JSON Web Token
A JSON Web Token (JWT) is a compact format for carrying claims between parties; its security depends on the protection and validation required by the protocol using it.
JWT is a format, not a purpose
Claims are statements such as an issuer, subject, intended audience, or expiration time. A JWT can carry them in signed, message-authenticated, encrypted, or nested forms. The specification also allows unsecured JWTs. Calling something a JWT does not tell you whether it is signed or confidential.
OpenID Connect ID tokens are JWTs. Some access tokens are JWTs too, but others use different formats. Start by identifying what the token is for.
Decoding is not validation
Decoding the payload of an unencrypted JWT lets you read its claims. It does not prove a trusted issuer created it, that it is still valid, or that your API should accept it. A signed token can also have readable claims; the signature does not hide them.
The receiving application needs to follow its protocol’s validation rules. Even a valid signature is not enough if the token was issued for a different recipient or purpose.
Check permitted algorithms, trusted keys and issuer, the intended audience, and required time limits. RFC 8725 calls for separate validation rules where different JWT types could be confused. Use a maintained library with explicit configuration, and keep live tokens out of public decoding tools.