This indicates an attack attempt to exploit a Remote Code Execution Vulnerability in Drupal Core.
The vulnerability is due to an input validation error when parsing a crafted HTTP request. A remote attacker could exploit this to execute arbitrary code within the context of target application, via a crafted HTTP request.
Drupal 8.5.x before 8.5.11
Drupal 8.6.x before 8.6.10
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor