What is an intermediate certificate?

An intermediate certificate sits between your certificate and the certificate authority’s root. The CA keeps its root keys offline, because a stolen root key would let an attacker impersonate any website, and instead signs day-to-day certificates with intermediates. Your certificate is signed by an intermediate; the intermediate is signed by the root; browsers trust the root.

This indirection is what makes the web PKI recoverable. If an intermediate key is compromised or mis-used, the CA can revoke that one intermediate and move issuance to another, without every trust store on earth having to remove a root. It is also why intermediates rotate on a schedule while roots last decades. Let’s Encrypt, for example, has replaced its full set of issuing intermediates more than once, and every rotation changes the chain your server is expected to send.

The operational catch is that the server, not the CA, delivers the intermediate to visitors. Your renewal tooling downloads a bundle containing the leaf and its intermediates, and the web server serves them together. When tooling writes the new leaf but keeps an old bundle, or a template hardcodes an intermediate that has since expired, visitors get a broken chain while the certificate itself is fine. Because some clients paper over the gap by fetching the missing intermediate themselves, the failure is intermittent and lands hardest on API clients, older devices, and anything that validates strictly.

An expired intermediate is one of the few failures that can take down many sites at once, since thousands of servers share the same intermediate. Checking what your server actually sends, rather than what sits on disk, is the only way to see it before your visitors do.