A threat is an event or activity that has the potential to cause harm to the information systems. A risk is the probability that a threat will materialize. A vulnerability, or weakness, is a lack of a safeguard, which may be exploited by a threat, causing harm to the information systems. Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 1: Access Control Systems (page 32).