diff --git a/django/contrib/sessions/backends/cache.py b/django/contrib/sessions/backends/cache.py
index 9ee8351930..9be47e00a1 100644
--- a/django/contrib/sessions/backends/cache.py
+++ b/django/contrib/sessions/backends/cache.py
@@ -27,7 +27,7 @@ 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):
@@ -49,6 +49,8 @@ class SessionStore(SessionBase):
             "It is likely that the cache is unavailable.")
 
     def save(self, must_create=False):
+        if self.session_key is None:
+            return self.create()
         if must_create:
             func = self._cache.add
         else:
@@ -60,7 +62,7 @@ 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 9257e2ba56..bc9a55fd9d 100644
--- a/django/contrib/sessions/backends/cached_db.py
+++ b/django/contrib/sessions/backends/cached_db.py
@@ -51,12 +51,12 @@ class SessionStore(DBStore):
                     logger = logging.getLogger('django.security.%s' %
                             e.__class__.__name__)
                     logger.warning(force_text(e))
-                self.create()
+                self._session_key = None
                 data = {}
         return data
 
     def exists(self, session_key):
-        if (KEY_PREFIX + session_key) in self._cache:
+        if session_key and (KEY_PREFIX + session_key) in self._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 30e2675b3b..0fba3ec178 100644
--- a/django/contrib/sessions/backends/db.py
+++ b/django/contrib/sessions/backends/db.py
@@ -26,7 +26,7 @@ class SessionStore(SessionBase):
                 logger = logging.getLogger('django.security.%s' %
                         e.__class__.__name__)
                 logger.warning(force_text(e))
-            self.create()
+            self._session_key = None
             return {}
 
     def exists(self, session_key):
@@ -43,7 +43,6 @@ class SessionStore(SessionBase):
                 # Key wasn't unique. Try again.
                 continue
             self.modified = True
-            self._session_cache = {}
             return
 
     def save(self, must_create=False):
@@ -53,6 +52,8 @@ 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 10d163acc4..41469c4a26 100644
--- a/django/contrib/sessions/backends/file.py
+++ b/django/contrib/sessions/backends/file.py
@@ -97,7 +97,7 @@ class SessionStore(SessionBase):
                     self.delete()
                     self.create()
         except (IOError, SuspiciousOperation):
-            self.create()
+            self._session_key = None
         return session_data
 
     def create(self):
@@ -108,10 +108,11 @@ 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/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.
diff --git a/docs/releases/1.7.9.txt b/docs/releases/1.7.9.txt
index a2101ad6f9..f402a4cdd5 100644
--- a/docs/releases/1.7.9.txt
+++ b/docs/releases/1.7.9.txt
@@ -6,6 +6,27 @@ Django 1.7.9 release notes
 
 Django 1.7.9 fixes several security issues and bugs in 1.7.8.
 
+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.
+
 Bugfixes
 ========
 
diff --git a/docs/releases/1.8.3.txt b/docs/releases/1.8.3.txt
index aa6e46a99c..c2283048a5 100644
--- a/docs/releases/1.8.3.txt
+++ b/docs/releases/1.8.3.txt
@@ -11,6 +11,27 @@ Also, ``django.utils.deprecation.RemovedInDjango20Warning`` was renamed to
 1.11 (LTS), 2.0 (drops Python 2 support). For backwards compatibility,
 ``RemovedInDjango20Warning`` remains as an importable alias.
 
+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.
+
 Bugfixes
 ========
 
diff --git a/tests/sessions_tests/tests.py b/tests/sessions_tests/tests.py
index 8a9e98e4a8..6af969f70c 100644
--- a/tests/sessions_tests/tests.py
+++ b/tests/sessions_tests/tests.py
@@ -178,6 +178,11 @@ class SessionTestsMixin(object):
         self.assertNotEqual(self.session.session_key, prev_key)
         self.assertEqual(list(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.
@@ -331,6 +336,21 @@ class SessionTestsMixin(object):
                 self.session.delete(old_session_key)
                 self.session.delete(new_session_key)
 
+    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('someunknownkey')
+        session.load()
+
+        self.assertFalse(session.exists(session.session_key))
+        # provided unknown key was cycled, not reused
+        self.assertNotEqual(session.session_key, 'someunknownkey')
+
 
 class DatabaseSessionTests(SessionTestsMixin, TestCase):