This indicates an attack attempt to exploit a Remote Code Execution Vulnerability in WordPress Project WordPress.
A remote, unauthenticated attacker could exploit this vulnerability by enticing an authenticated user to open a web page or link. Successful exploitation, in the worst case, could lead to execution of arbitrary PHP code.
WordPress Project WordPress prior to 5.1.1
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.