Introduction
A cURL error 35 occurs when the cURL function on your server is unable to establish a secure connection via SSL. This usually happens because of a mismatch in SSL certificates or protocols between your server and the service you are trying to reach. Since cURL often uses its own set of certificates bundled with PHP, an outdated environment is the most common cause.
Problem
You will see this error message when:
Your WordPress site fails to connect to an external API (like bol.com or Amazon).
You see the specific error:
Unknown SSL protocol error in connectionorcURL error 35in your logs.Synchronizations fail despite having a valid SSL certificate on your own website.
How to solve
To resolve the cURL 35 error, you need to ensure your server environment is up to date:
Update your PHP version: Older PHP versions use outdated SSL libraries. Ensure you are running PHP 8.1 or 8.2 for the best compatibility and security.
Update your cURL version: The cURL version on your server must support modern TLS protocols (like TLS 1.2 or 1.3). Check out this guide on how to update the cURL version.
Contact your hosting provider: If updating PHP doesn't fix it, ask your host to update the NSS or OpenSSL libraries on the server, as these handle the actual SSL handshake.
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