The output of a threat modeling session, or the creature from the bug lagoon

Wendy Nather has continued the twitter conversation which is now a set of blog posts. (My comments are threat modeling and risk assessment, and hers: “That’s not a bug, it’s a creature. “)

I think we agree on most things, but I sense a little semantic disconnect in some things that he says:

The only two real outputs I’ve ever seen from threat modeling are bugs and threat model documents. I’ve seen bugs work far better than documents in almost every case.

I consider the word “bug” to refer to an error or unintended functionality in the existing code, not a potential vulnerability in what is (hopefully) still a theoretical design. So if you’re doing whiteboard threat modeling, the output should be “things not to do going forward.”

As a result, you’re stuck with something to mitigate, probably by putting in extra security controls that you otherwise wouldn’t have needed. I consider this a to-do list, not a bug list.
(“That’s not a bug, it’s a creature. “, Wendy Nather)

I don’t disagree here, but want to take it one step further. I see a list of “things not to do going forward” and a “todo list” as an excellent start for a set of tests to confirm that those things happen or don’t. So you file bugs, and those bugs get tracked and triaged and ideally closed as resolved or fixed when you have a test that confirms that they ain’t happening. If you want to call this something else, that’s fine–tracking and managing bugs can be too much work. The key to me is that the “things not to do” sink in, and to to-do list gets managed in some good way.

And again, I agree with her points about probability, and her point that it’s lurking in people’s minds is an excellent one, worth repeating:

the conversation with the project manager, business executives, and developers is always, always going to be about probability, even as a subtext. Even if they don’t come out and say, “But who would want to do that?” or “Come on, we’re not a bank or anything,” they’ll be thinking it when they estimate the cost of fixing the bug or putting in the mitigations.

I simply think the more you focus threat modeling on the “what will go wrong” question, the better. Of course, there’s an element of balance: you don’t usually want to be movie plotting or worrying about Chinese spies replacing the hard drive before you worry about the lack of authentication in your network connections.