As computer systems become more complex and more widely distributed, it is becoming increasingly difficult to remove all vulnerabilities that can potentially be exploited by intruders. Intrusion tolerance is an emerging approach that aims to enable systems to continue functioning in spite of successful intrusions. Before intrusion tolerance is accepted as an approach to security, there must be quantitative techniques to measure its ef.cacy. However, there have been very few attempts at quantitative validation of intrusion-tolerant systems or, for that matter, of security in general. In this paper, we show that probabilistic validation through stochastic modeling is an attractive mechanism for evaluating intrusion tolerance. We demonstrate our approach by using stochastic activity networks to quantitatively validate an intrusion-tolerant replication management system. We characterize the intrusion tolerance provided by the system through several measures de.ned on the model, and study variations in these measures in response to changes in system parameters to evaluate the relative merits of various design choices.