--- modules\user\user.module
+++ modules\user\user.module

 function user_login_authenticate_validate($form, &$form_state) {
+  
+  // No additional validation of user credentials is needed
+  // when form_state['uid'] is set.
+  if (!empty($form_state['uid'])) {
+    return;
+  }
   $password = trim($form_state['values']['pass']);
   if (!empty($form_state['values']['name']) && !empty($password)) {
     // Do not allow any login from the current user's IP if the limit has been