Prioritization Heuristics

Even for the reports a vendor accepts as valid (in scope, credible, and valid), it is likely that the development team does not have time to address every report at the moment it arrives. Thus, if a report is found to be valid, the next question is how to allocate resources to the report. Most often this requires some measure of how significant the vulnerability is compared to other vulnerabilities that have been reported. In some scenarios, the vulnerability may be a critical flaw that requires immediate action, while other cases might indicate a very rare and hard-to-exploit vulnerability that should be given a low priority.

Beware Misuse of Scoring Systems

In our 2021 article Time to Change the CVSS?{:target="blank"}, we argued that many organizations misuse the Common Vulnerability Scoring System (CVSS) by using it as a risk metric, when it was never intended to be used that way. Since that time, CVSS version 4.0 has been released, and while it is a significant improvement over previous versions, it still has limitations. We have generally found that the information contained in the CVSS _vector is more useful than the score itself. This led us to develop the Stakeholder-Specific Vulnerability Categorization (SSVC) as a more flexible alternative.

There are a number of heuristics for evaluating the severity of vulnerabilities. Perhaps the most commonly known of these is the Common Vulnerability Scoring System (CVSS). This system allows a short standard description of the impact of a vulnerability and can be mapped to a score between 1.0 and 10.0 to help prioritization.

The Common Weakness Scoring System (CWSS) is a similar system that was intended to be used to assess the potential impact of a class of weaknesses. Whereas CVSS addresses the detailed impact of a specific vulnerability, CWSS could be used to evaluate the impact of a class of weaknesses.

While scoring systems like CVSS and CWSS can be useful at establishing relative severity among reports, care must be taken in their use since scores do not always map well onto a vendor's or deployer's priorities.

Stakeholder-Specific Vulnerability Categorization (SSVC)

The CERT/CC's Stakeholder-Specific Vulnerability Categorization (SSVC) provides a method for developing vulnerability management decision models that are tailored to the specific needs of different stakeholders.

Heuristic Evaluation

Vendors, Coordinators, and Deployers should ensure their analysts are trained in the chosen heuristic and understand its strengths and weaknesses so that its result can be overridden when necessary.

We do not, for example, recommend blind adherence to hard cutoffs such as "We only bother with reports that have a CVSS score greater than 7.0." No vulnerability scoring system is so precise.

Ideally, whatever prioritization scheme is used should also be made transparent to reporters so that the process is understood by all stakeholders. Transparency in this part of the process can help prevent frustration and confusion when CVD participants disagree on the priority given to a vulnerability.