Messages or signals should be provided in order to notify users (and system administrators) of potential threats to data security (i.e., of attempted intrusion by unauthorized users).
Additional Information:
For protecting data from unauthorized use, it may not be enough merely to resist intrusion. It may also be helpful if the computer can detect and report any intrusion attempts. In the face of persistent intrusion attempts, it may be desirable to institute countermeasures of some sort, such as changing user passwords or establishing other more stringent user authentication procedures.