This indicates an attack attempt to exploit a Buffer Overflow Vulnerability in Squid Project Squid.
An unauthenticated, remote attacker can exploit this vulnerability by sending a request for a URN with an NID corresponding to an attacker controlled host, then sending a crafted response to the request forwarded by Squid. Successful exploitation could result in the execution of arbitrary code under the security context of the squid process.
Squid Project Squid 3.x through 3.5.28
Squid Project Squid 4.x through 4.8
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.