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Two types of exploitability

Tom Alrich's picture
Supply chain Cyber Risk management - emphasis on SBOMs and VEX documents, Tom Alrich LLC

I provide consulting services in supply chain cybersecurity risk management and am now primarily focused on software bills of materials (SBOMs) and VEX (Vulnerability Exploitability eXchange). I...

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  • Jul 7, 2022


When software bills of materials (SBOMs) started being regularly used for vulnerability management purposes (I don’t believe there’s a historical marker for the day that this happened, but I’d nominate the day when Steve Springett posted the first version of Dependency-Track in 2012. This was based on the concept of a BOM, but this was before the term SBOM was even coined), this entailed changes in how organizations thought about software supply chain risks – if they were even thinking about them at that time (most probably weren’t).

The big change was that there was now a distinction between a vulnerability being present in a software product and its being exploitable in the product; previously, both words would have been considered to have the same meaning. However, once an organization could learn about vulnerabilities applicable to components in their software products (and the first users of Dependency-Track were developers. In fact, the great majority of users today are probably still developers), they discovered that many vulnerabilities that were applicable to a component, when it was considered as a standalone product, were not applicable after the component had been installed in their product; in fact, this was true for the great majority of component vulnerabilities.

The word that was used to describe the difference between the two cases was exploitable. If a vulnerability is exploitable in a product, this means it is not only physically present in the product, but it can be attacked by a hacker who is up to no good. If a vulnerability is physically present in a component but can’t be successfully attacked in the product itself, this means it isn’t exploitable in the product.

Of course, the VEX format was developed for precisely this purpose: for a supplier to notify their customers that, even though a component in one of their products (which is listed in an SBOM) is noted as subject to a particular vulnerability in the National Vulnerability Database (NVD), the product itself isn’t subject to this vulnerability. Therefore, the customer shouldn’t waste time looking for the vulnerability – and BTW, they shouldn’t tie up the supplier’s help lines asking about the vulnerability.

So, one of the supplier’s jobs is to continually search the NVD (and other vulnerability databases) for vulnerabilities in components they have installed in their products. They need to determine whether or not each component vulnerability is exploitable in the product itself, usually because of how it was incorporated into the product.

When the supplier discovers that one of these vulnerabilities isn’t exploitable, they should issue a VEX document stating that fact. The tooling on the user’s end (or at a third-party service provider that performs this service for the user) should maintain a list of exploitable vulnerabilities in each software product and version they utilize, and remove from this list any non-exploitable vulnerabilities. This is important, since probably over 90% of component vulnerabilities aren’t exploitable in the full product. Having an up-to-date list of only exploitable vulnerabilities will save staff members a lot of time wasted in searching for the non-exploitable ones, as well as calling the supplier to ask when they’ll be fixed.

However, I believe this determination of exploitability almost always must be made by the supplier. The supplier, or someone that knows how the product was put together, is the only entity that can reliably state whether or not a vulnerability is exploitable in their product. They wrote all the first-party code and installed all of the components (which nowadays make up about 90% of the code in the average software product). They can make judgments like, “There is no way that an attacker could ever reach this vulnerability to exploit it.” I don’t believe that, in most cases, any other entity can reliably make such a statement, even if they can review the source code[i]. I can attest that the CISA (formerly NTIA) VEX committee has always worked under the assumption that VEX documents will almost always be issued by the supplier of the product.

Yet, this will clearly not always be the case. For example, open source projects are staffed by volunteers who consider themselves coders. Yes, they should be concerned about vulnerabilities that turn out to be exploitable in the product they’ve developed, but I think it’s too much to expect them to spend a lot of time figuring out whether vulnerabilities are not exploitable. I’m not expecting too many open source communities to put out VEX documents of their own accord.

What about commercial products? A commercial supplier should in theory want to put out VEXes regularly, since a user who learns from a VEX that a vulnerability isn’t exploitable in a product they use won’t feel they need to call the supplier’s help desk with that question. In fact, the development of the VEX format was sparked by two very large suppliers who were concerned about this problem; one estimated they’d get literally a couple thousand unnecessary calls every month if they just put out SBOMS, without also putting out VEXes.

However, I’m also sure that a lot of commercial suppliers won’t put out VEXes for their products, even if they do put out SBOMs. Perhaps they won’t understand why VEX is important, or maybe they simply won’t want to invest the time required to learn about the format. This means there will definitely be a need for third parties to develop VEXes for both commercial and open source products.

However, I also believe that the VEXes produced by these third parties will be fundamentally different from those produced by the product suppliers themselves. This is because the third parties, no matter how expert, don’t understand exactly how a product (open source or commercial) was developed.

Does this mean we’re all SOL, when it comes to securing open source products and commercial software products whose suppliers don’t feel like producing VEXes? No, because there are two kinds of exploitability. The first is the absolute kind, which is what the supplier of the product can tell you about in a VEX. Only the supplier can make a categorical up-or-down statement whether or not a vulnerability is exploitable in a product.

But it’s still possible to make statements about exploitability that don’t have the categorical quality of the supplier’s statement. A good example is this excellent blog post by Walter Haydock. It’s aimed mostly at software developers rather than end users, so it might justifiably be considered overkill for the latter, especially given the large numbers of vulnerabilities they might identify when they start receiving SBOMs for most of the software that they operate.

The post lists five recommendations for determining exploitability of a vulnerability in a software product, including the new Exploit Prediction Scoring System (EPSS) scores, which are all about exploitability. These are all good recommendations, but it’s important to realize that now, we’re talking about the second type of exploitability. It doesn’t state whether a vulnerability can be exploited at all, but the likelihood that it will be. While knowing likelihood isn’t as good as a categorical black-or-white statement that a vulnerability isn’t exploitable, it’s certainly better than having no idea about whether a vulnerability is exploitable or not.

However, the difference between the two types of exploitability can be seen best in what they can be used for. Type 1 exploitability is used to determine whether or not the organization should make any effort at all to find and mitigate a particular vulnerability. If it’s exploitable and it’s a serious vulnerability, the organization should probably move heaven and earth (well, earth, anyway) to get it patched, including hounding the supplier day and night to develop a patch. And if a vulnerability isn’t exploitable, the organization doesn’t need to do anything at all.

But Type II exploitability is a question of prioritization of effort. If Vulnerability A is deemed more exploitable than Vulnerability B, then A should take precedence in the effort. As long as the organization goes after the vulnerabilities with the highest exploitability scores first, they don’t need to feel too bad if they don’t have time to mitigate the lower-scored vulnerabilities.

Thus, both types of exploitability are important. It’s always better to know whether a vulnerability is exploitable in the Type I sense or not, but in cases (like open source software) in which the best you’ll be able to get is an estimate of Type II exploitability, then by all means, look at that. And if, for example, an open source product has a serious vulnerability that also appears to be highly exploitable, you should certainly prioritize mitigating that vulnerability over mitigating a vulnerability that is Type 1 exploitable, yet still seems to be less serious (due to CVSS score, etc.).

One question you might have (I know I have it) is whether third parties that make statements about Type II exploitability should do so using VEX documents. In general, I don’t think they should, since the VEX format as of now is based entirely on Type 1 exploitability. The third party would have to assert that the vulnerability is exploitable or not; they wouldn’t be able to state the probability.

However, I’m also not against having a third party assert that a vulnerability is exploitable or not in a product, especially if the supplier hasn’t issued a VEX for this product at all. But the third party should utilize the “authors” field (in the CycloneDX VEX format) to identify themselves; they should also fill in the “email” field under “authors”, so users can get in touch with them if they have questions. Users will need to decide for themselves whether or not to believe the third party’s statement that the vulnerability in question is either completely exploitable or not exploitable at all in the product.

P.S. Speaking of exploitability, I want to raise one important point: Lots of vulnerabilities may not be exploitable in the Type 1 sense, when in fact they could be exploited if an attack started with another vulnerability, then moved on to the original one; this is known as a “chained vulnerabilities” attack. Does the fact that a seemingly non-exploitable vulnerability could in fact be exploitable in a chained attack mean we should call it exploitable?

There’s no right or wrong answer to that question, since it depends on how you define “exploitable”. However, I do know that the VEX concept, as it was developed in the last two years, only applies to vulnerabilities that are directly exploitable – i.e. not as part of a chained attack. The thinking is that, if you allow for chained attacks, then just about every vulnerability becomes exploitable in one way or another. It would be hard to prescribe any one mitigation, given the huge number of ways a chained attack could happen. Given that another vulnerability is the source of the chained attack, that vulnerability should be mitigated on its own.

I’ll admit that some security professionals don’t agree with me on this. They think it’s better to treat any vulnerability as exploitable, even if that only occurs through a chained attack. Do you want to know how I feel about this?....I didn’t think so, but I’ll tell you anyway: When SBOMs are widely available and regularly updated, end users will learn about lots of exploitable vulnerabilities, in software products they use every day; they probably wouldn’t have learned about all of these vulnerabilities otherwise. If they can handle that workload and want to learn about the exploitability of chained vulnerabilities as well, then they should do so.

Also, current users with high assurance use cases (e.g. the military) may feel it’s important to learn about chained vulnerability attacks as well. Go for it! But let’s not require that everybody else do the same.

Any opinions expressed in this blog post are strictly mine and are not necessarily shared by any of the clients of Tom Alrich LLC. If you would like to comment on what you have read here, I would love to hear from you. Please email me at

[i] There are some cases in which a third party could determine that a component vulnerability isn’t exploitable in the product itself, such as when the vulnerable module of a library has not been included with the product binaries.


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