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Defense-in-Depth with defense in breadth

You can find in the Cisco certification guide for the Cisco 640-553 this explanation of the Defense-in-Depth security approach :

“Cisco recommends multiple and overlapping solutions. These overlapping solutions target different aspects of security, such as securing against insider attacks and securing against technical attacks. These solutions should also be subjected to routine testing and evaluation. Security solutions should also overlap in a way that eliminates any single point of failure.

Defense in Depth is a design philosophy that achieves this layered security approach. The layers of security present in a Defense in Depth deployment should provide redundancy for one another while offering a variety of defense strategies for protecting multiple aspects of a network. Any single points of failure in a security solution should be eliminated, and weak links in the security solution should be strengthened.”

But if you consider attacks targeting the different aspect of security : Confidentiality, Integrity, and Availability, (CIA) things are not such simpler.

Read this article on “Observations on the effects of defense in depth on adversary behavior in cyber warfare”. They have built different networks with different number of security layers. Then a team tried to catch flags that correspond to the realization of an attack on the different CIA security aspects for each network.

The goal is to see the factor between the number of security layer in each network and the corresponding time for the attacker to successfully launch all the three attack ( Confidentiality, Integrity, Availability)

The result is that to launch a read or modify attack the workload that take two hours at configuration level one and two, take 26 hours at level 3. The big change is in the time to develop the attack instead of launching it.

You can see that in the following graph :



But what happens to availability attacks ?

More you have systems, more you have possibility to exploit a vulnerability from the chain of systems and to denial service to it.

Look at the following graph :




Less time to launch a denial of services attack with more layer of security.


Therefore, what to do ? Plans carefully your different layer and the overlaps between them , think about not only the number, but the scope covered by the layer think about “Defense-in-Depth” with “Defense-in-breadth”.


The following graph from the US Transport Security Agency is an example of the “Defense-in-Depth” with “Defense-in-breadth” security approach, you have multiple layer that’s overlaps with each other to form a large area of defense.

Graphic which shows layers of security used to ensure the security of the traveling public and the Nation's transportation system.

Source : http://www.tsa.gov/what_we_do/layers/index.shtm

In conclusion, adding security layers to a system does not necessarily guarantee increased assurance. Introducing new layers of security has the potential to introduce new vulnerabilities, or control surfaces, for sophisticated adversaries to exploit. Defensive layers must be analyzed to gain a thorough understanding of how they work together before they are integrated into an operational system.

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