Traditional security analyses are often geared towards cryptographic primitives or protocols. Although such analyses are absolutely necessary, they do not provide much insight for answering an equally important question: What is the security assurance of a physically or logically networked system when we consider it as a whole? This question is known to be notoriously difficult, and the state-of-the-art is that we know very little about it. In this paper, we make a step towards resolving it with a new modeling approach.