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Mastering JWT Tokens: From Basics to Best Practices

Code Crafter
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JSON Web Tokens (JWT) have become a cornerstone of modern web authentication and API authorization. They're compact, URL-safe and easy to work with, making them a popular choice for developers building secure web and mobile applications.

In this guide, we will take you on a journey from the basics of JWTs to advanced use cases and best practices.

Table of Contents

  • What is a JWT Token?
  • JWT Structure Explained
  • Why Use JWTs?
  • How JWT Authentication Works
  • Common Use Cases
  • JWT Implementation
  • JWT Security Best Practices

What is a JWT Token?

JWT (JSON Web Token) is a compact, URL-safe token used to securely transmit claims between two parties. It's digitally signed, allowing systems to verify authenticity without relying on a centralized session store. JWTs are widely used for authentication, authorization and secure information exchange in modern web applications.

JWT Structure

A JWT consists of three base64-encoded parts separated by dots (.)

<Header>.<Payload>.<Signature>

1. Header

Typically consists of two parts: the type of token and the signing algorithm

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

2. Payload

Contains the claims. Claims are statements about an entity (typically, the user) and additional metadata.

{
  "sub": "1234567890",
  "name": "John Doe",
  "admin": true,
  "exp": 1717040000
}

Claims can be registered, public, or private.

3. Signature

Used to verify the token was not altered. It's created by combining the encoded header, payload, a secret key and the algorithm specified. The JWT signature is calculated like this

HMACSHA256(
  base64UrlEncode(header) + "." +
  base64UrlEncode(payload),
  secret)

Then the resulting JWT looks like

JWT = base64urlEncode(header) + "." + base64urlEncode(payload) + "." + base64urlEncode(signature)

Why Use JWTs?

JWTs offer several advantages

  • ✅ Stateless Authentication
  • ✅ Scalability
  • ✅ Compact and Portable
  • ✅ Cross-domain support
  • ✅ Secure Information Exchange

How JWT Authentication Works

Here’s a step-by-step flow of JWT-based authentication

🔐 Step 1: Login Request

User provides credentials (e.g., username/password).

🔑 Step 2: Token Issued

Server verifies credentials and issues a signed JWT.

📦 Step 3: Token Stored Client-side

Token is usually stored in localStorage or cookies.

🔄 Step 4: Token Sent with Requests

Client sends the token in the Authorization header

Authorization: Bearer <token>

✅ Step 5: Token Verified

Server validates the token's signature and expiration before granting access.

Common Use Cases

Authentication (Login Sessions)

  • Use Case: After a user logs in successfully, the server creates a JWT and sends it to the client.
  • Client stores it: Usually in localStorage or cookies.
  • Sent on each request: Attached as an Authorization: Bearer <token> header.
  • Benefits: Stateless authentication (no need to store session in the backend).

Authorization

  • Use Case: Determine user access to specific resources (e.g., admin-only dashboards, role-based permissions).

Information Exchange

  • Use Case: Securely transmit user or session information between parties.
  • Example: Microservices can pass user context with JWTs for API-to-API communication.

Single Sign-On (SSO)

  • Use Case: JWTs are ideal for SSO implementations because they can securely carry user identity across different domains or services.
  • Example: Authenticate once in Service A → Access Services B, C, D using same JWT.

Stateless APIs (Token-based Authentication)

  • Use Case: RESTful APIs use JWTs to manage authentication without requiring a session store.
  • JWTs fit perfectly into HTTP headers and mobile APIs.

OAuth 2.0 / OpenID Connect

  • Use Case: JWTs (often in the form of ID tokens or access tokens) are standard in OAuth and OpenID Connect flows.
  • Example: Google, Facebook, Microsoft use JWT-based ID tokens for login.

Refresh Tokens

  • Use Case: Combine short-lived access tokens with long-lived refresh tokens.
  • JWT for access: Keeps user logged in with minimal security risk.

Cross-Service Communication in Microservices

  • Use Case: Pass identity/context between microservices using JWTs.
  • Benefits: No central auth server needed to re-check credentials.

Serverless &amp; Edge Architectures

  • Use Case: Stateless nature of JWTs makes them perfect for serverless apps (e.g., AWS Lambda, Cloudflare Workers).

Secure WebSocket Authentication

  • Use Case: Use JWT in the initial WebSocket handshake to authenticate clients.

JWT Implementation

A complete, structured guide to implementing JWT authentication in Java (Spring Boot). Full step by step guide with code &amp; explanation

👉 Read a full guide how to Implement JWT in Java (Step-by-Step)

JWT Security Best Practices

  • Use HTTPS Always : Always send tokens over HTTPS to prevent man-in-the-middle attacks.
  • Set Expiry (exp) : Always define an expiration (exp) for your JWTs tokens, should never live forever. Short-lived tokens reduce risk. For longer user sessions, pair them with a refresh token system to maintain both security and usability.
  • Keep JWT Payload Minimal : Only include essential information in the JWT payload. Avoid putting sensitive data like passwords, secrets or personal details
  • Rotate Secrets : Change your signing secret/key periodically.
  • Implement Refresh Tokens Securely : Use short-lived access tokens for better security and long-lived refresh tokens to maintain user sessions. Store refresh tokens securely using HTTP-only cookies and ensure they are revoked on logout or when suspicious activity is detected.
  • Never Store JWTs in LocalStorage : Avoid storing JWTs in LocalStorage, as it's susceptible to XSS attacks. Instead, use HTTP-only, Secure cookies to store tokens safely and protect user data from client-side threats.
  • Use Strong Signing Algorithms : Always sign JWTs using secure algorithms like HS256 or RS256. Avoid using the none algorithm, which can allow unsigned tokens and pose serious security risks.
  • Revoke Tokens on Logout or Breach : Since JWTs are stateless, revoking them isn’t straightforward. To handle this, consider using a blacklist or tracking token IDs (jti) so you can manually revoke tokens when a user logs out or if there’s any suspicious activity. It's an extra step, but crucial for maintaining security.
  • Prevent Token Leakage in URLs : Avoid sending JWTs in query parameters like ?token=abc123. They are easily exposed through browser history, server logs or third-party analytics. Instead, always pass tokens securely using the Authorization header.

Conclusion

JWTs are a powerful way to handle authentication and authorization in modern web applications. When used correctly, they can make your app more scalable, secure, and flexible.

Whether you're building a login system, securing APIs, or managing microservices communication, JWTs can be your go-to choice.