Guides & TutorialsTroubleshooting AdvancedTroubleshooting Unsupported Protocol Errors (TLS Support)

Troubleshooting Unsupported Protocol Errors (TLS Support)


An 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 on the Help > System Health Status page:

Your server is attempting a secure connection to using the outdated SSL protocol. As this is no-longer secure, most providers now require connections be made using the newer TLS protocols instead:

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

As a result, any restrictions to the cryptographic protocol WHMCS can use are as a result of the server configuration and not WHMCS itself.


To help resolve such errors it will be necessary to work with your server admin/hosting provider to ensure that remote cURL connections are made using the TLS 1. protocols by default, rather than the outdated SSL, TLS 1.0 and TLS 1.1 protocols.

There are a few items which can be checked quickly to help identify the cause of this error: