Standard Documentation

JWT Security: Decoding and Debugging Tokens Locally

Updated Jan 12, 2026•6 min read

JSON Web Tokens (JWT) are the invisible keys that power modern web authentication. From 'Login with Google' to complex microservice auth, JWTs are everywhere. But because they are Base64 encoded, they look like gibberish to the human eye. JWT Debugger is the tool that makes them readable.

The Anatomy of a JWT

A JWT is composed of three parts, separated by dots (.):

Header: Contains metadata about the token, such as the algorithm used for the signature (e.g., HS256 or RS256).
Payload: The 'meat' of the token. This contains the "Claims"—information about the user, their permissions, and the token's expiration date (exp).
Signature: The cryptographic proof that the token hasn't been tampered with.

⚠️ Security Alert

JWTs are NOT encrypted. They are only signed. Anyone who sees a JWT can decode the payload to see the user's ID or email. Never store sensitive secrets like passwords or credit card numbers inside a JWT payload.

Privacy-First Debugging

Many online JWT debuggers send your token to their servers to decode it. This is a massive security risk—if they log that token, they could potentially hijack your session. Our debugger is 100% Client-Side. We use the browser's native atob() function to decode the token locally. Your tokens never leave your browser.

Common Debugging Scenarios:

Checking Expiration: Is your token expired? Check the exp claim (a Unix timestamp) to see exactly when the session will end.
Verifying Scopes: Does the token contain the right permissions for your API? Inspect the scp or roles claims.
Troubleshooting 401 Errors: When an API returns 'Unauthorized,' the first step is always checking if the JWT being sent is actually valid and well-formed.

Professional Toolkit Integration

Our debugger supports color-coded JSON formatting, making it easy to distinguish between the Header (Red), Payload (Blue), and Signature (Purple). It's designed for speed, security, and clarity.