单选题 Most computer systems are 17 to two different groups of attacks: Insider attacks and outsider attacks. A system that is known to be 18 to an outsider attack by preventing 19 from outside can still be vulnerable to the insider attacks accomplished by abusive usage of 20 users. Detecting such abusive usage as well as attacks by outsides not only provides information on damage assessment, but also helps to prevent future attacks. These attacks are usually 21 by tools referred to as Intrusion Detection Systems.