Understanding JWT Tokens: Structure and Security

JSON Web Tokens (JWT) are a compact, URL-safe means of representing claims between two parties. They're widely used for authentication and information exchange.

JWT Structure

A JWT consists of three parts separated by dots: header.payload.signature

Header

{
  "alg": "HS256",
  "typ": "JWT"
}

Payload

Contains the claims — statements about the user and additional metadata.

Signature

Created by signing the header and payload with a secret key.

Common Claims

Claim	Description
`iss`	Issuer
`sub`	Subject
`exp`	Expiration time
`iat`	Issued at
`aud`	Audience

Security Best Practices

Always validate tokens server-side — never trust client-side validation alone

Set expiration times — use short-lived tokens with refresh tokens

Use strong signing algorithms — prefer RS256 over HS256 for distributed systems

Never store sensitive data in the payload — JWT payloads are only Base64-encoded, not encrypted

Try It Yourself

Decode and inspect any JWT token with our free JWT Decoder.