Intrusion Attacks
Attackers using well-known techniques can penetrate many networks. This often happens when attackers use known vulnerabilities in the network. In updateable systems, administrators may not have or take the time to install all the necessary patches in a large number of hosts. In addition, it is usually not possible to perfectly map an organization's policy on computer use to its access-control mechanisms and thus authorized users often can perform unauthorized actions.
Users may also demand network services and protocols that are known to be flawed and subject to attack. For example, a user might ask, "Why can't I just FTP the files