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Certificate pinning hardcodes which specific certificate (or CA) an application should trust, rather than accepting any certificate signed by a trusted root.
Why It Adds Security
It protects against a compromised or fraudulently issued certificate being accepted, even if that certificate technically traces back to a trusted CA — a defense against a narrow but real attack scenario.
Why It's Mostly Used in Mobile Apps
Websites rarely use pinning since it's hard to update at scale across every visitor's browser. Mobile apps, which developers fully control, can embed and update pinned certificates through app updates.
The Trade-Off
Pinning incorrectly, or forgetting to update a pin before a certificate renews, can lock out legitimate users entirely — it requires careful, ongoing maintenance.
