A method is introducted for detecting intrusions at the level of privileged processes. Evidence is given that short sequences of system calls executed by running processes are a good discriminator between normal and abnormal operating characteristics of several common UNIX programs. Normal behavior is collected in two ways: Synthetically, by exercising as many normal modes of usage of a program as possible, and in a live user environment by tracing the actual execution of the program. In the former case several types of intrusive behavior were studied; in the latter case, results were analyzed for false positives. 1 Introduction Modern computer systems are plagued by security vulnerabilities. Whether it is the latest UNIX buffer overflow or bug in Microsoft Internet Explorer, our applications and operating systems are full of security flaws on many levels. From the viewpoint of the traditional security paradigm, it should be possible to eliminate such problems through more exten...

This paper describes a specification-based approach to detect exploitations of vulnerabdities in securitycritical programs. The approach utilizes security specifications that describe the intended behavior of programs and scans audit trails for operations that are in violation of the specifications. We developed a formal framework for specifying the security-relevant behavior of programs, on which we based the design and implementation of a real-time intrusion detection system for a distributed system. Also, we wrote security specifications for 15 Unix setuid root programs. Our system detects attacks caused by monitored programs, including security violations caused by improper synchronization in distributed programs. Our approach encompasses attacks that exploit previously unknown vulnerabilities in security-critical programs. 1.

This paper presents a taxonomy of intrusion detection systems that is then used to survey and classify a number of research prototypes. The taxonomy consists of a classification first of the detection principle, and second of certain operational aspects of the intrusion detection system as such. The systems are also grouped according to the increasing difficulty of the problem they attempt to address. These classifications are used predictively, pointing towards a number of areas of future research in the field of intrusion detection. 1 Introduction There is currently a need for an up-to-date, thorough taxonomy and survey of the field of intrusion detection. This paper presents such a taxonomy, together with a survey of the important research intrusion detection systems to date and a classification of these systems according to the taxonomy. It should be noted that the main focus of this survey is intrusion detection systems, in other words major research efforts that have resul...

The Intrusion Detection System architectures commonly used in commercial and research systems have a number of problems that limit their congurability, scalability or efficiency. The most common shortcoming in the existing architectures is that they are built around a single monolithic entity that does most of the data collection and processing. In this paper, we review our architecture for a distributed Intrusion Detection System based on multiple independent entities working collectively. We call these entities Autonomous Agents. This approach solves some of the problems previously mentioned. We present the motivation and description of the approach, partial results obtained from an early prototype, a discussion of design and implementation issues, and directions for future work.

STATL is an extensible state/transition-based attack description language designed to support intrusion detection. The language allows one to describe computer penetrations as sequences of actions that an attacker performs to compromise a computer system. A STATL description of an attack scenario can be used by an intrusion detection system to analyze a stream of events and detect possible ongoing intrusions. Since intrusion detection is performed in different domains (i.e., the network or the hosts) and in different operating environments (e.g., Linux, Solaris, or Windows NT), it is useful to have an extensible language that can be easily tailored to different target environments. STATL defines domain-independent features of attack scenarios and provides constructs for extending the language to describe attacks in particular domains and environments. The STATL

This paper describes an expert system development toolset called the Production-Based Expert System Toolset (P-BEST) and how it is employed in the development of a modern generic signature-analysis engine for computer and network misuse detection. For more than a decade, earlier versions of P-BEST have been used in intrusion detection research and in the development of some of the most wellknown intrusion detection systems, but this is the first time the principles and language of P-BEST are described to a wide audience. We present rule sets for detecting subversion methods against which there are few defenses--- specifically, SYN flooding and buffer overruns---and provide performance measurements. Together, these examples and performance measurements indicate that P-BEST-based expert systems are well suited for real-time misuse detection in contemporary computing environments. In addition, the simplicity of the P-BEST language and its close integration with the C programming language ...

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. 1

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Security in computer systems is important so as to ensure reliable operation and to protect the integrity of stored information. Faults in the implementation of critical components can be exploited to breach security and penetrate a system. These faults must be identified, detected, and corrected to ensure reliability and safeguard against denial of service, unauthorized modification of data, or disclosure of information. We define a classification of security faults in the Unix operating system. We state the criteria used to categorize the faults and present examples of the different fault types. We present the design and implementation details of a prototype database to store vulnerability information collected from different sources. The data is organized according to our fault categories. The information in the database can be applied in static audit analysis of systems, intrusion detection, and fault detection. We also identify and describe software testing methods that should be effective in detecting different faults in our classification scheme.

To build survivable information systems (i.e., systems that continue to provide their services in spite of coordinated attacks), it is necessary to detect and isolate intrusions before they impact system performance or functionality. Previous research in this area has focussed primarily on detecting intrusions after the fact, rather than preventing them in the first place. We have developed a new approach based on specifying intended program behaviors using patterns over sequences of system calls. The patterns can also capture conditions on the values of system-call arguments. At runtime, we intercept the system calls made by processes, compare them against specifications, and disallow (or otherwise modify) those calls that deviate from specifications. Since our approach is capable of modifying a system call before it is delivered to the operating system kernel, it is capable of reacting before any damage-causing system call is executed by a process under attack. We present our specification language and illustrate its use by developing a specification for the ftp server. Observe that in our approach, every system call is intercepted and subject to potentially expensive operations for matching against many patterns that specify normal/abnormal behavior. Thus, minimizing the overheads incurred for pattern-matching is critical for the viability of our approach. We solve this problem by developing a new, low-overhead algorithm for matching runtime behaviors against specifications. A salient feature of our algorithm is that its runtime is almost independent of the number of patterns. In most cases, it uses a constant amount of time per system call intercepted, and uses a constant amount of storage, both independent of either the size or number of patterns. These benefits m...