This indicates an attack attempt against a denial-of-service vulnerability in Squid Proxy.
The vulnerability is caused by an error when the vulnerable software handles a specially crafted Hypertext Caching Protocol (HTCP) packet. It allows a remote attacker to cause a denial of service.
Squid Web Proxy Cache 3.0 PRE3 and previous versions
Denial of Service
Upgrade to Squid version 3.0.STABLE24: