Fixed #2550 -- Allow the auth backends to raise the PermissionDenied exception to completely stop the authentication chain. Many thanks to namn, danielr, Dan Julius, Łukasz Rekucki, Aashu Dwivedi and umbrae for working this over the years.

django/contrib/auth/__init__.py

@@ -1,6 +1,6 @@
 import re
 
-from django.core.exceptions import ImproperlyConfigured
+from django.core.exceptions import ImproperlyConfigured, PermissionDenied
 from django.utils.importlib import import_module
 from django.contrib.auth.signals import user_logged_in, user_logged_out, user_login_failed
 
@@ -60,6 +60,9 @@ def authenticate(**credentials):
         except TypeError:
             # This backend doesn't accept these credentials as arguments. Try the next one.
             continue
+        except PermissionDenied:
+            # This backend says to stop in our tracks - this user should not be allowed in at all.
+            return None
         if user is None:
             continue
         # Annotate the user object with the path of the backend.

django/contrib/auth/tests/auth_backends.py

@@ -6,7 +6,8 @@ from django.contrib.auth.models import User, Group, Permission, AnonymousUser
 from django.contrib.auth.tests.utils import skipIfCustomUser
 from django.contrib.auth.tests.custom_user import ExtensionUser
 from django.contrib.contenttypes.models import ContentType
-from django.core.exceptions import ImproperlyConfigured
+from django.core.exceptions import ImproperlyConfigured, PermissionDenied
+from django.contrib.auth import authenticate
 from django.test import TestCase
 from django.test.utils import override_settings
 
@@ -323,3 +324,37 @@ class InActiveUserBackendTest(TestCase):
     def test_has_module_perms(self):
         self.assertEqual(self.user1.has_module_perms("app1"), False)
         self.assertEqual(self.user1.has_module_perms("app2"), False)
+
+
+class PermissionDeniedBackend(object):
+    """
+    Always raises PermissionDenied.
+    """
+    supports_object_permissions = True
+    supports_anonymous_user = True
+    supports_inactive_user = True
+
+    def authenticate(self, username=None, password=None):
+        raise PermissionDenied
+
+
+class PermissionDeniedBackendTest(TestCase):
+    """
+    Tests that other backends are not checked once a backend raises PermissionDenied
+    """
+    backend = 'django.contrib.auth.tests.auth_backends.PermissionDeniedBackend'
+
+    def setUp(self):
+        self.user1 = User.objects.create_user('test', 'test@example.com', 'test')
+        self.user1.save()
+
+    @override_settings(AUTHENTICATION_BACKENDS=(backend, ) +
+            tuple(settings.AUTHENTICATION_BACKENDS))
+    def test_permission_denied(self):
+        "user is not authenticated after a backend raises permission denied #2550"
+        self.assertEqual(authenticate(username='test', password='test'), None)
+
+    @override_settings(AUTHENTICATION_BACKENDS=tuple(
+            settings.AUTHENTICATION_BACKENDS) + (backend, ))
+    def test_authenticates(self):
+        self.assertEqual(authenticate(username='test', password='test'), self.user1)