HMAC-SHA256 is a message authentication code built from a secret key and the SHA-256 hash function. It does not encrypt data. Instead, it produces a short tag that proves a message has not been altered and was created by someone who knows the shared secret. If the tag does not match, the message should be rejected.
In security systems, HMAC-SHA256 is often used to protect tokens, API requests, and SSO handshakes. It matters because a login or admin request may look valid on the surface, but without correct signature checking an attacker can reuse, forge, or relabel data across trust boundaries. Defenders must verify the exact bytes that were signed, use a strong secret key, and compare tags in constant time. Attackers often target implementation mistakes, such as inconsistent canonicalization, missing fields, or checking the wrong message before accepting authentication.