Abstract. Over the past decade many anomaly-detection techniques have been proposed and/or deployed to provide early warnings of cyberattacks, particularly of those attacks involving masqueraders and novel methods. To date, however, there appears to be no study which has identified a systematic method that could be used by an attacker to undermine an anomaly-based intrusion detection system. This paper shows how an adversary can craft an offensive mechanism that renders an anomaly-based intrusion detector blind to the presence of on-going, common attacks. It presents a method that identifies the weaknesses of an anomaly-based intrusion detector, and shows how an attacker can manipulate common attacks to exploit those weaknesses. The paper explores the implications of this threat, and suggests possible improvements for existing and future anomaly-based intrusion detection systems.

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Anomaly detection is a key element of intrusiondetection and other detection systems in which perturbations of normal behavior suggest the presence of intentionally or unintentionally induced attacks, faults, defects, etc. Because most anomaly detectors are based on probabilistic algorithms that exploit the intrinsic structure, or regularity, embedded in data logs, a fundamental question is whether or not such structure influences detection performance. If detector performance is indeed a function of environmental regularity, it would be critical to match detectors to environmental characteristics. In intrusion-detection settings, however, this is not done, possibly because such characteristics are not easily ascertained. This paper introduces a metric for characterizing structure in data environments, and tests the hypothesis that intrinsic structure influences probabilistic detection. In a series of experiments, an anomaly-detection algorithm was applied to a benchmark suite of 165 carefully calibrated, anomalyinjected datasets of varying structure. Results showed performance differences of as much as an order of magnitude, indicating that current approaches to anomaly detection may not be universally dependable.

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