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JWT Decoder & Security Analyzer

Decode Json Web Tokens (JWT) locally and run instant vulnerability audits. Detect authentication flaws, algorithm bypasses, and validation gaps.

100% Client-Side
Decoded in your browser. Tokens never leave your device.
Load Mock Token:

Paste Encoded Token

Decoded Header

Waiting for token...

Decoded Payload (Claims)

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Signature Hash Code

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JWT Authentication Flow

JWTs are stateless credentials. While they can be decoded client-side, the signature ensures cryptographic tamper-proofing. The backend must always verify this signature against a secure key.

The "alg: none" Exploit

Some legacy or misconfigured libraries process JWTs without verifying signatures when `"alg": "none"` is defined. Attackers exploit this to elevate privileges to administrative scopes.

JSON Web Key Sets (JWKS)

In modern B2B SaaS applications, public keys are dynamically exposed on a JWKS endpoint (e.g., `/.well-known/jwks.json`). This permits distributed API resource servers to verify token signatures asynchronously.