Intermediate CA (certificate authority)
An intermediate CA (certificate authority) sits between a root CA and the certificates that websites and services present. The root CA signs the intermediate's certificate, and the intermediate signs the day-to-day TLS certificates. A client such as a browser or curl trusts a site because it can walk the chain from the site certificate, through the intermediate, up to a root in its trust store.
Why intermediates exist
Root keys are the most valuable keys in public key infrastructure, so CAs keep them offline and use them rarely. Intermediates do the frequent signing. If an intermediate is lost or misused, it can be revoked without replacing the root that every device already trusts.
How a client checks the chain
The server sends its certificate plus the intermediate certificate.
The client checks that the intermediate signed the site certificate, and that a root in its trust store signed the intermediate.
Browsers also check revocation data and Certificate Transparency logs. Many command-line tools and language runtimes only check the chain.
When an intermediate is compromised
Anyone holding an intermediate's private key can issue a certificate for any domain, and every client that trusts the root will accept it. In October 2026, the private keys of eight eMudhra intermediate CAs turned out to be recoverable from their public certificates. Browsers blocked them within days, but tools such as curl, Python, Node.js, Go and Java kept trusting them until the two roots were untrusted or their CA bundles were updated. That is why the fix is to verify each trust store, not just the browser.
Related: mutual TLS.
