The Firewall Flaw
Firewalls, like security guards, eletric fences and alarm systems are a useful component of a security system. But firewalls are not secure within themselves any more than a bulletproof vest makes you bulletproof.
I have tried to come up with a number of analogies to explain what is wrong with relying purely on a firewall, but none explains it so clearly as this image that come through my inbox a day or two ago.

(This image arrived with no credit attached, if it is yours, add a comment so it can be credited properly)
