Today’s information systems face sophisticated attackers who combine multiple vulnerabilities to penetrate networks with devastating impact. The overall security of an enterprise network cannot be determined by simply counting the number of vulnerabilities. To more accurately assess the security of enterprise systems, one must understand how vulnerabilities can be combined and exploited to stage an attack. Composition of vulnerabilities can be modeled using probabilistic attack graphs, which show all paths of attacks that allow incremental network penetration. Attack likelihoods are propagated through the attack graph, yielding a novel way to measure the security risk of enterprise systems. This metric for risk mitigation analysis is used to maximize the security of enterprise systems. This methodology based on probabilistic attack graphs can be used to evaluate and strengthen the overall security of enterprise networks.