This indicates an attack attempt to exploit a Cross-Site Scripting Vulnerability in Squid Project Squid.
Thus, a remote attacker could exploit this vulnerability by enticing a Squid user to open a malicious link or web page. Successful exploitation could lead to arbitrary script code execution in 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 gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.