TLS

1. IMAPS connection fails with Rustls error
2. Access violation sending client cert with Schannel
3. Client cert handling with Issuer DN differs between backends
4. Client cert (MTLS) issues with Schannel
5. Schannel TLS 1.2 handshake bug in old Windows versions
6. CURLOPT_CERTINFO results in CURLE_OUT_OF_MEMORY with Schannel
7. mbedTLS and CURLE_AGAIN handling

Email protocols

1. IMAP SEARCH ALL truncated response
2. No disconnect command
3. AUTH PLAIN for SMTP is not working on all servers
4. APOP authentication fails on POP3
5. POP3 issue when reading small chunks

Command line

1. -T /dev/stdin may upload with an incorrect content length
2. -T - always uploads chunked

Build and portability issues

1. OS400 port requires deprecated IBM library
2. curl-config --libs contains private details
3. LDFLAGS passed too late making libs linked incorrectly
4. Cygwin: make install installs curl-config.1 twice
5. flaky CI builds
6. long paths are not fully supported on Windows
7. Unicode on Windows

Authentication

1. Digest auth-int for PUT/POST
2. MIT Kerberos for Windows build
3. NTLM in system context uses wrong name
4. NTLM does not support password with Unicode 'SECTION SIGN' character
5. libcurl can fail to try alternatives with --proxy-any
6. Do not clear digest for single realm
7. SHA-256 digest not supported in Windows SSPI builds
8. curl never completes Negotiate over HTTP
9. Negotiate on Windows fails
10. Negotiate authentication against Hadoop

FTP

1. FTP with ACCT
2. FTPS server compatibility on Windows with Schannel

SFTP and SCP

1. SFTP does not do CURLOPT_POSTQUOTE correct
2. Remote recursive folder creation with SFTP
3. libssh blocking and infinite loop problem
4. Cygwin: "WARNING: UNPROTECTED PRIVATE KEY FILE!"

Connection

1. --interface with link-scoped IPv6 address
2. Does not acknowledge getaddrinfo sorting policy
3. SOCKS-SSPI discards the security context

Internals

1. GSSAPI library name + version is missing in curl_version_info()
2. error buffer not set if connection to multiple addresses fails
3. HTTP test server 'connection-monitor' problems
4. Connection information when using TCP Fast Open
5. test cases sometimes timeout
6. CURLOPT_CONNECT_TO does not work for HTTPS proxy
7. WinIDN test failures
8. setting a disabled option should return CURLE_NOT_BUILT_IN

LDAP

1. OpenLDAP hangs after returning results
2. LDAP on Windows does authentication wrong?
3. LDAP on Windows does not work
4. LDAPS requests to Active Directory server hang

TCP/IP

1. telnet code does not handle partial writes properly
2. Trying local ports fails on Windows

CMake

1. cmake outputs: no version information available
2. uses -lpthread instead of Threads::Threads
3. generated .pc file contains strange entries
4. CMake build with MIT Kerberos does not work

Authentication

1. --aws-sigv4 does not handle multipart/form-data correctly

HTTP/2

1. HTTP/2 prior knowledge over proxy
2. HTTP/2 frames while in the connection pool kill reuse
3. ENHANCE_YOUR_CALM causes infinite retries
4. HTTP/2 + TLS spends a lot of time in recv

HTTP/3

1. connection migration does not work
2. quiche: QUIC connection is draining

RTSP

1. Some methods do not support response bodies

Known bugs intro

These are problems and bugs known to exist at the time of this release. Feel free to join in and help us correct one or more of these. Also be sure to check the changelog of the current development status, as one or more of these problems may have been fixed or changed somewhat since this was written.

TLS

IMAPS connection fails with Rustls error

curl issue 10457

Access violation sending client cert with Schannel

When using Schannel to do client certs, curl sets PKCS12_NO_PERSIST_KEY to avoid leaking the private key into the filesystem. Unfortunately that flag instead seems to trigger a crash.

See curl issue 17626

Client cert handling with Issuer DN differs between backends

When the specified client certificate does not match any of the server-specified DN fields, the OpenSSL and GnuTLS backends behave differently. The GitHub discussion may contain a solution.

See curl issue 1411

Client cert (MTLS) issues with Schannel

See curl issue 3145

Schannel TLS 1.2 handshake bug in old Windows versions

In old versions of Windows such as 7 and 8.1 the Schannel TLS 1.2 handshake implementation likely has a bug that can rarely cause the key exchange to fail, resulting in error SEC_E_BUFFER_TOO_SMALL or SEC_E_MESSAGE_ALTERED.

curl issue 5488

CURLOPT_CERTINFO results in CURLE_OUT_OF_MEMORY with Schannel

curl issue 8741

mbedTLS and CURLE_AGAIN handling

curl issue 15801

Email protocols

IMAP SEARCH ALL truncated response

IMAP SEARCH ALL truncates output on large boxes. "A quick search of the code reveals that pingpong.c contains some truncation code, at line 408, when it deems the server response to be too large truncating it to 40 characters"

https://curl.se/bug/view.cgi?id=1366

No disconnect command

The disconnect commands (LOGOUT and QUIT) may not be sent by IMAP, POP3 and SMTP if a failure occurs during the authentication phase of a connection.

AUTH PLAIN for SMTP is not working on all servers

Specifying --login-options AUTH=PLAIN on the command line does not seem to work correctly.

See curl issue 4080

APOP authentication fails on POP3

See curl issue 10073

POP3 issue when reading small chunks

CURL_DBG_SOCK_RMAX=4 ./runtests.pl -v 982

See curl issue 12063

Command line

-T /dev/stdin may upload with an incorrect content length

-T stats the path to figure out its size in bytes to use it as Content-Length if it is a regular file.

The problem with that is that on BSD and some other UNIX systems (not Linux), open(path) may not give you a file descriptor with a 0 offset from the start of the file.

See curl issue 12177

-T - always uploads chunked

When the < shell operator is used. curl should realize that stdin is a regular file in this case, and that it can do a non-chunked upload, like it would do if you used -T file.

See curl issue 12171

Build and portability issues

OS400 port requires deprecated IBM library

curl for OS400 requires QADRT to build, which provides ASCII wrappers for libc/POSIX functions in the ILE, but IBM no longer supports or even offers this library to download.

See curl issue 5176

curl-config --libs contains private details

curl-config --libs include details set in LDFLAGS when configure is run that might be needed only for building libcurl. Further, curl-config --cflags suffers from the same effects with CFLAGS/CPPFLAGS.

LDFLAGS passed too late making libs linked incorrectly

Compiling latest curl on HP-UX and linking against a custom OpenSSL (which is on the default loader/linker path), fails because the generated Makefile has LDFLAGS passed on after LIBS.

See curl issue 14893

Cygwin: make install installs curl-config.1 twice

curl issue 8839

flaky CI builds

We run many CI builds for each commit and PR on GitHub, and especially a number of the Windows builds are flaky. This means that we rarely get all CI builds go green and complete without errors. This is unfortunate as it makes us sometimes miss actual build problems and it is surprising to newcomers to the project who (rightfully) do not expect this.

See curl issue 6972

long paths are not fully supported on Windows

curl on Windows cannot access long paths (paths longer than 260 characters). However, as a workaround, the Windows path prefix \\?\ which disables all path interpretation may work to allow curl to access the path. For example: \\?\c:\longpath.

See curl issue 8361

Unicode on Windows

Passing in a Unicode filename with -o:

curl issue 11461

Passing in Unicode character with -d:

curl issue 12231

Windows Unicode builds use the home directory in current locale.

The Windows Unicode builds of curl use the current locale, but expect Unicode UTF-8 encoded paths for internal use such as open, access and stat. The user's home directory is retrieved via curl_getenv in the current locale and not as UTF-8 encoded Unicode.

See curl pull request 7252 and curl pull request 7281

Cannot handle Unicode arguments in non-Unicode builds on Windows

If a URL or filename cannot be encoded using the user's current code page then it can only be encoded properly in the Unicode character set. Windows uses UTF-16 encoding for Unicode and stores it in wide characters, however curl and libcurl are not equipped for that at the moment except when built with _UNICODE and UNICODE defined. Except for Cygwin, Windows cannot use UTF-8 as a locale.

https://curl.se/bug/?i=345 https://curl.se/bug/?i=731 https://curl.se/bug/?i=3747

NTLM authentication and Unicode

NTLM authentication involving Unicode username or password only works properly if built with UNICODE defined together with the Schannel backend. The original problem was mentioned in: https://curl.se/mail/lib-2009-10/0024.html and https://curl.se/bug/view.cgi?id=896

The Schannel version verified to work as mentioned in https://curl.se/mail/lib-2012-07/0073.html

Authentication

Digest auth-int for PUT/POST

We do not support auth-int for Digest using PUT or POST

MIT Kerberos for Windows build

libcurl fails to build with MIT Kerberos for Windows (KfW) due to its library header files exporting symbols/macros that should be kept private to the library.

NTLM in system context uses wrong name

NTLM authentication using SSPI (on Windows) when (lib)curl is running in "system context" makes it use wrong(?) username - at least when compared to what winhttp does. See https://curl.se/bug/view.cgi?id=535

NTLM does not support password with Unicode 'SECTION SIGN' character

Code point: U+00A7

https://en.wikipedia.org/wiki/Section_sign curl issue 2120

libcurl can fail to try alternatives with --proxy-any

When connecting via a proxy using --proxy-any, a failure to establish an authentication causes libcurl to abort trying other options if the failed method has a higher preference than the alternatives. As an example, --proxy-any against a proxy which advertise Negotiate and NTLM, but which fails to set up Kerberos authentication does not proceed to try authentication using NTLM.

curl issue 876

Do not clear digest for single realm

curl issue 3267

SHA-256 digest not supported in Windows SSPI builds

Windows builds of curl that have SSPI enabled use the native Windows API calls to create authentication strings. The call to InitializeSecurityContext fails with SEC_E_QOP_NOT_SUPPORTED which causes curl to fail with CURLE_AUTH_ERROR.

Microsoft does not document supported digest algorithms and that SEC_E error code is not a documented error for InitializeSecurityContext (digest).

curl issue 6302

curl never completes Negotiate over HTTP

Apparently it is not working correctly...?

See curl issue 5235

Negotiate on Windows fails

When using --negotiate (or NTLM) with curl on Windows, SSL/TLS handshake fails despite having a valid kerberos ticket cached. Works without any issue in Unix/Linux.

curl issue 5881

Negotiate authentication against Hadoop

curl issue 8264

FTP

FTP with ACCT

When doing an operation over FTP that requires the ACCT command (but not when logging in), the operation fails since libcurl does not detect this and thus fails to issue the correct command: https://curl.se/bug/view.cgi?id=635

FTPS server compatibility on Windows with Schannel

FTPS is not widely used with the Schannel TLS backend and so there may be more bugs compared to other TLS backends such as OpenSSL. In the past users have reported hanging and failed connections. It is likely some changes to curl since then fixed the issues. None of the reported issues can be reproduced any longer.

If you encounter an issue connecting to your server via FTPS with the latest curl and Schannel then please search for open issues or file a new issue.

SFTP and SCP

SFTP does not do CURLOPT_POSTQUOTE correct

When libcurl sends CURLOPT_POSTQUOTE commands when connected to an SFTP server using the multi interface, the commands are not being sent correctly and instead the connection is canceled (the operation is considered done) prematurely. There is a half-baked (busy-looping) patch provided in the bug report but it cannot be accepted as-is. See https://curl.se/bug/view.cgi?id=748

Remote recursive folder creation with SFTP

On this servers, the curl fails to create directories on the remote server even when the CURLOPT_FTP_CREATE_MISSING_DIRS option is set.

See curl issue 5204

libssh blocking and infinite loop problem

In the SSH_SFTP_INIT state for libssh, the ssh session working mode is set to blocking mode. If the network is suddenly disconnected during sftp transmission, curl is stuck, even if curl is configured with a timeout.

curl issue 8632

Cygwin: "WARNING: UNPROTECTED PRIVATE KEY FILE!"

Running SCP and SFTP tests on Cygwin makes this warning message appear.

curl issue 11244

Connection

--interface with link-scoped IPv6 address

When you give the --interface option telling curl to use a specific interface for its outgoing traffic in combination with an IPv6 address in the URL that uses a link-local scope, curl might pick the wrong address from the named interface and the subsequent transfer fails.

Example command line:

curl --interface eth0 'http://[fe80:928d:xxff:fexx:xxxx]/'

The fact that the given IP address is link-scoped should probably be used as input to somehow make curl make a better choice for this.

curl issue 14782

Does not acknowledge getaddrinfo sorting policy

Even if a user edits /etc/gai.conf to prefer IPv4, curl still prefers and tries IPv6 addresses first.

curl issue 16718

SOCKS-SSPI discards the security context

After a successful SSPI/GSS-API exchange, the function queries and logs the authenticated username and reports the supported data-protection level, but then immediately deletes the negotiated SSPI security context and frees the credentials before returning. The negotiated context is not stored on the connection and is therefore never used to protect later SOCKS5 traffic.

Internals

GSSAPI library name + version is missing in curl_version_info()

The struct needs to be expanded and code added to store this info.

See curl issue 13492

error buffer not set if connection to multiple addresses fails

If you ask libcurl to resolve a hostname like example.com to IPv6 addresses when you only have IPv4 connectivity. libcurl fails with CURLE_COULDNT_CONNECT, but the error buffer set by CURLOPT_ERRORBUFFER remains empty. Issue: curl issue 544

HTTP test server 'connection-monitor' problems

The connection-monitor feature of the HTTP test server does not work properly if some tests are run in unexpected order. Like 1509 and then 1525.

See curl issue 868

Connection information when using TCP Fast Open

CURLINFO_LOCAL_PORT (and possibly a few other) fails when TCP Fast Open is enabled.

See curl issue 1332 and curl issue 4296

test cases sometimes timeout

Occasionally, one of the tests timeouts. Inexplicably.

See curl issue 13350

CURLOPT_CONNECT_TO does not work for HTTPS proxy

It is unclear if the same option should even cover the proxy connection or if if requires a separate option.

See curl issue 14481

WinIDN test failures

Test 165 disabled when built with WinIDN.

setting a disabled option should return CURLE_NOT_BUILT_IN

When curl has been built with specific features or protocols disabled, setting such options with curl_easy_setopt() should rather return CURLE_NOT_BUILT_IN instead of CURLE_UNKNOWN_OPTION to signal the difference to the application

See curl issue 15472

LDAP

OpenLDAP hangs after returning results

By configuration defaults, OpenLDAP automatically chase referrals on secondary socket descriptors. The OpenLDAP backend is asynchronous and thus should monitor all socket descriptors involved. Currently, these secondary descriptors are not monitored, causing OpenLDAP library to never receive data from them.

As a temporary workaround, disable referrals chasing by configuration.

The fix is not easy: proper automatic referrals chasing requires a synchronous bind callback and monitoring an arbitrary number of socket descriptors for a single easy handle (currently limited to 5).

Generic LDAP is synchronous: OK.

See curl issue 622 and https://curl.se/mail/lib-2016-01/0101.html

LDAP on Windows does authentication wrong?

curl issue 3116

LDAP on Windows does not work

A simple curl command line getting ldap://ldap.forumsys.com returns an error that says no memory !

curl issue 4261

LDAPS requests to Active Directory server hang

curl issue 9580

TCP/IP

telnet code does not handle partial writes properly

It probably does not happen too easily because of how slow and infrequent sends are normally performed.

Trying local ports fails on Windows

This makes --local-port [range] to not work since curl cannot properly detect if a port is already in use, so it tries the first port, uses that and then subsequently fails anyway if that was actually in use.

curl issue 8112

CMake

cmake outputs: no version information available

Something in the SONAME generation seems to be wrong in the cmake build.

curl issue 11158

uses -lpthread instead of Threads::Threads

See curl issue 6166

generated .pc file contains strange entries

The Libs.private field of the generated .pc file contains -lgcc -lgcc_s -lc -lgcc -lgcc_s.

See curl issue 6167

CMake build with MIT Kerberos does not work

Minimum CMake version was bumped in curl 7.71.0 (#5358) Since CMake 3.2 try_compile started respecting the CMAKE_EXE_FLAGS. The code dealing with MIT Kerberos detection sets few variables to potentially weird mix of space, and ;-separated flags. It had to blow up at some point. All the CMake checks that involve compilation are doomed from that point, the configured tree cannot be built.

curl issue 6904

Authentication

--aws-sigv4 does not handle multipart/form-data correctly

curl issue 13351

HTTP/2

HTTP/2 prior knowledge over proxy

curl issue 12641

HTTP/2 frames while in the connection pool kill reuse

If the server sends HTTP/2 frames (like for example an HTTP/2 PING frame) to curl while the connection is held in curl's connection pool, the socket is found readable when considered for reuse and that makes curl think it is dead and then it is closed and a new connection gets created instead.

This is best fixed by adding monitoring to connections while they are kept in the pool so that pings can be responded to appropriately.

ENHANCE_YOUR_CALM causes infinite retries

Infinite retries with 2 parallel requests on one connection receiving GOAWAY with ENHANCE_YOUR_CALM error code.

See curl issue 5119

HTTP/2 + TLS spends a lot of time in recv

It has been observed that by making the speed limit less accurate we could improve this performance. (by reverting db5c9f4f9e0779) Can we find a golden middle ground?

See https://curl.se/mail/lib-2024-05/0026.html and curl issue 13416

HTTP/3

connection migration does not work

curl issue 7695

quiche: QUIC connection is draining

The transfer ends with error "QUIC connection is draining".

curl issue 12037

RTSP

Some methods do not support response bodies

The RTSP implementation is written to assume that a number of RTSP methods always get responses without bodies, even though there seems to be no indication in the RFC that this is always the case.

curl issue 12414