A method for assessing an impact of a security attack on a system includes defining a system affecting metric for an observation period as a fraction of time the system satisfies a defined specification, defining a resource failure based model and a resource usage based model for the system, obtaining results for each of a plurality of states of the resource failure based model and the resource usage based model, solving the resource failure based model and the resource usage based model and obtaining a term fraction of time each model spends on each of the plurality of states, obtaining a state probability according to the term fraction, and obtaining a measure of the system affecting metric according to the state probability.