[Dailydave] CVSS is the worst compression algorithm ever

Wim Remes wremes at gmail.com
Tue Jan 8 22:52:49 UTC 2019


Bruce really hits the nail on the head here. CVSS != Risk. To broaden that discussion and not waste too many words, I’ll reference FAIR (Factor Analysis of Information Risk, https://www.fairinstitute.org/what-is-fair <https://www.fairinstitute.org/what-is-fair>) to indicate where “Vulnerability” contributes to an eventual quantitative risk valuation. 

I also always considered CVSS scoring to be qualitative instead of quantitative and the numbers to be ordinal. That makes them fine for ranking vulnerability, but horrible to perform math on (Jet Fuel x Peanut Butter = Shiny — hi Alex Hutton!). 

That said, it all boils down to a point I’ve been rapping on about for a long long time now. Organizations should not expect third party penetration testers to make an accurate assessment of risk. The data provided by a third party penetration tester should feed into your risk management framework, that is also fed with internally acquired business data, to produce (or adjust) a risk valuation. It would be helpful if we, as consultants, wouldn’t pretend that we (a) can come up with any form of credible risk score during such assessments and (b) are delivering scoring that can help with prioritization in a business context without additional effort on the client side. On the other hand, clients that have a risk management framework that can actually take vulnerability scores and use them to generate risk scores should be clear in what they expect from us. If you are asked, whether in an RFP or an SoW, to produce a risk score for your findings at the very least you should be returning a question for asset valuation and threat community descriptions. 


> On 8 Jan 2019, at 18:33, Monroe, Bruce <bruce.monroe at intel.com> wrote:
> Hi Dave,
> I participate on the CVSS SIG being ran out of FIRST that is working on improvements to CVSS. So do a number of people out of CERT CC, NIST, MITRE along with a good representation of industry. A number of us provided feedback on this paper. CVSS is for scoring the severity of a vulnerability. CVSS does not = Risk.
> My understanding is there is a number of government entities that believe CVSS does = Risk and are using it in a vacuum for that purpose. While the CVSS score is a single component - you also must look at how the vulnerable component is deployed, controls in place, value of asset, patching windows, likelihood of exploit,ect…there is a lot that goes into determining risk.
> The fact that various USG entities is using CVSS wrong is an education issue imo. Yes CVSS has it’s issues with some of it’s elements being subjective eye of the beholder type items but that isn’t the reason for this paper…they’ve got USG people using it in a vacuum when it’s only a single element of determining your orgs risk due to a vulnerability. That isn’t a CVSS problem that’s a vulnerability management 101 problem.
> Regards,
> Bruce
> Intel PSIRT
> Opinions expressed are my own and may not reflect those of my employer.
>  <>From: Dailydave <dailydave-bounces at lists.immunityinc.com> On Behalf Of Dave Aitel
> Sent: Tuesday, January 08, 2019 8:14 AM
> To: dailydave at lists.immunityinc.com
> Subject: [Dailydave] CVSS is the worst compression algorithm ever
> I wanted to take a few minutes and do a quick highlight of a paper from CMU-CERT which I think most people have missed out on: https://resources.sei.cmu.edu/asset_files/WhitePaper/2018_019_001_538372.pdf <https://resources.sei.cmu.edu/asset_files/WhitePaper/2018_019_001_538372.pdf>
> Towards Improving CVSS - resources.sei.cmu.edu <https://resources.sei.cmu.edu/asset_files/WhitePaper/2018_019_001_538372.pdf>
> resources.sei.cmu.edu
> SOFTWARE ENGINEERING INSTITUTE | CARNEGIE MELLON UNIVERSITY REV-03.18.2016.0 Distribution Statement A: Approved for Public Release; Distribution Is Unlimited TOWARDS IMPROVING CVSS
> It's almost as funny a read as their previous best work on how "clientless HTTPS VPNs are insanely dumb <https://www.kb.cert.org/vuls/id/261869/> what were you thinking omg?" 
> They use a ton of big words in the paper to call CVSS out and give it a shellacking. Like most of you, we have extensive use of CVSS in our consulting practice and I've seen this stuff first hand. CVSS is of course just a buggy compression algorithm for taking complex qualitative data and then putting it on a number line. The paper has three angles here: 
> Qualitative mappings into quantitative numbers are a silly thing to do, like people trying to do "social science" by using SurveyMonkey.
> We're pretty sure that the compression algorithm is not, in fact, putting higher risk items as bigger numbers, which is the whole point of the thing.  
> Nobody is applying this in any sort of consistent way (which is probably impossible) which is ALSO the whole point of the thing.
> It's fine to have a lossy compression algorithm that emphasizes certain aspects of the input signal over others, of course, but an additional CERT/CC critique is we have no reason to think CVSS does this in any useful way. 
> There's definitely people in the CVSS process (who I will avoid calling out by name) who think ANY quantization is good. But read the paper and decide for yourself - because these are probably serious issues that are turning your entire risk org into a Garbage-In-Garbage-Out org...
> -dave
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