Understanding layered security and defense in depth

Understanding "layered security" and "defense in depth" and how they can be used to improve your own security is important for any system or network administrator.

Sometimes it seems like everybody talks about "layered security", "layered defense", or "defense in depth", but nobody really knows what it means.

The three phrases are often used interchangeably--but just as often, someone will use two of them to mean completely different things. There are actually two separate, but in some respects very similar, concepts that may be named by these phrases.

Layered security
A layered approach to security can be implemented at any level of a complete information security strategy. Whether you are the administrator of only a single computer, accessing the Internet from home or a coffee shop, or the go-to guy for a thirty thousand user enterprise WAN, a layered approach to security tools deployment can help improve your security profile.
In short, the idea is an obvious one: that any single defense may be flawed, and the most certain way to find the flaws is to be compromised by an attack--so a series of different defenses should each be used to cover the gaps in the others' protective capabilities.
Firewalls, intrusion detection systems, malware scanners, integrity auditing procedures, and local storage encryption tools can each serve to protect your information technology resources in ways the others cannot.
Security vendors offer what some call vertically integrated vendor stack solutions for layered security. A common example for home users is the Norton Internet Security suite, which provides (among other capabilities):
  1. an antivirus application
  2. a firewall application
  3. an anti-spam application
  4. parental controls
  5. privacy controls
Corporate vendors of security software are in an interesting position. In order to best serve their business goals, they must on one hand try to sell integrated, comprehensive solutions to lock customers into single-vendor relationships, and on the other, try to sell components of a comprehensive layered security strategy individually to those who are unlikely to buy their own integrated solution--and convince such customers that a best-of-breed approach is better than a vertically integrated stack approach to do it.

This contradictory set of needs has produced quite a few conflicting marketing pitches from security software vendors, and produces a lot of confusion among client bases at times. For this reason alone, it is no wonder that people are often at a loss to clearly articulate any reasonable, practical definition of "layered security".
The term "layered security" does not refer to multiple implementations of the same basic security tool.

Installing both ClamWin and AVG Free on the same MS Windows machine is not an example of layered security, even if it achieves some of the same benefit--making several tools each cover for the others' failings. This is a case of redundancy rather than layering; by definition, layered security is about multiple types of security measures, each protecting against a different vector for attack.

Defense in depth
Originally coined in a military context, the term "defense in depth" refers to an even more comprehensive security strategy approach than layered security. In fact, on might say that just as a firewall is only one component of a layered security strategy, layered security is only one component of a defense in depth strategy.

Layered security arises from the desire to cover for the failings of each component by combining components into a single, comprehensive strategy, the whole of which is greater than the sum of its parts, focused on technology implementation with an artificial goal of securing the entire system against threats.
Defense in depth, by contrast, arises from a philosophy that there is no real possibility of achieving total, complete security against threats by implementing any collection of security solutions. Rather, technological components of a layered security strategy are regarded as stumbling blocks that hinder the progress of a threat, slowing and frustrating it until either it ceases to threaten or some additional resources--not strictly technological in nature--can be brought to bear.

A layered security solution also assumes a singular focus on the origins of threats, within some general or specific category of attack. For instance, vertically integrated layered security software solutions are designed to protect systems that behave within certain common parameters of activity from threats those activities may attract, such as Norton Internet Security's focus on protecting desktop systems employed for common purposes by home users from Internet-borne threats.

Defense in depth, on the other hand, assumes a broader range of possibilities, such as physical theft followed by forensic recovery of data by unauthorized persons, incidental threats as a result of dangers that do not specifically target the protected systems, and even perhaps such exotic threats as van Eck phreaking.
Defense in depth strategies also include other security preparations than directly protective. They also address such concerns as:
  1. monitoring, alerting, and emergency response
  2. authorized personnel activity accounting
  3. disaster recovery
  4. criminal activity reporting
  5. forensic analysis
One of the most important factors in a well-planned defense in depth strategy is taking advantage of threat delay. By ensuring rapid notification and response when attacks and disasters are underway, and delaying their effects, damage avoidance or mitigation that cannot be managed by purely technological measures can be enacted before the full effects of a threat are realized.
For instance, while a honeypot system may not itself stop a malicious security cracker who has gained unauthorized access to a network indefinitely, it might facilitate notification of the breach to network security specialists and delay his progress long enough that the security specialists can identify and/or eject the intruder before any lasting damage is done.
Layered security vs. defense in depth
Layered security and defense in depth are two different concepts with a lot of overlap. They are not, however, competing concepts.
A good layered security strategy is extremely important to protecting your information technology resources. A defense in depth approach to security widens the scope of your attention to security and encourages flexible policy that responds well to new conditions, helping ensure you are not blindsided by unexpected threats.
Each of these strategic philosophies of security should inform your treatment of the other, so that normally overwhelming circumstances for a more narrow and brittle security strategy such as simultaneous attacks by independent threats, far greater intensity of attack than expected, and threats that seem to have strayed from their more common targets might all be effectively warded off. Both are worth understanding--and the first step to that is understanding how they differ from one another, how they are similar, and the relationship between them.