cURL Docs SSL Certificates
Details on Server SSL Certificates
SSL Certificate Verification
SSL is TLS
SSL is the old name. It is called TLS these days.
If libcurl was built with Schannel or Secure Transport support (the native SSL libraries included in Windows and Mac OS X), then this does not apply to you. Scroll down for details on how the OS-native engines handle SSL certificates. If you're not sure, then run "curl -V" and read the results. If the version string says "WinSSL" in it, then it was built with Schannel support.
It is about trust
This system is about trust. In your local CA cert bundle you have certs from trusted Certificate Authorities that you then can use to verify that the server certificates you see are valid. They're signed by one of the CAs you trust.
Which CAs do you trust? You can decide to trust the same set of companies your operating system trusts, or the set one of the known browsers trust. That's basically trust via someone else you trust. You should just be aware that modern operating systems and browsers are setup to trust hundreds of companies and recent years several such CAs have been found untrustworthy.
libcurl performs peer SSL certificate verification by default. This is done by using CA cert bundle that the SSL library can use to make sure the peer's server certificate is valid.
If you communicate with HTTPS, FTPS or other TLS-using servers using certificates that are signed by CAs present in the bundle, 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 don't install a CA cert bundle, if the server uses a certificate signed by a CA that isn't included in the bundle you use or if the remote host is an impostor impersonating your favorite site, and you want to transfer files from this server, do one of the following:
Neglecting to use one of the above methods when dealing with a server using a certificate that isn't signed by one of the certificates in the installed CA cert bundle, will cause SSL to report an error ("certificate verify failed") during the handshake and SSL will then refuse further communication with that server.
Certificate Verification with NSS
If libcurl was built with NSS support, then depending on the OS distribution, it is probably required to take some additional steps to use the system-wide CA cert db. RedHat ships with an additional module, libnsspem.so, which enables NSS to read the OpenSSL PEM CA bundle. This library is missing in OpenSuSE, and without it, NSS can only work with its own internal formats. NSS also has a new database format.
Starting with version 7.19.7, libcurl automatically adds the 'sql:' prefix to the certdb directory (either the hardcoded default /etc/pki/nssdb or the directory configured with SSL_DIR environment variable). To check which certdb format your distribution provides, examine the default certdb location: /etc/pki/nssdb; the new certdb format can be identified by the filenames cert9.db, key4.db, pkcs11.txt; filenames of older versions are cert8.db, key3.db, secmod.db.
Certificate Verification with Schannel and Secure Transport
If libcurl was built with Schannel (Microsoft's native TLS engine) or Secure Transport (Apple's native TLS engine) support, then libcurl will still perform peer certificate verification, but instead of using a CA cert bundle, it will use the certificates that are built into the OS. These are the same certificates that appear in the Internet Options control panel (under Windows) or Keychain Access application (under OS X). Any custom security rules for certificates will be honored.
Schannel will run CRL checks on certificates unless peer verification is disabled. Secure Transport on iOS will run OCSP checks on certificates unless peer verification is disabled. Secure Transport on OS X will run either OCSP or CRL checks on certificates if those features are enabled, and this behavior can be adjusted in the preferences of Keychain Access.