diff --git a/django/contrib/sessions/backends/cache.py b/django/contrib/sessions/backends/cache.py
index 467d5f1265..ffea036c5e 100644
--- a/django/contrib/sessions/backends/cache.py
+++ b/django/contrib/sessions/backends/cache.py
@@ -25,7 +25,7 @@ class SessionStore(SessionBase):
             session_data = None
         if session_data is not None:
             return session_data
-        self.create()
+        self._session_key = None
         return {}
 
     def create(self):
@@ -45,6 +45,8 @@ class SessionStore(SessionBase):
         raise RuntimeError("Unable to create a new session key.")
 
     def save(self, must_create=False):
+        if self.session_key is None:
+            return self.create()
         if must_create:
             func = self._cache.add
         else:
@@ -56,7 +58,7 @@ class SessionStore(SessionBase):
             raise CreateError
 
     def exists(self, session_key):
-        return (KEY_PREFIX + session_key) in self._cache
+        return session_key and (KEY_PREFIX + session_key) in self._cache
 
     def delete(self, session_key=None):
         if session_key is None:
diff --git a/django/contrib/sessions/backends/cached_db.py b/django/contrib/sessions/backends/cached_db.py
index ff6076df77..e15f88358c 100644
--- a/django/contrib/sessions/backends/cached_db.py
+++ b/django/contrib/sessions/backends/cached_db.py
@@ -30,11 +30,12 @@ class SessionStore(DBStore):
             data = None
         if data is None:
             data = super(SessionStore, self).load()
-            cache.set(self.cache_key, data, settings.SESSION_COOKIE_AGE)
+            if self.session_key:
+                cache.set(self.cache_key, data, settings.SESSION_COOKIE_AGE)
         return data
 
     def exists(self, session_key):
-        if (KEY_PREFIX + session_key) in cache:
+        if session_key and (KEY_PREFIX + session_key) in cache:
             return True
         return super(SessionStore, self).exists(session_key)
 
diff --git a/django/contrib/sessions/backends/db.py b/django/contrib/sessions/backends/db.py
index 3dd0d9516c..ad92baee92 100644
--- a/django/contrib/sessions/backends/db.py
+++ b/django/contrib/sessions/backends/db.py
@@ -20,7 +20,7 @@ class SessionStore(SessionBase):
             )
             return self.decode(force_unicode(s.session_data))
         except (Session.DoesNotExist, SuspiciousOperation):
-            self.create()
+            self._session_key = None
             return {}
 
     def exists(self, session_key):
@@ -37,7 +37,6 @@ class SessionStore(SessionBase):
                 # Key wasn't unique. Try again.
                 continue
             self.modified = True
-            self._session_cache = {}
             return
 
     def save(self, must_create=False):
@@ -47,6 +46,8 @@ class SessionStore(SessionBase):
         create a *new* entry (as opposed to possibly updating an existing
         entry).
         """
+        if self.session_key is None:
+            return self.create()
         obj = Session(
             session_key=self._get_or_create_session_key(),
             session_data=self.encode(self._get_session(no_load=must_create)),
diff --git a/django/contrib/sessions/backends/file.py b/django/contrib/sessions/backends/file.py
index 8ffddc4903..05dc194f77 100644
--- a/django/contrib/sessions/backends/file.py
+++ b/django/contrib/sessions/backends/file.py
@@ -56,11 +56,11 @@ class SessionStore(SessionBase):
                     try:
                         session_data = self.decode(file_data)
                     except (EOFError, SuspiciousOperation):
-                        self.create()
+                        self._session_key = None
             finally:
                 session_file.close()
         except IOError:
-            self.create()
+            self._session_key = None
         return session_data
 
     def create(self):
@@ -71,10 +71,11 @@ class SessionStore(SessionBase):
             except CreateError:
                 continue
             self.modified = True
-            self._session_cache = {}
             return
 
     def save(self, must_create=False):
+        if self.session_key is None:
+            return self.create()
         # Get the session data now, before we start messing
         # with the file it is stored within.
         session_data = self._get_session(no_load=must_create)
diff --git a/django/contrib/sessions/tests.py b/django/contrib/sessions/tests.py
index 7686bd254e..98271f4b0e 100644
--- a/django/contrib/sessions/tests.py
+++ b/django/contrib/sessions/tests.py
@@ -162,6 +162,11 @@ class SessionTestsMixin(object):
         self.assertNotEqual(self.session.session_key, prev_key)
         self.assertEqual(self.session.items(), prev_data)
 
+    def test_save_doesnt_clear_data(self):
+        self.session['a'] = 'b'
+        self.session.save()
+        self.assertEqual(self.session['a'], 'b')
+
     def test_invalid_key(self):
         # Submitting an invalid session key (either by guessing, or if the db has
         # removed the key) results in a new key being generated.
@@ -256,6 +261,20 @@ class SessionTestsMixin(object):
         encoded = self.session.encode(data)
         self.assertEqual(self.session.decode(encoded), data)
 
+    def test_session_load_does_not_create_record(self):
+        """
+        Loading an unknown session key does not create a session record.
+
+        Creating session records on load is a DOS vulnerability.
+        """
+        if self.backend is CookieSession:
+            raise unittest.SkipTest("Cookie backend doesn't have an external store to create records in.")
+        session = self.backend('deadbeef')
+        session.load()
+
+        self.assertFalse(session.exists(session.session_key))
+        # provided unknown key was cycled, not reused
+        self.assertNotEqual(session.session_key, 'deadbeef')
 
 class DatabaseSessionTests(SessionTestsMixin, TestCase):
 
diff --git a/docs/releases/1.4.21.txt b/docs/releases/1.4.21.txt
index 6ff4c6d115..da69b26564 100644
--- a/docs/releases/1.4.21.txt
+++ b/docs/releases/1.4.21.txt
@@ -5,3 +5,24 @@ Django 1.4.21 release notes
 *July 8, 2015*
 
 Django 1.4.21 fixes several security issues in 1.4.20.
+
+Denial-of-service possibility by filling session store
+======================================================
+
+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.