diff --git a/django/contrib/auth/middleware.py b/django/contrib/auth/middleware.py
index b2f392262a519..b34da60299d15 100644
--- a/django/contrib/auth/middleware.py
+++ b/django/contrib/auth/middleware.py
@@ -76,14 +76,7 @@ def process_request(self, request):
             # authenticated remote-user, or return (leaving request.user set to
             # AnonymousUser by the AuthenticationMiddleware).
             if request.user.is_authenticated():
-                try:
-                    stored_backend = load_backend(request.session.get(
-                        auth.BACKEND_SESSION_KEY, ''))
-                    if isinstance(stored_backend, RemoteUserBackend):
-                        auth.logout(request)
-                except ImportError:
-                    # backend failed to load
-                    auth.logout(request)
+                self._remove_invalid_user(request)
             return
         # If the user is already authenticated and that user is the user we are
         # getting passed in the headers, then the correct user is already
@@ -91,6 +84,11 @@ def process_request(self, request):
         if request.user.is_authenticated():
             if request.user.get_username() == self.clean_username(username, request):
                 return
+            else:
+                # An authenticated user is associated with the request, but
+                # it does not match the authorized user in the header.
+                self._remove_invalid_user(request)
+
         # We are seeing this user for the first time in this session, attempt
         # to authenticate the user.
         user = auth.authenticate(remote_user=username)
@@ -112,3 +110,17 @@ def clean_username(self, username, request):
         except AttributeError:  # Backend has no clean_username method.
             pass
         return username
+
+    def _remove_invalid_user(self, request):
+        """
+        Removes the current authenticated user in the request which is invalid
+        but only if the user is authenticated via the RemoteUserBackend.
+        """
+        try:
+            stored_backend = load_backend(request.session.get(auth.BACKEND_SESSION_KEY, ''))
+        except ImportError:
+            # backend failed to load
+            auth.logout(request)
+        else:
+            if isinstance(stored_backend, RemoteUserBackend):
+                auth.logout(request)
diff --git a/django/contrib/auth/tests/test_remote_user.py b/django/contrib/auth/tests/test_remote_user.py
index 5b743ac8c1952..45e96133ce6c6 100644
--- a/django/contrib/auth/tests/test_remote_user.py
+++ b/django/contrib/auth/tests/test_remote_user.py
@@ -125,6 +125,24 @@ def test_header_disappears(self):
         response = self.client.get('/remote_user/')
         self.assertEqual(response.context['user'].username, 'modeluser')
 
+    def test_user_switch_forces_new_login(self):
+        """
+        Tests that if the username in the header changes between requests
+        that the original user is logged out
+        """
+        User.objects.create(username='knownuser')
+        # Known user authenticates
+        response = self.client.get('/remote_user/',
+                                   **{self.header: self.known_user})
+        self.assertEqual(response.context['user'].username, 'knownuser')
+        # During the session, the REMOTE_USER changes to a different user.
+        response = self.client.get('/remote_user/',
+                                   **{self.header: "newnewuser"})
+        # Ensure that the current user is not the prior remote_user
+        # In backends that create a new user, username is "newnewuser"
+        # In backends that do not create new users, it is '' (anonymous user)
+        self.assertNotEqual(response.context['user'].username, 'knownuser')
+
     def tearDown(self):
         """Restores settings to avoid breaking other tests."""
         settings.MIDDLEWARE_CLASSES = self.curr_middleware
diff --git a/docs/releases/1.4.14.txt b/docs/releases/1.4.14.txt
index 6c140ee6dc134..811c3f67eaa93 100644
--- a/docs/releases/1.4.14.txt
+++ b/docs/releases/1.4.14.txt
@@ -38,3 +38,12 @@ if a file with the uploaded name already exists.
 underscore plus a random 7 character alphanumeric string (e.g. ``"_x3a1gho"``),
 rather than iterating through an underscore followed by a number (e.g. ``"_1"``,
 ``"_2"``, etc.).
+
+``RemoteUserMiddleware`` session hijacking
+==========================================
+
+When using the :class:`~django.contrib.auth.middleware.RemoteUserMiddleware`
+and the ``RemoteUserBackend``, a change to the ``REMOTE_USER`` header between
+requests without an intervening logout could result in the prior user's session
+being co-opted by the subsequent user. The middleware now logs the user out on
+a failed login attempt.
diff --git a/docs/releases/1.5.9.txt b/docs/releases/1.5.9.txt
index 232cbb0767987..5070f0eca1f6c 100644
--- a/docs/releases/1.5.9.txt
+++ b/docs/releases/1.5.9.txt
@@ -38,3 +38,12 @@ if a file with the uploaded name already exists.
 underscore plus a random 7 character alphanumeric string (e.g. ``"_x3a1gho"``),
 rather than iterating through an underscore followed by a number (e.g. ``"_1"``,
 ``"_2"``, etc.).
+
+``RemoteUserMiddleware`` session hijacking
+==========================================
+
+When using the :class:`~django.contrib.auth.middleware.RemoteUserMiddleware`
+and the ``RemoteUserBackend``, a change to the ``REMOTE_USER`` header between
+requests without an intervening logout could result in the prior user's session
+being co-opted by the subsequent user. The middleware now logs the user out on
+a failed login attempt.
diff --git a/docs/releases/1.6.6.txt b/docs/releases/1.6.6.txt
index c2ebdb9efb7fa..e52ea6b23f1e5 100644
--- a/docs/releases/1.6.6.txt
+++ b/docs/releases/1.6.6.txt
@@ -39,6 +39,15 @@ underscore plus a random 7 character alphanumeric string (e.g. ``"_x3a1gho"``),
 rather than iterating through an underscore followed by a number (e.g. ``"_1"``,
 ``"_2"``, etc.).
 
+``RemoteUserMiddleware`` session hijacking
+==========================================
+
+When using the :class:`~django.contrib.auth.middleware.RemoteUserMiddleware`
+and the ``RemoteUserBackend``, a change to the ``REMOTE_USER`` header between
+requests without an intervening logout could result in the prior user's session
+being co-opted by the subsequent user. The middleware now logs the user out on
+a failed login attempt.
+
 Bugfixes
 ========