Bypassing Firewalls: Tools and Techniques

This paper highlights a very important problem with network perimeter firewalls. The threat discussed is not exactly new, but neither is it widely recognised— even amongst network security professionals. Most commercial firewalls claim to be application layer devices, but they derive very little useful information about the context of the application traffic that passes through them. Malicious applications can misuse even the simplest protocols in a way that totally bypasses the firewall’s controls. This paper describes the methods of simple protocol tunnels, and shows how they can be applied. It also considers ways to counter this threat, and suggests that architectures based on military security principles and IPSec can improve security dramatically.