This indicates an attack attempt to exploit a Denial of Service Vulnerability in Squid Project Squid.
A remote, unauthenticated attacker can exploit this vulnerability by sending a crafted request to the target server. Successful exploitation will lead to the vulnerable service abnormally terminating, resulting in a denial-of-service condition.
Squid Project Squid 3.3.9 through 3.5.28
Squid Project Squid 4.x through 4.7
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.