Final answer:
Three classes of intruders in computer security are: clandestine user, misfeasor, and masquerader. Each type of intruder represents different methods and levels of access, from unauthorized to abuse of legitimate privileges.
Step-by-step explanation:
The three classes of intruders in the context of computer security are: clandestine user, misfeasor, and masquerader. A clandestine user gains system access without having a user account. A misfeasor is a legitimate user who accesses data, programs, or resources for which such access is not authorized, or who is authorized but misuses their privileges. The masquerader is someone who accesses the system by assuming the identity of a legitimate user.
When attempting to understand and quantify the causes of security breaches, it involves placing actions into one of three categories: accidental, deliberate, or both. The quantification process helps in developing preventative measures against such intrusions.