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Security Oct 21, 2026 8 min read

Securing Your APIs: A Deep Dive into JSON Web Tokens (JWT)

Explore the anatomy of a JWT, how to securely sign and decode them, and best practices for authentication in modern web applications.

Anatomy of a JSON Web Token

A JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object.

A JWT consists of three parts separated by dots (.):

  1. Header: Contains the token type (JWT) and the signing algorithm being used (e.g., HMAC SHA256 or RSA).
  2. Payload: Contains the claims. Claims are statements about an entity (typically, the user) and additional data.
  3. Signature: Used to verify that the sender of the JWT is who it says it is and to ensure that the message wasn't changed along the way.

Common Security Pitfalls

  • Exposing Sensitive Data: The payload is simply Base64Url encoded, not encrypted. Anyone can read the contents of a JWT payload. Never store passwords, SSNs, or other sensitive data in the payload.
  • Ignoring the exp Claim: Always set a short expiration time (exp) for access tokens.
  • Accepting the none Algorithm: Ensure your server strictly enforces the expected algorithm and does not accept tokens where the header specifies "alg": "none".

Need to decode a token to see what claims it holds? Use our offline JWT Debugger to safely inspect tokens without sending them to an external server.