The Evolution of Scoring Security Vulnerabilities: The Sequel

Back in April, I wrote a blog post about the new version of the Common Vulnerability Scoring System (CVSS). The changes made for CVSSv3 addressed some of the challenges that existed in CVSSv2. For example, CVSSv3 analyzes the scope of a vulnerability and identifies the privileges an attacker needs to exploit it. The CVSSv3 enhancements allow vendors to better analyze security vulnerability impact. The changes in CVSSv3 also help our customers more easily determine the urgency with which they need to respond to vulnerabilities

In my previous blog post, I shared the details of a study that analyzed the differences between CVSSv2 and CVSSv3 scores using scores provided by the National Vulnerability Database (NVD). I have continued to monitor the way vulnerabilities are scored using the new version of CVSS because Cisco will soon begin supporting the new version. In my previous study, back in April, I analyzed 745 vulnerabilities. I recently expanded the data set and this new analysis includes a total of 3862 vulnerabilities. I kept the scores and data vendor neutral and used only NVD’s CVSSv2 and CVSSv3 scores.

CVSSv3 Metrics

If you are not familiar with the CVSS metrics, you can read the CVSSv3 specification at FIRST’s website: https://www.first.org/cvss/specification-document. You can also use the CVSSv3 calculator: https:// www.first.org/cvss/calculator/3.0

FIRST has also published several examples of CVSSv2 vs. CVSSv3 scores at: https://www.first.org/cvss/examples

I have included screenshots of the Base, Temporal, and Environmental metrics from FIRST below for your reference.

Figure 1 – CVSSv3 Base Metrics

Figure 2 – CVSSv3 Temporal Metrics

Figure 3 – CVSSv3 Environmental Metrics

Study Results

The total number of vulnerabilities studied was 3862. These were vulnerabilities disclosed from January 1, 2016 thru October 6, 2016 and the source of the data is NVD.

The average base score increased from 6.5 (CVSSv2) to 7.4 (CVSSv3). This is illustrated in Figure 4.

Figure 4 – Average Base Score

Cisco adopted a Security Impact Rating (SIR) in 2015, which uses basically the same scale as the CVSSv3 qualitative severity rating scale. This was done to help organizations properly assess and prioritize their vulnerability management processes.

Figures 5 and 6 include high-level statistics for the qualitative severity differences between CVSSv2 and CVSSv3 scores for the vulnerabilities assessed in this study.

Figure 5 – Qualitative Metrics Change

Figure 6 – CVSSv2 vs. CVSSv3 Qualitative Metrics Distribution

There were several vulnerabilities whose base score decreased from a higher to a lower QM category when scored with CVSSv3. The following table depicts vulnerabilities for which the QM category increased (not just the score) when going from CVSSv2 to CVSSv3.

However, there were far more vulnerabilities whose CVSSv2 base score increased when scored with CVSSv3.

Seventy-four percent (74%) of the vulnerabilities that scored Low in CVSSv2 increased to Medium when scored with CVSSv3.

Figure 7– Low to Medium Change

The following table summarizes the top 3 Common Weaknesses Enumerators (CWEs) of the vulnerabilities that increased from Low to Medium when scored with CVSSv3.

Forty-four percent (44%) of the vulnerabilities that scored Medium in CVSSv2 increased to High when scored with CVSSv3.

Figure 8– Medium to High Change

The following table summarizes the top 3 CWEs of the vulnerabilities that increased from Medium to High when scored with CVSSv3.

Twenty-eight percent (28%) of the vulnerabilities that scored High in CVSSv2 increased to Critical when scored with CVSSv3.

Figure 9 – High to Critical Change

The following table summarizes the top 3 CWEs of the vulnerabilities that increased from High to Critical when scored with CVSSv3.

Why Should I Care?

One thousand seventy-seven (1077) vulnerabilities moved from Low or Medium to High or Critical. That is a 52% increase in High or Critical vulnerabilities.

As stated in our Security Vulnerability Policy in all of our security advisories:

“Cisco will provide an evaluation of the base vulnerability score, and in some instances, will provide a temporal vulnerability score. End users are encouraged to compute the environmental score based on their network parameters. The combination of all three scores should be considered the final score, which represents a moment in time and is tailored to a specific environment. Organizations are advised to use this final score to prioritize responses in their own environments. In addition, Cisco uses the Security Impact Rating (SIR) as a way to categorize vulnerability severity in a simpler manner. The SIR is based on the CVSS base score, adjusted by PSIRT to account for Cisco-specific variables, and will be included in every Cisco Security Advisory.”

Cisco takes a comprehensive approach to security and trust. Transparency and accountability in vulnerability management through Cisco’s Product Security Incident Response Team (PSIRT) is one of our core principles. This is why I want to share these results with you in anticipation of Cisco PSIRT using CVSSv3 in the first half of 2017.