Security is a very important aspect of distributed computing systems, especially in distributed information environments involving wide-area networks, such as internets. This paper addresses how a security breach, such as intrusion, propagates through a distributed or networked information system. In particular, the issue of how an intrusion, once it occurrs, propagates through a distributed system is addressed. Our approach has a strong resemblance to ripple effect analysis, which had been extensively studied in the area of software maintenance.