This indicates an attack attempt to exploit a Cross-Site Scripting vulnerability in Apache HTTP web server mod_proxy_balancer.
The vulnerability is due to a lack of input validation when handling crafted HTTP request. A remote attacker may be able to exploit this to execute arbitrary script code within the context of the application, via a crafted HTTP request.
Apache Software Foundation HTTP Server 2.2.x prior to 2.2.24
Apache Software Foundation HTTP Server 2.4.x prior to 2.4.4
System Compromise: Remote attackers can execute arbitrary script code in the context of the affected site.
Apply the most recent upgrade or patch from the vendor.