All posts by Daniel Stenberg

curl docs format evolution

I trust you have figured out already that I have the highest ambitions for the curl documentation. I want everything documented in a clear, easy-to-read and easy-to-find manner. This takes a lot of work and is not something that happens without effort.

Part of providing best-in-class documentation has in my mind always been to provide and ship man pages. And to make sure that those latest up-to-date man pages are always offered to users as good-looking webpages on the curl site.

Over twenty years ago I decided to render the web versions of the man pages by creating my own tool for that purpose: roffit. We still use it on the site today. It is a simple tool that converts nroff formatted man pages to HTML and adds cross-referencing links to other man pages. There is no concept of links in nroff, but I decided to use a combination of text and markup to use as a signal and it has worked out fine since. In part of course because I have then also subsequently made sure to format the curl man pages in this roffit-friendly way.


We started writing the curl man pages in the nroff format some time in the beginning of 1999. Mostly then because we did not know or have any tool that would convert nicely into nroff and later on to make sure we got things formatted the way we wanted. This approach works and can be used, but nroff is not a terribly human-friendly format and over time we decided that documenting several hundred command line options in a single nroff file was not ideal.

In November 2016 we revamped the system and started to generate the curl.1 man page at build time from multiple source files. Every command line option would then be documented in its own stand-alone file. Easier to read, easier to manage. Yet we controlled the output nroff format so that the web version would still look excellent. The individual files got .d extensions (for document) but were mostly still using nroff, and were mostly all appended to create the result.

Over time, we introduced more convenience formatting aids to the .d files so that we could write them using less and less nroff, and as a result they also grew more readable in their source format.

The .d file format never had its own name. It is just how we documented 250+ command line options for the curl command line tool.

The libcurl docs however remained authored in nroff. In 2014, the number of libcurl man pages exploded when we started doing a separate document for every libcurl option. We went from 59 man pages in the project to 270 between two releases. Since then we have added many more. Today in early 2024, we have a total of 496 individual man pages in the project. 493 of them concerns libcurl.


When curl started, markdown did not exist – that format was created first in 2004. The early documentation we created in the project that was not nroff, we made using plain text files. It was first in 2014 when we slowly started to convert some of our text documents into markdown.

Markdown has several upsides compared to plain old text. In particular it is easier to render good-looking web versions of them, which we do on the curl web site and it offers cross-document linking etc. Over the years we have converted almost all our text documents into markdown. Markdown is also a user-friend format as it is easy to read and easy to edit for contributors, new and old.


The move away from using nroff for the libcurl man pages came with the introduction of the “curldown” document format in early 2024. Meant to make it easy to generate the same kind of nroff files we previously hand-edited, but using “markdown”. I use quotes around it because it is not a full-fledged markdown support, but it looks similar enough to trick casual users and we decided to use “.md” file extensions to make editors, GitHub and more treat and show them as such.

The file format is inspired by the one we created for the command line tool: with a header holding meta-data and then simplified markdown. Easy to read, easy to edit – for everyone. No more strange nroff syntax to copy and paste. The fact that GitHub render the sources files clean and good-looking is also attractive.

Switching to a markdown lookalike format had some other side-effects: suddenly a lot of CI jobs we have running caught these files and had a go at them: spellchecks, prose checks, checks for capitalized words after periods and more. This improved the language significantly.

Also, generating the nroff files rather than editing them has more benefits than just avoiding the baroque syntax: it also allows us to change styles and escape certain sequences through-out all documentation with just a few fixes in the scripts producing the output.

After this 47,000 lines added and removed patch was merged, we have a completely new source format system for the libcurl docs, that generates the same style of nroff output we had previously. With improved language.

But then why not…

With the huge switch to “curldown” for libcurl options, it felt wrong to keep the old curl .d files for the curl.1 man page, seeing as they were almost there already. I did the work and converted them over to the same markdown lookalike style I did for libcurl: they are now also .md files and now they too get spell-checked and scrutinized much more.

I took this move one step further: previously we generated the single curl.1 man page using a fixed header file and a footer file that we prepended and appended to the nroff generated from the (converted) .d documentation.

Now, we instead have a mainpage.idx file that lists which (markdown) file names to include and convert to nroff. This allows us to split the two big headers and footers into several smaller markdown files, each with its own header. Like “DESCRIPTION”, “URL”, and “GLOBBING”. Separating them makes them easier to manage for contributors and the index file makes it easy to change the order of sections etc.

We end up with a curl.1 man page that looks quite similar to before, but with improve language and easier format to edit the documentation going forward.

When I type this, we have 836 files with .md extensions in the curl git repository. At the time of the most recent release, we had 61.


I’m sure it does not stop here. I already have some ideas on how to improve this setup further.

I’m considering doing something to completely avoid the nroff step for the website HTML versions of the man pages and instead go straight from markdown.

The curl tool has the manpage built-in, (shown with the -M option), and I would like to fix the build to do this without using nroff.

I have some more CI jobs to add to verify and check the language in the markdown files. Mostly to make sure the language is consistent.

curl is a CNA

The curl project has been accepted as a CVE Numbering Authority (CNA) for vulnerabilities in all products directly made or managed by the project. If I’m counting correctly, we are the 351st CNA.

The official announcement from Mitre states: curl is now a CVE Numbering Authority (CNA) for all products made and managed by the curl project. This includes curl, libcurl, and trurl.

In plain English, this means that we will reserve and manage our own CVEs in the future directly against the CVE database with no middle man, and also that we have a scope for CVEs that is our territory: curl and libcurl. No one else can now register CVEs for our products – without involving us. (There’s an appeals process so someone can still actually file CVEs for issues even if we say no, but at least there’s a process where both sides will argue their points.)

We do not particularly want to be a CNA but we hope that this move will make it harder to file more stupid curl CVEs in the future.

emails I received, the collection

I have since at least 2009 posted occasional emails I received on this blog. Often they are emails from people who found my email address somewhere, thinking I am involved in the product or service where they found me. In cars, games, traces after breaches, apps and more.

They range from plain weird to fun to scary and threatening.

Somehow they help me remember that the world we live in is super complicated.

The ones I have shown here on the blog are only a small subset of those that I have received. And of all the ones I have received, I have not kept all, or at least not stored them in ways that makes it easy for me to find them now.

Also: for products such as Cisco Anyconnect and Mobdro I have received dozens of emails so I just selected a few examples to include

Out of the ones that I could find, I have dug up seventy-four different messages, converted them to markdown and added them to a brand new GitHub repository: bagder/emails.

I plan to add future email correspondence that meet the criteria as well.


The top image for this blog post is a joke back from when I received the subject: urgent warning email which talked about the “hacking ring”.

PSL in curl

PSL in this context stands for Public Suffix List. It is a list of known public suffixes in domain names. A public suffix is a name under which Internet users can (or historically could) directly register names – other than the top-level domains. One of the most commonly known and used such suffixes is “”, but the complete list is rather long and exhaustive.

Public suffixes are important when working with HTTP cookies, because the only way an HTTP client can accurately deny setting super cookies is to know about every existing PSL domain and deny servers to set cookies for those domains.

The use of PSL is recommend in RFC 6265: “If feasible, user agents SHOULD use an up-to-date public suffix list” (and yes, there mere fact that it speaks of a list and not the list of course highlights how problematic this is)

A super cookie is a cookie that is set for a “too wide” domain, making it apply across multiple sites in ways cookies are not supposed to function. If we allow a super cookie set for “”, that cookie could get sent to a huge amount of different domains.

If you ask me, this is one of the ugliest parts of cookie functionality. And there are a few such parts to select from.

The exact list of names present in the PSL varies over time. New names are added, old names are removed. This of course makes aging clients over time increasingly likely to unwittingly accept cookies for domain names that have since been added to the list. Or block cookies for domains that are no longer in the list!

curl, cookies and PSL

curl has supported cookies since 1998, but it has only supported PSL since 2015 (7.46.0). PSL support in curl is powered by the libpsl library.

Support for this external library is entirely optional in the build and lots of users deliberately decide to build curl without it. Leaving it out reduces footprint, avoids an extra dependency etc. Lots of users who build curl won’t use cookies or do not care about the risk for super cookies.

When you invoke curl -V, to get to see the versions and features of your currently installed curl tool, look for the PSL keyword among the features to learn whether your curl build supports Public Suffix List handling.

cookies are cross-client

Since cookies are sent to clients / browsers typically in a completely client-agnostic way, setting cookies with “illegal” domains make them get discarded and not used. This makes sites tend to not try super cookies for normal scenarios as those cookies will just be rejected by most clients. This fact sometimes save the non-PSL compliant clients.

Of course, servers can also just send a flood of cookies to the client trying different domain attributes – as clients will just silent discard the illegal ones and keep the legal ones and then use them later according to their own best abilities.

Force a decision

Based on the realization that people have been building and shipping curl without PSL support perhaps without fully understanding what they have ducked for, I have just merged a change to curl’s configure script. This change will ship in curl 8.6.0.

The configure script checks for libpsl by default (and has done so for a long time) and will use the library if detected, but starting now it will exit with an error if the detection fails to highlight this fact in a much more noticeable way. The users who want to keep building without it can just add --without-libpsl to hush up the message. Previously, a failed detection only worked as a hint to configure that it should build curl without PSL support – completely fine, but maybe not always done so with the user being totally knowledgeable of what that means.

This change still does not necessarily mean that the person getting the configure error understands PSL in depth, but it forces them to make a decision: Willingly build without it, or fix the build to use it. Your choice!

Not a security issue (in curl)

This (now past) lenient take, to allow users to build curl easily without PSL support, was reported as a curl security problem.

I disagree with that take, as I believe it is the responsibility of everyone building curl to make sure that the sum of the components is what they think it should be – the curl build procedure is not designed for nor attempting to make sure that the final product is hardened security wise (whatever that could be). PSL support is optionally built-in – by design.

This said: users who mistakenly use curl without PSL support might of course end up in what could be seen as a potential security problem. I do however not think that it is a security problem in the curl project itself.

Funding Stefan’s curl work

The curl fund sponsors curl development in Q1 2024 . The curl fund consists entirely and only of money donated to the project by companies and individuals.

Thank you sponsors!

These two funded projects are first out in 2024.

Support for OpenSSL’s QUIC API

Designated lead developer: Stefan Eissing

In OpenSSL‘s recent 3.2 release, they introduced support for client-side QUIC. OpenSSL has taken a different API approach than what just about all other TLS libraries have done (including all the OpenSSL forks): they implement a full QUIC stack.

The result is that curl’s QUIC (and by extension HTTP/3) support that already works with three different backends does not work with OpenSSL, but only with the OpenSSL family of forks (BoringSSL, libressl, AWS-LC and quictls), wolfSSL and GnuTLS. Also, OpenSSL just shipped this, while the other TLS libraries have shipped their QUIC APIs for years already.

This design choice of OpenSSL significantly hampers HTTP/3 adoption with curl (and others) since OpenSSL remains the by far most popular TLS library to use with curl.

To improve this situation going forward, Stefan Eiussing will work on adding support for OpenSSL’s QUIC in curl using nghttp3 on top. Experience from other QUIC libraries says that they usually don’t have all the methods we need on their first attempt, and since this is OpenSSL’s first attempt and nobody from them has asked us for feedback, we expect a high risk of at least minor problems. Finding those issues earlier is better than later, in case we need to work with the OpenSSL team to improve things.

Ideally, no major problems are identified and we have HTTP/3 support with OpenSSL in official curl builds later in the year.

Refactoring the HTTP Handler

Designated lead developer: Stefan Eissing

The second half of Stefan’s assignment is to work on generally improving the state of HTTP and handling the different HTTP versions in the libcurl source code.

We want to make curl’s HTTP handler focus on generic HTTP processing of requests and responses only and separate the serialization on the wire into version specific parts. This puts HTTP 1.x, 2 and 3 implementations on par with each other. Other 1.x implementations can then be configured in (e.g. the Rust based “hyper” module) without needing to replace the core HTTP handling of curl.


Stefan has already started. We expect and hope these two projects to be mostly done and landed by the end of Q1 2024. Stefan has an established presence in the project and has already previously done lots of good development work. We are in good hands with Stefan behind the wheel for this.

I personally will of course assist, help out, review, participate in design decisions and generally cheer on, while continuing to do my regular “chores” and curl work: security, releases, maintenance, customer and user support for example.

My upcoming FOSDEM 2024

I attended FOSDEM for the first time back in 2010. I have since been back and attended every single physical version of the conference since then (remember that it skipped a few years in the COVID days).

FOSDEM is my favorite conference no doubt.

I did a presentation in the embedded dev room in 2010. I have in fact talked in front of audiences almost every year and some years I did it more than once.

Stickers and coasters

If you are interested in getting a curl sticker or two, this is a great opportunity. I will bring a senseless amount of curl stickers in different flavors and sizes so that hopefully everyone who wants one can get one. Find me at FOSDEM to get one. Or find the wolfSSL stand (K building level 1), where I will stock up stickers and also spend time every once in a while.

You can find me at the wolfSSL booth following my talks. Saturday 11:30 – 13:00 and Sunday 11:00 – 13:00.

I will also bring fancy PCB-style coasters. You must have a commit merged in curl’s source repository to be eligible to one of those beauties receive from me.

Feel free to email or DM me on Mastodon or something for syncing.

Broom not included: curling the modern way

That’s the title of my talk in the Network devroom. It is scheduled to take place at 10:50 on Saturday February 3rd. In room UB5.230.

The talk is set to take 20 minutes (including questions). My presentation abstract from when I still naively thought I could get 40 minutes describes the talk like this:

Everyone uses curl, the Swiss army knife of Internet transfers. Earlier this year we celebrated curl’s 25th birthday, and while this tool has provided a solid set of command line options for decades, new ones are added over time This talk is a look at some of the most powerful and interesting additions to curl done in recent years. The perhaps lesser known curl tricks that might enrich your command lines, extend your “tool belt” and make you more productive. Also trurl, the recently created companion tool for URL manipulations you maybe did not realize you want.

I will have to make some tough decisions on what of all this that I can actually include…

You too could have made curl

This talk has been accepted on the main track, but is not yet scheduled.

The talk is 50 minutes. It happens at 10:00 Sunday February 4th in room K1.105 (La Fontaine).

Daniel has taken the curl project to run in some 20 billion installations. He talks about what it takes to succeed with Open Source: patience, time, ups and downs, cooperation, fighting your impostor syndrome – all while having fun. There’s no genius or magic trick behind successful open source. You can do it. The talk will of course be spiced up with anecdotes, experiences and stories from Daniel’s 25 years of leading the curl project.


I proposed a talk titled “HTTP/3 – why and where are we” in the Web Performance devroom but it was not accepted.

I will update this post with more info as such becomes available.

The I in LLM stands for intelligence

I have held back on writing anything about AI or how we (not) use AI for development in the curl factory. Now I can’t hold back anymore. Let me show you the most significant effect of AI on curl as of today – with examples.

Bug Bounty

Having a bug bounty means that we offer real money in rewards to hackers who report security problems. The chance of money attracts a certain amount of “luck seekers”. People who basically just grep for patterns in the source code or maybe at best run some basic security scanners, and then report their findings without any further analysis in the hope that they can get a few bucks in reward money.

We have run the bounty for a few years by now, and the rate of rubbish reports has never been a big problem. Also, the rubbish reports have typically also been very easy and quick to detect and discard. They have rarely caused any real problems or wasted our time much. A little like the most stupid spam emails.

Our bug bounty has resulted in over 70,000 USD paid in rewards so far. We have received 415 vulnerability reports. Out of those, 64 were ultimately confirmed security problems. 77 of the report were informative, meaning they typically were bugs or similar. Making 66% of the reports neither a security issue nor a normal bug.

Better crap is worse

When reports are made to look better and to appear to have a point, it takes a longer time for us to research and eventually discard it. Every security report has to have a human spend time to look at it and assess what it means.

The better the crap, the longer time and the more energy we have to spend on the report until we close it. A crap report does not help the project at all. It instead takes away developer time and energy from something productive. Partly because security work is consider one of the most important areas so it tends to trump almost everything else.

A security report can take away a developer from fixing a really annoying bug. because a security issue is always more important than other bugs. If the report turned out to be crap, we did not improve security and we missed out time on fixing bugs or developing a new feature. Not to mention how it drains you on energy having to deal with rubbish.

AI generated security reports

I realize AI can do a lot of good things. As any general purpose tool it can also be used for the wrong things. I am also sure AIs can be trained and ultimately get used even for finding and reporting security problems in productive ways, but so far we have yet to find good examples of this.

Right now, users seem keen at using the current set of LLMs, throwing some curl code at them and then passing on the output as a security vulnerability report. What makes it a little harder to detect is of course that users copy and paste and include their own language as well. The entire thing is not exactly what the AI said, but the report is nonetheless crap.

Detecting AI crap

Reporters are often not totally fluent in English and sometimes their exact intentions are hard to understand at once and it might take a few back and fourths until things reveal themselves correctly – and that is of course totally fine and acceptable. Language and cultural barriers are real things.

Sometimes reporters use AIs or other tools to help them phrase themselves or translate what they want to say. As an aid to communicate better in a foreign language. I can’t find anything wrong with that. Even reporters who don’t master English can find and report security problems.

So: just the mere existence of a few give-away signs that parts of the text were generated by an AI or a similar tool is not an immediate red flag. It can still contain truths and be a valid issue. This is part of the reason why a well-formed crap report is harder and takes longer to discard.

Exhibit A: code changes are disclosed

In the fall of 2023, I alerted the community about a pending disclosure of CVE-2023-38545. A vulnerability we graded severity high.

The day before that issue was about to be published, a user submitted this report on Hackerone: Curl CVE-2023-38545 vulnerability code changes are disclosed on the internet

That sounds pretty bad and would have been a problem if it actually was true.

The report however reeks of typical AI style hallucinations: it mixes and matches facts and details from old security issues, creating and making up something new that has no connection with reality. The changes had not been disclosed on the Internet. The changes that actually had been disclosed were for previous, older, issues. Like intended.

In this particular report, the user helpfully told us that they used Bard to find this issue. Bard being a Google generative AI thing. It made it easier for us to realize the craziness, close the report and move on. As can be seen in the report log, we did have to not spend much time on researching this.

Exhibit B: Buffer Overflow Vulnerability

A more complicated issue, less obvious, done better but still suffering from hallucinations. Showing how the problem grows worse when the tool is better used and better integrated into the communication.

On the morning of December 28 2023, a user filed this report on Hackerone: Buffer Overflow Vulnerability in WebSocket Handling. It was morning in my time zone anyway.

Again this sounds pretty bad just based on the title. Since our WebSocket code is still experimental, and thus not covered by our bug bounty it helped me to still have a relaxed attitude when I started looking at this report. It was filed by a user I never saw before, but their “reputation” on Hackerone was decent – this was not their first security report.

The report was pretty neatly filed. It included details and was written in proper English. It also contained a proposed fix. It did not stand out as wrong or bad to me. It appeared as if this user had detected something bad and as if the user understood the issue enough to also come up with a solution. As far as security reports go, this looked better than the average first post.

In the report you can see my first template response informing the user their report had been received and that we will investigate the case. When that was posted, I did not yet know how complicated or easy the issue would be.

Nineteen minutes later I had looked at the code, not found any issue, read the code again and then again a third time. Where on earth is the buffer overflow the reporter says exists here? Then I posted the first question asking for clarification on where and how exactly this overflow would happen.

After repeated questions and numerous hallucinations I realized this was not a genuine problem and on the afternoon that same day I closed the issue as not applicable. There was no buffer overflow.

I don’t know for sure that this set of replies from the user was generated by an LLM but it has several signs of it.

Ban these reporters

On Hackerone there is no explicit “ban the reporter from further communication with our project” functionality. I would have used it if it existed. Researchers get their “reputation” lowered then we close an issue as not applicable, but that is a very small nudge when only done once in a single project.

I have requested better support for this from Hackerone. Update: this function exists, I just did not look at the right place for it…


As these kinds of reports will become more common over time, I suspect we might learn how to trigger on generated-by-AI signals better and dismiss reports based on those. That will of course be unfortunate when the AI is used for appropriate tasks, such as translation or just language formulation help.

I am convinced there will pop up tools using AI for this purpose that actually work (better) in the future, at least part of the time, so I cannot and will not say that AI for finding security problems is necessarily always a bad idea.

I do however suspect that if you just add an ever so tiny (intelligent) human check to the mix, the use and outcome of any such tools will become so much better. I suspect that will be true for a long time into the future as well.

I have no doubts that people will keep trying to find shortcuts even in the future. I am sure they will keep trying to earn that quick reward money. Like for the email spammers, the cost of this ends up in the receiving end. The ease of use and wide access to powerful LLMs is just too tempting. I strongly suspect we will get more LLM generated rubbish in our Hackerone inboxes going forward.


Hacker news


Image by Haider Mahmood from Pixabay

The curl activity of 2023

We have the full commit history of all curl source code since late December 1999. The reason we don’t have the history from before that moment is simply because I did not bother with that when I imported our code into Sourceforge back then. Just me being sloppy.

We know exactly when and who authored every change done to curl since then.

Development activity has gone both up and down over time since then. The number of new commit authors has increased slowly over time.


In the record year of 2004, when we still used CVS for version control, we had no CI infrastructure and I wrote 91% of all the commits myself. We made 2102 commits in the curl source tree that year. Back then without CI we did many more follow-up “oops” commits when we had to repair breakages we discovered post commit in our “autobuilds”. The quality of every individual commit was much lower then than today.

Looking through the commits and at the official history logs, there is no spectacular or special events that happened that year. It was just a period of intense development and general improvements of the code. The project was still young. We did seven curl releases in 2004, which is below the all time annual average.

The slowest (in terms of number of commits) recorded year so far was 2000, when I alone committed 100% of the 709 commits. The first year we have saved code history from.

The second most active year has been 2014 with 1745 commits. Also a year that does not really stand out when you look at features or other special things in the project. It was probably pushed that high due to an increased activity by others than me. I was just the second most commit author in curl that year. We did six releases in 2014.

In 2023 we celebrated curl’s official 25th birthday.

Today, we have merged more commits into the curl git during 2023 than we did in 2014 and 2023 is now officially the most active curl year commit-wise since 2004. With only a few days left of 2023, we can also conclude that we cannot reach the amount of 2004.


The commits of course end up in releases. This year, the shipped twelve new versions, which is over the average amount. The increased frequency is due to a modified policy where we are more likely to do follow-up patch releases when we find regressions in a release. In an effort to reduce the amount of known problems existing in the latest release.


Counting these twelve releases of 2023, we released 1195 bugfixes during the year. Big and small. Code and docs. In build procedures and in the products. The rate of bugfixes has increased tremendously over the last decade. These days we average at over 3 bugfixes per day, compared to less than one ten years ago.

As a comparison: in the summary of the curl year 2012, we counted 199 bugfixes.

This is also partly explained by us keeping better records these days.


As I am typing this post, we count 187 commit authors of the year 2023 which is the exact same number we had in 2021 and a few more than last year. Out of all the committers this year, 123 authored a merged commit that for the first time. In 2021 that number was 135 so clearly we had a larger amount of return committers this year even if the total amount ended up the same.

Looking at the most active commit authors, this year broke another trend. In 2023, two authors did 50% of all the commits. As the graph below shows, I did more than 50% of the commits for the last few years. This year, Dan Fandrich made 12.6% of the commits and I did less than half of them; together we did >50%.

The number of authors needed to reach 70% and 95% of the commits show a little more varied story:

Or why not, the number of people who authored ten or more commits within the same calendar year:

My own activities

Out of the 1888 commits done in curl so far this year, I personally have done 847 (44.9%) of them.

According to GitHub, I have never had a more active year before than 2023. This increase seems unsustainable. I should probably then also add that a lot of these contributions were not done in the curl source tree repo. I was also fairly active in the repositories for the curl website, everything curl, trurl and a few others.

This year I approach the end of my fifth year working on curl full time. I maintain that I am living the dream: I work from home with my own open source “invention” with a team of awesome people. I can’t think of a better job.


I stick to my old mantra: I cannot and I will not try to tell and foresee the future. I know that there will be more internet transfers, more protocols, more challenges, more bugs, more APIs, faster computers and quite likely more users, more customers and more security problems. But the details and how exactly all that is going to play out, I have no idea.

I hope we maintain a level of paying curl support customers so that I can keep living this life. I wish we get business to a degree to hire a second full time curl developer. I cross my fingers that companies and organizations will continue to pay up to get features added to curl and to sponsor maintenance, security and development in financial ways.

I have no plans to step off the curl train any time soon.


Image by Thomas Wolter from Pixabay

Making it harder to do wrong

You know I spend all my days working on curl and related matters. I also spend a lot of time thinking on the project; like how we do things and how we should do things.

The security angle of this project is one of the most crucial ones and an area where I spend a lot of time and effort. Dealing with and assessing security reports, handling the verified actual security vulnerabilities and waiving off the imaginary ones.

150 vulnerabilities

The curl project recently announced its 150th published security vulnerability and its associated CVE. 150 security problems through a period of over 25 years in a library that runs in some twenty billion installations? Is that a lot? I don’t know. Of course, the rate of incoming security reports is much higher in modern days than it was decades ago.

Out of the 150 published vulnerabilities, 60 were reported and awarded money through our bug-bounty program. In total, the curl bug-bounty has of today paid 71,400 USD to good hackers and security researchers. The monetary promise is an obvious attraction to researchers. I suppose the fact that curl also over time has grown to run in even more places, on more architectures and in even more systems also increases people’s interest in looking into and scrutinize our code. curl is without doubt one of the world’s most widely installed software components. It requires scrutiny and control. Do we hold up our promises?

curl is a C program running in virtually every internet connect device you can think of.


Another noticeable trend among the reports the last decade is that we are getting way more vulnerabilities reported with severity level low or medium these days, while historically we got more ones rated high or even critical. I think this is partly because of the promise of money but also because of a generally increased and sharpened mindset about security. Things that in the past would get overlooked and considered “just a bug” are nowadays more likely to get classified as security problems. Because we think about the problems wearing our security hats much more now.


Every time we publish a new CVE people will ask about when we will rewrite curl in a memory-safe language. Maybe that is good, it means people are aware and educated on these topics.

I will not rewrite curl. That covers all languages. I will however continue to develop it, also in terms of memory-safety. This is what happens:

  1. We add support for more third party libraries written in memory-safe languages. Like the quiche library for QUIC and HTTP/3 and rustls for TLS.
  2. We are open to optionally supporting a separate library instead of native code, where that separate library could be written in a memory-safe language. Like how we work with hyper.
  3. We keep improving the code base with helper functions and style guides to reduce risks in the C code going forward. The C code is likely to remain with us for a long time forward, no matter how much the above mention areas advance. Because it is the mature choice and for many platforms still the only choice. Rust is cool, but the language, its ecosystem and its users are rookies and newbies for system library level use.

Step 1 and 2 above means that over time, the total amount of executable code in curl gradually can become more and more memory-safe. This development is happening already, just not very fast. Which is also why number 3 is important and is going to play a role for many years to come. We move forward in all of these areas at the same time, but with different speeds.

Why no rewrite

Because I’m not an expert on rust. Someone else would be a much more suitable person to lead such a rewrite. In fact, we could suspect that the entire curl maintainer team would need to be replaced since we are all old C developers maybe not the most suitable to lead and take care of a twin project written in rust. Dedicated long-term maintainer internet transfer library teams do not grow on trees.

Because rewriting is an enormous project that will introduce numerous new problems. It would take years until the new thing would be back at a similar level of rock solid functionality as curl is now.

During the initial years of the port’s “beta period”, the existing C project would continue on and we would have two separate branches to maintain and develop, more than doubling the necessary work. Users would stay on the first version until the second is considered stable, which will take a long time since it cannot become stable until it gets a huge amount of users to use it.

There is quite frankly very little (if any) actual demand for such a rewrite among curl users. The rewrite-it-in-rust mantra is mostly repeated by rust fans and people who think this is an easy answer to fixing the share of security problems that are due to C mistakes. Typically, the kind who has no desire or plans to participate in said venture.

C is unsafe and always will be

The C programming language is not memory-safe. Among the 150 reported curl CVEs, we have determined that 61 of them are “C mistakes”. Problems that most likely would not have happened had we used a memory-safe language. 40.6% of the vulnerabilities in curl reported so far could have been avoided by using another language.

Rust is virtually the only memory-safe language that is starting to become viable. C++ is not memory-safe and most other safe languages are not suitable for system/library level use. Often because how they fail to interface well with existing C/C++ code.

By June 2017 we had already made 51 C mistakes that ended up as vulnerabilities and at that time Rust was not a viable alternative yet. Meaning that for a huge portion of our problems, Rust was too late anyway.

40 is not 70

In lots of online sources people repeat that when writing code with C or C++, the share of security problems due to lack of memory-safety is in the range 60-70% of the flaws. In curl, looking per the date of when we introduced the flaws (not reported date), we were never above 50% C mistakes. Looking at the flaw introduction dates, it shows that this was true already back when the project was young so it’s not because of any recent changes.

If we instead count the share per report-date, the share has fluctuated significantly over time, as then it has depended on when people has found which problems. In 2010, the reported problems caused by C mistakes were at over 60%.

Of course, curl is a single project and not a statistical proof of any sort. It’s just a 25 year-old project written in C with more knowledge of and introspection into these details than most other projects.

Additionally, the share of C mistakes is slightly higher among the issues rated with higher severity levels: 51% (22 of 43) of the issues rated high or critical was due to C mistakes.

Help curl authors do better

We need to make it harder to write bad C code and easier to write correct C code. I do not only speak of helping others, I certainly speak of myself to a high degree. Almost every security problem we ever got reported in curl, I wrote. Including most of the issues caused by C mistakes. This means that I too need help to do right.

I have tried to learn from past mistakes and look for patterns. I believe I may have identified a few areas that are more likely than others to cause problems:

  1. strings without length restrictions, because the length might end up very long in edge cases which risks causing integer overflows which leads to issues
  2. reallocs, in particular without length restrictions and 32 bit integer overflows
  3. memory and string copies, following a previous memory allocation, maybe most troublesome when the boundary checks are not immediately next to the actual copy in the source code
  4. perhaps this is just subset of (3), but strncpy() is by itself complicated because of the padding and its not-always-null-terminating functionality

We try to avoid the above mentioned “problem areas” like this:

  1. We have general maximum length restrictions for strings passed to libcurl’s API, and we have set limits on all internally created dynamic buffers and strings.
  2. We avoid reallocs as far as possible and instead provide helper functions for doing dynamic buffers. In fact, avoiding all sorts of direct memory allocations help.
  3. Many memory copies cannot be avoided, but if we can use a pointer and length instead that is much better. If we can snprintf() a target buffer that is better. If not, try do the copy close to the boundary check.
  4. Avoid strncpy(). In most cases, it is better to just return error on too long input anyway, and then instead do plain strcpy or memcpy with the exact amount. Ideally of course, just using a pointer and the length is sufficient.

These helper functions and reduction of “difficult functions” in the code are not silver bullets. They will not magically make us avoid future vulnerabilities, they should just ideally make it harder to do security mistakes. We still need a lot of reviews, tools and testing to verify the code.

Clean code

Already before we created these helpers we have gradually and slowly over time made the code style and the requirements to follow it, stricter. When the source code looks and feels coherent, consistent, as if written by a single human, it becomes easier to read. Easier to read becomes easier to debug and easier to extend. Harder to make mistakes in.

To help us maintain a consistent code style, we have tool and CI job that runs it, so that obvious style mistakes or conformance problems end up as distinct red lines in the pull request.

Source verification

Together with the strict style requirement, we also of course run many compilers with as many picky compiler flags enabled as possible in CI jobs, we run fuzzers, valgrind, address/memory/undefined behavior sanitizers and we throw static code analyzers on the code – in a never-ending fashion. As soon as one of the tools gives a warning or indicates that something could perhaps be wrong, we fix it.

Of course also to verify the correct functionality of the code.

Data for this post

All data and numbers I speak of in this post are publicly available in the curl git repositories: curl and curl-www. The graphs come from the curl web site dashboard. All graph code is available.

curl 8.5.0

Release presentation


the 253rd release
2 changes
56 days (total: 9,392)

188 bug-fixes (total: 9,734)
266 commits (total: 31,427)
0 new public libcurl function (total: 93)
0 new curl_easy_setopt() option (total: 303)

0 new curl command line option (total: 258)
78 contributors, 43 new (total: 3,039)
40 authors, 19 new (total: 1,219)
2 security fixes (total: 150)


cookie mixed case PSL bypass

(CVE-2023-46218) This flaw allows a malicious HTTP server to set “super cookies” in curl that are then passed back to more origins than what is otherwise allowed or possible. This allows a site to set cookies that then would get sent to different and unrelated sites and domains.

It could do this by exploiting a mixed case flaw in curl’s function that verifies a given cookie domain against the Public Suffix List (PSL). For example a cookie could be set with domain=co.UK when the URL used a lower case hostname, even though is listed as a PSL domain.

HSTS long file name clears contents

(CVE-2023-46219) When saving HSTS data to an excessively long file name, curl could end up removing all contents, making subsequent requests using that file unaware of the HSTS status they should otherwise use.


We have only logged two changes for this cycle.

gnutls supports CURLSSLOPT_NATIVE_CA

If you use libcurl built to use GnuTLS, you too can now set this bit and get to use the Windows CA store directly from within libcurl instead of having to provide a separate PEM file. When your application runs on Windows. libcurl already previously support this for OpenSSL and wolfSSL.

See the CURLOPT_SSL_OPTIONS documentation for details.

HTTP3 with ngtcp2 no longer experimental

The first HTTP/3 code in curl was merged into git in 2019. Now we ship HTTP/3 support non-experimental for the first time. curl supports HTTP/3 using several different backends but this news is only for HTTP/3 built to use ngtcp2 + nghttp3. The other HTTP/3 backends remain experimental for the time being.

Note that in order to be able to use ngtcp2 you need to build with a TLS library that offers the necessary API. This means you need to use one of the following libraries: wolfSSL, quictls, BoringSSL, libressl, AWS-LC or GnuTLS. Let me stress that OpenSSL is not in that list.


As usual, here I have collected a few of my favorite fixes from this cycle.

improved IPFS and IPNS URL support

Turned out there were some IPFS URLs that curl did not manage properly.

doh: use PIPEWAIT when HTTP/2 is attempted

This makes curl prefer doing DoH requests multiplexed over a single connection rather than doing them as two separate connections. Most DoH lookups do one request for an IPv4 response and a separate one for IPv6.

duphandle: several OOM cleanups

If libcurl runs out of memory in the middle of the curl_easy_duphandle function, it could previously do several mistakes, including free-twice.

hostip: show the list of IPs when resolving is done

The verbose mode now shows the full list of IP addresses that was resolved from the name, before it continues to try to connect to them in a serial fashion.

aws-sigv4: canonicalize valueless query params

Turns out there was yet another glitch in the sigv4 logic that made curl send the wrong checksum for URLs using “valueless” query parameters.

hyper: temporarily remove HTTP/2 support

The hyper integration for HTTP/2 was incorrect and has therefore been disabled for now. hyper support in curl is still experimental.

drop vc10, vc11 and vc12 projects from dist, add vc14.20

We generate less project files for older Visual Studio versions but we added one for a recent version. Going forward, we might soon drop them completely from the tarballs since they can now be generated with cmake.

openssl: include SIG and KEM algorithms in verbose

curl now presents more details from the TLS handshake in the verbose output when using OpenSSL.

openssl: make CURLSSLOPT_NATIVE_CA import Windows intermediate CAs

The keyword here is intermediate. Previously, curl would ignore those which would lead to handshake errors because that is not what users expect when you use the native CA store.

openssl: when a session-ID is reused, skip OCSP stapling

When trying to do verify status, the more technical name for OCSP stapling, after a connection has been established using a session-ID, it would return error.

make SOCKS5 use the CURLOPT_IPRESOLVE choice

There are just too many combinations, but we find it likely that a user that asks for a specific IP version probably also wants it for the traffic over the SOCKS proxy.

tool: support bold headers in Windows

The feature that has been provided on other systems since 2018 now comes to Windows.

make the carriage return fit wide progress bars

The progress bar set with -#/--progress-bar had its math off by one, which made it not output the carriage return character when the terminal width was wider than 256 columns, making the output wrong.

url: find scheme with a “perfect hash”

The internal function that scans the list of supported URL schemes no longer iterates through the list but instead uses a “perfect hash”. Although much faster, that was hardly a function that caused performance problems before so it is not likely to actually be measurable.

use ALPN “http/1.1” for HTTP/1.x, including HTTP/1.0

To interoperate better with legacy servers, curl now sends http/1.1 in the ALPN field even when it wants to speak HTTP/1.0. This, because http/1.1 and h2 were the original ALPN codes and some of the old servers that support ALPN from those days don’t know about http/1.0 for ALPN.

all libcurl man pages examples compile cleanly

All (almost five hundred) libcurl man pages have EXAMPLE sections showing their use. Starting now, all of those sections are test-compiled as part of the CI builds to catch mistakes.