Tool

JWT Fuzzer

Generate JWT security test variants for development and education.

Last updated July 1, 2026

Development and education only. Use only on tokens you own. Never test against production systems without authorization.

JWT Token

Runs entirely in your browser — no data sent to servers. Privacy policy

How to Use This Tool

Paste a JWT token you own (development or test environment only).

Click Generate Test Variants to create tampered versions for security testing.

Use variants to verify your API rejects invalid signatures, expired tokens, and algorithm confusion attacks.

Never use against production systems without authorization.

Code Examples

// Always reject alg:none
jwt.verify(token, secret, { algorithms: ['HS256'] });

// Reject expired tokens
if (payload.exp < Date.now() / 1000) throw new Error('Expired');

// Reject tampered payloads (signature won't match)
jwt.verify(tamperedToken, secret, { algorithms: ['HS256'] });

Frequently Asked Questions

Is JWT fuzzing legal?

Only test systems you own or have written authorization to test. Unauthorized security testing may violate laws and terms of service.

What attacks does this simulate?

Tampered payloads, algorithm confusion (alg:none), expired tokens, and invalid signatures — common JWT vulnerability patterns.

Should my API accept alg:none?

Never. Always explicitly specify allowed algorithms in jwt.verify() and reject tokens with alg:none or unexpected algorithms.

What is JWT fuzzing?

JWT fuzzing generates intentionally malformed or tampered tokens — wrong signatures, expired claims, or algorithm confusion — to verify your API rejects them. Use only on systems you own or are authorized to test.

How do I test JWT security in development?

Paste a test token, generate variants, and confirm your API returns 401 for tampered payloads, expired exp, alg:none, and invalid signatures. Pair this with explicit algorithm allowlists in your JWT library.