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Cisco’s important VEX announcement

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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  • Sep 14, 2023

Warning: The post below starts on an optimistic note but ends on a pessimistic one. Parental discretion is advised.

Yesterday, Cisco made an important announcement, although people who aren’t VEX addicts probably didn’t see it and if they did, it didn’t register as important. I had been looking for the short announcement about VEX that Cisco made a few weeks ago and found it odd that it didn’t appear in searches on Google or the Cisco web site. However, I see now that yesterday’s announcement filled in the details of the previous announcement, which was just a placeholder for the real thing.

The announcement was written by Omar Santos of Cisco, who is also the leader of the OASIS CSAF project. CSAF is the leading general vulnerability reporting format, and the VEX “profile” in CSAF adapts that format to the more limited needs of VEX (mainly, it limits the large number of status designations in the full CSAF format to four: “affected” (i.e., exploitable), “not affected”, “fixed” and “under investigation”).

The announcement is of the “Cisco Vulnerability Repository” (CVR), although I want to point out that name is a mistake. The software world has enough vulnerabilities that we don’t need to have Cisco create a repository to protect them 😊! The best name would be “Cisco VEX Repository” (which doesn’t require a change of acronym, BTW), since that’s what it is. A second-best name would be “Cisco Vulnerability Advisory Repository” or just “Vulnerability Advisory Repository”.

In any case, why do I think this is a significant announcement? For one thing, this is just the second announcement by a major software or device supplier (Cisco is both, of course) that they will be producing VEX documents for customers regularly. The first announcement was by Red Hat in February; however, within the last two months Red Hat changed their mind about VEX (nothing wrong with that, of course!) and renamed the documents they were calling VEX back to their previous name: security advisories.

However, I believe that RH will soon announce VEX documents that are a lot like the ones Cisco just announced – since they are big supporters of CSAF themselves and they have a seat on the CSAF board.

The announcement lists three types of vulnerabilities that will trigger VEX notifications:

  • Vulnerabilities found in the Cybersecurity and Infrastructure Security Agency (CISA)’s Known Exploited Vulnerabilities (KEV) Catalog
  • Vulnerabilities that Cisco has determined to be high-risk
  • Vulnerabilities whose status has been requested using CVR 

This clarifies what Cisco's VEX strategy is (and I think it's a good one): Instead of putting out VEX documents that discuss the status of multiple vulnerabilities in a particular product (which is the strategy that a lot of people have been thinking of), this lists the status (exploitable or not) of one significant vulnerability in one or more products. The example in the announcement utilizes the "product family" capability of CSAF, since it applies to the entire Catalyst 9800 Series family of wireless controllers.

Perhaps the biggest problem with VEX is that user organizations already face a huge backlog of unpatched software vulnerabilities. If they are to learn about new ones, they need to know just the ones that pose a significant risk to them. Meanwhile, software suppliers who have looked at creating VEXes (and very few have even looked at it, I’m sure) have realized that determining whether a particular component vulnerability is exploitable in a particular product/version is hard and time-intensive (unless they want to adapt the “shortcut” I described in my most recent post, which will require them to patch a lot more component vulnerabilities than otherwise. I don’t know whether that’s a good trade-off or not, of course; that will depend on lots of different factors).

Cisco is clearly quite aware of this problem, which is why they have stated up front that they will only consider certain component vulnerabilities for remediation, meaning they won’t even bother to determine whether any others are exploitable or not. I think other suppliers should emulate what Cisco has done here: state very clearly which vulnerabilities they will consider for remediation and which they won’t. Their customers should appreciate this, since they won’t be burdened with a lot of patches that mitigate little risk (of course, the risk to Cisco in this strategy is that a high-risk vulnerability like log4shell will slip through their net and not be patched. But given the level of awareness of vulnerabilities in general nowadays, it’s likely that any such vulnerability will quickly be pointed to by their customers).

Regarding Cisco’s three criteria for considering a vulnerability:

  1. Almost by definition, the KEV catalog is a list of the most risky vulnerabilities
  2. Cisco’s own list of high-risk vulnerabilities is important, since presumably Cisco knows best which risks to its products are high ones; but,
  3. I especially like the third category: "vulnerabilities whose status has been requested using CVR". This gives Cisco’s customers a voice in deciding which vulnerabilities to consider for remediation. Moreover, notice the announcement doesn’t say that customers will only be able to access VEX information for products they own (in fact, it doesn’t sound like Cisco would even have any way to implement that policy, since they’re reporting VEX information by vulnerability, not by product). And because a large percentage of all organizations in the world are customers of at least one Cisco product or service, in theory they will all be able both to make suggestions for vulnerabilities to address and to see all VEX notifications (although I admit I’m guessing this is the case).

So I think this announcement might set a pattern for a lot of other organizations; at least I hope it does.

But there’s a downside to this announcement as well. It can be summarized in three questions:

1. Omar Santos is chairman of the CSAF committee. Pete Allor of Red Hat is one of the most active participants on that committee. Oracle has also started putting out CSAF VEX documents, and they also have a long history with CSAF and its predecessor format CVRF. That’s great.

However, CSAF is an extremely complex format, as a quick skim through the 100-page-plus[i] draft specification will quickly reveal. Even more importantly, figuring out how to fill out the mandatory “product tree” and “branches” fields requires close study of about 15 pages of very dense text within the CSAF format, and even then there are so many options available that a supplier that doesn’t benefit from long experience with CVRF/CSAF is unlikely to devote the time to do that. After all, nobody has a gun to their head, requiring them to produce VEX documents at all, let alone CSAF VEXes. Why go through the aggravation?

Aha, but you don’t need to know all of CSAF to create a VEX in that format. What about the VEX profile? That limits the fields you need to put in the document (which is good), but it omits mention of the need for the “baseline” elements required in every CSAF document, including product tree and branches.

2. Then, what about a tool to create a VEX document in CSAF? There are no VEX-specific tools; in fact, I know of no tools that create any type of CSAF document – meaning they ask the supplier questions about what they want to include (products, vulnerabilities, etc.) and then put them into a CSAF document, requiring no direct CSAF knowledge on the VEX author’s side. The only CSAF tool I know of is Secvisogram. This is a good editor, but it requires knowledge of the CSAF format. Again, it will be quite a hard row for an organization to come up to speed on creating CSAF documents of any type, unless they have somebody that can devote a substantial block of their time to learning the format and creating documents with it – using Secvisogram, faute de mieux.

3. But let’s say a supplier has created a CSAF VEX document? What are their customers going to do with it? Are there low-cost, commercially-supported tools available that will ingest SBOMs and CSAF VEX documents, then provide the user with a continually updated list of exploitable component vulnerabilities in a product/version they use, which has a recent SBOM? No, there aren’t. How about open source tools that do that? Again, no.

So, how about any tools that utilize CSAF documents for security analysis purposes? Here is the official list of CSAF tools. Do you see anything that meets that criterion?[ii] I don’t.

In fact, I’ve asked several large suppliers that produce CSAF documents (whether VEX or just vulnerability advisories – which is the main use case for CSAF, of course) what their users do with them. They honestly don’t know – except that some customers may have developed their own tooling for that purpose.

So, here are the two big problems with CSAF VEX (these each have several parts, and a small number of those parts also apply to the CycloneDX VEX format. However, I know it is many times easier both to create and to utilize CDX VEX documents than CSAF VEX documents):

  1. Any software or device supplier that wants to learn how to create VEX documents in CSAF needs to be prepared to spend a lot of time doing so. I doubt many suppliers will be interested in doing that.
  2. Those suppliers that distribute VEX documents would be much better off developing a PDF format and sending PDF VEXes out by email (or just putting them on a portal).  I know one very large device maker that is doing that with SBOMs: They produce both JSON-based and PDF-based SPDX SBOMs and make them both available on their portal. They report much better uptake for the PDF files.

So – as often happens in this blog – we have run up into one of the “showstopper” problems for SBOMs and VEX: the lack of tooling, especially the lack of low cost, commercially supported tools that are essential for widespread use of both SBOMs and VEX documents by organizations whose primary business isn’t software development (i.e., at least 98% of organizations, public and private, worldwide).

Can this problem be solved? It can be partially mitigated, but substantial mitigation (I won’t even use the word “full”) also requires addressing the other showstopper problems, especially the naming problem. One partial mitigation would be a “playbook” for CSAF VEX, like the “playbooks” that were developed during the NTIA SBOM initiative (here are the two most important of these: for producers and for consumers).

I know that a draft playbook for Cyclone DX VEX has already been developed. However, I don’t know of any current attempt to develop a playbook for CSAF VEX. One is sorely needed.

I told you this post would be depressing, but you didn’t believe me!

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] I’m guessing at the page count, since the spec doesn't have page numbers.

[ii] I believe there are a few vulnerability scanning tools that can ingest CSAF files and use the information in them to remove non-exploitable vulnerabilities from scanning results. This is certainly a useful result, but it has nothing to do with SBOMs or product components.


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