As became apparent after the tragic events of September 11, 2001, terrorist organizations and other criminal groups are increasingly using the legitimate ways of Internet access to conduct their malicious activities. Such actions cannot be detected by existing intrusion detection systems that are generally aimed at protecting computer systems and networks from some kind of "cyber attacks". Preparation of an attack against the human society itself can only be detected through analysis of the content accessed by the users. The proposed study aims at developing an innovative methodology for abnormal activity detection, which uses web content as the audit information provided to the detection system. The new behavior-based detection method learns the normal behavior by applying an unsupervised clustering algorithm to the contents of publicly available web pages viewed by a group of similar users. In this paper, we represent page content by the well-known vector space model. The content models of normal behavior are used in real-time to reveal deviation from normal behavior at a specific location on the net. The detection algorithm sensitivity is controlled by a threshold parameter. The method is evaluated by the tradeoff between the detection rate (TP) and the false positive rate (FP).