1.1.6 Think Chains, Not Certificates
Although we spend a lot of time talking about server certificates, in practice we need complete and valid certificate chains to establish secure connections. Because this is something server operators have to configure manually, mistakes are rife. Most commonly, you will see TLS servers with just the leaf certificate or a set of certificates that don’t actually form a valid chain.
An invalid certificate chain may render the server certificate invalid, causing a browser warning. To make things worse, this problem is often difficult to diagnose because some browsers try hard to fix it and others don’t. This is a good example of a problem that should be diagnosed by an independent assessment tool.