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Audit trail or log analysis is the art of extracting meaningful information and drawing conclusions about security posture from computer-generated audit records. Log analysis is not a science by a long shot, at least not currently; reliance on individual analysts skills and intuition as well as pure luck play too large a role in this endeavor for log analysis to qualify as a scientific pursuit. This definition of log analysis may sound dry, but the important words are "meaningful conclusions." Simply looking at logs does not constitute analysis, as it rarely yields anything other than an intense sense of boredom and desperation. In the case of a single- user machine with little activity, almost any previously unseen log record is suspicious, but it's not so easy in real life.
Let's consider some general tenets of log analysis. First, even some seemingly straightforward logs (such as an intrusion detection logfile with a successful attack alert) need analysis and correlation with other information sources. Correlation means the manual or automated process of establishing relationships between seemingly unrelated events happening on the network. Events that happen on different machines at different times could have some sort of (often obscure) relationship. Is the target vulnerable to the detected attack? Is this IDS rule a frequent cause of false positives? Is someone on your staff testing a vulnerability scanner on your network? Answers to those and many other similar questions might be needed before activating the response plan upon seeing the IDS alert. Connection attempts, crashed services, and various system failures often require multiple levels of correlation with other information sources in order to extract meaningful data.
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