Node.jsIntermediate9 min read2026-03-01
JWT Authentication Explained: Under the Hood
Understand JSON Web Token structure: Header, Payload, Signature, hashing algorithms, and XSS/CSRF mitigation.
Prerequisites
- Basic web security and HTTP concepts
1. Token Anatomy: Header.Payload.Signature
A JWT is composed of three Base64URL-encoded strings separated by periods.
text
// Example JWT structure:
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkphbmUgRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5cBest Practices & Architecture Advice
- Always store sensitive access tokens in memory or HttpOnly, Secure, SameSite=Strict cookies to protect against XSS.
- Sign tokens using strong cryptographic algorithms (HMAC-SHA256 with 256-bit keys or asymmetric RSA/EdDSA).
Common Mistakes to Watch Out For
- •Storing confidential data like passwords in JWT payloads; the payload is only encoded, not encrypted!
Frequently Asked Questions
How do I invalidate a JWT before it expires?
Since JWTs are stateless, instant revocation requires either maintaining a distributed Redis token blacklist or rotating user token version counters.
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