In a Risk Matrix, risk levels are determined by the intersection of the likelihood of an event occurring and the impact that event would have if it did occur. When the probability of an attack is very low, it means that the event is unlikely to happen. However, if the impact of that attack is major, it suggests that the event would have significant consequences if it did occur.
The combination of a very low probability with a major impact typically results in a low risk level. This is because the overall risk is mitigated by the low chance of the event happening, despite the potential for a significant impact. Therefore, even though the impact is major, the risk level is kept low due to the very low likelihood of occurrence.
References: The EC-Council's Certified SOC Analyst (CSA) program covers the concepts of risk assessment and the use of Risk Matrices. The CSA study materials and courses provide detailed explanations on how to evaluate and categorize risks based on their probability and impact, aligning with industry-standard practices123.