JWT Best Practices: How to Implement JSON Web Tokens Securely

JSON Web Tokens (JWT) are a powerful way to handle user authentication. But if not used properly, they can expose your app to major security risks.

Let’s walk through the right way to use JWTs securely — in interviews and in real projects.

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What is a JWT?

A JWT is a Base64-encoded token that stores claims (like user ID, issuer, etc.) in a compact form.

It’s not encrypted — anyone can decode it. So never include passwords or secret data inside the token.

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Why Security Matters

• A weak JWT setup can lead to data leaks or unauthorized access.
• Many developers misuse JWTs by trusting them blindly or storing them unsafely.

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Common Mistakes

• Storing tokens in localStorage or sessionStorage
• Trusting the algorithm (alg) sent by the token
• Not checking iss (issuer) and aud (audience) fields
• Keeping tokens valid for too long

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Best Practices

• Store JWTs in HTTP-only, Secure cookies
• Accept only trusted algorithms like RS256
• Validate critical claims (iss, aud)
• Set short expiration times (exp) and use refresh tokens
• Use the phantom token pattern to avoid exposing real tokens

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What is Phantom Token Pattern?

• Client receives a reference token
• Backend uses it to fetch the actual JWT
• Real token stays hidden from the user

This approach adds an extra layer of control and security — especially in microservice or API gateway setups.

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Final Thoughts

JWTs are useful but must be handled with care. Think of them like passports — easy to carry, but dangerous if forged.

Stick to the best practices, and you’ll be well-prepared for both technical interviews and production-ready security.

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References

• Auth0 – JWT Security Best Practices
• Curity – Phantom Token Pattern