This indicates an attack attempt against a Cross-Site Scripting vulnerability in WordPress.
The vulnerability is due to insufficient validation of comments when the vulnerable software handles a maliciously crafted request. It allows a remote attacker to execute arbitrary script codes via a crafted HTTP request.
WordPress Project WordPress prior to 4.2.1
System Compromise: Remote attackers can execute arbitrary script code within the context of the target user's browser.
Apply the most recent upgrade or patch from the vendor