Likelihood of occurrence
According to the National Institute of Standards and Technology (NIST), the likelihood of occurrence is based on the probability that a particular threat is capable of exploiting a particular vulnerability, with possible ratings of Low, Medium, or High.
- High: the potential adversary is highly skilled and motivated and the measures that have been put in place to protect against the vulnerability are insufficient.
- Medium: the potential adversary is motivated and skilled but the measures put in place to protect against the vulnerability may impede their success.
- Low: the potential adversary is unskilled or lacks motivation and there are measures in place to protect against the vulnerability that are partially or completely effective.
The level of impact is determined by evaluating the amount of harm that could occur if the vulnerability in question were exploited or otherwise taken advantage of.
- High: taking advantage of the vulnerability could result in very significant financial losses, serious harm to the mission or reputation of the organization, or even serious injury, including loss of life.
- Medium: taking advantage of the vulnerability could lead to financial losses, harm to the mission or reputation of the organization, or human injury.
- Low: taking advantage of the vulnerability could result in some degree of financial loss or impact to the mission and reputation of the organization.
Once the likelihood of occurrence and impact have been determined, you can then determine the overall risk rating, which is defined as a function of the two ratings. The overall risk can be rated Low, Medium, or High, which provides guidance to those responsible for securing and maintaining the systems in question.
- High: There is a strong requirement for additional measures to be implemented to protect against the vulnerability. In some cases, the system may be allowed to continue operating but a plan must be designed and implemented as soon as possible.
- Medium: There is a requirement for additional measures to be implemented to protect against the vulnerability. A plan to implement the required measures must be done in a timely manner.
- Low: The owner of the system will determine whether to implement additional measures to protect against the vulnerability or they can opt to accept the risk instead and leave the system unchanged.