🐍 A toolkit for testing, tweaking and cracking JSON Web Tokens
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ticarpi Update to jwt_tool v1.1
== UPDATE: jwt_tool.py is now at v1.1 ==
[Bugfixes, new features, and improved speed]

New features:  
* Create new header claims  
* Sign with key file (kid)  
* Check signature against key file (kid)  
* Output as standard and URL-safe tokens  

* Fix broken base64 decoding when certain ASCII chars present  
* Fix broken signature checking/brute-forcing on URL-safe tokens  
* Many other minor tweaks and improvements
Latest commit d30ee10 Jun 9, 2018
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README.md Update README.md Jun 8, 2018
jwt_tool.py Update to jwt_tool v1.1 Jun 8, 2018


The JSON Web Token Toolkit

jwt_tool.py is a toolkit for validating, forging and cracking JWTs (JSON Web Tokens).

Its functionality includes:

  • Checking the validity of a token
  • Testing for the RS/HS256 public key mismatch vulnerability
  • Testing for the alg=None signature-bypass vulnerability
  • Testing the validity of a secret/key/key file
  • Identifying weak keys via a High-speed Dictionary Attack
  • Forging new token header and payload values and creating a new signature with the key or via another attack method


This tool is written for pentesters, who need to check the strength of the tokens in use, and their susceptibility to known attacks.
It may also be useful for developers who are using JWTs in projects, but would like to test for stability and for known vulnerabilities, when using forged tokens.


This tool is written natively in Python 2.x using the common libraries.

Customised wordlists are recommended for the Dictionary Attack option.
As a speed reference, an Intel i5 laptop can test ~1,000,000 passwords per second on HMAC-SHA256 signing. YMMV.


Installation is just a case of downloading the jwt_tool.py file (or git cloneing the repo).
(chmod the file too if you want to add it to your $PATH and call it from anywhere.)


$ python jwt_tool.py <JWT> (filename)

The first argument should be the JWT itself, followed by a filename/filepath (for cracking the token, or for use as a key file).

For example:
$ python jwt_tool.py eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.aqNCvShlNT9jBFTPBpHDbt2gBB1MyHiisSDdp8SQvgw /usr/share/wordlists/rockyou.txt

The toolkit will validate the token and list the header and payload values.
It will then provide a menu of your available options.
Note: signing the token is currently supported using HS256, HS384, HS512 algorithms

Input is in either standard or url-safe JWT format, and the resulting tokens are output in both formats for your ease of use.

Further Reading