An advanced technique with real risks.
WHAT IT IS
Configuring a client to accept only a specific certificate or key for a given host, rejecting any other even if validly issued.
WHERE IT APPEARS
Mobile applications connecting to their own backend Some payment integrations High-security internal systems
WHY IT EXISTS
It protects against a validly issued but unauthorised certificate, such as one obtained through a compromised authority or a corporate inspection proxy.
WHY IT IS RISKY
When your certificate is renewed with a new key, every pinned client stops connecting until updated. With a mobile app, that means an app store release and users who have not updated.
This has broken major services.
IF YOU PIN
Pin to an intermediate rather than the leaf certificate where possible Include a backup pin for a key you have not yet deployed Have a plan for emergency rotation
FOR MOST SITES
You will never need this. It is mentioned because people occasionally ask whether they should. The answer is almost always no.
