Intrusion detection has emerged as an important approach to network security. In this paper, we adopt an anomaly detection approach by detecting possible intrusions based on user profiles built from normal usage data. In particular, user profiles based on Unix shell commands are modeled using two different types of behavioral models. The dynamic modeling approach is based on hidden Markov models (HMM) and the principle of maximum likelihood, while the static modeling approach is based on event occurrence frequency distributions and the principle of minimum cross entropy. The novelty detection approach is adopted to estimate the model parameters using normal training data only. To determine whether a certain behavior is similar enough to the normal model and hence should be classified as normal, we use a scheme that can be justified from the perspective of hypothesis testing. Our experimental results show that static modeling outperforms dynamic modeling for this application. Moreover, the static modeling approach based on cross entropy is similar in performance to instance-based learning reported previously by others for the same dataset but with much higher computational and storage requirements than our method.
展开▼
