This indicates an attack attempt to exploit a Cross-Site Scripting Vulnerability in Squid Project Squid.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able exploit this to execute arbitrary script code within the context of the target user's browser.
Squid Project Squid 2.x through 2.7.STABLE9
Squid Project Squid 3.x through 3.5.28
Squid Project Squid 4.x through 4.7
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.