Top JWT Interview Questions and Answers for 2025 | Complete Guide

This guide will walk you through the most commonly asked JWT interview questions, with clear, beginner-friendly explanations. Let's boost your confidence and help you nail that interview!

What is JWT? Overview and Key Concepts

JWT, or JSON Web Token, is a compact, URL-safe means of representing claims securely between two parties. It's primarily used for authentication and authorization in web applications and APIs.

What are the key responsibilities in a JWT implementation?

A JWT implementation typically involves:

• User authentication through credentials
• Token generation with proper signing
• Secure token storage
• Token validation on protected resources
• Handling token expiration and refresh
• Ensuring proper security measures

Why use JWT for authentication?

JWT offers several advantages:

• Stateless authentication: No need to store session information on the server
• Compact size: Efficient for transmission in HTTP headers
• Self-contained: Contains all necessary information about the user
• Cross-platform support: Works with any programming language
• Secure by design: Digitally signed to prevent tampering

Common JWT Interview Questions with Answers

1. What is JWT and how does it work?

JWT (JSON Web Token) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties. The information is digitally signed, so it can be verified and trusted.

A typical JWT workflow:

1. User logs in with credentials
2. Server validates credentials and creates a JWT token
3. Token is sent back to the client
4. Client stores the token (typically in local storage or cookies)
5. Client includes the token in requests to protected resources
6. Server validates the token signature before processing requests

2. What is the structure of a JWT token?

A JWT token consists of three parts separated by dots:

1. Header: Contains the algorithm used for signing and token type
2. Payload: Contains claims or user information
3. Signature: Ensures the token hasn't been altered

The format looks like: xxxxx.yyyyy.zzzzz (header.payload.signature)

3. What types of claims can be included in JWT?

JWT claims are categorized into three types:

• Registered claims: Predefined claims like 'exp' (expiration time), 'iss' (issuer), 'sub' (subject), and 'aud' (audience)
• Public claims: Custom claims that conform to a public namespace
• Private claims: Custom claims agreed upon between parties

4. How is a JWT token secured?

JWT tokens are secured through digital signatures. The signature is created using:

1. The encoded header
2. The encoded payload
3. A secret key (for HMAC algorithms) or private/public key pair (for RSA/ECDSA)

This signature ensures that the token hasn't been tampered with. However, JWT doesn't encrypt the payload by default, so sensitive information shouldn't be included unless encryption is implemented.

5. What are the differences between JWT and OAuth?

This is a common interview question:

• JWT is a token format that defines how information is securely transmitted
• OAuth is an authorization protocol that may use JWT as a token format

OAuth includes server-side and client-side storage, whereas JWT alone is stateless. OAuth2 provides proper logout capabilities, while basic JWT authentication doesn't inherently support logout functionality.

6. How do you handle JWT token expiration?

To handle JWT token expiration:

1. Set an appropriate expiration time in the token payload (exp claim)
2. Implement token validation on the server to check expiration
3. Create a token refresh mechanism using refresh tokens
4. Implement proper error handling for expired tokens on the client side

7. What is a refresh token in JWT authentication?

A refresh token is a long-lived token used to obtain new access tokens when the current access token expires. Benefits include:

• Improved security as access tokens can be short-lived
• Better user experience as users don't need to re-authenticate frequently
• Ability to revoke access by invalidating refresh tokens

8. Why is JWT considered stateless?

JWT is stateless because the server doesn't need to keep track of session information. All necessary user information is contained within the token itself. This allows for:

• Scalability across multiple servers
• Reduced database lookups
• Simplified architecture for distributed systems

9. What are common security concerns with JWT?

Key security concerns include:

• Token theft: JWT tokens can be stolen if not properly secured
• XSS and CSRF attacks: Can compromise tokens stored in browsers
• Secret key management: Poor key management can compromise security
• Token storage: Improper storage in client-side applications
• Lack of revocation: Basic JWT implementations don't support token revocation

10. How do you implement JWT in a Spring Boot application?

For implementing JWT in Spring Boot:

1. Add JWT dependencies (like jjwt library)
2. Create a JWT utility class for token generation and validation
3. Configure Spring Security with a JWT filter
4. Implement authentication endpoints
5. Configure protected resources

Bonus Tips for JWT Interview Success

Preparation Essentials

• Practice implementation: Write code to create and validate JWT tokens
• Understand security concepts: CSRF, XSS, and token security
• Stay updated: Be aware of the latest JWT specifications and security best practices

On Interview Day

• Arrive with confidence in your JWT knowledge
• Listen carefully to questions before answering
• If unsure, walk through your thought process
• Relate JWT concepts to real-world application scenarios
• Be ready to discuss tradeoffs in different authentication approaches

Final Thoughts‍

Mastering JWT concepts is valuable for many tech roles, from backend developers to security engineers. By understanding these common interview questions and their answers, you're well on your way to demonstrating your expertise.
Remember that interviewers are often looking for your understanding of the underlying concepts rather than memorized definitions. Be prepared to discuss how JWT fits into real-world authentication and authorization scenarios, and you'll be set for success!
Good luck with your interview preparation, and don't hesitate to revisit this guide whenever you need a refresher on JWT concepts.