Modeling and Quantification of Security Attributes of Software Systems

Quite often failures in network based services and server systems may not be accidental, but rather caused by deliber-ate security intrusions. We would like such systems to either completely preclude the possibility of a security intrusion or design them to be robust enough to continue functioning de-spite security attacks. Not only is it important to prevent or tolerate security intrusions, it is equally important to treat security as a QoS attribute at par with, if not more impor-tant than other QoS attributes such as availability and per-formability. This paper deals with various issues related to quantifying the security attribute of an intrusion tolerant system, such as the SITAR system. A security intrusion and the response of an intrusion tolerant system to the attack is modeled as a random process. This facilitates the use of stochastic modeling techniques to capture the attacker be-havior as well as the system’s response to a security intru-sion. This model is used to analyze and quantify the security attributes of the system. The security quantification analy-sis is first carried out for steady-state behavior leading to measures like steady-state availability. By transforming this model to a model with absorbing states, we compute a se-curity measure called the “mean time (or effort) to security failure ” and also compute probabilities of security failure due to violations of different security attributes