Bruce Schneier, author of Applied Cryptography, has proposed a different method for analyzing information security risks: attack trees. An attack tree, quite simply, is a visual representation of possible attacks against a given target. The attack goal (target) is called the root node; the various subgoals necessary to reach the goal are called leaf nodes.
 Schneier, Bruce. "Attack Trees: Modeling Security Threats." Dr. Dobbs' Journal: Dec 1999.
To create an attack tree, you must first define the root node. For example, one attack objective might be "Steal ABC Corp.'s Customers' Account Data." Direct means of achieving this could be as follows:
Obtain backup tapes from ABC's file server.
Intercept email between ABC Corp. and their customers.
Compromise ABC Corp.'s file server from over the Internet.
These three subgoals are the leaf nodes immediately below our root node (Figure 1-3).
Next, for each leaf node, you determine subgoals that achieve that leaf node's goal. These become the next "layer" of leaf nodes. This step is repeated as necessary to achieve the level of detail and complexity with which you wish to examine the attack. Figure 1-4 shows a simple but more-or-less complete attack tree for ABC Corp.
No doubt, you can think of additional plausible leaf nodes at the two layers in Figure 1-4, and additional layers as well. Suppose for the purposes of our example, however, that this environment is well secured against internal threats (which, incidentally, is seldom the case) and that these are therefore the most feasible avenues of attack for an outsider.
In this example, we see that backup media are most feasibly obtained by breaking into the office. Compromising the internal file server involves hacking through a firewall, but there are three different avenues to obtain the data via intercepted email. We also see that while compromising ABC Corp.'s SMTP server is the best way to attack the firewall, a more direct route to the end goal is simply to read email passing through the compromised gateway.
This is extremely useful information: if this company is considering sinking more money into its firewall, it may decide based on this attack tree that their money and time is better spent securing their SMTP gateway (although we'll see in Chapter 2 that it's possible to do both without switching firewalls). But as useful as it is to see the relationships between attack goals, we're not done with this tree yet.
After an attack tree has been mapped to the desired level of detail, you can start quantifying the leaf nodes. For example, you could attach a " cost" figure to each leaf node that represents your guess at what an attacker would have to spend to achieve that leaf node's particular goal. By adding the cost figures in each attack path, you can estimate relative costs of different attacks. Figure 1-5 shows our example attack tree with costs added (dotted lines indicate attack paths).
In Figure 1-5, we've decided that burglary, with its risk of being caught and being sent to jail, is an expensive attack. Nobody will perform this task for you without demanding a significant sum. The same is true of bribing a system administrator at the ISP: even a corruptible ISP employee will be concerned about losing her job and getting a criminal record.
Hacking is a bit different, however. Hacking through a firewall takes more skill than the average script kiddie has, and it will take some time and effort. Therefore, this is an expensive goal. But hacking an SMTP gateway should be easier, and if one or more remote users can be identified, the chances are good that the user's home computer will be easy to compromise. These two goals are therefore much cheaper.
Based on the cost of hiring the right kind of criminals to perform these attacks, the most promising attacks in this example are hacking the SMTP gateway and hacking remote users. ABC Corp., it seems, had better take a close look at their perimeter network architecture, their SMTP server's system security, and their remote-access policies and practices.
Cost, by the way, is not the only type of value you can attach to leaf nodes. Boolean values such as "feasible" and "not feasible" can be used: a "not feasible" at any point on an attack path indicates that you can dismiss the chance of an attack on that path with some safety. Alternatively, you can assign effort indices, measured in minutes or hours. In short, you can analyze the same attack tree in any number of ways, creating as detailed a picture of your vulnerabilities as you need to.
Before we leave the subject of attack tree threat modeling, I should mention the importance of considering different types of attackers. The cost estimates in Figure 1-5 are all based on the assumption that the attacker will need to hire others to carry out the various tasks. These costs might be computed very differently if the attacker is himself a skilled system cracker; in such a case, time estimates for each node might be more useful.
So, which type of attacker should you model against? As many different types as you realistically think you need to. One of the great strengths of this method is how rapidly and easily attack trees can be created; there's no reason to quit after doing only one.