Index: trunk/django/contrib/auth/__init__.py
===================================================================
--- trunk/django/contrib/auth/__init__.py       (revision 3596)
+++ trunk/django/contrib/auth/__init__.py       (working copy)
@@ -1,4 +1,4 @@
-from django.core.exceptions import ImproperlyConfigured
+from django.core.exceptions import ImproperlyConfigured, PermissionDenied

 SESSION_KEY = '_auth_user_id'
 BACKEND_SESSION_KEY = '_auth_user_backend'
@@ -35,6 +35,9 @@
         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.