Should I Start Threat Modeling from Assets?

by adam on April 15, 2014

A couple of reviewers have commented that they have different perspective on assets. For example, in a review I very much appreciated, Gunnar Peterson says:

I have slightly a different perspective on Shostack’s view on assets. The book goes into different views that launch the threat model, the approach advocated for in the book is very software-centric. I have no quibble on that, especially if you work for a software company. But for a financial institution, a supply chain, a healthcare company, I think it works less well to assume a software first view of the world. Its simpler in those companies to get traction by looking at the assets in play – accounts, confidential data, supplier access, and so on. for one thing, you can engage a wider portion of the organization. For another you have an easier time linking the resultant threats to a business impact.

The TL;DR response is consultants may like asset-centered approaches more than others, and the closer you are to the code, the less helpful they are. Previously, I wrote (page 39):

Focusing on assets appears to be a common-sense approach to threat modeling, to the point where it seems hard to argue with. Unfortunately, much of the time, a discussion of assets does not improve threat modeling. However, the misconception is so common that it’s important to examine why it doesn’t help.

There’s no direct line from assets to threats, and no prescriptive set of steps… That discussion, at best, results in a list of things to look for in your software or operational model, so why not start by creating such a model? Once you have a list of assets, that list is not (ahem) a stepping stone to finding threats; you still need to apply some methodology or approach. Finally, assets may help you prioritize threats, but if that’s your goal, it doesn’t mean you should start with or focus on assets. [Note that this sentence could be more clear as “assets may help you prioritize threats, but that doesn’t mean you should start with, or focus on, assets.’]

Let me say a bit more, since this perspective keeps coming up in both reviews and private conversations. Financial institutions are a great example, because, after all, they hold money as an asset which attackers want and that they want to defend, so we can avoid definitional issues. We can over-simplify and say that money is THE asset that the bank has. Also, I’ve done a fair amount of work threat modeling for financial institutions, so I have a basis for discussing how those systems are really built.

So, let’s take a retail bank as an example. (In contrast to an investment bank, a retail bank does most of its business with consumers and businesses, providing things like loans and money management.) The bank has a general ledger (GL) system. It’s often still on a mainframe, and it is the only place where money really moves from one account to another. So we should start threat modeling there, right? Maybe.

Let’s look at the rest of the business. It turns out that there are pretty much two types of systems at the bank: those that touch the GL as a matter of course, and the desktops and laptops that people use to do their day jobs. In other words, pretty much everything that the bank has deployed, from the web front ends that show you your balance through to the inter-bank transfer systems like ACH and credit cards through the systems that calculate taxes have access to the GL. That access is often mediated through several layers of control, protocol translation and the like, but the access is there. Worse, even the systems that you’d hope are read-only (the tax calculators) can often move money around (manage deductions, enforce IRS mandatory withholding, etc). Now, I’m modeling, that is to say simplifying. But in this instance, I don’t think I’m dramatically over-simplifying.

If you go and enumerate everywhere the money is, or everywhere that there’s access to the GL, what you eventually get to (in essence) is a model of the software. It’s more expensive than a software-mainly model because you need to draw in additional people who can explain the business processes, and add a layer of model that covers how money moves.

Now, it may be that if you come in consulting at the senior management level (and I think Gunnar often does), that you start from the money because the senior management folks have no idea how their systems are actually plugged together, and figuring it out requires following the business processes, and reporting will require going back along that path of following the money. So here, I think the difference is one of intended audience: the book is primarily for people building or operating software, not consultants. If we make it accessible, those folks can and should learn to threat model. It’s easier than git.

People building software or systems at a financial institution, a supply chain, or a healthcare company should start from the software they’re building because it’s what they know best. Another way to say this is that they are surrounded by layers of business analysts, architects, project managers, and other folks who translate between the business requirements (including assets) and software and system requirements.

Gunnar and I agree that assets are a great tool to link “the resultant threats to a business impact.” It’s my experience (as a consultant and at my current employer) that the assets come out in risk discussion after you’ve identified a software issue.

So to answer the question: it depends who you are. If you know the software, you should start there.

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