Threat Modeling At a Startup
I’ve been threat modeling for a long time, and at Microsoft, had the lovely opportunity to put some rigor into not only threat modeling, but into threat modeling in a consistent, predictable, repeatable way. Because I did that work at Microsoft, sometimes people question how it would work for a startup, and I want to address that from having done it. I’ve done threat modeling for startups like Zero Knowledge (some of the output is in the Freedom security whitepaper), and while a consultant or advisor, for other startups who decided against the open kimono approach.
Those threat modeling sessions have taken anywhere from a few hours to a few days, and the same techniques that we developed to make threat modeling more effective can be used to make it more efficient. One key aspect is to realize that what security folks call threat modeling is comprised of several related tasks, and each of them can be done in different ways. Even more important, only one of the steps is purely focused on threats.
I’ve seen a four-question framework work at all sorts of organization. The four questions are: “what are we building/deploying,” “what could go wrong”, “what are we going to do about it” and “did we do an acceptable job at this?”
The “what are we building” question is about creating a shared model of the system. This can be done on a napkin or a whiteboard, or for a large complex system which has grown organically, can involve people working for months. In each case, there’s value beyond security. Your code is cleaner, more consistent, more maintainable, and sometimes even faster when everyone agrees on what’s running where. Discussing trust boundaries can be quick and easy, and understanding them can lead to better marshaling and fewer remote calls. Of course, if your code is spaghetti and no one’s ever bothered to whiteboard it, this step can involve a lot of discussion and debate, because it will lead to refactoring. Maybe you can launch without that refactoring, maybe it’s important to do it now. It can be hard to tell if no one knows how your system is put together.
Understanding “what could go wrong” can happen in as little as an hour for people new to security using the Elevation of Privilege game. You should go breadth-first here, getting an understanding of where the threats might cluster, and if you need to dig deeper.
What are you going to do about it? You’re going to track issues as bugs. (Startups should not spend energy thinking about bugs and flaws as two separate things with their own databases.) If your bugs are post-its, that’s great. If you’re using a database, fine. Just track the issues. Some of them will get turned into tests. If you’re using Test-Driven Development, threat modeling is a great way to help you think about security testing and where it needs to focus. (If you’re not using TDD, it’s still a great way to focus your security testing.)
Total time spent for a startup can be as low as a few hours, especially if you’ve been thinking about software architecture and boundaries along the way. I’ve never spent more than a few days on threat modeling with a small company, and never heard it called a poor use of time. Of course, it’s easy to say that each thing that someone wants a startup to do “only takes a little time,” but all those things can add up to a substantial distraction from the core product. However, a media blow-up or a Federal investigation can also distract you, and distract you for a lot longer. A few hours of threat modeling can help you avoid the sorts of problems that distracted Twitter or Snapchat. The landscape continues to change rapidly post-Snowden, and threat modeling is the fastest way to consider what security investments a startup should be making.
[Update, Feb 5: David Cowan has a complimentary document, “Security for Startups in 10 Steps. He provides context and background on Techcrunch.]