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 packet. 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 3.2.x and prior
Squid Project Squid 3.3.x
Squid Project Squid 3.4.x
Squid Project Squid 3.5.x prior to 3.5.16
Squid Project Squid 4.0.x prior to 4.0.8
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.