Update issue of Drupal 8.34
As you can read on https://www.drupal.org/project/drupal/releases/8.3.9 Drupal 8.3.9 fixes SA-CORE-2018-002 (and hence probably also SA-CORE-2018-001). However, there has been several later issues - see https://www.drupal.org/security - and only Drupal 8.5 is supported. Quote from the latest 8.5.6 release: 


  Versions of Drupal 8 prior to 8.5.x are end-of-life and do not receive
  security coverage


In other words, you better upgrade to Drupal 8.5.6 and fix/upgrade the modules that no longer works.