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Troubleshooting Unsupported Protocol Errors (TLS Support)

Introduction

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Your server is attempting a secure connection to using the outdated SSL protocol.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.

Troubleshooting

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provider to ensure that remote cURL connections are made using the TLS 1.1 or 1.2 protocols by default, rather than the outdated SSL and TLS 1.0 protocols.

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