This indicates an attempt to exploit a Denial of Service vulnerability in Squid.
The vulnerability is due to input validation when the vulnerable software handles a malformed request. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system, via crafted packets.
Squid Project Squid 3.x prior to 3.5.15
Squid Project Squid 4.0.x prior to 4.0.7
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.