CURLOPT_PROXY_SSL_OPTIONS - HTTPS proxy SSL behavior options

#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXY_SSL_OPTIONS,
                          long bitmask);

Pass a long with a bitmask to tell libcurl about specific SSL behaviors. Available bits:

Tells libcurl to not attempt to use any workarounds for a security flaw in the SSL3 and TLS1.0 protocols. If this option is not used or this bit is set to 0, the SSL layer libcurl uses may use a work-around for this flaw although it might cause interoperability problems with some (older) SSL implementations. WARNING: avoiding this work-around lessens the security, and by setting this option to 1 you ask for exactly that. This option is only supported for Secure Transport and OpenSSL.
Tells libcurl to disable certificate revocation checks for those SSL backends where such behavior is present. This option is only supported for Schannel (the native Windows SSL library), with an exception in the case of Windows' Untrusted Publishers block list which it seems cannot be bypassed. (Added in 7.44.0)
Tells libcurl to not accept "partial" certificate chains, which it otherwise does by default. This option is only supported for OpenSSL and fails the certificate verification if the chain ends with an intermediate certificate and not with a root cert. (Added in 7.68.0)
Tells libcurl to ignore certificate revocation checks in case of missing or offline distribution points for those SSL backends where such behavior is present. This option is only supported for Schannel (the native Windows SSL library). If combined with CURLSSLOPT_NO_REVOKE, the latter takes precedence. (Added in 7.70.0)
Tell libcurl to use the operating system's native CA store for certificate verification. If you set this option and also set a CA certificate file or directory then during verification those certificates are searched in addition to the native CA store.

Works with wolfSSL on Windows, Linux (Debian, Ubuntu, Gentoo, Fedora, RHEL), macOS, Android and iOS (added in 8.3.0), with GnuTLS (added in 8.5.0) or on Windows when built to use OpenSSL (Added in 7.71.0).

Tell libcurl to automatically locate and use a client certificate for authentication, when requested by the server. This option is only supported for Schannel (the native Windows SSL library). Prior to 7.77.0 this was the default behavior in libcurl with Schannel. Since the server can request any certificate that supports client authentication in the OS certificate store it could be a privacy violation and unexpected. (Added in 7.77.0)


All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.

All TLS backends support this option.

int main(void)
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode res;
    curl_easy_setopt(curl, CURLOPT_URL, "");
    curl_easy_setopt(curl, CURLOPT_PROXY, "https://proxy");
    /* weaken TLS only for use with silly proxies */
    res = curl_easy_perform(curl);

Added in 7.52.0

Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.


April 26 2024 libcurl