Can I store sensitive data in a JWT?

Absolutely NOT. The payload of a JWT is completely readable by anyone who decodes the string. It is mathematically signed, not encrypted. Never put passwords, credit cards, or secret keys inside a JWT payload.

Where should a React app store the JWT?

This is highly debated. `localStorage` is vulnerable to Cross-Site Scripting (XSS) attacks. Many modern architectures store the short-lived Access Token in `localStorage` or memory, but store the long-lived Refresh Token in a strict `HttpOnly` cookie to prevent JavaScript access.

How do I invalidate a JWT if someone's account is hacked?

Because JWTs are stateless, you cannot 'delete' them from the server. This is exactly why the Access Token must have a short lifespan (e.g., 5 minutes). To immediately lock an account, you must blacklist the Refresh Token in the database or change the user's password, which prevents them from generating new Access Tokens.

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API Security & JWT

Master API Security. Learn the critical difference between stateful and stateless authentication, decode the cryptographic architecture of JSON Web Tokens (JWT), and learn how to configure CORS for frontend applications.

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