This indicates an attack attempt against a denial-of-service vulnerability in Squid.
The vulnerability is caused by an assertion error that occurs when the vulnerable software handles a malicious TRACE request. A remote attacker may exploit this to crash the program via a crafted TRACE request.
Squid versions prior to 2.6.STABLE12
Denial of Service: Remote attackers can crash vulnerable systems.
Upgrade to Squid version 2.6.STABLE12 :
Or apply the patch :