This indicates an attack attempt to exploit a Denial-of-Service vulnerability in Squid proxy server.
The vulnerability is caused by an error when generating an error page. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system, via a crafted HTTP request.
Squid Project Squid 3.2.x prior to 3.2.9
Squid Project Squid 3.3.x prior to 3.3.3
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.