Ensures all access to the users membership provider is done with our extension method

src/Umbraco.Web/Security/WebSecurity.cs

@@ -135,7 +135,7 @@ namespace Umbraco.Web.Security
         /// <returns></returns>
         internal bool ValidateBackOfficeCredentials(string username, string password)
         {
-            var membershipProvider = Membership.Providers[UmbracoSettings.DefaultBackofficeProvider];
+            var membershipProvider = MembershipProviderExtensions.GetUsersMembershipProvider();
             return membershipProvider != null && membershipProvider.ValidateUser(username, password);
         }