There is something about having your product installed in over twenty billion instances all over the world and even out of the globe. In my case it helps me remain focused on and committed to working on the security aspects of curl. Ideally, we will never have our heartbleed moment.

Security is also a generally growing concern in the world around us and Open Source security perhaps especially so. This is one reason why NVD making things up is such a big problem.

The National Vulnerability Database (NVD) has a global presence. They host and share information about security vulnerabilities. If you search for a CVE Id using your favorite search engine, it is likely that the first result you get is a link to NVD’s page with information about that specific CVE. They take it upon themselves to educate the world about security issues. A job that certainly is needed but also one that puts a responsibility and requirement on them to be accurate. When they get things wrong they help distributing misinformation. Misinformation makes people potentially draw the wrong conclusions or act in wrong, incomplete or exaggerated ways.

Low or Medium severity issues

There are well-known, recognized and reputable Open Source projects who by policy never issue CVEs for security vulnerabilities they rank severity low or medium. (I will not identify such projects here because it is not the point of this post.)

Such a policy successfully avoids the risk that NVD will greatly inflate their issues since they can already only be high or critical. But is it helping the users and the ecosystem at large?

In the curl project we have a policy which makes us register a CVE for every single reported or self-detected problem that can have a security impact. Either at will or by mistake. This includes a fair amount of low and medium issues. The amount of low and medium issues as a total of all issues increases over time as we keep finding issues, but the really bad ones are less frequently reported.

As we have all data recorded and stored, we can visualize this development over time. Below is a graph showing the curl vulnerability and severity level trends since 2010.

Out of the 145 published curl security vulnerabilities so far, 28% have been rated severity high or critical while 104 of them were set low or medium by the curl security team.

I think this trend is easy to explain. It is because of two separate developments:

1. We as a project have matured and have learned over time how to test better, write code better to minimize risk and we have existed for a while to have a series of truly bad flaws already found (and fixed). We make less serious bugs these days.
2. Since 2010, lots of more people look for security problems and these days we are much better better at identifying problems as security related and we have better tools, while for a few years ago the same problem would just have become “a bug fix”.

Deciding severity

When a security problem is reported to curl, the curl security team and the reporter collaborate. First to make sure we understand the full width of the problem and its security impact. What can happen and what is required for that badness to trigger? Further, we assess what the likeliness that this can be done on purpose or by mistake and how common those situations and required configurations might be. We know curl, we know the code but we also often go back and double-check exactly what the documentation says and promises to better assess what users should be expected to know and do, and what is not expected from them etc. And we re-read the involved code again and again.

curl is currently a little over 160,000 lines of feature packed C code (excluding blank lines). It might not always be straight forward to a casual observer exactly how everything is glued together even if we try to also document internals to help you find deeper knowledge.

I think it is fair to say that it requires a certain amount of experience and time spent with the code to be able to fully understand a curl security issue and what impact it might have. I believe it is difficult or next to impossible for someone without knowledge about how it works to just casually read our security advisories and try to second-guess our assessments and instead make your own.

Yet this is exactly what NVD does. They don’t even ask us for help or for clarifications of anything. They think they can assess the severity of our problems without knowing curl nor fully understanding the reported issues.

A case to prove my point

In March 2023 we published a security advisory for the problem commonly referred to as CVE-2023-27536.

This is potentially a security problem, probably never hurts anyone and is in fact quite unlikely to ever cause a problem. But it might. So after deliberating we accepted it and ranked it severity low.

Bear with me here. I’ll spend two paragraphs revealing some details from the internal libcurl engine:

The problem is of a kind we have had several times in the past: curl has a connection pool and when a user makes a subsequent request which this particular option modified (compared to how it was when the previous connection was setup) it would wrongly reuse the first connection thinking they had the exact same properties.

The second would then accidentally get the wrong rights because it was setup differently. Still, the first connection would need the correct credentials and everything and so would the second one, it would just differ depending on what “GSSAPI delegation” that is allowed.

NVD ranks this

The person or team at NVD whose job it is to make up stuff for security vulnerabilities ranked this as CRITICAL 9.8. Almost as bad as it gets apparently. 10 is the max as you might recall.

When realizing this, at the end of May, I first fell off my chair in shock by this insanity, but after a quick recovery I emailed them (again) and complained (yet again) on setting this severity for *27536. I used the word “ridiculous” in my email to describe their actions. Why and who benefits from them scaremongering the world like this? It makes no sense. On the contrary, this is bad for everyone.

As a reaction to my complaint, someone at NVD went back and agreed to revise the CVSS string they had set and suddenly it was “only” ranked HIGH 7.2. I say “someone” because they never communicate with names and never sign the emails which whomever I talk to. They are just “NVD”.

I objected to their new CVSS string as well. It is just not a high severity security problem!

In my new argument I changed two particular details in the CVSS string (compared to the one they insisted was good) and presented arguments for that. For your pleasure, I include my exact wording below. (Some emphasis is added here for display purposes.)

How I motivated a downgrade

I could possibly live with: AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:N/A:N (4.4) - even if that means Medium and we argue Low.

These are two changes and my motivations:

Attack complexity high - because how this requires that you actually have a working first communication and then do a second is slightly changed and you would expect the second to be different but in reality it accidentally reuse the first connection and therefore gives different/elevated rights.

It is a super-niche and almost impossible attack and there has been no report ever of anyone having suffered from this or even the existence of an application that actually would enable it to happen.

It is more likely to only happen by mistake by an application, but it also seems unlikely to ever be used by an application in a way that would trigger it since having the same user credentials with different values for GSS delegation and assume different access levels seems … weird.

This almost impossible chance of occurring is the primary reason we think this is a Low severity. With CVSS, it seems impossible to reach Low.

Privileges Required high - because the only way you can trigger this flaw is by having full privileges for the *same* user credentials that is later used again but with changed GSS API delegation set. While the previous connection is still live in the connection pool.

It would also only be an attack or a flaw if that second transfer actually assumes to have different access properties, which is probably debatable if users of the API would expect or not

CVSS still sucks

CVSS is a crap system so using this single-dimension number it seems next to impossible to actually get severity low report.

NVD wants “public sources”

NVD does not just take my word for how curl works. I mean, I only wrote a large chunk of it and am probably the single human that knows most about its internals and how it works. I also wrote the patch for this issue, I wrote the connection pool logic and I understand the problem exactly. Nope, just because I say so does not make it true.

My claims above about this issue can of course be verified by reading the publicly available source code and you can run tests to reproduce my claims. Not to mention that the functionality in question is documented.

But no.

They decided to agree to one of my proposed changes, which further downgraded the severity to MEDIUM 5.9. Quite far away from their initial stance. I think it is at least a partial victory.

For the second change to the CVSS string I requested, they demand that I provide more information for them. In their words:

There is no publicly available information about the CVE that clarifies your statement so we must request clarification from you and additionally have this detail added to the HackerOne report or some other public interface for transparency purposes prior to making changes to the CVSS vector.

… which just emphasizes exactly what I have stated already in this post. They set a severity on this without understanding the issue, with no knowledge of the feature that gets this wrong and without clues about what is actually necessary to trigger this flaw in the first place.

For people intimately familiar with curl internals, we actually don’t have to spell out all these facts with excruciating details. We know how the connection pool works, how the reuse of connections should work and what it means when curl gets it wrong. We have also had several other issues in this areas in the past. (It is a tricky area to get right.)

But it does not make this CVE more than a Low severity issue.

Conclusion

This issue is now stuck at this MEDIUM 5.9 at NVD. Much less bad than where they started. Possibly Low or Medium does not make a huge difference out there in the world.

I think it is outrageous that I need to struggle and argue for such a big and renowned organization to do right. I can’t do this for every CVE we have reported because it takes serious time and energy, but at the same time I have zero expectation of them getting this right. I can only assume that they are equally lost and bad when assessing security problems in other projects as well.

A completely broken and worthless system. That people seem to actually use.

It is certainly tempting to join the projects that do not report Low or Medium issues at all. If we would stop doing that, at least NVD would not shout wolf and foolishly claim they are critical.

My response

That is a ridiculous request.

I'm stating *verifiable facts* about the flaw and how curl is vulnerable to it. The publicly available information this is based on is the actual source code which is openly available. You can also verify my claims by running code and checking what happens and then you'd see that my statements match what the code does.

The fact that you assess the severity of this (and other) CVE without understanding the basic facts of how it works and what the vulnerability is, just emphasizes how futile your work is: it does not work. If you do not even bother to figure these things out then of course you cannot set a sensible severity level or CVSS score. Now I understand your failures much better.

We in the curl project's security team already know how curl works, we understand this vulnerability and we set the severity accordingly. We don't need to restate known facts. curl functionality is well documented and its source code has always been open and public.

If you have questions after having read that, feel free to reach out to the curl security team and we can help you. You reach us at security@curl.se

I recommend that you (NVD) always talk to us before you set CVSS scores for curl issues so that we can help guide you through them. I think that could make the world a better place and it would certainly benefit a world of curl users who trust the info you provide.

 / Daniel

Recently I have received curious questions from users, customers and bystanders.

Can you explain the seemingly increased CVE activity in curl over like the last year or so?

(data and stats for this post comes mostly from the curl dashboard)

Frequency

In 2022 we have already had 14 CVEs reported so far, and we will announce the 15th when we release curl 7.85.0 at the end of August. Going into September 2022, there have been a total of 18 reported CVEs in the last 12 months.

During the whole of 2021 we had 13 CVEs reported – and already that was a large amount and the most CVEs in a single year since 2016.

There has clearly been an increased CVE issue rate in curl as of late.

Finding and fixing problems is good

While every reported security problem stings my ego and makes my soul hurt since it was yet another mistake I feel I should have found or not made in the first place, the key take away is still that it is good that they are found and reported so that they can be fixed properly. Ideally, we also learn something from each such report and make it less likely that we ever introduce that (kind of) problem again.

That might be the absolutely hardest task around each CVE. To figure out what went wrong, detect a pattern and then lock it down. It’s almost amusing how all bugs look a like a one-off mistake with nothing to learn from…

Ironically, the only way we know people are looking really hard at curl security is when someone reports security problems. If there was no reports at all, can we really be sure that people are scrutinizing the code the way we want?

Who is counting?

Counting the amount of CVEs and giving that a meaning, or even comparing the number between projects, is futile and a bad idea. The number does not say much and comparing two projects this way is impossible and will not tell you anything. Every project is unique.

Just counting CVEs disregards the fact that they all have severity levels. Are they a dozen “severity low” or are they a handful of critical ones? Even if you take severity into account, they might have gotten entirely different severity for virtually the same error, or vice versa.

Further, some projects attract more scrutinize and investigation because they are considered more worthwhile targets. Or perhaps they just pay researchers more for their findings. Projects that don’t get the same amount of focus will naturally get fewer security problems reported for them, which does not necessarily mean that they have fewer problems.

Incentives

The curl bug-bounty really works as an incentive as we do reward security researchers a sizable amount of money for every confirmed security flaw they report. Recently, we have handed over 2,400 USD for each Medium-severity security problem.

In addition to that, finding and getting credited for finding a flaw in a widespread product such as curl is also seen as an extra “feather in the hat” for a lot of security-minded bug hunters.

Over the last year alone, we have paid about 30,000 USD in bug bounty rewards to security researchers, summing up a total of over 40,000 USD since the program started.

Who’s looking?

We have been fortunate to have received the attention of some very skilled, patient and knowledgeable individuals. To find security problems in modern curl, the best bug hunters both know the ins and outs of the curl project source code itself while at the same time they know the protocols curl speaks to a deep level. That’s how you can find mismatches that shouldn’t be there and that could lead to security problems.

The 15 reports we have received in 2022 so far (including the pending one) have been reported by just four individuals. Two of them did one each, the other two did 87% of the reporting. Harry Sintonen alone reported 60% of them.

Hardly any curl security problems are found with source code analyzers or even fuzzers these days. Those low hanging fruits have already been picked.

We care, we act

Our average response time for security reports sent to the curl project has been less than two hours during 2022, for the 56 reports received so far.

We give each report a thorough investigation and we spend a serious amount of time and effort to really make sure we understand all the angles of the claim, that it really is a security problem, that we produce the best possible fix for it and not the least: that we produce a mighty fine advisory for the issue that explains it to the world with detail and accuracy.

Less than 8% of the submissions we get are eventually confirmed actual security problems.

As a general rule all security problems we confirm, are fixed in the pending next release. The only acceptable exception would be if the report arrives just a day or two from the next release date.

We work hard to make curl more secure and to use more ways of writing secure code and tools to detect mistakes than ever, to minimize the risk for introducing security flaws.

Judge a project on how it acts

Since you cannot judge a project by the number of CVEs that come out of it, what you should instead pay more focus on when you assess the health of a software project is how it acts when security problems are reported.

Most problems are still very old

In the curl project we make a habit of tracing back and figuring out exactly in which release each and every security problem was once introduced. Often the exact commit. (Usually that commit was authored by me, but let’s not linger on that fact now.)

One fun thing this allows us to do, is to see how long time the offending code has been present in releases. The period during which all the eyeballs that presumably glanced over the code missed the fact that there was a security bug in there.

On average, curl security problems have been present an extended period of time before there are found and reported.

On average for all CVEs: 2,867 days

The average time bugs were present for CVEs reported during the last 12 months is a whopping 3,245 days. This is very close to nine years.

How many people read the code in those nine years?

The people who find security bugs do not know nor do they care about the age of a source code line when they dig up the problems. The fact that the bugs are usually old could be an indication that we introduced more security bugs in the past than we do now.

Finding vs introducing

Enough about finding issues for a moment. Let’s talk about introducing security problems. I already mentioned we track down exactly when security flaws were introduced. We know.

With this, we can look at the trend and see if we are improving over time.

The project has existed for almost 25 years, which means that if we introduce problems spread out evenly over time, we would have added 4% of them every year. About 5 CVE problems are introduced per year on average. So, being above or below 5 introduced makes us above or below an average year.

Bugs are probably not introduced as a product of time, but more as a product of number of lines of code or perhaps as a ratio of the commits.

75% of the CVE errors were introduced before March 2014 – and yet the code base “only” had 102,000 lines of code at the end of that period. At that time, we had done 61% of the git commits (17684). One CVE per 189 commits.

15% of the security problems were introduced during the last five years and now we are at 148,000 lines of code. Finding needles in a growing haystack. With more code than ever before, we introduce bugs at a lower-than average rate. One CVE per 352 commits. This probably is also related to the ever-growing number of tests and CI jobs that help us detect more problems before we merge them.

The commit rate has remained between 1,100 and 1,700 commits per year since 2007 with ups and downs but no obvious growing or declining trend.

Harry Sintonen

Harry reported a large amount of the recent curl CVEs as mentioned above, and is credited for a total of 17 reported curl CVEs – no one else is even close to that track record. I figured it is apt to ask him about the curl situation of today, to make sure this is not just me hallucinating things up. What do you think is the reason for the increased number of CVEs in curl in 2022?

Harry replied:

1. The news of good bounties being paid likely has been attracting more researchers to look at curl.
2. I did put considerable effort in doing code reviews. I’m sure some other people put in a lot of effort too.
3. When a certain before unseen type of vulnerability is found, it will attract people to look at the code as a whole for similar or surrounding issues. This quite often results in new, similar CVEs bundling up. This is kind of a clustering effect.

It’s worth mentioning, I think, that even though there has been more CVEs found recently, they have been found (well most of them at least) as part of the bug bounty program and get handled in a controlled manner. While it would be even better to have no vulnerabilities at all, finding and handling them in controlled manner is the 2nd best option.

This might be escaping a random observer who just looks at the recent amount of CVEs and goes “oh this is really bad – something must have gone wrong!”. Some of the issues are rather old and were only found now due to the increased attention.

Conclusion

We introduce CVE problems at a slower rate now than we did in the past even though we have gotten problems reported at a higher than usual frequency recently.

The way I see it, we are good. I suppose the future will tell if I am right.