curl 7.86.0 with WebSocket

Welcome to another curl release. You know the drill…


the 211th release
2 changes
56 days (total: 8,986)

192 bug-fixes (total: 8,337)
314 commits (total: 29,331)
3 new public libcurl function (total: 91)
1 new curl_easy_setopt() option (total: 300)

0 new curl command line option (total: 248)
74 contributors, 43 new (total: 2,733)
42 authors, 17 new (total: 1,082)
4 security fixes (total: 130)
Bug Bounties total: 46,180 USD

Release presentation


This release contains fixes for four separate security vulnerabilities.

CVE-2022-32221: POST following PUT confusion

When doing HTTP(S) transfers, libcurl might erroneously use the read callback (CURLOPT_READFUNCTION) to ask for data to send, even when the CURLOPT_POSTFIELDS option has been set, if the same handle previously was used to issue a PUT request which used that callback.

This flaw may surprise the application and cause it to misbehave and either send off the wrong data or use memory after free or similar in the subsequent
POST request.

The problem exists in the logic for a reused handle when it is changed from a PUT to a POST.

CVE-2022-35260: netrc parser out-of-bounds access

curl can be told to parse a .netrc file for credentials. If that file ends in a line with consecutive non-white space letters and no newline, curl could read past the end of the stack-based buffer, and if the read works, write a zero byte possibly beyond its boundary.

This will in most cases cause a segfault or similar, but circumstances might also cause different outcomes.

If a malicious user can provide a custom .netrc file to an application or otherwise affect its contents, this flaw could be used as denial-of-service.

CVE-2022-42915: HTTP proxy double-free

If curl is told to use an HTTP proxy for a transfer with a non-HTTP(S) URL, it sets up the connection to the remote server by issuing a CONNECT request to the proxy, and then tunnels the rest of protocol through.

An HTTP proxy might refuse this request (HTTP proxies often only allow outgoing connections to specific port numbers, like 443 for HTTPS) and instead return a non-200 response code to the client.

Due to flaws in the error/cleanup handling, this could trigger a double-free in curl if one of the following schemes were used in the URL for the transfer: dict, gopher, gophers, ldap, ldaps, rtmp, rtmps, telnet

CVE-2022-42916: HSTS bypass via IDN

curl’s HSTS check could be bypassed to trick it to keep using HTTP.

Using its HSTS support, curl can be instructed to use HTTPS directly instead of using an insecure clear-text HTTP step even when HTTP is provided in the URL. This mechanism could be bypassed if the host name in the given URL uses IDN characters that get replaced to ASCII counterparts as part of the IDN conversion. Like using the character UTF-8 U+3002 (IDEOGRAPHIC FULL STOP) instead of the common ASCII full stop (U+002E) ..

Like this: http://curl?se?


This time around we add one and we remove one.

NPN support removed

curl no longer supports using NPN for negotiating HTTP/2. The standard way for doing this has been ALPN for a long time and the browsers removed their support for NPN several years ago.

WebSocket API

There is an experimental WebSocket API included in this release. It comes in the form of three new functions and a new setopt option to control behavior. The possibly best introduction to this new API is in everything curl.

I am very interested in feedback on the API.


Here some of the fixed issues from this cycle that I think are especially worthy to highlight.

aws_sigv4 header computation

The sigv4 code got a significant overhaul and should now do much better than before. This is a fairly complicated setup and there are more improvements coming for future releases.

curl man page details multi-use for each option

Every command line option is documented in its own file, which is then used as input when the huge curl.1 man page is generated. Starting now, each such file needs to specify how the option functions when specified more than once. So from now on, this information is mentioned in the man page for all supported options.

deprecate builds with small curl_off_t

Starting in this release, we deprecate support for building curl for systems without 64 bit data types. Those systems are extremely rare this days and we believe it makes sense to finally simplify a few internals when it hurts virtually no one. This is still only deprecated so users can still build on such systems for a short while longer if they really want to.

the ngtcp2 configure option defaults to ‘no’

You need to explicitly ask for ngtcp2 to be enabled in the build.

reject cookie names or content with TAB characters

Cookies with tabs in names or content are not interoperable and they caused issues when curl saved them to disk, so we decided to rather reject them.

for builds with gcc + want warnings, set gnu89 standard

Just to make better sure we maintain compatibility.

use -O2 as default optimize for clang in configure

It was just a mistake that it did not already do this.

warn for –ssl use, considered insecure

To better highlight for users that this option merely suggests for curl that it should use TLS for the protocol, while --ssl-reqd is the one that requires TLS.

ctype functions converted to macros-only

We replaced the entire function family with macros.

100+ documentation spellfixes

After a massive effort and new CI jobs, we now regularly run a spellcheck on most man pages and as a result we fixed lots of typos and we should now be able to better maintain properly spelled documentation going forward.

make nghttp2 less picky about field whitespace in HTTP/2

If built with a new enough nghttp2 library, curl will now ask it to be less picky about trailing white space after header fields. The protocol spec says they should cause failure, but they are simply too prevalent in live servers responses for this to be a sensible behavior by curl.

use the URL-decoded user name for .netrc parsing

This regression made curl not URL decode the user name provided in a URL properly when it later used a .netrc file to find the corresponding password.

make certinfo available for QUIC

The CURLOPT_CERTINFO option now works for QUIC/HTTP/3 transfers as well.

make forced IPv4 transfers only use A queries

When asking curl to use IPv4-only for transfers, curl now only resolves IPv4 names. Out in the wide world there is a significant share of systems causing problems when asking for AAAA addresses so having this option to avoid them is needed.

schannel: when importing PFX, disable key persistence

Some operations when using the Schannel backend caused leftover files on disk afterward. It really makes you wonder who ever thought designing such a thing was a good idea, but now curl no longer triggers this effect.

add and use Curl_timestrcmp

curl now uses this new constant-time function when comparing secrets in the library in an attempt to make it even less likely for an outsider to be able to use timing as a feedback as to how closely a guessed user name or password match the secret ones.

curl: prevent over-queuing in parallel mode

The command line tool would too eagerly create and queue up pending transfers in parallel mode, making a command line with millions of transfers easily use ridiculous amounts of memory.

url parser: extract scheme better when not guessing

A URL has a scheme and we can use that fact to detect it better and more reliable when not using the scheme-guessing mode.

fix parsing URL without slash with CURLU_URLENCODE

When the URL encode option is set when parsing a URL, the parser would not always properly handle URLs with queries that also lacked a slash in the path. Like

url parser: leaner with fewer allocs

The URL parser is now a few percent faster and makes significantly fewer memory allocations and it uses less memory in total.

url parser: reject more bad characters from the host name field

Another step on the journey of making the parser stricter.

wolfSSL: fix session management bug

The session-id cache handling could trigger a crash due to a missing reference counter.


We have several pull-requests in the pipe that will add changes to trigger a minor number bump.


We are planning to remove the following features in a future-:

  • support for systems without 64 bit data type
  • support for the NSS TLS library

If you depend on one of those features, yell at us on the mailing list!

Funded curl improvements

I am happy to announce that curl receives funding from The Sovereign Tech Fund. This funding is directed towards three specific projects that we have identified as interesting and worthwhile to push forward as ways to improve curl and the life of curl users.

This “investment” will fund two developers to work on curl over a period of six months: Stefan Eissing and myself. The three projects are explained at some detail below. Of course everyone and anyone is welcome to join in and help out with these projects. Everything will be done in the open, as usual.

At the end of the period, we will produce some kind of report or summary of how things turned out.

The three projects we are getting funded have been especially created and crafted (by me) to be good solid projects that we really want to see done. This funding is different than many others we have gotten over the years in that we got to decide and plan what we wanted done. These are things that are meant to improve curl as a project and to generally make Internet transfers better and more powerful for a vast amount of users.

Project 1 – known bugs cleanup

The curl project currently lists 120+ items as known bugs (up from 77 just two years ago).

The items in that list are reported problems that were recognized as problems at the time of their submission but as nobody worked on the issues at the time they were added to this list. The list includes everything from smaller irks up to big things that will either take a long time  to fix or be (almost) impossible to address.

There is a good chance that the list will be extended during the project period just because some new bugs fall into the description mentioned above.

This project is an effort to go through as many as possible to make sure they are correctly categorized/described and work on fixing the issues or whatever is necessary to get them off the list.

The process would entail an initial proper research round to extend the description and increase the understanding of each entry, followed by a rough assessment of the amount of work it would take to fix them. Possibly with a 1 (easy) to 5 (extreme) scale.

The action would then be to address the issues, possibly in an easy to hard order. Addressing the issues could be to fix the code to remove the issue, dismiss it as not actually intended to work or document it as not working or even moving it over to the TODO document if it is more of a good idea for the future.

The goal being to reduce the list to zero entries and thus polish off numerous rough corners and annoyances in the project.

This will be done by Daniel Stenberg over a period of 6 months.

Project 2 – HTTP/3

Make HTTP/3 release-ready.

curl features experimental HTTP/3 and QUIC support already since a few years back, but there are several details still lacking:

  • known bugs
  • proper multiplexing: doing multiple transfers to the same host should be able to reuse an existing connection and multiplex over that, just as curl already does when using HTTP/2
  • HTTP/3 support for the test suite (and CI jobs) need to be done for us to be able to consider the support release ready. Cooperation can be had with QUIC libraries such as ngtcp2 to consider where/how some of the testing is best performed.
  • considerations for 0-RTT connection establishments (if anything needs to be done)
  • support for early data: to send off the HTTP request to the server faster.
  • connection migration:, a QUIC feature that allows a server to move over a live connection from one server to another without disruption
  • fallback to h1/h2 if the QUIC connection fails. The failure rate for QUIC connections are still in the 3-7% rate generally, so having a good fallback mechanism or documented for how applications can go back to an older HTTP version instead, is important.
  • HTTPS RR. This fairly new DNS resource record might contain information about the target server’s support for HTTP/3. If such a record is provided, curl can avoid superfluous round-trips to get the Alt-Svc header and rather connect directly to the HTTP/3 server.

All features and changes need to be documented. Functionality needs to be verified by test cases. Interop with real world servers is of course implied and assumed.

Stefan Eissing will spend 4 months on this project.

Project 3 – HTTP/2 over proxy

curl has provided support for doing network transfers via HTTP proxies since decades, and this is a very commonly used feature and network setup.

curl however only supports using HTTP version 1 over proxies. This makes applications less effective as it sometimes leads to many more TCP connections being used than otherwise would be necessary if HTTP/2 could be used. In particular when applications behind a proxy operate against many different hosts on the other side of the proxy.

It can also be noted that in many enterprise setups, this kind of HTTP proxy is used for all kinds of network operations through the use of the CONNECT method, so this functionality is not limited to plain HTTP(S), it should work for all TCP based protocols libcurl supports and which already work over HTTP/1 proxies.

The project will require adding new options to enable this functionality, to both the library and the command line tool. With associated documentation.

It will require creating or extending the HTTP/2 support in the curl test suite so that this new functionality can be verified and proven to work at a satisfactory level.

Considerations must be taken so that this work does not close the door for future extending this to support HTTP/3 over proxy. Time permitting, work should be taken to pave the road for that or even perhaps gently start the work to support that as well.

Stefan Eissing will spend 2 months on this project.

The outcome(s)

I hope and presume that the results of these projects will appear as a stream of pull-requests for curl that will be done and managed through-out the project period and not saved up to the end or anything. The review, test and merge process of these pull requests will follow our normal and standard project guidelines and procedures.

The projects are fully packed and (over) ambitious. There is a high risk that we will not be able to complete all the details for these projects within the time frame. But we will try.

Thanks Sovereign Tech Fund for this.

Deviating from specs

tldr: we do not particular keep track nor document curl’s exact spec compliance. I cannot fathom how we could.

Today, in October 2022, curl and libcurl combined consist of nearly 150,000 lines of source code (not counting blank lines). 19% of those are comments.

This source code pile was carefully crafted with the sole purpose of performing Internet transfers using one or more of the 28 separate supported protocols. (There are 28 different supported URL schemes, it can be discussed if they are also 28 protocols or not.)

Which specs does curl use

It was recently proposed to me that we should document which RFCs curl adheres to and follows, and what deviants there are. In the name of helping the users understand what to expect from curl and educating the world how curl will behave.

This is indeed a noble idea and a worthy goal. We do not want to surprise users. We want them to know.

It was suggested that it might have a security impact if curl would deviate from a spec and if this is not documented clearly, users could be mislead.

What specs

curl speaks TCP/IP (and UDP or QUIC at times), it does DNS and DNS-over-HTTPS, it speaks over proxies and it speaks a range of various application protocols to perform what asked of it. There are literally hundreds of RFCs to read to catch up on all the details.

A while ago I collected the what I consider most important RFCs to read to figure out how curl works and why. That is right now 149 specification documents at a total of over 300,000 lines of text. (It was not done very scientifically.)

Counting the words in these 149 documents, they add up to a total of many more words than the entire Harry Potter series, and the Lord of the Rings series (including the Hobbit) is far behind together with War and Peace: 1.6 megawords.

Luckily, specs are mostly reference literature and we rarely have to read through them all to start our journey, but we often need to go back to check details.

Everything changes over time

The origins of curl trace back to late 1996 and it has been in constant development since then. curl, the Internet and the specifications have all changed significantly over these years.

The specifications that were around when we started have generally been updated multiple times, while we struggle to maintain behavior and functionality for our users. It is hard to spot and react to minor changes in specification updates. They might have been done to clarify a situation, but sometimes such a clarification ends up triggering a functionality change in our code.

Sometimes an update to a spec is even largely ignored by fellow protocol implementers out there in the world, and for the sake of interoperability we too then need to adjust our interpretations so that we work similarly to our peers.

Expectations from users change as values and terms are established in people’s minds rather than in specs. For example: what exactly is the “URL” you see in the browser’s top bar?

Over time, other tools and programs that also work on URLs and on the Internet, gradually change as they too development and slowly morph into the new beings we did not foresee decades ago. This change perceptions and expectations in the user base at large.

The always changing nature of the Internet creates interoperability challenges ever so often: out of the blue a team of protocol implementers can decide to interpret an existing term or a passage in a specification differently one day. When the whole world takes a turn like that, we are sometimes forced to follow along as that is then the new world view.

Another complication is also that curl uses (several) third party libraries for parts of its operations, and some of those details are of course also covered by RFCs.


Our primary guidelines when performing Internet transfers are:

  1. Follow established standard protocol specifications
  2. Security is a first-tier property
  3. Interop widely
  4. Maintain behavior for existing features

As you can figure out for yourself, these four bullet points often collide with each other. Checking off all four is not always possible. They can be hard enough on their own.

Protocol specifications

There are conflicting specifications. Specifications vary over time. They can be hard to interpret to figure out exactly what they say one should do.


Increasing security might at the same time break existing use cases for existing users. It might violate what the specs say. It might add friction in the ability to interoperate with others. It might not even be allowed according to specifications.


This often mean to not follow specifications they way we want to read them, because apparently others do not read them the same way or sometimes they just disregard what the specs say. At times, it is hard to increase security levels by default because it would hamper interop with others.

Maintain behavior

The scripts written 15 years ago that use curl should continue working. The applications written to use libcurl can upgrade libcurl and its Internet transfers just continue. We do not break existing established behaviors. This may very well conflict both with interop and protocol updates, and sometimes it is hard to tighten the security because it would hurt a certain share of existing users.

How does curl deviate from which specs?

I consider this question more or less impossible to answer to, to document and to keep accurate over time. At least it would be a huge and energy-consuming effort both to get the list done but it would also be a monster task to maintain. And it would involve a lot of gray zones.

What is important to me is not what RFCs curl follows nor what or how it deviates from them. I have also basically never gotten that question from a user.

Users want reliable Internet transfers that are secure and interoperate correctly and conveniently with other “players” out there. They want consistent behavior and backwards compatibility.

If you use curl to perform feature X over protocol Y version Z, does it matter which set of RFCs that this would touch and does anyone care about the struggles we have been through when we implemented this set? How many users can even grasp or follow the implication of mentioning that for RFC XYX section A.B we decided to disregard a SHOULD NOT at times?

And how on earth would we keep that up-to-date when we do bugfixes and RFCs are updated down the line?

No one else documents this

The browsers have several hundreds of paid engineers on staff involved and they do not provide documentation like this. Neither does any curl alternative or competitor to my knowledge.

I don’t know of any tool or software anywhere that offer such a deviance documentation and I can perfectly understand and sympathize with why that is so.

There is a tab in my cookie

An HTTP cookie, is just a name + value pair sent from the server to the client. That pair is stored and is sent back to the server in subsequent requests when conditions match.

Cookies were first invented and used in the 1990s. Sources seem to agree that the first browser to support them, was Netscape 0.9beta released in September 1994. Internet Explorer added support in October 1995.

After many years and several failed specification attempts, they were eventually documented in RFC 6265 in 2011. They have been debated, criticized and misunderstood since virtually forever. Mostly because of the abuse/tracking they (used to?) allow in browsers. Less so because of how they actually work over the wire.


curl has supported cookies since the 1990s as well (October 1998 to be specific), and it is a frequently used feature among curl users everywhere. Not the least because the login pattern of the web has become HTTP POST with credentials with a session cookie returned on success – and curl is often used to mimic or reproduce such operations to allow for automated processes and more.

For curl, it is important to remain interoperable and compatible with cookies the same way browsers do them so that users can keep doing these things.

What to accept

Not very long ago, I blogged about a cookie change we had to do because curl’s former liberal attitude towards what a cookie might contain turned out to be a possible vector for mischief. That flaw was basically a direct result of curl never totally adapting to the language in RFC 6265 because we typically do not change what seems to work and has not been reported to be wrong.

In that curl change, we started rejecting incoming cookies that contain “control codes” but we let ASCII code 9 through. ASCII code 9 is the tab character. Generally considered white space. We let it through because we checked the source code for two major browsers and they since they do, curl does as well.

Tab where?

But accepting tabs in the cookie line is one thing. The next question then comes where exactly in the cookie line should it be acceptable?

In the currently ongoing security audit for curl, our friends at Trail of Bits figured out that if a cookie is sent to curl with a tab in the name (literally inside the name and not before or after, like foo<tab>bar) curl would save that wrongly if saved to a file. This, because curl saves cookies in a tab-separated file format, the so called Netscape cookie file format, and it has no escape mechanism or anything. A tab in the cookie name causes the cookie to later get treated wrongly when the file is later loaded again.

Interop status

The file format curl uses for saving cookies is the same as the original format Netscape and then early Firefox used back in the old days. Since this format does not support tabs, I believe it is reasonable to assume the early browsers did not accept tabs in names or content.

I checked how (current) Chrome and Firefox handle cookies like this by creating a test page that sends cookies to the browser.

Chrome rejects cookies with tabs in name or content. They are simply not accepted or stored.

Firefox rejects cookies with tabs in name, but strangely enough it strips tabs from the content and otherwise let them through.

(Safari doesn’t work on Linux so I ignored that)

This, even if my reading of the RFC 6265 seems to say that they should be fine. They should be kept when “internal” to a name or content. I believe curl followed the spec here better than the browsers.

But clearly tabs in cookie names or content is not an interoperable concept on the web.

Adding or removing

With all this in my luggage, I decided to bring this question to the team working on the cookie spec update. The rfc6265bis effort.

I figured that this non-interoperable state and support situation could be worth highlighting or perhaps make a bit stricter in the spec update.

In that issue on GitHub, I was instead informed that recently changed language in their RFC draft rather made browser implementers keen on adding support for this kind of tabs in cookies.

Instead of admitting defeat and documenting that tabs in cookie names and values do not work correctly, we would rather continue limping along pretending this will work.

The HTTP community have supported cookies for almost thirty years without tabs working correctly inside cookies.

Adding (clearer) support for them in a spec would be good in the sense that more defined behavior is a good thing, but since we have decades of this non or perhaps spotty support the already deployed software and long tail of clients will not adapt to any such new wording rapidly. Even if accepted in the new spec, it will take ages until cookies could be done interoperable with tabs inside.

I believe we are better off just documenting that tabs SHOULD NOT be used in cookie name or content as they will not interop. Because that is the truth and will be for a long time no matter what we do today or tomorrow.

My comments in that issue at least seem to have brought reason for maybe reconsider the draft wording. Maybe.


Someone might ask the excellent question “why would anyone want to use a tab in the name or content?”, but quite frankly, I cannot think of any good or legitimate reason other than maybe laziness or a lack of proper filtering.

There is no sound technical reason why this needs to be done.

An executive decision

To fix the breakage curl does when saving cookies like this, and to align better with existing browsers, we decided to rather make curl reject incoming cookies that have tabs in names or content. Starting in the next release: 7.86.0.

We needed to do something. I believe going with the more strict approach is the better one here and now.

If the rfc6265bis draft ends up ultimately keeping the language “encouraging” tab support and browser authors decide to follow, then I presume we will have reason to revisit this decision later on and perhaps take curl in the other direction instead. I think we can handle that, even if I believe it would be the wrong thing for the ecosystem.


Cookie image by StockSnap from Pixabay