What it is

The certificate chain couldn't be fully validated — an incomplete chain or an unknown intermediate authority. Depending on their browser, some visitors may see a security warning.

How to fix it

Check that your server sends every intermediate certificate, not just the leaf certificate — your certificate provider's install guide usually covers this.

In depth

Your website's security certificate has a missing piece in its chain of trust, which is like having an ID card that can't be verified all the way back to the issuing authority. When visitors come to your site, their browsers check that your certificate is legitimate by following a chain of verification, starting with your certificate and going back to a trusted root authority—but one or more intermediate certificates in that chain are missing. Some visitors, especially those using older browsers, will see a scary security warning instead of your site, which damages trust and can hurt your business. The fix is straightforward: you need to contact your certificate provider and ask them how to install the complete certificate chain on your server, which usually means adding intermediate certificate files that they provide. Most certificate providers have step-by-step installation guides that walk you through exactly which files to upload to your hosting account and where to put them. Once you upload all the pieces of the chain, your visitors will see your site normally without warnings.