单选题
Most computer systems are {{U}} {{U}} 66
{{/U}} {{/U}}to two different groups of attacks: Insider attacks and outsider
attacks. A system that is known to be {{U}} {{U}} 67 {{/U}}
{{/U}}to an outsider attack by preventing {{U}} {{U}} 68 {{/U}}
{{/U}}from outside can still be vulnerable to the insider attacks accomplished by
abusive usage of {{U}} {{U}} 69 {{/U}} {{/U}}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 {{U}} {{U}} 70 {{/U}} {{/U}}by tools referred to as
Intrusion Detection Systems.