Tag Archives: release

curl 8.1.0 – http2 over proxy

We are back with the first release since that crazy March day when we did two releases on the same day. First 8.0.0 shipped that bumped the major version for the first time in decades. Then curl 8.0.1 followed just hours after, due to a serious mess-up in the factory lines.

Release video presentation


the 217th release
3 changes
58 days (total: 9,189)

185 bug-fixes (total: 9,006)
322 commits (total: 30,367
0 new public libcurl function (total: 91)
0 new curl_easy_setopt() option (total: 302)

1 new curl command line option (total: 251)
64 contributors, 35 new (total: 2,875)
37 authors, 17 new (total: 1,142)
4 security fixes (total: 145)


We disclose four new curl security vulnerabilities today, three of them at severity Low and one of them at Medium. This also means that 3,840 USD was awarded as bug bounties in this release cycle.

UAF in SSH sha256 fingerprint check

[CVE-2023-28319] libcurl offers a feature to verify an SSH server’s public key using a SHA 256 hash. When this check fails, libcurl would free the memory for the fingerprint before it returns an error message containing the (now freed) hash.

siglongjmp race condition

[CVE-2023-28320] libcurl provides several different backends for resolving host names, selected at build time. If it is built to use the synchronous resolver, it allows name resolves to time-out slow operations using alarm() and siglongjmp().

When doing this, libcurl used a global buffer that was not mutex protected and a multi-threaded application might therefore crash or otherwise misbehave.

IDN wildcard match

[CVE-2023-28321] curl supports matching of wildcard patterns when listed as “Subject Alternative Name” in TLS server certificates. curl can be built to use its own name matching function for TLS rather than one provided by a TLS library. This private wildcard matching function would match IDN (International Domain Name) hosts incorrectly and could as a result accept patterns that otherwise should mismatch.

more POST-after-PUT confusion

[CVE-2023-28322] 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 second transfer.


This release has only three real changes. One bigger and two smaller:

HTTP/2 over proxy

libcurl can now negotiate and use HTTP/2 when it is told to use a HTTPS proxy (details in the CURLOPT_PROXYTYPE man page), and the command line tool can of course switch it on using the --proxy-http2 option. Explained more in this blog post.

refuse to resolve the .onion TLD

When a host name ending with .onion is passed on to the name resolver functions, they will cause an error and will not be resolved. Like RFC 7686 tells us.

curl’s -w option can now output URL components

The list of variables was extended by a whole range of new ones. Possibly best learned by checking out the writeout section in everything curl.


The official counter says we did more than 180 bugfixes in his release cycle. Here follows some of my favorites:

checksrc fixes

We made it better at checking the code style for three distinct code situations – and then updated the source code accordingly.

cmake fixes

  • bring in the network library on Haiku
  • do not add zlib headers for OpenSSL
  • make config version 8 compatible with 7
  • set SONAME for SunOS too

only do transfer-encoding compression if asked to

Transfer encodings other than “chunked” are rarely used. Up until now libcurl would still activate automatic decompression if such was used, even if it was not asked for by the application.

bring back support for SFTP path ending in /~

A regression made a URL that ends with /~ no longer make a directory listing because the URL does not end with a slash. Now we bring back that behavior, even if it goes a little against the standard behavior.

never allocate dynbufs larger than “too big”

The general dynamic buffer system no longer allocates more memory than what the specific buffer is allowed to grow to. An optimization.

various gskit compile errors in OS400

Makes curl build fine there again.

enforce a maximum DNS cache size independent of timeout value

The DNS cache entries are purged on age only (default 60 seconds). With this new code, libcurl limits caps the maximum total amount of DNS cache entries to 30,000.

libssh2: fix crash in keyboard callback

Better SCP and SFTP when built with libssh2.

libssh: tell it to use SFTP non-blocking

Better SCP and SFTP when built with libssh.

add multi-ignore logic to multi_socket_action

The improved signal ignore logic for curl_multi_perform in 8.0.0 is now also done for curl_multi_socket_action. For better performance.

remove PENDING + MSGSENT handles from the main linked list

Not yet activated transfers and the transfers that are already completed, are now moved away off the main linked list. For performance.

runtests: prepare for parallel

Lots of cleanups and smaller fixes have been merged during this cycle in preparation for the pending introduction of parallel tests.

verify socketpair with a random value

The custom socketpair implementation used for platforms without a native one, was changed to use truly random values when verifying that the pipe works.

Fix ‘Location:’ formatting for early VTE terminals

The special terminal highlighting of the URL that is shown in the Location: header is now disabled for some terminals that can’t display it properly.

urlapi polish

Several different bugs and improvements were made. Including:

  • cleanups and performance improvements
  • detect and error on illegal IPv4 addresses
  • prevent setting invalid schemes
  • URL encoding for the URL missed the fragment

enhanced WebSocket en-/decoding

Parts of the websocket parser code was rewritten to fix bugs.

curl 8.0.1 because I jinxed it

Right. I said in the 8.0.0 blog post that it might be a good release. It was. Apart form the little bug that caused it to crash in several test cases.

So now we shipped curl 8.0.1, which is almost identical apart from a single commit that was reverted.

Exactly why this was not discovered in our tests and CI jobs before the release we have yet to figure out, but it is certainly more than just a little disturbing.

My deepest apologies for this.

curl 8.0.0 is here

Exactly one month since the previous release, we are happy to give you curl 8.0.0 released on curl’s official 25th birthday.

This a major version number bump but without any ground-breaking changes or fireworks. We decided it was about time to reset the minor number down to more a manageable level and doing it exactly on curl’s 25th birthday made it extra fun. There is no API nor ABI break in this version.

This is likely the best curl release we ever made.

Release video presentation

curl 25 years celebrations

Note the additional event happening later on March 20. and the Fossified podcast episode on curl 25 years.


the 215th release
1 changes
28 days (total: 9,131)

130 bug-fixes (total: 8,820)
189 commits (total: 30,042)
0 new public libcurl function (total: 91)
0 new curl_easy_setopt() option (total: 302)

0 new curl command line option (total: 250)
42 contributors, 23 new (total: 2,841)
21 authors, 5 new (total: 1,125)
6 security fixes (total: 141)


We disclose six new vulnerabilities today, five of them at severity Low and one of them at Medium.

CVE-2023-27533: TELNET option IAC injection

curl supports communicating using the TELNET protocol and as a part of this it offers users to pass on user name and “telnet options” for the server negotiation.

Due to lack of proper input scrubbing and without it being the documented functionality, curl would pass on user name and telnet options to the server as provided. This could allow users to pass in carefully crafted content that pass on content or do option negotiation without the application intending to do so. In particular if an application for example allows users to provide the data or parts of the data.

CVE-2023-27534: SFTP path ~ resolving discrepancy

curl supports SFTP transfers. curl’s SFTP implementation offers a special feature in the path component of URLs: a tilde (~) character as the first path element in the path to denotes a path relative to the user’s home directory. This is supported because of wording in the once proposed to-become RFC draft that was to dictate how SFTP URLs work.

Due to a bug, the handling of the tilde in SFTP path did however not only replace it when it is used stand-alone as the first path element but also wrongly when used as a mere prefix in the first element.

Using a path like /~2/foo when accessing a server using the user dan (with home directory /home/dan) would then quite surprisingly access the file /home/dan2/foo.

This can be taken advantage of to circumvent filtering or worse.

CVE-2023-27535: FTP too eager connection reuse

libcurl would reuse a previously created FTP connection even when one or more options had been changed that could have made the effective user a very different one, thus leading to the doing the second transfer with wrong credentials.

libcurl keeps previously used connections in a connection pool for subsequent transfers to reuse if one of them matches the setup. However, several FTP settings were left out from the configuration match checks, making them match too easily. The settings in questions are CURLOPT_FTP_ACCOUNT, CURLOPT_FTP_ALTERNATIVE_TO_USER, CURLOPT_FTP_SSL_CCC and CURLOPT_USE_SSL level.

CVE-2023-27536: GSS delegation too eager connection re-use

libcurl would reuse a previously created connection even when the GSS delegation (CURLOPT_GSSAPI_DELEGATION) option had been changed that could have changed the user’s permissions in a second transfer.

libcurl keeps previously used connections in a connection pool for subsequent transfers to reuse if one of them matches the setup. However, this GSS delegation setting was left out from the configuration match checks, making them match too easily, affecting krb5/kerberos/negotiate/GSSAPI transfers.

CVE-2023-27537: HSTS double-free

libcurl supports sharing HSTS data between separate “handles”. This sharing was introduced without considerations for do this sharing across separate threads but there was no indication of this fact in the documentation.

Due to missing mutexes or thread locks, two threads sharing the same HSTS data could end up doing a double-free or use-after-free.

CVE-2023-27538: SSH connection too eager reuse still

libcurl would reuse a previously created connection even when an SSH related option had been changed that should have prohibited reuse.

libcurl keeps previously used connections in a connection pool for subsequent transfers to reuse if one of them matches the setup. However, two SSH settings were left out from the configuration match checks, making them match too easily.


There is only one actual “change” in this release. This is the first curl release to drop support for building on a systems that lack a working 64 bit data type. curl now requires that ‘long long‘ or an equivalent exists.


This release cycle was half the length of a regular one but yet we managed to merge an impressive amount of bugfixes. Below I highlight a few that I think deserve a special mention.

build: drop the use of XC_AMEND_DISTCLEAN

A strange description but this change removed an old autotools macro that made configure sometimes “balloon” Makefiles to several gigabytes.

connect: fix time_connect and time_appconnect timer statistics

A regression after the new happy eyeball h2/h3 connect approach was introduced.

curl.1: list all “global options”

Command line options that survive the use of --next are called “global options” and the man page now lists all of them for easier identification.

To accomplish this, there is a new metadata “tag” for this purpose to mark the global options in their corresponding docs files.

ftp: active mode with SSL, add the filter

Regression: FTPS in active mode did not setup the data connection correctly.

replaced sscanf() in several parsers

From 24 occurrences of sscanf() calls in the code in the previous release, down to just 4 left.

headers: make curl_easy_header and nextheader return different buffers

http2 bugfixes

  • error handling during parallel operations
  • fix http2 prior knowledge when reusing connections
  • RST and GOAWAY better recognize partial transfers
  • avoid upload busy loop

http: don’t send 100-continue for short PUT requests

Now aligns with and behaves more similarly to how curl has treated POST for a long time.

http: fix unix domain socket use in https connects

A regression.

multi: make multi_perform ignore/unignore signals less often

When iterating over a potentially long list of individual transfers to “take care of”, we can avoid many ignore + unignore sequences by retaining the previous state when possible.

multi: remove PENDING + MSGSENT handles from the main linked list

To speed up the handling of large amounts of easy handles added to a multi handle that are either pending or already completed, those easy handles are now moved out of the main linked list to separate queues.

rand: use arc4random as fallback when available

Makes curl built without a TLS library get better random, assuming the platform supports it.

urlapi: ‘%’ is illegal in host names

The URL parser would wrongly accept a stand-alone percent as part of a host name. It remains accepted for percent-encoded host names and as separator between an IPv6 address and a zone id.

urlapi: parse IPv6 literals without ENABLE_IPV6

To make the URL parser behavior more consistent, it can now parse and deal with IPv6 addresses perfectly fine and the same way even if IPv6 connectivity does not actually work.

binding to an interface with host name using c-ares

Works again!

7.88.1 the second final one

Things did not work out the way we had planned. The 7.88.0 release that was supposed to be the last curl version 7 release contained a nasty bug that made us decide that we better ship an update once that is fixed. This is the update. The second final version 7 release.

Release presentation


the 214th release
0 changes
5 days (total: 9,103)

25 bug-fixes (total: 8,690)
32 commits (total: 29,853)
0 new public libcurl function (total: 91)
0 new curl_easy_setopt() option (total: 302)

0 new curl command line option (total: 250)
19 contributors, 7 new (total: 2,819)
10 authors, 1 new (total: 1,120)
0 security fixes (total: 135)


As this is a rushed patch-release, there is only a small set of bugfixes merged in this cycle. The following notable bugs were fixed.

http2 multiplexed data corruption

The main bug that triggered the patch release. In some circumstances , when data was delivered as a HTTP/2 multiplexed stream, curl would get it wrong and cause the saved data to be corrupt. It would get the wrong data from the internal buffer.

This was not a new bug, but recent changes made it more likely to trigger.

make connect timeouts use full duration

In some cases curl would only allow half the given timeout period when doing connects.

runtests: fix “uninitialized value $port”

Running the test suite with verbose mode enabled, it would error out with this message. Since a short while back, we consider warnings in the test script fatal so this then aborts all the tests.

tests: make sure gnuserv-tls has SRP support before using it

The test suite uses gnuserv-tls to verify SRP authentication. It will only use this tool if found at startup, but due to recent changes in the GnuTLS project that ships this tool, it now builds with SRP disabled by default and thus can’t be used for this test. Now, the test script also checks that it actually supports SRP before trying to use it.

setopt: allow HTTP3 when HTTP2 is not defined

A regression made it impossible to ask for HTTP/3 if the build did not also support HTTP/2.

socketpair: allow EWOULDBLOCK when reading the pair check bytes

The fix in 7.88.0 turned out to cause occasional hiccups (on Windows at least) and this is a follow-up improvement for the verification of the socketpair emulation. When we create the pair and verify that it works, we must make sure that the code handles EWOULDBLOCK correctly.

curl 7.88.0 seven stops here

Welcome to the final and last release in the series seven. The next release is planned and intended to become version 8.


the 213th release
5 changes
56 days (total: 9,098)

173 bug-fixes (total: 8,665)
250 commits (total: 29,821)
0 new public libcurl function (total: 91)
0 new curl_easy_setopt() option (total: 302)

1 new curl command line option (total: 250)
78 contributors, 41 new (total: 2,812)
42 authors, 18 new (total: 1,119)
3 security fixes (total: 135)

Release presentation


This time we bring you three security fixes. All of them covering cases for which we have had problems reported and fixed before, but these are new subtle variations.



While we count over 140 individual bugfixes merged for this release, here follows a curated subset of some of the more interesting ones.

http/3 happy eyeballs

When asking for HTTP/3, curl will now also try older HTTP versions with a slight delay so that if HTTP/3 does not work, it might still succeed with and use an older version.

update all copyright lines and remove year ranges

Mentioned separately.

allow up to 10M buffer size

An application can now set drastically larger download buffers. For high speed/localhost transfers of some protocols this might sometimes make a difference.

curl: output warning at –verbose output for debug-enabled version

To help users realize when they use a debug build of curl, it now outputs a warning at the top of the --verbose output. We strongly discourage users to ship or use such builds in production.

websocket: multiple bugfixes

WebSocket support remains an experimental feature in curl but it is getting better. Several smaller and bigger bugs were squashed. Please continue to try it and report any problems and we can probably consider removing the experimental label soon.

dict: URL decode the entire path always

If you used a DICT URL it would sometimes do wrong as it previously only URL decoded parts of the path when using it. Now it correctly decodes the entire thing.

URL-encode/decode much faster

The libcurl functions for doing these conversions were sped up significantly. In the order of 3x and 7x.

haxproxy: send before TLS handhshake

The haproxy details are now properly sent before the TLS handshake takes place.

HTTP/[23]: continue upload when state.drain is set

Fixes a stalling problem when data is being uploaded and downloaded at the same time.

http2: aggregate small SETTINGS/PRIO/WIN_UPDATE frames

Optimizes outgoing frames for HTTP/2 into doing more in fewer sends.

openssl: store the CA after first send (ClientHello)

By changing the order of things, curl is better off spending CPU cycles while waiting for the server’s response and thereby making the entire handshake process complete faster.

curl: repair –rate

A regression in 7.87.0 made this feature completely broken. Now back on track again.

HTTP/2 much faster multiplexed transfers

By improving the handling of multiple concurrent streams over a single connection, curl now performs such transfers much faster than before. Sometimes an almost 3x speedup.

noproxy: support for space-separated names is deprecated

The parser that parses the “noproxy” string accepts plain space (without comma) as separators, while hardly any other tool or library does. This matters because it can be set in an environment variable. This accepted space-only separation is now marked as deprecated.

nss: implement data_pending method

The NSS backend was improved to work better for cases when the socket has been drained of data and only the NSS internal buffers has it, which could lead to curl getting stalled or losing data. Note: NSS support is marked for removal later in 2023.

socketpair: allow localhost MITM sniffers

curl has an internal socketpair emulation function for Windows. The way it worked did not allow MITM sniffers, but instead return error if such a thing was detected. It turns out too many users run tools on Windows that do this, so we have changed the logic to accept their presence and use.

tests-httpd: infra to run curl against an apache httpd

An entirely new line of tests that opens up new ways to test and verify our HTTP implementations in ways we could not do before. It uses pytest and an apache httpd server with special test modules.

curl: fix hiding of command line secrets

A regression.

curl: fix error code on bad URL

If you would use an invalid URL for upload, curl would erroneously report the problem as “out of memory” which unsurprisingly greatly confused users.

curl 7.87.0


the 212th release
5 changes
56 days (total: 9,042)

155 bug-fixes (total: 8,492)
238 commits (total: 29,571)
0 new public libcurl function (total: 91)
2 new curl_easy_setopt() option (total: 302)

1 new curl command line option (total: 249)
83 contributors, 40 new (total: 2,771)
42 authors, 20 new (total: 1,101)
2 security fixes (total: 132)
Bug Bounties total: 48,580 USD

Release presentation

At 10:00 CET (9:00 UTC) on December 21, Daniel live-streams the release presentation on twitch. This paragraph will later be replaced by a link to the YouTube version of that video.


Two security advisories this time around, severity low and medium.

CVE-2022-43551: Another HSTS bypass via IDN

The HSTS logic could be bypassed if the host name in the given URL first 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). Then in a subsequent request, it does not detect the HSTS state and makes a clear text transfer. Because it would store the info IDN encoded but look for it IDN decoded.

CVE-2022-43552: HTTP Proxy deny use-after-free

When an HTTP PROXY denied to tunnel SMB or TELNET, curl would use a heap-allocated struct after it had been freed in its transfer shutdown code path.



curl’s 249th command line option adds data to the query part of the URL.


Tell libcurl to not wait for any DNS threads on exit.


New and easier way to signal write callback errors.


libcurl can now cache the CA store in memory, as I blogged about separately.

feature names added to curl_version_info_data

The struct returned by curl_version_info now returns all built-in features listed by name. This is a preparation to allow applications to adapt slowly and get ready for the future moment when the features can no longer fit in in the 32 bit fields previously used for this purpose.


Better base64

The encoder now allocates the output using a more appropriate size, and both the encoder and decoder implementations are much faster.


We fixed a few issues in the hyper backend and are down to just 12 remaining disabled tests to address.

gen.pl: fix the linkifier

This script generates the curl.1 man page and make sure to properly mark references correctly, so that the man page can get rendered as we webpage with correct links etc on the website. This time we made it work better and therefore more cross-references in the man page is now linked correctly in the web version.

tool: override the numeric locale and set “C” by force

In previous curl versions it mistakenly used the locale when parsing floating point numbers, which then made the tool hard to use in scripts which would run in multiple locales. An example is the timeout option specified with -m / --max-time as number of seconds with a fraction. Now it requires the decimal separator to always be a dot/period independently of the user’s locale.

tool: timeout in the read callback

The command line tool can now timeout reading data better, for example when using telnet:// with a timeout option and the user does not press any key and nothing happens over the network.

curl_get_line: allow last line without newline char

Because of a somewhat lazy recent fix, the .netrc parsed and other users of the nternal curl_get_line() function would ignore the last line if it did not end with a newline. This is no more.

support growing FTP files with CURLOPT_IGNORE_CONTENT_LENGTH

If this option is set, also known as --ignore-content-length on the command line, curl will not complain if the size grows from the moment the FTP transfer starts until it ends. Thus allowing it to grow while being transferred.

do not send PROXY more than once

The HAproxy protocol line could get sent more than once and thus break stuff.

feature deprecation warnings in gcc

A number of outdated libcurl options and functions are now tagged as deprecated, which will cause compiler warnings when used in application code for users of gcc 6.1 or later. Deprecated here means that we recommend using other, more modern, alternatives.

parse numbers with fixed known base 10

In several places in curl and libcurl source code we would allow numbers to be specified using octal or hexadecimal while decimal was the only expected and documented base. In order to minimize surprises and for consistency, we now limited them as far as possible to only accepting decimal numbers.

rewind BEFORE request instead of AFTER previous

When curl is used to send a request, for example a POST, and there is reason for it to send it again, like if there is a redirect or an ongoing authentication process, it would previously rewind the stream at the end of that transfer first transfer in order to have it done when the next transfer is about to get done. Now, it instead does the rewind first in the second request. This, because there are times when the second request are not done, and the rewind may not work. So, such a failing rewind can be avoided by not doing it until it is strictly necessary.


Several independent regressions were fixed – in spite of the new set of test cases added for testing this feature in the previous release. Noproxy is the support for the NO_PROXY environment variable and related options.

openssl: prefix errors with ‘[lib]/[version]:’

To help users understand errors and their origins a little better, libcurl will now prefix error messages originating from OpenSSL (and forks) with the name of the flavor and its version number.

RTSP auth works again

This functionality was broken a few versions back and now it has finally been fixed again.

runtests: –no-debuginfod now disables DEBUGINFOD_URLS

valgrind and gdb support downloading stuff at the moment of need if this environment variable is set. Previously the curl test running script would unset that variable unconditionally, but now it will not and instead offer an option that unsets it – for the cases where the environment variable causes problems (such as performance slowdowns).

HTTP/3 tests

We finally have the first infrastructure merged for doing and running HTTP/3 specific tests in the curl test suite. Now we can better avoid regressions going forward. This is only the beginning and I expect us to expand and grow these tests going forward.

determine the correct fopen option for -D

When saving response headers into a dedicated file with curl’s -D, –dump-header option, curl would be inconsistent about when to create a new file and when to append do it. Now it acts exactly as documented.

better error message for -G with bad URL

Several users figured out curl showed misleading error messages when -G was used in combination with a malformed URL. This is now improved.

repair IDN for proxies

A recent fix we landed for IDN for host names accidentally simultaneously broke it for proxies…

cmake: set the soname on the shared library

Using cmake to build libcurl as a shared library on Linux and several other systems, will now set the SONAME number correctly in the same style and with the same number that the autotools build uses.

WebSocket polish

  • fixes for partial frames and buffer updates
  • now returns CURLE_NOT_BUILT_IN when websockets support is not built in
  • returns error properly when the connection is closed

TLS goes connection filters => more HTTPS-proxy

As a direct result of the internal refactor and introduction of connection filters also for TLS, curl now supports HTTPS-proxy for a wider selection of TLS backends than previously.


Release image by @sny@mas.to

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 https://example.com?moo.

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!

curl 7.85.0 for you

Welcome to a new curl release, the result of a slightly extend release cycle this time.

Release presentation


the 210th release
3 changes
65 days (total: 8,930)

165 bug-fixes (total: 8,145)
230 commits (total: 29,017)
0 new public libcurl function (total: 88)
2 new curl_easy_setopt() option (total: 299)

0 new curl command line option (total: 248)
79 contributors, 38 new (total: 2,690)
44 authors, 22 new (total: 1,065)
1 security fixes (total: 126)
Bug Bounties total: 40,900 USD


We have yet another CVE to disclose.

control code in cookie denial of service

CVE-2022-35252 allows a server to send cookies to curl that contain ASCII control codes. When such cookies subsequently are sent back to a server, they will cause 400 responses from servers that downright refuse such requests. Severity: low. Reward: 480 USD.


This release counts three changes. They are:

schannel backend supports TLS 1.3

For everyone who uses this backend (which include everyone who uses the curl that Microsoft bundles with Windows) this is great news: now you too can finally use TLS 1.3 with curl. Assuming that you use a new enough version of Windows 10/11 that has the feature present. Let’s hope Microsoft updates the bundled version soon.


These are two new options meant to replace and be used instead of the options with the same names without the “_STR” extension.

While working on support for new future protocols for libcurl to deal with, we realized that the old options were filled up and there was no way we could safely extend them with additional entries. These new functions instead work on text input and have no limit in number of protocols they can be made to support.

QUIC support via wolfSSL

The ngtcp2 backend can now also be built to use wolfSSL for the TLS parts.


This was yet again a cycle packed with bugfixes. Here are some of my favorites:

asyn-thread: fix socket leak on OOM

Doing proper and complete memory cleanup even when we exist due to out of memory is sometimes difficult. I found and fixed this very old bug.

cmdline-opts/gen.pl: improve performance

The script that generates the curl.1 man page from all its sub components was improved and now typically executes several times faster then before. curl developers all over rejoice.

configure: if asked to use TLS, fail if no TLS lib was detected

Previously, the configure would instead just silently switch off TLS support which was not always easy to spot and would lead to users going further before they eventually realize this.

configure: introduce CURL_SIZEOF

The configure macro that checks for size of variable types was rewritten. It was the only piece left in the source tree that had the mention of GPL left. The license did not affect the product source code or the built outputs, but it caused questions and therefore some friction we could easily avoid by me completely writing away the need for the license mention.

close the happy eyeballs loser connection when using QUIC

A silly memory-leak when doing HTTP/3 connections on dual-stack machines.

treat a blank domain in Set-Cookie: as non-existing

Another one of those rarely used and tiny little details about following what the spec says.

configure: check whether atomics can link

This, and several other smaller fixes together improved the atomics support in curl quite a lot since the previous version. We conditionally use this C11 feature if present to make the library initialization function thread-safe without requiring a separate library for it.

digest: fix memory leak, fix not quoted ‘opaque’

There were several fixes and cleanups done in the digest department this time around.

remove examples/curlx.c

Another “victim” of the new license awareness in the project. This example was the only file present in the repository using this special license, and since it was also a bit convoluted example we decided it did not really have to be included.

resolve *.localhost to

curl is now slightly more compliant with RFC 6761, follows in the browsers’ footsteps and resolves all host names in the “.localhost” domain to the fixed localhost addresses.

enable obs-folded multiline headers for hyper

curl built with hyper now also supports “folded” HTTP/1 headers.

libssh2:+libssh make atime/mtime date overflow return error

Coverity had an update in August and immediately pointed out these two long-standing bugs – in two separate SSH backends – related to time stamps and 32 bits.

curl_multi_remove_handle closes CONNECT_ONLY transfer

When an applications sets the CONNECT_ONLY option for a transfer within a multi stack, that connection was not properly closed until the whole multi handle was closed even if the associated easy handle was terminated. This lead to connections being kept around unnecessarily long (and wasting resources).

use pipe instead of socketpair on apple platforms

Apparently those platform likes to close socketpairs when the application is pushed into the background, while pipes survive the same happening… This is a change that might be preferred for other platforms as well going forward.

use larger dns hash table for multi interface

The hash table used for the DNS cache is now made larger for the multi interface than when created to be used by the easy interface, as it simply is more likely to be used by many host names then and then it performs better like this.

reject URLs with host names longer than 65535 bytes

URLs actually have no actual maximum size in any spec and neither does the host name within one, but the maximum length of a DNS name is 253 characters. Assuming you can resolve the name without DNS, another length limit is the SNI field in TLS that is an unsigned 16 bit number: 65535 bytes. This implies that clients cannot connect to any SNI-using TLS protocol with a longer name. Instead of checking for that limit in many places, it is now done early.

reduce size of several struct fields

As part of the repeated iterative work of making sure the structs are kept as small as possible, we have again reduced the size of numerous struct fields and rearranged the order somewhat to save memory.


The next release is planned to ship on October 25, 2022.

The curl release cycle

In the curl project we do timed releases and we try to do them planned and scheduled long in advance. The dates are planned. The content is not.

I have been the release manager for every single curl release done. 209 releases at current count. Having this role means I make sure things are in decent shape for releases and I do the actual mechanic act of running the release scripts etc on the release days.

Since 2014, we make releases on Wednesdays, every eight weeks. We sometimes adjust the date slightly because of personal events (meaning: if I have a vacation when the release is about to happen, we can move it), and we have done several patch releases within a shorter time when the previous release proved to have a serious enough problem to warrant an out-of-schedule release. Even more specifically, I make the releases available at or around 8 am (central euro time) on the release days.

The main objective is to stick to the 56 day interval.

It probably goes without saying but let me be clear: curl is a software project that quite evidently will never be done or complete. It will keep getting fixes, improvements and features for as long as it lives, and as long as it lives we keep making new releases.

Timed releases

We decided to go with a fixed eight weeks release frequency quite arbitrarily (based on past release history and what felt “right”) but it has over time proved to work well. It is short enough for everyone to never have to wait very long for the next release, and yet it is long enough to give us time for both merging new features and having a period of stabilization.

Timed releases means that we ship releases on the predetermined release date with all the existing features and bugfixes that have landed in the master branch in time. If a change is not done in time and it is not merged before the release, it will simply not be included in the release but will get a new opportunity to get included and shipped in the next release. Forever and ever.

I am a proponent of timed releases compared to feature based ones for projects like curl. For the simplicity of managing the releases, for long term planning, for user communication and more.

The cycle has three windows

Within the eight week release cycle, there are three distinct and different windows or phases.

1 – release margin

The cycle can be said to start at the day of a release. The release is created straight from the master git branch. It gets signed, uploaded, blogged about and the news of it is shouted across the globe.

This day also starts the “release margin window”. In this window we still accept and merge bugfixes into the master branch, but we do not merge changes or new features.

The five day margin this speaks of, is that this window gives us a few calendar days to assess and get a feel for how the previous release is being received. Is there a serious bug reported? Did we somehow royally mess up? If we did an important enough snafu, we may decide to do a follow-up patch release soon and if we do, we are in a better position if we have not merged any changes yet into the release branch.

If we decide on doing a patch release, we skip the feature window and go directly to feature freeze.

2 – feature window

If we survived the margin window without anything alarming happening, we open the feature window. On the Monday following the preceding release.

This is the phase during which we merge changes and new features in addition to the regular normal bugfixes (assuming there are any of course). Things that warrant the minor version number for the next release to get bumped.

The feature window is open 23 days. Ideally, people have already been working and polishing on their pull-requests for a while before this window opens and then the work can get merged fairly quickly – and presumably painless.

What exactly can be considered a change and what is a bugfix can of course become a matter of opinion and discussion, but we tend to take the safer approach when in doubt.

3 – feature freeze

In the 28 days before the pending release we only merge bugfixes into the master branch. All features and changes are queued up and will have to wait until the feature window opens again. (Exceptions might be made for experimental and off-by-default features.)

This phase is of course intended for things to calm down, to smooth out rough edges, to fix any leftover mistakes done in the previous feature merges. With the mindset that the pending next release will be the best, most stable, shiniest, glorious release we ever did. With even fewer bugs than before. The next release is always our best release yet.

The release cycle as an image

(Yes, if you squint and roll your chair away from the screen a bit, it looks like the Chrome logo!)

Release frequency graph

The graph below shows the number of days between all curl releases ever done. You can see the eight week release cycle introduced in 2014 visible in the graph, and you can also easily spot that we have done much quicker releases than eight weeks a number of a times since then – all of them actually a sign of some level of failure since that means we felt urged to do a patch release sooner than we had previously anticipated.

curl 7.84.0 inside every box

Welcome to take the next step with us in this never-ending stroll.

Release presentation


the 209th release
8 changes
47 days (total: 8,865)

123 bug-fixes (total: 7,980)
214 commits (total: 28,787)
0 new public libcurl function (total: 88)
2 new curl_easy_setopt() option (total: 297)

1 new curl command line option (total: 248)
51 contributors, 20 new (total: 2,652)
35 authors, 13 new (total: 1,043)
4 security fixes (total: 125)
Bug Bounties total: 34,660 USD


This is another release in which scrutinizing eyes have been poking around and found questionable code paths that could be lead to insecurities. We announce four new security advisories this time – all found and reported by Harry Sintonen. This bumps mr Sintonen’s curl CVE counter up to 17; the number of security problems in curl found and reported by him alone.

CVE-2022-32205: Set-Cookie denial of service

A malicious server can serve excessive amounts of Set-Cookie: headers in a HTTP response to curl and curl stores all of them. A sufficiently large amount of (big) cookies make subsequent HTTP requests to this, or other servers to which the cookies match, create requests that become larger than the threshold that curl uses internally to avoid sending crazy large requests (1048576 bytes) and instead returns an error.

CVE-2022-32206: HTTP compression denial of service

curl supports “chained” HTTP compression algorithms, meaning that a server response can be compressed multiple times and potentially with different algorithms. The number of acceptable “links” in this “decompression chain” was unbounded, allowing a malicious server to insert a virtually unlimited number of compression steps.

CVE-2022-32207: Unpreserved file permissions

When curl saves cookies, alt-svc and hsts data to local files, it makes the operation atomic by finalizing the operation with a rename from a temporary name to the final target file name.

In that rename operation, it might accidentally widen the permissions for the target file, leaving the updated file accessible to more users than intended.

CVE-2022-32208: FTP-KRB bad message verification

When curl does FTP transfers secured by krb5, it handles message verification failures wrongly. This flaw makes it possible for a Man-In-The-Middle attack to go unnoticed and even allows it to inject data to the client.


We have no less than eight different changes logged this time. Two are command line changes and the rest are library side.


This new command line option rate limits the number of transfers per time period.

deprecate --random-file and --egd-file

These are two options that have not been used by anyone for an extended period of time, and starting now they have no functionality left. Using them has no effect.

curl_global_init() is threadsafe

Finally, and this should be conditioned to say that the function is only thread-safe on most platforms.

curl_version_info: adds CURL_VERSION_THREADSAFE

The point here is that you can check if global init is thread-safe in your particular libcurl build.

CURLINFO_CAPATH/CAINFO: get default CA paths

As the default values for these values are typically figured out and set at build time, applications might appreciate being able to figure out what they are set to by default.


For libssh2 enabled builds, you can now set a callback for hostkey verification.


The libcurl version of the change mentioned above for the command line. The CURLOPT_RANDOM_FILE and CURLOPT_EGDSOCKET options no longer do anything. They most probably have not been used by any application for a long time.

unix sockets to socks proxy

You can now tell (lib)curl to connect to a SOCKS proxy using unix domain sockets instead of traditional TCP.


We merged way over a hundred bugfixes in this release. Below are descriptions of some of the fixes I think are particularly interesting to highlight and know about.

improved cmake support for libpsl and libidn2

more powers to the cmake build

address cookie secure domain overlay

Addressed issues when identically named cookies marked secure are loaded over HTTPS and then again over HTTP and vice versa. Cookies are complicated.

make repository REUSE compliant

Being REUSE compliant makes we now have even better order and control of the copyright and licenses used in the project.

headers API no longer EXPERIMENTAL

The header API is now officially a full member of the family.

reject overly many HTTP/2 push-promise headers

curl would accept an unlimited number of headers in a HTTP/2 push promise request, which would eventually lead to out of memory – starting now it will instead reject and cancel such ridiculous streams earlier.

restore HTTP header folding behavior

curl broke the previous HTTP header behavior in the 7.83.1 release, and it has now been restored again. As a bonus, the headers API supports folded headers as well. Folding headers being the ones that are the rare (and deprecated) continuation headers that start with a whitespace.

skip fake-close when libssh does the right thing

Previously, libssh would, a little over-ambitiously, close our socket for us but that has been fixed and curl is adjusted accordingly.

check %USERPROFILE% for .netrc on Windows

A few other tools apparently look for and use .netrc if found in the %USERPROFILE% directory, so by making curl also check there, we get better cross tool .netrc behavior.

support quoted strings in .netrc

curl now supports quoted strings in .netrc files so that you can provide spaces and more in an easier way.

many changes in ngtcp2

There were lots of big and small changes in the HTTP/3 backend powered by ngtcp2.

provide a fixed fake host name in NTLM

curl no longer tries to provide the actual local host name when doing NTLM authentication to reduce information leakage. Instead, curl now uses the same fixed fake host name that Firefox uses when speaking NTLM: WORKSTATION.

return error from “lethal” poll/select errors

A persistent error in select() or poll() could previously be ignored by libcurl and not result in an error code returned to the user, making it loop more than necessary.

strcase optimizations

The case insensitive string comparisons were optimized.

maintain path-as-is after redirects

After a redirect or if doing multi-stage authentication, the --path-as-is status would be dropped.

support CURLU_URLENCODE for curl_url_get

This is useful when for example you ask the API to accept spaces in URLs and you want to later extract a valid URL with such an embedded space URL encoded

Coming next

7.85.0 is scheduled to ship on August 31, 2022.