September 3, 2008
We are now up to the CVSS “Environmental Metrics” group. According to the CVSS documentation, this group ‘captures the characteristics of a vulnerability that are associated with a user’s environment’. This group is also optional from a scoring perspective and is intended to be completed by someone familiar with the environment the vulnerability resides within.
In “Post 1” I mentioned that CVSS does not take into consideration “threat event frequency” or how often I expect to get attacked nor does it take into consideration “loss event frequency”; how often I expect to realize a loss. The “environmental metric” does not fill this void either – but there is still value in being able to quickly analyze vulnerability in the context of these metrics – again, as contributing factors to various FAIR risk taxonomy elements.
FAIR & CVSS
Collateral Damage Potential. This metric measures the potential for loss of life or physical assets through damage or theft of property. Now real quick, I scoffed when I saw the loss of life – and none of the risk issues I have ever dealt with ever involved estimating loss of life. However, there are real life examples of software defects (essentially vulnerabilities) that have loss of human life implications. Take a look at “Geekonomics” by David Rice, there is some fascinating information in the book that will give you a whole new perspective on vulnerabilities. Getting back on track, the collateral damage metric maps very well to the “probable loss magnitude (PLM)” branch of the FAIR taxonomy. I do not want to dive into PLM right now – but let me state this – the word potential is not the same as probable, nor does it imply expected loss. So with the CVSS metric it could be very easy for someone to err on the side of a worst case loss versus choosing a value that best resembles expected loss. Either way, with CVSS this would just result in the CVSS score being raised. I would prefer to see a value in terms of dollars; whether it is monetary ranges or actual expected loss amounts based off simulations.
Target Distribution. This metric measures the proportion of vulnerable systems. I like this metric and I think it can be very useful as a contributing factor to the FAIR taxonomy element “threat event frequency”; specifically “threat contact” and possibly “threat capability”. The number and placement of vulnerable systems in my environment could directly factor into how often or what type if threat agents I expect to come into contact with the vulnerable systems – let alone attack them. Remember, within FAIR – attacking an asset with a vulnerability does not guarantee loss. We have to take into consideration the ability of the attacker to overcome the control resistance applied to the asset.
Security Requirements. These metrics enable the analyst to customize the CVSS score based on the importance of the affected IT asset to a user’s organization in terms of confidentiality (CR), integrity (IR), and availability (AR). Possible values include: LOW, MEDIUM, HIGH, or NOT DEFINED. These metrics were designed to work with the CVSS “Base Metrics” group; specifically the CIA Impact metrics. So if the vendor analyst states that a vulnerability has a Confidentiality Impact, and the analyst for the organization that has the vulnerable asset states that her or his organization has a Confidentiality Requirement – then the CVSS score could increase. Sounds pretty straightforward – seems to map nicely into the PLM branch of the FAIR taxonomy. Specifically, as contributing factors to estimating loss should the vulnerability be exploited and a loss occur.
It is too bad that the CVSS environmental metrics are optional. I understand why they are and regardless of CVSS generating a score and not taking into account loss event frequency – just imagine how much more informed a security folks and decision makers could be if they took a few more minutes to analyze a given vulnerability and the CVSS score that was provided to them from a vendor in light of these metrics.
In the next (and final) CVSS post, I will share some final thoughts on CVSS and finally put a nail in what was not intended to be a series of posts. Thanks for reading!
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Posted by Chris Hayes
September 3, 2008
If you missed the first two posts, the links to Post 1 and Post 2 are on the right hand side of your browser screen.
Let’s get to it…
We are now up to the CVSS “Temporal Metrics”. According to the documentation, these are optional and are meant to be completed by the vendor vulnerability analyst more so then the end user. As to be expected, all three of these can be considered as contributing factors to assessing a vulnerability for risk.
FAIR & CVSS
Exploitability. This metric measures the current state of exploit techniques or code availability (possible values are: Unproven, Proof-of-concept, Functional, High, and Not Defined). So the key words here are “current state”. Thus indicating that state changes for better or for worse over time. So in the world of FAIR, this would seem to map nicely to “threat capability” and “control resistance”. “Threat capability” because the exploit methods may be limited to a very small percentage of the threat community; a fraction of the percentage of the threat population as a whole (the community is a subset of the population). “Control resistance” because the exploit may only be possible if the attacker has local system access. Heck, in some cases, security controls could be entirely absent and the system is no more vulnerable then if there were a tens of thousands of dollars worth of controls (paying dollars to protect pennies).
Remediation Level. According to CVSS, this metric is an important factor for prioritization. Possible values for this metric include: Official fix, Temporary Fix, Workaround, Unavailable, and Not Defined. These choices are more on how the vendor would score it. Seems like there is room for us, the risk assessors to provide our own value. This metric maps well to control resistance in the FAIR taxonomy. Specifically, how resistant are my security controls against the overall threat population? Just because the vendor may not have a solution for us, there could be off-setting controls that do not require a vendor solution for the short term. I appreciate the context this metric was developed within – but do not be fooled into taking logic back into your own hands when it comes to these metrics. We all hear and preach about security in depth – take an opportunity to leverage those investments when analyzing vulnerability from a risk assessment perspective.
Report Confidence. This metric measures the degree of confidence in the existence of the vulnerability and the credibility of the known details. Provided values include: Unconfirmed, Uncorroborated, Confirmed and Not Defined. This may be the one metric that when a vulnerability is first disclosed, only the vendor or other industry experts can accurately select a value for. But within days, weeks or months of public disclosure – pretty much any non-“wolf crying” security expert can do some simple Google searches and validate a provided CVSS metric score or update one on their own. From my perspective, this metric maps well to FAIR’s “threat event frequency” and “threat capability” taxonomy elements. For “threat event frequency”; specifically, “threat contact” and “threat action”. For “threat capability” because the exploit methods may be limited to a very small percentage of the threat community; a fraction of the percentage of the threat population as a whole (the community is a subset of the population). How often do I think a threat agent comes into contact with a vulnerable system and how often do I think they will attempt to exploit the vulnerability once they do come in contact it. So, “report confidence” is a contributing factor to possibly a higher threat contact or threat action – but does not necessarily guarantee that your threat event frequency is going to be higher or that every threat agent in a threat community or threat population is capable of exploiting the vulnerability – especially in light of other security controls that may be present in your environment.
I will do two more posts on the CVSS framework; the “environmental metrics” group and then a summary post. Thanks for reading!
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Posted by Chris Hayes
