All posts by Daniel Stenberg

Network, Web

Workshop Basel day three

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See also: day one, day two.

There is only one thing that is better than two days of HTTP workshop, and that is of course three days of HTTP workshop. The final day of this edition of the series started out with us again shuffling around where we parked ourselves around the big table. Except Mr captain of course who once again got to herd us forward through another day from the same seat.

Why MOQ is going to replace HTTP live streaming

MOQ (Media over QUIC transport) is not HTTP, but it uses QUIC so it is at least tangentially interesting and it involves a lot of the same people so this status update still felt welcome and suitable. Compared to existing HTTP based solutions, MOQ is supposed to offer less complexity and lower latency. The moon landing was broadcasted with less latency than current live-streamed TV and maybe MOQ can make us come close to those numbers again. In MOQ clients subscribe to a track that then contains a lot of objects that are delivered. It’s not the request + response approach of HTTP. The fact that this is not HTTP of course brings a lot of questions and well, doubts, and we lingered on various aspects of this topic for quite a while.

Reverse HTTP

My prize for the best slides of the HTTP workshop 2026 goes to [redacted] for the excellent use of potato images in their presentation.

PTTH is HTTP spelled backwards, commonly pronounced as PoTaToH. A client sets up the connection but the actual HTTP request is sent from the server to the client. One of the intended use cases for this, is to allow an origin server to connect to the CDN proxy and then be able to deliver traffic to the world, rather than to have the CDN connect to the origin the way they usually do. Apparently most CDNs already have custom and proprietary solutions for exactly this kind of feature, so maybe doing it in a standard way instead makes sense?

Resumable uploads

The draft explains the new proposed way to continue a previously interrupted upload over HTTP. The upload request gets a Location: header back for the resource being uploaded, and if it gets stopped prematurely, a client can then HEAD that resource, figure out the size and then do a second upload (using the PATCH method) request that tells the server that this transfer should start at offset X.

Exactly how this should be supported in browser’ upload forms seemed a little bit uncertain. For my own sake I can see a challenge to implement this nicely for curl in particular when the upload is using formpost upload (curl’s -F flag) which after all still is a very common way to do uploads on the current web. I’ll return to this topic at a later time when I written an implementation to test…

io_uring vs. multithreaded server runtimes vs HTTP mismatch

io_uring is a Linux asynchronous I/O framework that avoids the overhead of traditional system calls. It uses two shared ring buffers between user space and the kernel, allowing applications to batch I/O operations with zero-copy efficiency.

The feature is disabled by Google in ChromeOS, Android and in production Google servers which certainly holds back some use of it.

io_uring can be helpful to speed up things, but might be complicated to use in existing software architectures and the presentation went into some details on why this is so.

Modern UDP I/O for Firefox in Rust

A walk-through of some of the recent developments and improvements in Firefox’s UDP networking stack. Going from single datagrams to the modern ways to ship large chunks of data offloaded to the kernel to speed things up. Upload throughput in Firefox is up 60-90% over the last 11 releases. Lots of fun graphs and metrics were shown. This work is based on the quinn-udp stack.

Rollout of Happy Eyeballs v3 in Firefox

Happy Eyeballs v3 is coming and Firefox is implementing it. It now takes into account many more data sources than before, including alt-svc and HTTPS-RR and races connections against each other to use the one that connects first. There are some recommended timers in the specification and parts of the discussion was around how maybe the timers could instead be tightened a bit, and maybe the delay between the subsequent attempts could then use an exponential backoff instead sticking to a fixed interval?

(I know I’ll discuss some of these details with my curl hacker friends and see what we should adjust… curl already supports most of the Happy Eyeballs v3 specification.)

Shorter ones

As we approached the end of the day a few shorter topics were ventilated to give us a little more to consider before going home:

  • Why is there no UTF8 in URIs? “If we would do it again, we would have allowed UTF8 in there” was said by someone who was there in the mid 1990s…
  • Optimistic DNS is a draft. Use stale DNS cache data while getting the new. Connection remains alive for 120 seconds while DNS data is often not cached for even 30 seconds. No one in the room seemed to hate it. Let’s do this!
  • The journey to QUERY. One of the primary authors of the RFC took us through what it took to make it happen. It was sixteen years since the most previous registered HTTP method and maybe this was the last one ever?

The end for this time

With this, the seventh HTTP workshop had ended. Again a very fine event. This time graciously sponsored and arranged by Adobe. Thank you everyone!

The general idea is to continue with these events roughly every second year and I support this. The HTTP workshops are definitely one of my favorite events.

Credits

The top image on this post was used in the final presentation and the author told me he is aware of the AI errors in there, “of which there are at least two”.

Network, Web

Workshop Basel day two

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If you missed it. I already described day one.

Caffeinated and ready, we all gathered in the same spacious room as yesterday, but seated in new places as “suggested” by our captain. Some of us even remembered to move over the name tags we wrote yesterday to our new seats.

No time was wasted on introductions today. We dove straight in at the deep end.

How AI is changing how HTTP is implemented.

Is the future of software that we check-in the AI prompts in the git repository and trust it to generate the correct code? Are specifications the new level o

f abstraction for source code? These questions triggered long discussions with a huge mix of opinions and experiences getting shared about how AI is used, should be used and could be used now and in the future. 

Observations and Measurements of HTTP/2 During Large-Scale Web Crawls

The Common Crawl spidering upgraded to using HTTP/2 for their scan and as an end result, I believe 61% of the responses used HTTP/2 and the entire round ended a few percent faster than before, which when you traverse a few billion URLs really makes a difference. They apparently use a locally patched version of Apache Nutch for this.

HTTP/1.1 behavior divergence

The HTTP probe project runs a lot of tests on HTTP/1 servers and compares how they behave in a lot of different aspects and then generates these awesome tables. Looks like something for every server implementer team to have a look at and decide what of these red boxes that should rather be converted into green alternatives.

Request smuggling test suite 

HTTP Zoll is a new test suite for intermediaries that tests intermediaries (what we often call proxies) for a large amount of request and response smuggling issues. Some real world problems found were discussed and as this project aims at going Open Source words were expressed on what kind of precautions and checks that maybe should be done first. I hope we get to hear more about this project soon.

Server performance & measurement

The HTTP Arena is another project that does performance and measurements. They test HTTP server frameworks and present the results in various ways on their site.

Increase and evolve HTTP/3 & QUIC

In this presentation, we were presented with different HTTP/3 deployment numbers from different sources and the associated reasoning around why they differ but then more importantly. what can and should be done to increase HTTP/3 usage. 

Anti-virus interceptions, enterprise blocks and server-side performance not yet on par with TCP were mentioned as reasons for holding back the numbers.

Reasons for using HTTP/3 include use cases that encourage QUIC adoption: WebTransport, Media over QUIC and MASQUE (HTTP/3 proxies and HTTP/3 proxies over older HTTP proxies). 

Using HTTPS-RR for upgrade was promoted, as every alt-svc response that is returned with an ALPN using h3 should perhaps also offer h3 over DNS. Why doesn’t your server announce its h3 support over HTTPS-RR?

QUIC v2 is deployed on an amazing 0.003% of all QUIC v1 domains and there was a discussion why this is so and the common sentiment in the room seemed to be that very few saw a reason for deploying v2 and several expressed a concern that doing so might in fact introduce issues. Someone (you can probably guess who) in the room increased that number a lot by quietly mentioning that haxproxy.org certainly supports it.

QMUX

QUIC multiplexing over bi-directional streams is a proposal on how to do QUIC-style multiplexing over TLS (or anything else really). It has been adopted by the IETF QUIC working group and there was a somewhat extended discussion about what the HTTPbis group should or should not do with it. The biggest interest might be for data center use, but is that then something IETF should bother about? This is not the first time I blog about this, and even if there did not seem to be a strong demand or need for this, it also did not seem to be completely dead. I bet we will hear more about this later.

Multiplexed proxying: challenges in H2 and H3

Doing a TLS terminating MITM proxy has its challenges and we were given some insights and experiences on the challenges of doing HTTP/2 and HTTP/3 to the server.

The browsers refuse to do HTTP/3 when they detect custom CA certs installed, which apparently is mostly because of lots of past bad experiences with anti-virus software that in particular seems to break QUIC and for users it is not obvious where the blame should go. This then makes browsers not do HTTP/3 over any MITM proxy.

Some time was spent on how allowing different clients to the proxy uses a shared h2 connection to the target server is complicated and not used, even though in theory it should be possible. An argument was made that it could even lead to worse performance than when using HTTP/1 but I could not quite follow that reasoning. I’m sure I missed some subtle detail in that explanation.

Making the Web QUICer with Rapid Start

When the afternoon is running late and we have been promised beer and snacks after the final talk, what is better than a hard core technical presentation with lots of graphs and numbers showing how QUIC performance can be improved by tweaking the congestion control algorithm and send more data in the startup phase of a new QUIC connections? This new approach is called Rapid Start and it looks like a promising and yet simple improvement. According to experiments done on real world traffic, the time to last byte was reduced by 14.7% on average. Not bad at all.

Drinks and food

Our meeting sponsor Adobe graciously sponsored drinks and food so we got to linger around for a few extra hours and talk even more HTTP and networking until the personal firmly insistent they needed us to leave the room and we instead continued solving world problems elsewhere. Topics around the table included the famous HTTP/2 spec coin flip, the QUIC spin bit, the SCONE situation for QUIC, the timeline behind the QUERY method and many more great stories.

Thanks for the beer!

Now we can’t wait for day three.

Network, Web

Workshop Basel day one

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On this hot summer’s day in Basel, Switzerland, the seventh HTTP workshop started. These events tend to work roughly the same way and the people in the room are also to large extent familiar and known since previous editions. Forty people in a meeting room, where we take turns in doing short talks on HTTP and networking topics, with the following question and discussion session. The rules for the meetings are explicitly Chatham rules, which means that everything I write about the meeting will be sufficiently fuzzy and without many company or personal names. This is not the kind of meeting that can be easily summed up in a short blog post anyway. You really should be here.

Present in the room were representatives from all the world’s most prominent and used HTTP deployments: clients, browsers, CDNs, proxies and servers. I’m happy to say that there were also several first-timers. We like fresh blood.

(If you think I’m being overly brief or vague about specifics in this post; that is partially on purpose but primarily because I’m a lousy note-taker and mostly write this up after a busy day that also may have involved beer.)

After a round of introductions, we started.

Extending REST for State synchronization

REST is a set of constraints, and in this presentation it was argued that it can or maybe even should be extended to do more. A number of recent applications like Mastodon/ActivityPub, Bluesky/AT, Matrix, Nostr, IndieWeb, all currently use HTTP to do state synchronization but they all do it differently in their own unique ways. Can REST and maybe HTTP be adjusted to help this for improved interoperability?

Last-Modified header use over time

Looking at the Common Crawl data and comparing data over time, it was observed that responses use the Last-Modified header field more now than they did in the past, and there were great follow-up speculations on why this is so. Data also shows that a large share of these headers present dates that are almost identical to the time the requests were issued.

How is HTTP used in the world?

With the cc-lint tool, data was gathered on how HTTP is actually used today, proving that there is work to be done: deprecated headers are used, some headers are done wrong, and many are overly big. This indicates that there are well used both servers and clients out there that would benefit from cleanup. It probably also shows that doing HTTP correctly and all the correct headers is far from an easy task.

AI-bots’ use of HTTP

Another presentation showed data, this time from a well-known CDN, on the impact the existing AI scraper bots have on the Internet from their point of view. It showed that roughly half of the requests and half of the bandwidth are spent by scraper bots. A long discussion followed where the numbers were questioned as maybe the numbers look like this because a sufficiently large number of the “bad AI scrapers” appear as regular users to the classifiers. Speculations of different kinds were made. 

The Apple HTTP stack two years later

As a follow-up from a presentation from a previous HTTP workshop we got to learn how the journey on developing their new HTTP stack has progressed and several fun adventures and lessons from that were shared with the audience.

Why new HTTP APIs?

A look into new HTTP API development at Apple. Some discussions and lessons learned from creating new APIs for both servers and clients.

Android Networking

We got an excellent walk-through of some details and internals of the Android networking stack. Emphasis was perhaps especially put on ECH and QUIC connection migration, and the final “don’t tell us when your connection closed” led to a long new discussion on how we really should fix the problem: when connection has been left idle for a long time and it is closed by the server, the client (mobile phones) don’t want to be told. This, because getting that RST and more, just wakes up the radio and more on the phone only to tell it to go back to sleep. It was theorized that if we could get rid of this unnecessary battery waste, the accumulated gain across billions of devices would make a serious dent.

Day one world problem solving

Several additional HTTP related problems were of course also subsequently solved as we then wandered into the city for dinner and maybe a beer. Of course yours truly returned back to his hotel room in good time to be able to write up this blog post.

The best part of these workshops might be the (no pun intended) networking and discussions had completely outside of the agenda.

End of day one. Two more to come,

cURL and libcurl, Open Source, Security

Do excellent vulnerability reports

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Over the years, we have received, read and handled way over one thousand vulnerability reports filed against curl. We have seen most kinds.

It is time for me to try to help future reporters by providing a short guide on how to submit a truly excellent vulnerability report to an Open Source project.

Researchers

We tend to call everyone who reports a security problem a security researcher, because by the act of the submission itself they fulfill the definition. There are however many different kinds of people who submit reports; from the most rookie youngster with limited experience, to the multi-decade experienced senior in the field.

Most reports submitted to a project like curl come from reporters who never submitted anything to the project before and are completely previously unknown. Many reporters use hacker handles or pseudonyms, so there is not a lot to learn about the person behind the report either. We don’t know the reporters’ age, experience level, employer, sex or on which continent they live. But also: none of those things matter.

When you submit a vulnerability report, consider telling the project how you want to get credited, should they consider your report real.

There is a potentially almost unlimited amount of security researchers that can find problems in a project. The project receiving your report only has a limited small number of overloaded maintainers that take care of the reports. Consider this imbalance. Make your report as easy as possible for the team to manage.

Finding

To us maintainers who receive a steady stream of vulnerability reports, it rarely matters exactly how the problem was detected. Whether you fell over it by accident, you found it by reading every single line of source code or if an AI pointed it out to you, it has little relevance to the security team. The team primarily cares about if the problem is real and if it is, how serious the impact is.

Really?

If the problem is documented, then it likely isn’t a vulnerability. This is a common theme in curl: people report that they can find something strange or peculiar to happen when they do something, only to have one of us point out that the action is either documented to have that side-effect, or the action was done in spite of clear warnings in the documentation.

To make a good vulnerability report, you should make sure you understand what the software is supposed to do – and what the documentation says its limitations and conditions are. A good Open Source project has those things documented.

Where

Figure out where and how to submit your report. If you found several problems, it is considered polite to ask the team how they want to receive the rest. As separate individual submissions or maybe as a curated list. Perhaps paced at a slow rate to avoid overflow.

Never circumvent the submission method suggested by the project. That is impolite.

Consider the initial submitting of the issue to be the first step in a multi-step communication process with the project that will continue for as long as at least one of your reported issues has not been resolved or dismissed. This can be days, weeks or in some cases even months.

Expect responses and follow-up questions. Be prepared to clarify, expand and maybe provide more code and reasoning. Remember that you submit vulnerability reports in order to help and improve the project.

Report

These days people like to create enormously long and detailed reports that have all the details, often explained three times and with several embedded lists using bullet points describing impact and providing more or less good analysis attempts.

Your first paragraph of the report should be a human-written, brief explainer of what the problem is and what badness it leads to. You should be able to explain that in just a few sentences. It is a reality-check, because if you can’t do this, if you don’t understand the flaw enough yourself to write such a paragraph, then you have homework to do. Figure it out, then come back and write the intro paragraph.

Having a quality intro saves a lot of time for the security team receiving your report.

Be aware that the Open Source project you contact may be overloaded, on vacation or seeing your report as yet another duplicate they already saw reported seven times.

Be helpful and respect that you add a load to a small team that probably consists of volunteers working on this in their spare time.

Even if you have used a lot of or just a little AI when finding the issue and writing up the report, you must make sure that you communicate as a human. With your human communication skills.

Reproducer

Your report should contain a reproducer. Ideally a fully contained and stand-alone script or source code that the security team can build and run to see the vulnerability trigger.

A reproducer helps prove to the team that the problem is real or maybe already an accepted risk or behavior. It is also convenient for the developers to first understand and reproduce the issue, and then they can convert the reproducer into a project test case for the pending fix.

Without providing a reproducer in your report, you instead push that work to the receiving end. We still need the reproducer. We still need a test case.

Patch

Provide a patch for the problem.

If you can figure out a way to fix the code to make your finding no longer trigger, that is great information for the security team and such a patch usually helps them understand the issue better and get a speedier result. It reduces the load.

Sure, such a patch is often perhaps not perfect and it can usually be improved and expanded as the developers have a different view and a more nuanced understanding of the problem and the software architecture involved. It still helps. Getting 80% towards the target is still valuable.

Versions

Usually you should look for vulnerabilities in the latest version of the software, often even using an up-to-date git repository. Whatever version you used to find it, you need to specify that in your report.

If the problem turns out to be real, which your report claims and you should never report anything if you don’t think so, it is then also immediately interesting to know when this problem first appeared. Which is the earliest version of the software that you can trigger this problem with?

The project will want to know this to write up a proper advisory for the issue. You can help figuring this out by bisecting etc.

Collaborate

Remain available after your initial submission.

In the curl project at least, we want to work with the reporter to make sure we get every angle and detail right. First, when trying to understand and assess the initial report and agreeing on a severity for it.

Then, we jointly produce and agree to a remedy (patch) for the problem, which ideally means taking the reporter’s version and massaging it into perfection.

If the problem is serious enough, there could be reasons to discuss a rushed patch release at an earlier date than the pending release would otherwise happen on. To reduce the time users in the wild remain vulnerable.

Finally, we collaborate on the description and explainer for the problem that goes into the security advisory.

Advisory

For every CVE that is registered and assigned to a particular vulnerability, there needs to be a detailed security advisory written. It should ideally describe the issue, how it triggers, what it means, the impact, the affected version ranges and more. Everything related to the vulnerability that we can think might help users.

Your job as a security researcher is to make sure the description in the advisory matches your finding, your understanding of the problem and that the description is understandable.

Learn

For every confirmed security report, the receiving project will try to learn from it and fix code and practices to avoid making the same mistake again.

As a reporter, your job is to learn from the submission experience and try to improve your reporting procedure and approach for the next time.

Then submit your next report!

cURL and libcurl

A curl mountain movie

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One of my favorite visuals for known vulnerabilities in curl is the mountain. It shows how many currently known vulnerabilities were present in the code through-out curl’s history.

In the end of June 2026 it looks like this:

Over time we get more vulnerabilities reported. Since every flaw has a version range during which the problem existed and with more issues that have overlapping version ranges, the mountain grows. It changes shape every time we do a release or we publish a new vulnerability.

At this moment in time, curl version 7.34.0 is the release that contains the most number of known vulnerabilities: 101. The worst one ever if you will. Out of a total of 206.

The mountain uses different colors for different severity levels of the published vulnerabilities, as the legend in the top-left of the image explains.

To illustrate the ever-changing nature of the shape and size, I wrote a script that renders the mountain the way it looked at specific dates in the past up until today. More specifically, the script renders one image for every month since curl started (March 1998). I then turned these 340 individual images into a little movie that shows how it grew into today’s shape. At four months/second.

The data for this come from vuln.pm and the curl git repository. The graph rendering is based on the dashboard scripts. All images put into a movie with ffmpeg of course.

The 2016 drop

Several people have asked what happened in 2016 that caused the notable drop. A slope if you will.

If we zoom in on that, we can spot that curl 7.51.0 has eleven fewer vulnerabilities than the version before that. This release was the first one after the 2016 Cure53 code audit, but other than that there is no clear distinct process or obvious code changes that explain this trend shift.

Lots of other graphs show just the ordinary pace and growth in various project areas. It was still fairly early days CI-wise but had been running at least a few CI jobs per commit for a few years already by then.

curl was adopted into the OSS-Fuzz project in July 2017, which since then makes us find some issues better, but the drop looks like it happened before then.

We had already been analyzing the code regularly on Coverity since a few years.

Better tooling? New compiler options? We simply don’t know.

Future

As we keep announcing more vulnerabilities going forward, things will continue to change. Maybe I will come back and make another movie in five years?

cURL and libcurl

Trailing dots are the worst

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Trailing dots after hostnames in URLs remain my worst enemies. I wrote about several problems with them in the past that involved those nasty things. They are still painful. When we shipped curl 8.21.0 on June 24 2026 we fixed at least three brand new problems that involved trailing dots. C’mon, follow me down the trailing dot rabbit hole, episode two. I can just feel that there will be a third episode as well in a future…

IPv4 numerical address

Let’s for a second imagine that you create a URL that uses a numerical IPv4 address. Not entirely uncommon. For example lots of people use 127.0.0.1 in local tests etc. Used everywhere since the dawn of time.

Now imagine that you add a trailing dot to this hostname, like “192.168.0.1.”. What does the trailing dot even mean here?

This particular trailing dot caused a problem in curl. To figure out if curl should allow wildcard certificates when connecting to a TLS server, it needs to know if the given hostname is a numerical IP or a hostname. The check uses inet_pton() on the provided hostname extracted from the URL – which incidentally returns false for an IPv4 address that ends with dot! So if it isn’t a numerical address it is a hostname and then we allow wildcards… Argh.

I decided to solve this particular problem like this: if the address is a valid IPv4 address and there is only a single dot afterwards, that dot is “swallowed” as part of the regular IPv4 normalization process that curl always does for IPv4 addresses when parsing URLs. This way, a numerical IPv4 address with a trailing dot will never be passed on to curl internals anymore. And the meaning of the trailing dot for this use case is clear: it is a mistake so we get rid of it. (This also seems to be what browsers do.) Shipping in curl 8.21.0.

This choice has already been reported problematic by at least one user who expected a transfer for a URL like this to return error… I suppose this means that the jury is still out on what the best approach for this trailing dot is.

Double trailing dots HSTS

What could be more fun than trailing dots if not two trailing dots!

Two trailing dots is not possible to use as a hostname when resolving hostnames using DNS. It is an illegal name and causes an error. But as curl provides other ways to populate the DNS cache with a provided name, and you can provide names in /etc/hosts etc you can make curl work with URLs where the hostname has two trailing dots. Or rather, you could up until recently until I made sure it is properly banned always because of the trouble they cause internally.

A double-dot is correctly treated as a host with a trailing dot, but it turns out that in for example the HSTS logic that became problematic as removing the trailing dot for some functions would still have a trailing dot there when there were two of them to begin with… and it would get confused and act up.

No more double trailing dots. One is annoying enough. Shipping in curl 8.21.0.

Cookie domain

HTTP cookies are basically name/value pairs set by the server and held by the client to get sent back to the server again in later communications. The server can specify for which domain a cookie should apply to, so that it can be used across multiple domains. (Yes, it is a little crazy,)

To prevent the server from being able to set the cookie on a too wide domain cookie clients check if the specified domain is Public Suffic Domain (PSL) or not. A server is not allowed to set cookies for PSL domains, as that allows it to create “super cookies” that work across domains in ways that are not allowed. Cookies attempted to get set for such a name should be rejected.

In libcurl we check domains against the PSL using the libpsl library.

Turns out this too could be tricked by trailing dots. If you communicate with the URL “example.co.uk.” (with a trailing dot) and it sets a cookie for for “co.uk.” (with a trailing dot), the internal check would ask libpsl about the PSL status and… it did not work with trailing dots. The exact same process without trailing dots correctly says it is a PSL and the cookie is refused. But with the trailing dots present it was fooled and curl would allow the cookie to get stored and later sent back to such a host…

This particular issue ended up considered a vulnerability known as CVE-2026-8924. Fix shipped in curl 8.21.0.

We should consider these things

Yes, you can of course quite correctly argue that none of these things are actually new or sudden changes. Trailing dots are there, they have always been there and people will continue to use them in the future. I’m not blaming anyone else. I’m just expressing my frustration.

Trailing dots are the worst.

cURL and libcurl, Security

a CVE dispute

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A few years years ago the curl project signed up and became a CNA. This means that we are masters of and can allocate our own CVE identifiers. For any security problems within our territory, it is we who decides if the issue should get a CVE or not. No more bogus CVEs.

57 CVEs

During these years we have published fifty-seven separate security vulnerabilities with their associated CVE identifiers. Getting a CVE for an issue is easy and really quickly done when you are a CNA. No hassle, no friction and as we are a small and lean security team it just works as smoothly as you could ask. Just an API call and we have new number.

Being a CNA is low maintenance, as there really is nothing extra we need to do. We already had an established and proven process for receiving, managing and assessing vulnerability reports before we became a CNA since we are a responsible and well-run Open Source project. Becoming a CNA just made the process easier as we now don’t need to involve any outsider at all.

Assess

For every report we work hard to first assess and decide if the issue is actually a vulnerability or a security problem at all.

If we deem that there is a security problem in there, we then grade it into LOW, MEDIUM, HIGH or CRITICAL. Since we don’t know how users use curl or libcurl we cannot take that into account but rather observe and set a severity of the problem from a pure curl point of view.

It’s a rough indication how we see the problem but of course every user that actually are affected by the problem might rate it differently.

Lower than LOW

For a rare few issues we can imagine that there could be a minuscule risk but because of the set of extreme requirements and convoluted steps to get there, we deem the risk so small that in practice no user is likely to ever reach it. Internally we tend to call that an issue with a severity level lower than LOW. Issues we believe we serve humanity better by not issuing a CVE for. To avoid the security dance when it seems unnecessary.

The cost of a CVE

libcurl is installed in somewhere around thirty billion instances on the globe. If we imagine that at least a sizeable portion of those installs are managed by people who want to make sure they use a secure version, it means that every CVE we publish trigger activities in many security teams all over the world, leading to a significant number of patches and subsequent software updates.

Every CVE thus has this huge cost tied to it. A cost that does not land on us and we don’t really see or feel it, but a cost on the ecosystem I believe we should not ignore. We should act responsibly. Never ignore real problems of course, but also to make sure we don’t ring the alarm for theoretical problems that will not trigger any vulnerability.

The dispute

Our first ever CVE dispute since we became a CNA reached us on February 10th, 2026 for a report submitted to us two months earlier. The reporter thinks we should have assigned their reported problem a CVE but we think not. Now they want to force the issue to get a CVE anyway, by escalating the situation to MITRE.

Yes, it makes you wonder why it is that important to have this as a CVE, but I will avoid speculations for now.

I replied to MITRE explaining that we considered and debated the issue and we remain happy with our previous decision. I linked them the original report and discussion to show them.

Hostname with a leading dot

The issue is quite technical (of course) but is based on a bug in curl’s function that checks if the used hostname matches a wildcard provided in a certificate.

First: the user must use a hostname in a URL with a leading dot, like https://.example.com/

This name is not possible to use with DNS (it is an illegal name there), but you can provide an IP address for it in your /etc/hosts file or similar, but still this condition is already making this issue really niche.

Why would a user ever do this? Well, there could be a redirect to such a host name from a malicious server if the application allows redirects but getting the address for the host is still a challenge and mostly requires a local attacker present add that.

Then: if curl can find an address for the illegal DNS hostname, the site curl connects to, also needs to have a wildcard certificate for the name *.example.com where the tail of the wildcard needs to match the name in the URL.

If curl was built to use an OpenSSL flavor or Schannel for TLS (remember that curl supports many different TLS backends), it then calls the Curl_cert_hostcheck() function to check if the wildcard covers the used hostname.

This function had a bug. The above mention combination then erroneously would return TRUE. A match. When in reality it is not a match according to the spec.

We fixed this problem on December 8, 2025, and we added unit tests for exactly this scenario to make sure that the problem doesn’t come back. For all security issues at several below HIGH, we fix them asap so that was just our normal procedure. We then continued to discuss if this was worthy of a CVE or not.

Lower than LOW

It should be extremely rare that anyone uses a dot prefixed name, unless you are in an internal and controlled environment where you use something else than DNS for resolving.

It is not possible to trick an application to use a dot prefixed arbitrary name as it will fail to resolve.

The explicitly set, weirdly dot prefixed name, then needs to connect to a host that has a wildcard set for that same name and an attacker manage to run this impostor host and can now serve the application malicious data because curl did not properly reject the connection because of the wildcard mismatch.

A series of highly unlikely conditions that all need to be fulfilled for this to become a vulnerability. A lower than LOW situation. Too unlikely; no CVE.

Again in May

On May 28, we were again contacted by MITRE in the same case, asking again for our rationale for not giving this issue a CVE. We responded with virtually the same wording as before and linking again to the same original Hackerone issue and discussion thread. It’s all public information really.

Again in June

On June 15, we were again contacted by MITRE asking for the reasoning behind our decision to not give a CVE for this issue.

We replied with similar wording again. Linking to the same issue, again.

This seems like a great system.

Verdict

On June 24 we finally got the verdict. It is not considered a security vulnerability.

Hello Yuhao,

Thank you for your participation in the CVE dispute process regarding the reported issue affecting curl through 8.17.0.

The MITRE TL-Root has completed its review of the information provided by all parties involved,
including the materials submitted by you and the response from the responsible CNA. Based on this
review, the MITRE TL-Root has determined that a CVE ID will not be assigned for the reported issue.

CNA Determination (Summary):

"This is a bug, now fixed in the master branch. It is not considered a security vulnerability because of how it requires a local attacker with privileges present to make it so."

After evaluating the available evidence and the CNA’s assessment, the MITRE TL-Root agrees with this determination and considers the matter resolved. As the adjudicating authority in this dispute process, the decision of the MITRE TL-Root represents the final determination for this case.

We appreciate your engagement with the CVE Program and your efforts to responsibly report and coordinate security issues.

Respectfully,

MITRE TL-Root
cURL and libcurl

curl 8.21.0

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Release presentation

Numbers

the 275th release
6 changes
56 days (total: 10,817)
276 bugfixes (total: 14,187)
531 commits (total: 39,077)
0 new public libcurl function (total: 100)
0 new curl_easy_setopt() option (total: 308)
1 new curl command line option (total: 274)
102 contributors, 69 new (total: 3,731)
45 authors, 26 new (total: 1,489)
18 security fixes (total: 206)

Security

As mentioned before, the security report volume has been intense lately. We publish eighteen new curl vulnerabilities this time. A new project record for a single release and for the total number of vulnerabilities published within the same calendar year.

As always, we have document each vulnerability in detail and I encourage you to read up on the details.

Severity Medium

Severity Low

Changes

The huge focus on vulnerability reports during this release cycle made us merge fewer new features than we wanted, but here are the ones we still managed to get to:

  • curl: named globs
  • curl: named globs in output file name for uploads
  • HTTP/3 proxy CONNECT and MASQUE CONNECT-UDP support
  • removed HTTP/2 stream dependency tracking
  • removed support for CURLAUTH_DIGEST_IE
  • added support for SHA256 host public keys with libssh

Bugfixes

We again manage to land more than 250 separate bugfixes, and they are all detailed in the changelog.

Pending removals

Planned upcoming removals include:

  • local crypto implementations
  • NTLM
  • SMB
  • TLS-SRP support

If you are concerned about any of these, speak up on the curl-library list ASAP.

Next release

Unless we messed up this one and need to do a patch release, the pending next release is scheduled to happen on September 2. This release cycle is extended by two weeks due to the summer of bliss.

cURL and libcurl, Web

QUERY with curl

2 Comments

RFC 10008 is brand new a specification detailing the new HTTP method called QUERY:

This specification defines the QUERY method for HTTP. A QUERY requests that the request target process the enclosed content in a safe and idempotent manner and then respond with the result of that processing. This is similar to POST requests but can be automatically repeated or restarted without concern for partial state changes

A GET with body

For all practical purposes you can think of QUERY as a way to send a GET with a body. It looks exactly like POST, but done with another verb.

Contrary to POST, QUERY requests are idempotent – they can be retried or repeated when needed, for instance after a connection failure.

curl it

You can use curl to do HTTP requests with QUERY just fine. curl offers the --request option (also known as -X in the short form) that you can use like this:

curl -d "data to send" -X QUERY https://example.com/

But redirects!

There is one little caveat to remember with this curl option that changes the method. When also asking curl to follow any possible redirects, it is important that you use a new enough curl version because you want the --follow option. Not the old --location/-L one.

Why? Because the old option changes the HTTP method on all subsequent requests independently of what the server responds, which in many cases is not what you want.

The newer --follow option instead acts according to what the HTTP response code suggests in should do. Stick to the same method again, or maybe switch to GET in the following request.

Why?

Why or when would you use this? First of course you only want to use this if the server supports it, but the spec offers some reasons why this might be a good choice:

  • avoid or circumvent URL size limits. Somewhere around 8000 bytes they start to no longer work reliably because servers and intermediaries set limits.
  • expressing certain kinds of data in the URL is inefficient because encoding overhead
  • URLs are more likely to be logged than request content

cURL and libcurl, Security

curl summer of bliss

14 Comments

The curl project will not accept or otherwise handle any vulnerability reports during the month of July 2026. We call it the curl summer of bliss.

curl’s submission form on Hackerone will be paused starting July 1, 2026.

Summer of bliss starts: July 1, 2026. 00:00 CEST

Submissions resume: August 3 2026. 09:00 CEST

The security email address will also be a dead end, as we will not process or otherwise care about security or vulnerability reports sent to us that way either.

Whatever issue you find that you feel a need to report to the curl project during this month has to wait. curl’s Hackerone form opens for submissions again on Monday August 3.

We do not accept vulnerability reports over email in general, and this fact remains during and after our vacation.

Vacation for real

The curl maintainers will use this time of less pressure to take in some extra air and to enjoy the summer. Maybe stroll outside a bit more. Breath. Some of us may spend some of this time to see other places.

We may get some extra time to spend on fixing bugs or working on new code. Fun stuff!

Side-effects

As a direct side-effect of this summer of bliss, to allow us some more time to handle the issues that might have piled up for us in early August, we also push the release date of 8.22.0 two weeks into the future. Now scheduled to happen on September 2, 2026.

Vulnerability rate

As previously mentioned, we have been under a huge pressure for the last four months or so. Now we need some rest. We do not expect this deluge to be over.

GitHub

curl’s issue and pull-request trackers on GitHub remain open and active like normal.

You too?

If you and your Open Source projects also want to participate in the summer of bliss 2026: just do it and let us know! I would of course encourage you to do so. To take care of yourself as a top priority.

The bad guys won’t rest

Probably not. But we will.

But what if there is an emergency

Then we get to read about it in August. Or you get a support contract and we get to read about it earlier.

Contracts excluded

Everyone with a paid support contracts will of course still get full and appropriate service even during this period.

Daniel, in a relaxed state.

Credits

The ice cream image was made by fotografierende from Pixabay

Discussed

On hacker news.