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Detecting Intruders in Computer Systems
 by Teesa F. Lunt.

Abstract
Although a computer system's primary defense is its access controls, computer system access controls cannot be relied upon in most cases to safeguard against a penetration or insider attack. Even the most secure systems are vulnerable to abuse by insiders who misuse their privileges and audit trails may be the only means of detecting authorized but abusive user activity. While many computer systems collect audit data, most do not have any capability for automated analysis of that data. Moreover, many systems collect large volumes of data that are not necessarily security relevant. To address the need for automated security analysis of audit trails, SRI is developing a real-time intrusion-detection expert system (NIDES). NIDES is an independent system that runs on its own workstation and processes audit data characterizing user activity received from a target system. NIDES provides a system-independent mechanism for real-time detection of security violations, whether they are initiated by outsiders who attempt to break into a system or by insiders who attempt to misuse their privileges. NIDES detects masqueraders by keeping statistical profiles of past user behavior and raising an alarm when observed activity departs from established patterns of use for individual users. NIDES also includes expert-system rules that characterize certain types of intrusions. NIDES raises an alarm if observed activity matches any of its encoded intrusion scenarios.
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