--- modules/user/user.module
+++ modules/user/user.module
@@ -2081,6 +2081,11 @@ function user_login_name_validate($form, &$form_state) {
  * is set to the matching user ID.
  */
 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