Opinion

Where Defense in Depth Falls Short

For IT security practitioners, hearing about defense in depth can conjure up images of clutter. Here to cut through that clutter, point out the shortcomings and offer a better way is CSO columnist Ariel Silverstone.

By Ariel Silverstone, CISSP

October 06, 2009 — CSO —

Defense in Depth is arguably the most time-tested principle in security and applies to the physical security as well as online. It builds on a concept of a hardened “core” where one places their “crown jewels.” This core is then surrounded by castle walls and motes, with ever-increasing generality of defense.

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It's a great concept, but comes at a price. Just as the area covered is wider from layer to layer, so is the cost associated with protecting against more plentiful and less and less specific threats. A firewall typically acts as the last line of defense on the enterprise perimeter but has to protect against a great many varieties of threats, while a server-room door has to “only” be concerned with physical access.

The Server in The Castle

Another flaw in the Defense in Depth design is its inherent difficulty to implement vis-à-vis the three basic tenets of security: confidentiality, integrity and availability. Why? Because most forms of defense create increasing confidentiality, but make integrity more difficult to implement and manage. Any increase in defense, of course, makes the concept of availability that much harder to provide to the users.

A difficulty that I myself encountered many times is the applicability of Defense in Depth to my “layer 8” problem – the users. If users are not trained properly, if they are not aware of information protection needs, methods, and the “why?” of it, they become a liability, rather than an asset, towards data security. If you are like me, you find the need to increase our moat-to-user-ratio on an ongoing base harder to design, implement, manage, and pay for. Many of us resign ourselves to the proverbial “this is reality” and define our demarcation line as a physical device, such as a router, an access point, a firewall or a webserver. There are potentially two things “wrong” with doing so:

We are basically saying “we are a target just waiting to be attacked” and
We allow most barbarians (in the form of rogue traffic, networks and devices) to hit our gates

If we continue to do so, we will have approached a mathematical certainty of being hacked, or at least DDoS’ed out of the Net. I really prefer NOT to draw analogies here to the real world, and we all know which those are.