This article presents a general model of multistate system that can function at different levels of performances. The measures of risk associated with undesirable system behavior are defined on the basis of an acceptability function that represents the required relation between the system performance and a demand. A reliability block diagram method based on a universal generating function technique is presented as an efficient tool for evaluating the risk measures for complex multistate series–parallel systems. The risk associated with combination of internal failures and external attack is considered and a defense strategy methodology is discussed.