Network intrusion detection systems often rely on matching patterns that are gleaned from known attacks. While this method is reliable and rarely produces false alarms, it has the obvious disadvantage that it cannot detect novel attacks. An alternative approach is to learn a model of normal traffic and report deviations, but these anomaly models are typically restricted to modeling IP addresses and ports, and do not include the application payload where many attacks occur. We describe a novel approach to anomaly detection.
We extract a set of attributes from each event (IP packet or TCP connection),including strings in the payload, and induce a set of conditional rules which have a very low probability of being violated in a nonstationary model of the normal network traffic in the training data. In the 1999 DARPA intrusion detection evaluation data set, we detect about 60% of 190 attacks at a false alarm rate of 10 per day (100 total). We believe that anomaly detection can work because most attacks exploit software or configuration errors that escaped field testing, so are only exposed under unusual consitions.
Though our rule learning techniques are applied to network intrusion detection, they are general enough for detecting anomalies in other applications.