What is a root certificate?

A root certificate is the trust anchor of the web PKI: a self-signed certificate belonging to a certificate authority, shipped inside operating systems and browsers. Every certificate chain a browser accepts must end at one of these roots. There is no signature above a root; it is trusted because a root program decided to include it.

The root programs are run by the trust store owners: Mozilla, Google, Apple, and Microsoft each maintain one, and Mozilla’s is fully public. A CA that wants its certificates accepted must pass audits and follow each program’s policy, and a CA that breaks the rules can be removed. Removal is not theoretical: browsers have distrusted major CAs, and every certificate chaining to the removed roots stops working in that browser on the announced date.

Roots live for decades, but they do expire, and old devices make that messy. A phone that last updated its trust store years ago does not have the newer roots, so CAs bridge the gap with cross-signatures: a new root signed by an old one, so that old clients can still build a path they trust. When the old root finally expires, servers still serving the cross-signed chain can break old clients, which is what happened around several high-profile root expiries.

For an operator, the practical points are three. You never install a root on your server; you serve the leaf and intermediates only. Your certificate’s trust depends on decisions root programs make about your CA, which is a reason to watch CA news. And clients with stale trust stores fail in ways your own testing will not show, because your browser is not their browser.