What is OCSP?
The Online Certificate Status Protocol checks whether one certificate has been revoked without downloading a whole certificate revocation list. The client sends the certificate's serial number, with identifiers for its issuer, to an OCSP responder run by or for the CA. The responder returns a signed answer: good, revoked or unknown. The responder's address is listed in the certificate's Authority Information Access extension. OCSP is defined in RFC 6960, and requests usually travel over plain HTTP, because the response is signed and cannot be altered without detection.
What an OCSP response looks like
An administrator can query a responder with openssl ocsp -issuer issuing-ca.pem -cert portal.pem -url followed by the responder address from the certificate. For a certificate revoked after its key leaked, the result looks like this:
Response verify OK
portal.pem: revoked
This Update: Oct 23 14:20:53 2026 GMT
Next Update: Oct 30 14:20:53 2026 GMT
Reason: keyCompromise
Revocation Time: Oct 23 14:20:52 2026 GMT
"Response verify OK" means the responder's signature checked out. Every response has a validity window, and clients may cache it until the Next Update time.
The weaknesses of plain OCSP
- Privacy: each check tells the CA which site the user is visiting.
- Speed and availability: the client has to reach the responder before it can finish trusting a site, which adds a round trip and a dependency.
- Soft-fail: browsers that check OCSP typically accept the certificate anyway when the responder is unreachable, so an attacker who can block OCSP traffic can hide a revocation.
OCSP stapling and Must-Staple
OCSP stapling moves the query to the server. The web server fetches a signed OCSP response for its own certificate on a schedule and attaches, or staples, it to the TLS handshake. Because the CA signed the response, the server cannot forge a good status, and clients never contact the CA, which removes the privacy and speed problems. Running openssl s_client -connect portal.example.com:443 -status shows the stapled response, or "OCSP response: no response sent" when the server does not staple. A certificate can also carry the OCSP Must-Staple extension, which tells clients that honor it to reject the certificate if no stapled response arrives, closing the soft-fail gap.
OCSP vs. CRL, and where public PKI is heading
A CRL is a complete list that clients download and cache, which is private but slow to update. OCSP answers about one certificate at a time with near-real-time status, at the cost of privacy and an extra dependency. In public web PKI the balance has shifted: the CA/Browser Forum made OCSP optional for public TLS CAs while keeping CRLs mandatory, Let's Encrypt shut down its OCSP service in 2025, and browsers increasingly use revocation lists their vendors compile from CRLs. Enterprise CAs, such as Active Directory Certificate Services with its Online Responder role, still commonly run OCSP.
OCSP on the CompTIA Security+ exam
OCSP is named in SY0-801 objective 1.3, together with CRLs, as a way to check certificate status. Wording such as "real-time revocation status", "check one certificate without downloading the full list" or "good, revoked or unknown" points to OCSP. "The web server provides revocation status during the handshake so browsers don't contact the CA" points to OCSP stapling. "A signed list of revoked serial numbers published on a schedule" is a CRL instead. Another distractor is certificate pinning, which fixes which certificate or key a client will accept but says nothing about revocation.