A cURL Error code 35 Unknown SSL protocol or Unsupported SSL protocol error may occur when WHMCS attempts to connect to a remote service such as a payment gateway or registrar via a secure connection.

You may also see a Secure TLS Support in cURL warning at Configuration > System Health (Help > System Health Status prior to WHMCS 8.0):

When you see these errors, the server is attempting a secure connection using an outdated SSL protocol. Due to security risks, most providers now require connections to use up-to-date TLS protocols.

For more information, see:

• Stripe Documentation
• Authorize.net Documentation
• PayPal Documentation
• Nominet Documentation

WHMCS is cryptographic protocol agnostic and we do not specify a protocol version when establishing cURL connections to external services. cURL will auto-negotiate the best available cryptographic protocol based on the server configuration and the service being connected to.

Any restrictions to the cryptographic protocol WHMCS can use are a result of the server configuration and not WHMCS itself.

Work with your hosting provider or system administrator to ensure that remote cURL connections use TLS 1. protocols by default. Ensure that they do not use outdated SSL, TLS 1.0, and TLS 1.1 protocols.

To help identify the cause of this error, you can:

• Update to an active version of WHMCS.
• Ensure that WHMCS is using a supported PHP version for your version of WHMCS.
• Ensure that the version of cURL on your server can support the correct cryptographic protocol for the service you want to connect to.
• Ensure that the SSL library on your server can support the correct cryptographic protocol for the service you want to connect to.
• Test your website using a service like ssltest. Ensure that your required protocol displays at the top of the list: