Anomaly-based intrusion detection (AID) techniques are useful for detecting novel intrusions into computing resources. One of the most successful AID detectors proposed to date is stide, which is based on analysis of system call sequences. In this paper, we present a detailed formal framework to analyze, understand and improve the performance of stide and similar AID techniques. Several important properties of stide-like detectors are established through formal proofs, and validated by carefully conducted experiments using test datasets. Finally, the framework is utilized to design two applications to improve the cost and performance of stide-like detectors which are based on sequence analysis. The first application reduces the cost of developing AID detectors by identifying the critical sections in the training dataset, and the second application identifies the intrusion context in the intrusive dataset, that helps to fine-tune the detectors. Such fine-tuning in turn helps to improve detection rate and reduce false alarm rate, thereby increasing the effectiveness and efficiency of the intrusion detectors.
@cite_0: Anomaly based intrusion detection has been held out as the best (perhaps only) hope for detecting previously unknown exploits. We examine two anomaly detectors based on the analysis of sequences of system calls and demonstrate that the general information hiding paradigm applies in this area also. Given even a fairly restrictive definition of normal behavior, we were able to devise versions of several exploits that escape detection. This is done in several ways: by modifying the exploit so that its manifestations match "normal," by making a serious attack have the manifestations of a less serious but similar attack, and by making the attack look like an entirely different attack. We speculate that similar attacks are possible against other anomaly based IDS and that the results have implications for other areas of information hiding.
@cite_15: Audit trail patterns generated on behalf of a Unix process canb e used to model the process behavior. Most of the approaches proposed so far use a table of fixed-length patterns to represent the process model. However, variable-length patterns seem to be more naturally suited to model the process behavior, but they are also more difficult to construct. In this paper, we present a novel technique to build a table of variable-length patterns. This technique is based on Teiresias, an algorithm initially developed for discovering rigid patterns in unaligned biological sequences. We evaluate the quality of our technique in a testbed environment, and compare it with the intrusion-detection system proposed by [8], which is based on fixed-length patterns. The results achieved with our novel method are significantly better than those obtained with the original method based on fixed-length patterns.
@cite_10: Intrusion detection systems rely on a wide variety of observable data to distinguish between legitimate and illegitimate activities. We study one such observable-sequences of system calls into the kernel of an operating system. Using system-call data sets generated by several different programs, we compare the ability of different data modeling methods to represent normal behavior accurately and to recognize intrusions. We compare the following methods: simple enumeration of observed sequences; comparison of relative frequencies of different sequences; a rule induction technique; and hidden Markov models (HMMs). We discuss the factors affecting the performance of each method and conclude that for this particular problem, weaker methods than HMMs are likely sufficient.
@cite_12: We examine several host-based anomaly detection systems and study their security against evasion attacks. First, we introduce the notion of a mimicry attack, which allows a sophisticated attacker to cloak their intrusion to avoid detection by the IDS. Then, we develop a theoretical framework for evaluating the security of an IDS against mimicry attacks. We show how to break the security of one published IDS with these methods, and we experimentally confirm the power of mimicry attacks by giving a worked example of an attack on a concrete IDS implementation. We conclude with a call for further research on intrusion detection from both attacker's and defender's viewpoints.
Following the publication of stide, several research studies have been published with criticisms and suggestions of improvement of stide @cite_12 @cite_0 @cite_10 @cite_15 . Under our proposed framework, they can be interpreted in a logical way to determine their basic foundations.