ACK Tunneling Trojans

Source: Security Protocols

The following is the complete paper published by: Arne Vidstrom.
Trojans normally use ordinary TCP or UDP communication between their client and server parts. Any firewall between the attacker and the victim that blocks incoming traffic will usually stop all Trojans from working. ICMP tunneling has existed for quite some time now, but if you block ICMP in the firewall, you will be safe from that. This paper describes another concept that is called ACK Tunneling. ACK Tunneling works through firewalls that do not apply their rule sets on TCP ACK segments (ordinary packet filters belong to this class of firewalls).