Amid so many recent high-profile hacks and data breaches, security experts are fond of pointing out that there’s no such thing as perfect security. It’s true! But it also invites the question: Why doesn’t literally everything get hacked all the time? The answer has to do with the relative incentives and the costs of infiltrating a given network. And one of the concepts underlying that calculus is the idea of an “attack surface.” Here’s an example.