CVE-2015-5963

@@ -128,6 +128,13 @@ def clear(self):
         self.accessed = True
         self.modified = True
 
+    def is_empty(self):
+        "Returns True when there is no session_key and the session is empty"
+        try:
+            return not bool(self._session_key) and not self._session_cache
+        except AttributeError:
+            return True
+
     def _get_new_session_key(self):
         "Returns session key that isn't being used."
         # Todo: move to 0-9a-z charset in 1.5
@@ -230,7 +237,7 @@ def flush(self):
         """
         self.clear()
         self.delete()
-        self.create()
+        self._session_key = None
 
     def cycle_key(self):
         """

@@ -58,4 +58,4 @@ def flush(self):
         """
         self.clear()
         self.delete(self.session_key)
-        self.create()
+        self._session_key = None

@@ -14,30 +14,38 @@ def process_request(self, request):
     def process_response(self, request, response):
         """
         If request.session was modified, or if the configuration is to save the
-        session every time, save the changes and set a session cookie.
+        session every time, save the changes and set a session cookie or delete
+        the session cookie if the session has been emptied.
         """
         try:
             accessed = request.session.accessed
             modified = request.session.modified
+            empty = request.session.is_empty()
         except AttributeError:
             pass
         else:
-            if accessed:
-                patch_vary_headers(response, ('Cookie',))
-            if modified or settings.SESSION_SAVE_EVERY_REQUEST:
-                if request.session.get_expire_at_browser_close():
-                    max_age = None
-                    expires = None
-                else:
-                    max_age = request.session.get_expiry_age()
-                    expires_time = time.time() + max_age
-                    expires = cookie_date(expires_time)
-                # Save the session data and refresh the client cookie.
-                request.session.save()
-                response.set_cookie(settings.SESSION_COOKIE_NAME,
-                        request.session.session_key, max_age=max_age,
-                        expires=expires, domain=settings.SESSION_COOKIE_DOMAIN,
-                        path=settings.SESSION_COOKIE_PATH,
-                        secure=settings.SESSION_COOKIE_SECURE or None,
-                        httponly=settings.SESSION_COOKIE_HTTPONLY or None)
+            # First check if we need to delete this cookie.
+            # The session should be deleted only if the session is entirely empty
+            if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
+                response.delete_cookie(settings.SESSION_COOKIE_NAME,
+                    domain=settings.SESSION_COOKIE_DOMAIN)
+            else:
+                if accessed:
+                    patch_vary_headers(response, ('Cookie',))
+                if (modified or settings.SESSION_SAVE_EVERY_REQUEST) and not empty:
+                    if request.session.get_expire_at_browser_close():
+                        max_age = None
+                        expires = None
+                    else:
+                        max_age = request.session.get_expiry_age()
+                        expires_time = time.time() + max_age
+                        expires = cookie_date(expires_time)
+                    # Save the session data and refresh the client cookie.
+                    request.session.save()
+                    response.set_cookie(settings.SESSION_COOKIE_NAME,
+                            request.session.session_key, max_age=max_age,
+                            expires=expires, domain=settings.SESSION_COOKIE_DOMAIN,
+                            path=settings.SESSION_COOKIE_PATH,
+                            secure=settings.SESSION_COOKIE_SECURE or None,
+                            httponly=settings.SESSION_COOKIE_HTTPONLY or None)
         return response

@@ -150,6 +150,7 @@ def test_flush(self):
         self.session.flush()
         self.assertFalse(self.session.exists(prev_key))
         self.assertNotEqual(self.session.session_key, prev_key)
+        self.assertIsNone(self.session.session_key)
         self.assertTrue(self.session.modified)
         self.assertTrue(self.session.accessed)
 
@@ -432,6 +433,75 @@ def test_no_httponly_session_cookie(self):
         self.assertNotIn('httponly',
                          str(response.cookies[settings.SESSION_COOKIE_NAME]))
 
+    def test_session_delete_on_end(self):
+        request = RequestFactory().get('/')
+        response = HttpResponse('Session test')
+        middleware = SessionMiddleware()
+
+        # Before deleting, there has to be an existing cookie
+        request.COOKIES[settings.SESSION_COOKIE_NAME] = 'abc'
+
+        # Simulate a request that ends the session
+        middleware.process_request(request)
+        request.session.flush()
+
+        # Handle the response through the middleware
+        response = middleware.process_response(request, response)
+
+        # Check that the cookie was deleted, not recreated.
+        # A deleted cookie header looks like:
+        #  Set-Cookie: sessionid=; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-Age=0; Path=/
+        self.assertEqual(
+            'Set-Cookie: %s=; expires=Thu, 01-Jan-1970 00:00:00 GMT; '
+            'Max-Age=0; Path=/' % settings.SESSION_COOKIE_NAME,
+            str(response.cookies[settings.SESSION_COOKIE_NAME])
+        )
+
+    @override_settings(SESSION_COOKIE_DOMAIN='.example.local')
+    def test_session_delete_on_end_with_custom_domain(self):
+        request = RequestFactory().get('/')
+        response = HttpResponse('Session test')
+        middleware = SessionMiddleware()
+
+        # Before deleting, there has to be an existing cookie
+        request.COOKIES[settings.SESSION_COOKIE_NAME] = 'abc'
+
+        # Simulate a request that ends the session
+        middleware.process_request(request)
+        request.session.flush()
+
+        # Handle the response through the middleware
+        response = middleware.process_response(request, response)
+
+        # Check that the cookie was deleted, not recreated.
+        # A deleted cookie header with a custom domain looks like:
+        #  Set-Cookie: sessionid=; Domain=.example.local;
+        #              expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-Age=0; Path=/
+        self.assertEqual(
+            'Set-Cookie: %s=; Domain=.example.local; expires=Thu, '
+            '01-Jan-1970 00:00:00 GMT; Max-Age=0; Path=/' % (
+                settings.SESSION_COOKIE_NAME,
+            ),
+            str(response.cookies[settings.SESSION_COOKIE_NAME])
+        )
+
+    def test_flush_empty_without_session_cookie_doesnt_set_cookie(self):
+        request = RequestFactory().get('/')
+        response = HttpResponse('Session test')
+        middleware = SessionMiddleware()
+
+        # Simulate a request that ends the session
+        middleware.process_request(request)
+        request.session.flush()
+
+        # Handle the response through the middleware
+        response = middleware.process_response(request, response)
+
+        # A cookie should not be set.
+        self.assertEqual(response.cookies, {})
+        # The session is accessed so "Vary: Cookie" should be set.
+        self.assertEqual(response['Vary'], 'Cookie')
+
 
 class CookieSessionTests(SessionTestsMixin, TestCase):
 

@@ -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.

@@ -197,12 +197,17 @@ You can edit it multiple times.
 
     .. method:: flush
 
-      Delete the current session data from the session and regenerate the
-      session key value that is sent back to the user in the cookie. This is
-      used if you want to ensure that the previous session data can't be
-      accessed again from the user's browser (for example, the
+      Deletes the current session data from the session and deletes the session
+      cookie. This is used if you want to ensure that the previous session data
+      can't be accessed again from the user's browser (for example, the
       :func:`django.contrib.auth.logout()` function calls it).
 
+      .. versionchanged:: 1.4.22
+
+          Deletion of the session cookie was added. Previously, the behavior
+          was to regenerate the session key value that was sent back to the
+          user in the cookie, but this was a denial-of-service vulnerability.
+
     .. method:: set_test_cookie
 
       Sets a test cookie to determine whether the user's browser supports

@@ -142,6 +142,13 @@ def clear(self):
         self.accessed = True
         self.modified = True
 
+    def is_empty(self):
+        "Returns True when there is no session_key and the session is empty"
+        try:
+            return not bool(self._session_key) and not self._session_cache
+        except AttributeError:
+            return True
+
     def _get_new_session_key(self):
         "Returns session key that isn't being used."
         while True:
@@ -268,7 +275,7 @@ def flush(self):
         """
         self.clear()
         self.delete()
-        self.create()
+        self._session_key = None
 
     def cycle_key(self):
         """

@@ -79,7 +79,7 @@ def flush(self):
         """
         self.clear()
         self.delete(self.session_key)
-        self.create()
+        self._session_key = None
 
 
 # At bottom to avoid circular import

@@ -18,32 +18,40 @@ def process_request(self, request):
     def process_response(self, request, response):
         """
         If request.session was modified, or if the configuration is to save the
-        session every time, save the changes and set a session cookie.
+        session every time, save the changes and set a session cookie or delete
+        the session cookie if the session has been emptied.
         """
         try:
             accessed = request.session.accessed
             modified = request.session.modified
+            empty = request.session.is_empty()
         except AttributeError:
             pass
         else:
-            if accessed:
-                patch_vary_headers(response, ('Cookie',))
-            if modified or settings.SESSION_SAVE_EVERY_REQUEST:
-                if request.session.get_expire_at_browser_close():
-                    max_age = None
-                    expires = None
-                else:
-                    max_age = request.session.get_expiry_age()
-                    expires_time = time.time() + max_age
-                    expires = cookie_date(expires_time)
-                # Save the session data and refresh the client cookie.
-                # Skip session save for 500 responses, refs #3881.
-                if response.status_code != 500:
-                    request.session.save()
-                    response.set_cookie(settings.SESSION_COOKIE_NAME,
-                            request.session.session_key, max_age=max_age,
-                            expires=expires, domain=settings.SESSION_COOKIE_DOMAIN,
-                            path=settings.SESSION_COOKIE_PATH,
-                            secure=settings.SESSION_COOKIE_SECURE or None,
-                            httponly=settings.SESSION_COOKIE_HTTPONLY or None)
+            # First check if we need to delete this cookie.
+            # The session should be deleted only if the session is entirely empty
+            if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
+                response.delete_cookie(settings.SESSION_COOKIE_NAME,
+                    domain=settings.SESSION_COOKIE_DOMAIN)
+            else:
+                if accessed:
+                    patch_vary_headers(response, ('Cookie',))
+                if (modified or settings.SESSION_SAVE_EVERY_REQUEST) and not empty:
+                    if request.session.get_expire_at_browser_close():
+                        max_age = None
+                        expires = None
+                    else:
+                        max_age = request.session.get_expiry_age()
+                        expires_time = time.time() + max_age
+                        expires = cookie_date(expires_time)
+                    # Save the session data and refresh the client cookie.
+                    # Skip session save for 500 responses, refs #3881.
+                    if response.status_code != 500:
+                        request.session.save()
+                        response.set_cookie(settings.SESSION_COOKIE_NAME,
+                                request.session.session_key, max_age=max_age,
+                                expires=expires, domain=settings.SESSION_COOKIE_DOMAIN,
+                                path=settings.SESSION_COOKIE_PATH,
+                                secure=settings.SESSION_COOKIE_SECURE or None,
+                                httponly=settings.SESSION_COOKIE_HTTPONLY or None)
         return response

@@ -159,6 +159,7 @@ def test_flush(self):
         self.session.flush()
         self.assertFalse(self.session.exists(prev_key))
         self.assertNotEqual(self.session.session_key, prev_key)
+        self.assertIsNone(self.session.session_key)
         self.assertTrue(self.session.modified)
         self.assertTrue(self.session.accessed)
 
@@ -589,6 +590,75 @@ def test_session_save_on_500(self):
         # Check that the value wasn't saved above.
         self.assertNotIn('hello', request.session.load())
 
+    def test_session_delete_on_end(self):
+        request = RequestFactory().get('/')
+        response = HttpResponse('Session test')
+        middleware = SessionMiddleware()
+
+        # Before deleting, there has to be an existing cookie
+        request.COOKIES[settings.SESSION_COOKIE_NAME] = 'abc'
+
+        # Simulate a request that ends the session
+        middleware.process_request(request)
+        request.session.flush()
+
+        # Handle the response through the middleware
+        response = middleware.process_response(request, response)
+
+        # Check that the cookie was deleted, not recreated.
+        # A deleted cookie header looks like:
+        #  Set-Cookie: sessionid=; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-Age=0; Path=/
+        self.assertEqual(
+            'Set-Cookie: {0}=; expires=Thu, 01-Jan-1970 00:00:00 GMT; '
+            'Max-Age=0; Path=/'.format(settings.SESSION_COOKIE_NAME),
+            str(response.cookies[settings.SESSION_COOKIE_NAME])
+        )
+
+    @override_settings(SESSION_COOKIE_DOMAIN='.example.local')
+    def test_session_delete_on_end_with_custom_domain(self):
+        request = RequestFactory().get('/')
+        response = HttpResponse('Session test')
+        middleware = SessionMiddleware()
+
+        # Before deleting, there has to be an existing cookie
+        request.COOKIES[settings.SESSION_COOKIE_NAME] = 'abc'
+
+        # Simulate a request that ends the session
+        middleware.process_request(request)
+        request.session.flush()
+
+        # Handle the response through the middleware
+        response = middleware.process_response(request, response)
+
+        # Check that the cookie was deleted, not recreated.
+        # A deleted cookie header with a custom domain looks like:
+        #  Set-Cookie: sessionid=; Domain=.example.local;
+        #              expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-Age=0; Path=/
+        self.assertEqual(
+            'Set-Cookie: {}=; Domain=.example.local; expires=Thu, '
+            '01-Jan-1970 00:00:00 GMT; Max-Age=0; Path=/'.format(
+                settings.SESSION_COOKIE_NAME,
+            ),
+            str(response.cookies[settings.SESSION_COOKIE_NAME])
+        )
+
+    def test_flush_empty_without_session_cookie_doesnt_set_cookie(self):
+        request = RequestFactory().get('/')
+        response = HttpResponse('Session test')
+        middleware = SessionMiddleware()
+
+        # Simulate a request that ends the session
+        middleware.process_request(request)
+        request.session.flush()
+
+        # Handle the response through the middleware
+        response = middleware.process_response(request, response)
+
+        # A cookie should not be set.
+        self.assertEqual(response.cookies, {})
+        # The session is accessed so "Vary: Cookie" should be set.
+        self.assertEqual(response['Vary'], 'Cookie')
+
 
 class CookieSessionTests(SessionTestsMixin, TestCase):
 

@@ -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.

@@ -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.

@@ -226,12 +226,18 @@ You can edit it multiple times.
 
     .. method:: flush()
 
-      Delete the current session data from the session and regenerate the
-      session key value that is sent back to the user in the cookie. This is
-      used if you want to ensure that the previous session data can't be
-      accessed again from the user's browser (for example, the
+      Deletes the current session data from the session and deletes the session
+      cookie. This is used if you want to ensure that the previous session data
+      can't be accessed again from the user's browser (for example, the
       :func:`django.contrib.auth.logout()` function calls it).
 
+      .. versionchanged:: 1.7.10
+
+          Deletion of the session cookie was added. Previously, the behavior
+          was to regenerate the session key value that was sent back to the
+          user in the cookie, but this could be a denial-of-service
+          vulnerability.
+
     .. method:: set_test_cookie()
 
       Sets a test cookie to determine whether the user's browser supports

@@ -36,7 +36,7 @@ def process_response(self, request, response):
             else:
                 if accessed:
                     patch_vary_headers(response, ('Cookie',))
-                if modified or settings.SESSION_SAVE_EVERY_REQUEST:
+                if (modified or settings.SESSION_SAVE_EVERY_REQUEST) and not empty:
                     if request.session.get_expire_at_browser_close():
                         max_age = None
                         expires = None

@@ -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.

@@ -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.

@@ -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
 ========
 

@@ -678,6 +678,23 @@ def test_session_delete_on_end_with_custom_domain(self):
             str(response.cookies[settings.SESSION_COOKIE_NAME])
         )
 
+    def test_flush_empty_without_session_cookie_doesnt_set_cookie(self):
+        request = RequestFactory().get('/')
+        response = HttpResponse('Session test')
+        middleware = SessionMiddleware()
+
+        # Simulate a request that ends the session
+        middleware.process_request(request)
+        request.session.flush()
+
+        # Handle the response through the middleware
+        response = middleware.process_response(request, response)
+
+        # A cookie should not be set.
+        self.assertEqual(response.cookies, {})
+        # The session is accessed so "Vary: Cookie" should be set.
+        self.assertEqual(response['Vary'], 'Cookie')
+
 
 # Don't need DB flushing for these tests, so can use unittest.TestCase as base class
 class CookieSessionTests(SessionTestsMixin, unittest.TestCase):

@@ -36,7 +36,7 @@ def process_response(self, request, response):
             else:
                 if accessed:
                     patch_vary_headers(response, ('Cookie',))
-                if modified or settings.SESSION_SAVE_EVERY_REQUEST:
+                if (modified or settings.SESSION_SAVE_EVERY_REQUEST) and not empty:
                     if request.session.get_expire_at_browser_close():
                         max_age = None
                         expires = None

@@ -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.

@@ -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.

@@ -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
 ========
 

@@ -660,6 +660,23 @@ def test_session_delete_on_end_with_custom_domain(self):
             str(response.cookies[settings.SESSION_COOKIE_NAME])
         )
 
+    def test_flush_empty_without_session_cookie_doesnt_set_cookie(self):
+        request = RequestFactory().get('/')
+        response = HttpResponse('Session test')
+        middleware = SessionMiddleware()
+
+        # Simulate a request that ends the session
+        middleware.process_request(request)
+        request.session.flush()
+
+        # Handle the response through the middleware
+        response = middleware.process_response(request, response)
+
+        # A cookie should not be set.
+        self.assertEqual(response.cookies, {})
+        # The session is accessed so "Vary: Cookie" should be set.
+        self.assertEqual(response['Vary'], 'Cookie')
+
 
 # Don't need DB flushing for these tests, so can use unittest.TestCase as base class
 class CookieSessionTests(SessionTestsMixin, unittest.TestCase):