A significant security problem for networked systems is hostile, or at least unwanted, trespass being unauthorized login or use of a system, by local or remote users; or by software such as a virus, worm, or Trojan horse.
One of the two most publicized threats to security is the intruder (or hacker or cracker), which Anderson identified three classes of:
• Masquerader: An individual who is not authorized to use the computer (outsider)
• Misfeasor: A legitimate user who accesses unauthorized data, programs, or resources (insider)
• Clandestine user: An individual who seizes supervisory control of the system and uses this control to evade auditing and access controls or to suppress audit collection (either)
Intruder attacks range from the benign (simply exploring net to see what is there); to the serious (who attempt to read privileged data, perform unauthorized modifications, or disrupt system)