JWT Decoder & Security Auditor

JWT Decoder & Security Auditor | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Security
Input types: textarea, text
Output type: html
Sample coverage: 4
API ready: Yes

Overview

The JWT Decoder & Security Auditor is a specialized tool designed to decode JSON Web Tokens, verify HS256 or RS256 signatures, and identify potential security risks. By pasting your token and optional cryptographic keys, you can instantly inspect headers and payloads, validate expiration dates, and uncover vulnerabilities related to sensitive claims or weak algorithms.

When to use

Debugging authentication failures or unexpected authorization behaviors in web applications.
Auditing third-party or internally generated JWTs for security misconfigurations like missing expiration dates.
Verifying token signatures during API development using HMAC secrets or RSA public keys.

How it works

1Paste your encoded JWT into the primary input field.
2Optionally, provide an HMAC secret or RSA public key to enable cryptographic signature verification.
3The tool decodes the token's header and payload into readable JSON formats.
4It automatically scans the claims to flag security risks, such as expired tokens, weak algorithms, or exposed sensitive data.

Use cases

Full-stack developers troubleshooting login flows and inspecting user roles embedded in token payloads.
Security engineers performing penetration testing to check if an application accepts tokens with the 'none' algorithm.
API integrators validating that identity providers are issuing tokens with correct issuer (iss) and audience (aud) claims.

Examples

1. Debugging an HS256 Authentication Token

Backend Developer

Background

A backend developer is building a Node.js API and users are reporting random logouts.

Problem

Need to verify if the issued JWTs are expiring too quickly and if the signature matches the server's secret.

How to use

Paste the user's JWT into the JWT Token field and enter the server's secret into the HMAC Secret field.

Outcome

The tool decodes the payload, revealing an 'exp' claim set to only 5 minutes, and confirms the signature is valid, pinpointing the short expiration as the root cause.

2. Auditing an RS256 Identity Token

Security Analyst

Background

A security analyst is reviewing a third-party SSO integration that uses asymmetric encryption.

Problem

Ensure the token is properly signed by the identity provider and doesn't leak sensitive PII in the payload.

How to use

Paste the JWT into the primary field and the provider's PEM-formatted public key into the RSA Public Key PEM field.

Outcome

The tool verifies the RS256 signature successfully but flags a security risk because the user's plain-text internal ID and email are exposed in the custom claims.

FAQ

Does this tool store my JWTs or secret keys?

No, all decoding and auditing processes happen locally in your browser. Your tokens and keys are never sent to a server.

Which signature algorithms are supported for verification?

The tool currently supports verifying signatures for HS256 (using an HMAC secret) and RS256 (using an RSA public key).

Can I decode a JWT without providing a secret key?

Yes. You can decode the header and payload without a key, but signature verification requires the corresponding secret or public key.

What kind of security risks does the auditor flag?

It checks for missing or past expiration dates (exp), weak or 'none' algorithms in the header, and potentially sensitive information stored in plaintext claims.

Why is my RS256 signature verification failing?

Ensure you are pasting the complete RSA public key in PEM format, including the -----BEGIN PUBLIC KEY----- and -----END PUBLIC KEY----- tags.

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Debugging an HS256 Authentication Token

2. Auditing an RS256 Identity Token

FAQ

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