The built-in session backends now create a session record only if the session is actually modified; empty session records are not created. Thus this potential DoS is now only possible if the site chooses to expose a session-modifying view to anonymous users.

As each built-in session backend was fixed separately (rather than a fix in the core sessions framework), maintainers of third-party session backends should check whether the same vulnerability is present in their backend and correct it if so.

Acknowledgements:

Red Hat would like to thank the upstream Django project for reporting this issue.