TLS Certificate Verification

Native vs file based

If curl was built with Schannel support, then curl uses the Windows native CA store for verification. On Apple operating systems, it is possible to use Apple's "SecTrust" services for certain TLS backends, details below. All other TLS libraries use a file based CA store by default.

Verification

Every trusted server certificate is digitally signed by a Certificate Authority, a CA.

In your local CA store you have a collection of certificates from trusted certificate authorities that TLS clients like curl use to verify servers.

curl does certificate verification by default. This is done by verifying the signature and making sure the certificate was crafted for the server name provided in the URL.

If you communicate with HTTPS, FTPS or other TLS-using servers using certificates signed by a CA whose certificate is present in the store, you can be sure that the remote server really is the one it claims to be.

If the remote server uses a self-signed certificate, if you do not install a CA cert store, if the server uses a certificate signed by a CA that is not included in the store you use or if the remote host is an impostor impersonating your favorite site, the certificate check fails and reports an error.

If you think it wrongly failed the verification, consider one of the following sections.

Skip verification

Tell curl to not verify the peer with -k/--insecure.

We strongly recommend this is avoided and that even if you end up doing this for experimentation or development, never skip verification in production.

Use a custom CA store

Get a CA certificate that can verify the remote server and use the proper option to point out this CA cert for verification when connecting - for this specific transfer only.

With the curl command line tool: --cacert [file]

If you use the curl command line tool without a native CA store, then you can specify your own CA cert file by setting the environment variable CURL_CA_BUNDLE to the path of your choice. SSL_CERT_FILE and SSL_CERT_DIR are also supported.

If you are using the curl command line tool on Windows, curl searches for a CA cert file named curl-ca-bundle.crt in these directories and in this order:

1. application's directory
2. current working directory
3. Windows System directory (e.g. C:\Windows\System32)
4. Windows Directory (e.g. C:\Windows)
5. all directories along %PATH%

curl 8.11.0 added a build-time option to disable this search behavior, and another option to restrict search to the application's directory.

Use the native store

In several environments, in particular on Microsoft and Apple operating systems, you can ask curl to use the system's native CA store when verifying the certificate. Depending on how curl was built, this may already be the default.

With the curl command line tool: --ca-native.

Modify the CA store

Add the CA cert for your server to the existing default CA certificate store.

Usually you can figure out the path to the local CA store by looking at the verbose output that curl -v shows when you connect to an HTTPS site.

Change curl's default CA store

The default CA certificate store curl uses is set at build time. When you build curl you can point out your preferred path.

Extract CA cert from a server

curl -w %{certs} https://example.com > cacert.pem

The certificate has BEGIN CERTIFICATE and END CERTIFICATE markers.

Get the Mozilla CA store

Download a version of the Firefox CA store converted to PEM format on the CA Extract page. It always features the latest Firefox bundle.

Native CA store

Windows + Schannel

If curl was built with Schannel, then curl uses the certificates that are built into the OS. These are the same certificates that appear in the Internet Options control panel (under Windows). Any custom security rules for certificates are honored.

Schannel runs CRL checks on certificates unless peer verification is disabled.

Apple + OpenSSL/GnuTLS

When curl is built with Apple SecTrust enabled and uses an OpenSSL compatible TLS backend or GnuTLS, the default verification is handled by that Apple service. As in:

curl https://example.com

You may still provide your own certificates on the command line, such as:

curl --cacert mycerts.pem https://example.com

In this situation, Apple SecTrust is not used and verification is done only with the trust anchors found in mycerts.pem. If you want both Apple SecTrust and your own file to be considered, use:

curl --ca-native --cacert mycerts.pem https://example.com

Other Combinations

How well the use of native CA stores work in all other combinations depends on the TLS backend and the OS. Many TLS backends offer functionality to access the native CA on a range of operating systems. Some provide this only on specific configurations.

Specific support in curl exists for Windows and OpenSSL compatible TLS backends. It tries to load the certificates from the Windows "CA" and "ROOT" stores for transfers requesting the native CA. Due to Window's delayed population of those stores, this might not always find all certificates.

HTTPS proxy

curl can do HTTPS to the proxy separately from the connection to the server. This TLS connection is handled and verified separately from the server connection so instead of --insecure and --cacert to control the certificate verification, you use --proxy-insecure and --proxy-cacert. With these options, you make sure that the TLS connection and the trust of the proxy can be kept totally separate from the TLS connection to the server.