OCSP stapling

OCSP stapling is a TLS feature where the server fetches a signed, timestamped statement from the certificate authority saying "this certificate is not revoked", and attaches — staples — it to the TLS handshake.

Why it exists

A certificate can be revoked before it expires: a key leaks, a domain changes hands, a CA discovers it issued in error. Browsers need a way to find out.

The original mechanism, plain OCSP, had the browser ask the CA directly. That is slow, it fails badly when the CA is unreachable, and it tells the CA which sites every visitor is browsing.

Stapling moves the lookup to the server, which does it periodically rather than per visitor. The response is signed by the CA, so the server cannot forge it.

What goes wrong

The usual failure is a server configured to staple that quietly stops — the cached response expires, the CA is briefly unreachable, and the handshake continues without a staple. Nothing breaks visibly. You simply lose the property you thought you had.

The other is Must-Staple, a flag some certificates carry that tells browsers to refuse a handshake with no staple. Excellent for security and a foot-gun: a stapling failure becomes an outage rather than a degradation.

Related

The SSL certificate checker reports the TLS configuration of any host, and IT Watch grades the same configuration continuously on every monitored site.

See also: Certificate Transparency, SAN.