curl / Docs / Project / Known Bugs

Known Bugs

This is the document 'docs/KNOWN_BUGS' from the curl release archive.

HTTP

1.2 hyper is slow
1.5 Expect-100 meets 417

TLS

2.3 Unable to use PKCS12 certificate with Secure Transport
2.4 Secure Transport will not import PKCS#12 client certificates without a password
2.5 Client cert handling with Issuer DN differs between backends
2.7 Client cert (MTLS) issues with Schannel
2.11 Schannel TLS 1.2 handshake bug in old Windows versions
2.12 FTPS with Schannel times out file list operation
2.13 CURLOPT_CERTINFO results in CURLE_OUT_OF_MEMORY with Schannel

Email protocols

3.1 IMAP SEARCH ALL truncated response
3.2 No disconnect command
3.3 POP3 expects "CRLF.CRLF" eob for some single-line responses
3.4 AUTH PLAIN for SMTP is not working on all servers
3.5 APOP authentication fails on POP3

Build and portability issues

5.1 OS400 port requires deprecated IBM library
5.2 curl-config --libs contains private details
5.3 building for old macOS fails with gcc
5.5 cannot handle Unicode arguments in non-Unicode builds on Windows
5.6 cygwin: make install installs curl-config.1 twice
5.9 Utilize Requires.private directives in libcurl.pc
5.11 configure --with-gssapi with Heimdal is ignored on macOS
5.12 flaky CI builds
5.13 long paths are not fully supported on Windows
5.14 Windows Unicode builds use homedir in current locale
5.15 Unicode on Windows

Authentication

6.1 NTLM authentication and unicode
6.2 MIT Kerberos for Windows build
6.3 NTLM in system context uses wrong name
6.5 NTLM does not support password with § character
6.6 libcurl can fail to try alternatives with --proxy-any
6.7 Do not clear digest for single realm
6.9 SHA-256 digest not supported in Windows SSPI builds
6.10 curl never completes Negotiate over HTTP
6.11 Negotiate on Windows fails
6.12 cannot use Secure Transport with Crypto Token Kit
6.13 Negotiate against Hadoop HDFS

FTP

7.1 FTP upload fails if remebered dir is deleted
7.3 FTP with NOBODY and FAILONERROR
7.4 FTP with ACCT
7.11 FTPS upload data loss with TLS 1.3
7.12 FTPS directory listing hangs on Windows with Schannel

SFTP and SCP

9.1 SFTP does not do CURLOPT_POSTQUOTE correct
9.2 wolfssh: publickey auth does not work
9.3 Remote recursive folder creation with SFTP
9.4 libssh blocking and infinite loop problem
9.5 cygwin: "WARNING: UNPROTECTED PRIVATE KEY FILE!"

SOCKS

10.3 FTPS over SOCKS

Internals

11.2 error buffer not set if connection to multiple addresses fails
11.4 HTTP test server 'connection-monitor' problems
11.5 Connection information when using TCP Fast Open

LDAP

12.1 OpenLDAP hangs after returning results
12.2 LDAP on Windows does authentication wrong?
12.3 LDAP on Windows does not work
12.4 LDAPS requests to ActiveDirectory server hang

TCP/IP

13.2 Trying local ports fails on Windows

CMake

15.1 cmake outputs: no version information available
15.2 support build with GnuTLS
15.3 unusable tool_hugehelp.c with MinGW
15.6 uses -lpthread instead of Threads::Threads
15.7 generated .pc file contains strange entries
15.8 libcurl.pc uses absolute library paths
15.11 ExternalProject_Add does not set CURL_CA_PATH
15.13 CMake build with MIT Kerberos does not work

aws-sigv4

16.1 aws-sigv4 does not sign requests with * correctly
16.6 aws-sigv4 does not behave well with AWS VPC Lattice

HTTP/2

17.2 HTTP/2 frames while in the connection pool kill reuse
17.3 ENHANCE_YOUR_CALM causes infinite retries

HTTP/3

18.1 connection migration does not work

RTSP

19.1 Some methods do not support response bodies


1. HTTP

1.2 hyper is slow

When curl is built to use hyper for HTTP, it is unnecessary slow.

https://github.com/curl/curl/issues/11203

1.5 Expect-100 meets 417

If an upload using Expect: 100-continue receives an HTTP 417 response, it ought to be automatically resent without the Expect:. A workaround is for the client application to redo the transfer after disabling Expect:. https://curl.se/mail/archive-2008-02/0043.html

2. TLS

2.3 Unable to use PKCS12 certificate with Secure Transport

See https://github.com/curl/curl/issues/5403

2.4 Secure Transport will not import PKCS#12 client certificates without a password

libcurl calls SecPKCS12Import with the PKCS#12 client certificate, but that function rejects certificates that do not have a password. https://github.com/curl/curl/issues/1308

2.5 Client cert handling with Issuer DN differs between backends

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

See https://github.com/curl/curl/issues/1411

2.7 Client cert (MTLS) issues with Schannel

See https://github.com/curl/curl/issues/3145

2.11 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.

https://github.com/curl/curl/issues/5488

2.12 FTPS with Schannel times out file list operation

"Instead of the command completing, it just sits there until the timeout expires." - the same command line seems to work with other TLS backends and other operating systems. See https://github.com/curl/curl/issues/5284.

2.13 CURLOPT_CERTINFO results in CURLE_OUT_OF_MEMORY with Schannel

https://github.com/curl/curl/issues/8741

3. Email protocols

3.1 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

3.2 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.

3.3 POP3 expects "CRLF.CRLF" eob for some single-line responses

You have to tell libcurl not to expect a body, when dealing with one line response commands. Please see the POP3 examples and test cases which show this for the NOOP and DELE commands. https://curl.se/bug/?i=740

3.4 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 https://github.com/curl/curl/issues/4080

3.5 APOP authentication fails on POP3

See https://github.com/curl/curl/issues/10073

4. Command line

5. Build and portability issues

5.1 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 https://github.com/curl/curl/issues/5176

5.2 curl-config --libs contains private details

"curl-config --libs" will 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.

5.3 building for old macOS fails with gcc

Building curl for certain old macOS versions fails when gcc is used. We command using clang in those cases.

See https://github.com/curl/curl/issues/11441

5.5 cannot handle Unicode arguments in non-Unicode builds on Windows

If a URL or filename cannot be encoded using the user's current codepage 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. And, 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

5.6 cygwin: make install installs curl-config.1 twice

https://github.com/curl/curl/issues/8839

5.9 Utilize Requires.private directives in libcurl.pc

https://github.com/curl/curl/issues/864

5.11 configure --with-gssapi with Heimdal is ignored on macOS

... unless you also pass --with-gssapi-libs

https://github.com/curl/curl/issues/3841

5.12 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 https://github.com/curl/curl/issues/6972

5.13 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 https://github.com/curl/curl/issues/8361

5.14 Windows Unicode builds use homedir 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 https://github.com/curl/curl/pull/7252 and

 https://github.com/curl/curl/pull/7281

5.15 Unicode on Windows

Passing in a unicode filename with -o:

https://github.com/curl/curl/issues/11461

Passing in unicode character with -d:

https://github.com/curl/curl/issues/12231

6. Authentication

6.1 NTLM authentication and unicode

NTLM authentication involving unicode user name 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 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

6.2 MIT Kerberos for Windows build

libcurl fails to build with MIT Kerberos for Windows (KfW) due to KfW's library header files exporting symbols/macros that should be kept private to the KfW library. See ticket #5601 at https://krbdev.mit.edu/rt/

6.3 NTLM in system context uses wrong name

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

6.5 NTLM does not support password with § character

https://github.com/curl/curl/issues/2120

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

When connecting via a proxy using --proxy-any, a failure to establish an authentication will cause 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 will not proceed to try authentication using NTLM.

https://github.com/curl/curl/issues/876

6.7 Do not clear digest for single realm

https://github.com/curl/curl/issues/3267

6.9 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).

https://github.com/curl/curl/issues/6302

6.10 curl never completes Negotiate over HTTP

Apparently it is not working correctly...?

See https://github.com/curl/curl/issues/5235

6.11 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.

https://github.com/curl/curl/issues/5881

6.12 cannot use Secure Transport with Crypto Token Kit

https://github.com/curl/curl/issues/7048

6.13 Negotiate authentication against Hadoop HDFS

https://github.com/curl/curl/issues/8264

7. FTP

7.1 FTP upload fails if remebered dir is deleted

curl's FTP code assumes that the directory it entered in a previous transfer still exists when it comes back to do a second transfer, and does not respond well if it was indeed deleted in the mean time.

https://github.com/curl/curl/issues/12181

7.3 FTP with NOBODY and FAILONERROR

It seems sensible to be able to use CURLOPT_NOBODY and CURLOPT_FAILONERROR with FTP to detect if a file exists or not, but it is not working: https://curl.se/mail/lib-2008-07/0295.html

7.4 FTP with ACCT

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

7.11 FTPS upload data loss with TLS 1.3

During FTPS upload curl does not attempt to read TLS handshake messages sent after the initial handshake. OpenSSL servers running TLS 1.3 may send such a message. When curl closes the upload connection if unread data has been received (such as a TLS handshake message) then the TCP protocol sends an RST to the server, which may cause the server to discard or truncate the upload if it has not read all sent data yet, and then return an error to curl on the control channel connection.

Since 7.78.0 this is mostly fixed. curl will do a single read before closing TLS connections (which causes the TLS library to read handshake messages), however there is still possibility of an RST if more messages need to be read or a message arrives after the read but before close (network race condition).

https://github.com/curl/curl/issues/6149

7.12 FTPS directory listing hangs on Windows with Schannel

https://github.com/curl/curl/issues/9161

9. SFTP and SCP

9.1 SFTP does not do CURLOPT_POSTQUOTE correct

When libcurl sends CURLOPT_POSTQUOTE commands when connected to a SFTP server using the multi interface, the commands are not being sent correctly and instead the connection is "cancelled" (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

9.2 wolfssh: publickey auth does not work

When building curl to use the wolfSSH backend for SFTP, the publickey authentication does not work. This is simply functionality not written for curl yet, the necessary API for make this work is provided by wolfSSH.

See https://github.com/curl/curl/issues/4820

9.3 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 https://github.com/curl/curl/issues/5204

9.4 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 will be stuck, even if curl is configured with a timeout.

https://github.com/curl/curl/issues/8632

9.5 cygwin: "WARNING: UNPROTECTED PRIVATE KEY FILE!"

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

https://github.com/curl/curl/issues/11244

10. SOCKS

10.3 FTPS over SOCKS

libcurl does not support FTPS over a SOCKS proxy.

11. Internals

11.2 error buffer not set if connection to multiple addresses fails

If you ask libcurl to resolve a hostname like example.com to IPv6 addresses only. But you only have IPv4 connectivity. libcurl will correctly fail with CURLE_COULDNT_CONNECT. But the error buffer set by CURLOPT_ERRORBUFFER remains empty. Issue: https://github.com/curl/curl/issues/544

11.4 HTTP test server 'connection-monitor' problems

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

See https://github.com/curl/curl/issues/868

11.5 Connection information when using TCP Fast Open

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

See https://github.com/curl/curl/issues/1332 and https://github.com/curl/curl/issues/4296

12. LDAP

12.1 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 https://github.com/curl/curl/issues/622 and

 https://curl.se/mail/lib-2016-01/0101.html

12.2 LDAP on Windows does authentication wrong?

https://github.com/curl/curl/issues/3116

12.3 LDAP on Windows does not work

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

https://github.com/curl/curl/issues/4261

12.4 LDAPS requests to ActiveDirectory server hang

https://github.com/curl/curl/issues/9580

13. TCP/IP

13.2 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 will try the first port, use that and then subsequently fail anyway if that was actually in use.

https://github.com/curl/curl/issues/8112

15. CMake

15.1 cmake outputs: no version information available

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

https://github.com/curl/curl/issues/11158

15.2 support build with GnuTLS

15.3 unusable tool_hugehelp.c with MinGW

see https://github.com/curl/curl/issues/3125

15.6 uses -lpthread instead of Threads::Threads

See https://github.com/curl/curl/issues/6166

15.7 generated .pc file contains strange entries

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

See https://github.com/curl/curl/issues/6167

15.8 libcurl.pc uses absolute library paths

The libcurl.pc file generated by cmake contains things like Libs.private: /usr/lib64/libssl.so /usr/lib64/libcrypto.so /usr/lib64/libz.so. The autotools equivalent would say Libs.private: -lssl -lcrypto -lz

See https://github.com/curl/curl/issues/6169

15.11 ExternalProject_Add does not set CURL_CA_PATH

CURL_CA_BUNDLE and CURL_CA_PATH are not set properly when cmake's ExternalProject_Add is used to build curl as a dependency.

See https://github.com/curl/curl/issues/6313

15.13 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.

https://github.com/curl/curl/issues/6904

16. aws-sigv4

16.1 aws-sigv4 does not sign requests with * correctly

https://github.com/curl/curl/issues/7559

16.6 aws-sigv4 does not behave well with AWS VPC Lattice

https://github.com/curl/curl/issues/11007

17. HTTP/2

17.2 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 will be found readable when considered for reuse and that makes curl think it is dead and then it will be 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.

17.3 ENHANCE_YOUR_CALM causes infinite retries

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

See https://github.com/curl/curl/issues/5119

18. HTTP/3

18.1 connection migration does not work

https://github.com/curl/curl/issues/7695

19. RTSP

19.1 Some methods do not support response bodies

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

https://github.com/curl/curl/issues/12414