[1.8.x] Fixed DoS possiblity in contrib.auth.views.logout()

Thanks Florian Apolloner and Carl Meyer for review.

This is a security fix.

docs/releases/1.4.22.txt

@@ -9,3 +9,21 @@ Django 1.4.22 fixes a security issue in 1.4.21.
 It also fixes support with pip 7+ by disabling wheel support. Older versions
 of 1.4 would silently build a broken wheel when installed with those versions
 of pip.
+
+Denial-of-service possibility in ``logout()`` view by filling session store
+===========================================================================
+
+Previously, a session could be created when anonymously accessing the
+:func:`django.contrib.auth.views.logout` view (provided it wasn't decorated
+with :func:`~django.contrib.auth.decorators.login_required` as done in the
+admin). This could allow an attacker to easily create many new session records
+by sending repeated requests, potentially filling up the session store or
+causing other users' session records to be evicted.
+
+The :class:`~django.contrib.sessions.middleware.SessionMiddleware` has been
+modified to no longer create empty session records.
+
+Additionally, the ``contrib.sessions.backends.base.SessionBase.flush()`` and
+``cache_db.SessionStore.flush()`` methods have been modified to avoid creating
+a new empty session. Maintainers of third-party session backends should check
+if the same vulnerability is present in their backend and correct it if so.

docs/releases/1.7.10.txt

@@ -5,3 +5,21 @@ Django 1.7.10 release notes
 *August 18, 2015*
 
 Django 1.7.10 fixes a security issue in 1.7.9.
+
+Denial-of-service possibility in ``logout()`` view by filling session store
+===========================================================================
+
+Previously, a session could be created when anonymously accessing the
+:func:`django.contrib.auth.views.logout` view (provided it wasn't decorated
+with :func:`~django.contrib.auth.decorators.login_required` as done in the
+admin). This could allow an attacker to easily create many new session records
+by sending repeated requests, potentially filling up the session store or
+causing other users' session records to be evicted.
+
+The :class:`~django.contrib.sessions.middleware.SessionMiddleware` has been
+modified to no longer create empty session records.
+
+Additionally, the ``contrib.sessions.backends.base.SessionBase.flush()`` and
+``cache_db.SessionStore.flush()`` methods have been modified to avoid creating
+a new empty session. Maintainers of third-party session backends should check
+if the same vulnerability is present in their backend and correct it if so.

docs/releases/1.8.4.txt

@@ -6,6 +6,19 @@ Django 1.8.4 release notes
 
 Django 1.8.4 fixes a security issue and several bugs in 1.8.3.
 
+Denial-of-service possibility in ``logout()`` view by filling session store
+===========================================================================
+
+Previously, a session could be created when anonymously accessing the
+:func:`django.contrib.auth.views.logout` view (provided it wasn't decorated
+with :func:`~django.contrib.auth.decorators.login_required` as done in the
+admin). This could allow an attacker to easily create many new session records
+by sending repeated requests, potentially filling up the session store or
+causing other users' session records to be evicted.
+
+The :class:`~django.contrib.sessions.middleware.SessionMiddleware` has been
+modified to no longer create empty session records.
+
 Bugfixes
 ========