JWT Structure Analysis

A JSON Web Token consists of three parts: Header, Payload, and Signature.

Header

Declares the type and signing algorithm.

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

Payload

Contains standard claims (iss, exp) and custom data.

{
  "sub": "1234567890",
  "name": "John Doe",
  "iat": 1516239022
}

Signature

Signs the first two parts using a secret to prevent tampering.

Security Risks and Prevention

1. Algorithm Confusion: Enforce alg verification and ban none.
2. Sensitive Data Leakage: Payload is Base64 encoded, not encrypted. Never store passwords.
3. Replay Attacks: Use jti (unique ID) and short expiration times (exp).

Best Practices

• HTTPS Only: Encrypt transport to prevent eavesdropping.
• Short-lived Token + Refresh Token: Balance security with user experience.
• Server-side Blocklist: Use Redis to maintain revoked tokens for logout scenarios.

Summary

JWT is the cornerstone of modern stateless authentication, but its security relies heavily on correct implementation and strict configuration.