This indicates an attack attempt against an Information Disclousre vulnerability in Drupal Core.
The vulnerability is due to insufficient access control when downloading a full configuration export. A remote attacker may be able to read arbitrary files in the targeted system via a crafted HTTP request.
Drupal Drupal 8.x prior to 8.1.10
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Apply the latest update from the vendor.