I'm excited about Exploit Prediction Scoring System (EPSS)! Most Information Security and IT professionals will tell you that one of their top pain points is vulnerability management. Keeping systems updated feels like a hamster wheel of work: update after update, yet always behind. It’s simply not possible to update all the systems all the time, so prioritization is needed. Common Vulnerability Scoring System (CVSS) provides a way to rank vulnerabilities, but at least from the risk analyst perspective, something more is needed. EPSS is what we’ve been looking for.

Hi CVSS. It’s not you, it’s me

Introduced in 2007, CVSS was the first mainstream model to tackle the vulnerability ranking problem and provide an open and easy-to-use model that offers a ranking of vulnerabilities. Security, risk, and IT people could then use the scores as a starting point to understand how vulnerabilities compare with each other, and by extension, prioritize system management.

CVSS takes a weighted scorecard approach. It combines base metrics (access vector, attack complexity, and authentication) with impact metrics (confidentiality, integrity, availability). Each factor is weighted and added together, resulting in a combined score of 0 through 10, with 10 being the most critical and needing urgent attention.

So, what’s the problem? Why do we want to break up with CVSS? Put simply, it’s a little bit of you, CVSS – but it’s mostly me (us). CVSS has a few problems: there are better models than a weighted scorecard ranked on an ordinal scale, and exploit complexity has seriously outgrown the base/impact metrics approach. Despite the problems, it’s a model that has served us well over the years. The problem lies with us; the way we use it, the way we've shoehorned CVSS into our security programs way beyond what it was ever intended to be. We’ve abused CVSS.

We use it as a de facto vulnerability risk ranking system. Keep in mind that risk, which is generally defined as an adverse event that negatively affects objectives, is made up of two components: the probability of a bad thing happening, and the impact to your objectives if it does. Now go back up and read what the base and impact metrics are: it’s not risk. Yes, they can be factors that comprise portions of risk, but a CVSS score is not risk on its own.

CVSS was never meant to communicate risk.

The newly released v3.1 adds more metrics on the exploitability of vulnerabilities, which is a step in the right direction. But, what if we were able to forecast future exploitability?

Why I like EPSS

If we want to change the way things are done, we can browbeat people with complaints about CVSS and tell them it’s broken, or we can make it easy for people to use a better model. EPSS does just that. I first heard about EPSS after Blackhat 2019 when Michael Roytman and Jay Jacobs gave a talk and released an accompanying paper describing the problem space and how their model solves many issues facing the field. In the time since, an online EPSS calculator as been released. After reading the paper and using the calculator on several real-world risk analysis, I’ve come to the conclusion that EPSS is easier and much more effective than using CVSS to prioritize remediation efforts based on risk. Some of my main takeaways on EPSS are:

• True forecasting methodology: The EPSS calculation returns a probability of exploit in the next 12 months. This is meaningful, unambiguous – and most importantly – information we can take action on.

• A move away from the weighted scorecard model. Five inputs into a weighted scorecard is not adequate to understand the full scope of harm a vulnerability can (or can’t) cause, considering system and exploit complexities.

• Improved measurement: The creators behind EPSS created a model that inspects the attributes of a current vulnerability and compares it with the attributes of vulnerabilities in the past and whether or not they've been successfully exploited. This is the best indicator we have that will tell us whether not something is likely to be exploitable in the future. This will result in (hopefully) better vulnerability prioritization. This is an evolution from CVSS which measures attributes that may not be correlated to a vulnerability’s chance of exploit.

• Comparisons: When using an ordinal scale, you can only make comparisons between items on that scale. By using probabilities, EPSS allows the analyst to compare anything: a system update, another risk that has been identified outside of CVSS, etc.

In a risk analysis, EPSS significantly improves assessing the probability side of the equation. In some scenarios, a risk analyst can use this input directly, leaving only magnitude to work on. This speeds up the time to perform risk assessments over CVSS. Using CVSS as an input to help determine the probability of successful exploit requires a bit of extra work. For example, I would check to see if a Metasploit package was available, combine with past internal incident data and ask a few SME’s for adjustment. Admittedly crude and time-consuming, but it worked. I don't have to do this anymore.

There’s a caution to this, however. EPSS informs the probability portion only of a risk calculation. You still need to calculate magnitude by cataloging the data types on the system and determine the various ways your company could be impacted if the system was unavailable or the data disclosed.

Determining the probability of a future event is always a struggle, and EPSS significantly reduces the amount of work we have to do. I’m interested in hearing from other people in Information Security – is this significant for you as well? Does this supplement, or even replace, CVSS? If not, why?

Further Reading and Links:

• Information on CVSS

• Exploit Prediction Scoring System | paper

• Exploit Prediction Scoring System (EPSS) | BlackHat 2019 slides

• Webinar: Predictive Vulnerability Scoring System | SIRA webinar series, behind member wall

• EPSS Calculator from Kenna Research