This indicates an attack attempt against a Cross-Site Scripting vulnerability in Wordpress.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to execute arbitrary script code within the context of the application, via crafted HTTP requests.
WordPress versions 4.3 and earlier
System Compromise: Remote attackers can execute arbitrary script code on vulnerable systems.
Apply the most recent upgrade or patch from the vendor.