This indicates an attack attempt to exploit a File Inclusion vulnerability in the WordPress blogging software plugin Advanced Custom Fields.
The vulnerability is due to insufficient sanitizing of user supplied inputs when handling crafted HTTP requests. A remote attacker can include arbitrary files and execute them within the context of the application via a crafted HTTP request.
Advanced Custom Fields plug-in 3.5.1 and earlier versions.
System Compromise: Remote attackers can gain control of vulnerable systems.
Currently we are unaware of any vendor supplied patch or updates available for this issue.