Jwt_tool – Toolkit For Validating, Forging, Scanning & Tampering JWTs (JSON Web Tokens).


Its functionality includes:

  • Checking the validity of a token
  • Testing for known exploits:
    • (CVE-2015-2951) The alg=none signature-bypass vulnerability
    • (CVE-2016-10555) The RS/HS256 public key mismatch vulnerability
    • (CVE-2018-0114) Key injection vulnerability
    • (CVE-2019-20933/CVE-2020-28637) Blank password vulnerability
    • (CVE-2020-28042) Null signature vulnerability
  • Scanning for misconfigurations or known weaknesses
  • Fuzzing claim values to provoke unexpected behaviours
  • Testing the validity of a secret/key file/Public Key/JWKS key
  • Identifying weak keys via a High-speed Dictionary Attack
  • Forging new token header and payload contents and creating a new signature with the key or via another attack method
  • Timestamp tampering
  • RSA and ECDSA key generation, and reconstruction (from JWKS files)
  • …and lots more!


This tool is written for pentesters, who need to check the strength of the tokens in use, and their susceptibility to known attacks. A range of tampering, signing and verifying options are available to help delve deeper into the potential weaknesses present in some JWT libraries.
It has also been successful for CTF challengers – as CTFs seem keen on JWTs at present.
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 3 (version 3.6+) using the common libraries, however various cryptographic funtions (and general prettiness/readability) do require the installation of a few common Python libraries.
(An older Python 2.x version of this tool is available on the legacy branch for those who need it, although this is no longer be supported or updated)


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

$ git clone https://github.com/ticarpi/jwt_tool
$ python3 -m pip install termcolor cprint pycryptodomex requests

On first run the tool will generate a config file, some utility files, logfile, and a set of Public and Private keys in various formats.

Custom Configs

  • To make best use of the scanning options it is strongly advised to copy the custom-generated JWKS file somewhere that can be accessed remotely via a URL. This address should then be stored in jwtconf.ini as the “jwkloc” value.
  • In order to capture external service interactions – such as DNS lookups and HTTP requests – put your unique address for Burp Collaborator (or other alternative tools such as RequestBin) into the config file as the “httplistener” value.
    Review the other options in the config file to customise your experience.

Colour bug in Windows

To fix broken colours in Windows cmd/Powershell: uncomment the below two lines in jwt_tool.py (remove the “# ” from the beginning of each line)
You will also need to install colorama: python3 -m pip install colorama

# import colorama
# colorama.init()


The first argument should be the JWT itself (unless providing this in a header or cookie value). Providing no additional arguments will show you the decoded token values for review.
$ python3 jwt_tool.py <JWT>

The toolkit will validate the token and list the header and payload values.

Additional arguments

The many additional arguments will take you straight to the appropriate function and return you a token ready to use in your tests.
For example, to tamper the existing token run the following:
$ python3 jwt_tool.py eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.aqNCvShlNT9jBFTPBpHDbt2gBB1MyHiisSDdp8SQvgw -T

Many options need additional values to set options.
For example, to run a particular type of exploit you need to choose the eXploit (-X) option and select the vulnerability (here using “a” for the alg:none exploit):
$ python3 jwt_tool.py eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.aqNCvShlNT9jBFTPBpHDbt2gBB1MyHiisSDdp8SQvgw -X a

Extra parameters

Some options such as Verifying tokens require additional parameters/files to be provided (here providing the Public Key in PEM format):
$ python3 jwt_tool.py eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.aqNCvShlNT9jBFTPBpHDbt2gBB1MyHiisSDdp8SQvgw -V -pk public.pem

Sending tokens to a web application

All modes now allow for sending the token directly to an application.
You need to specify:

  • target URL (-t)
  • a request header (-rh) or request cookies (-rc) that are needed by the application (at least one must contain the token)
  • (optional) any POST data (where the request is a POST)
  • (optional) any additional jwt_tool options, such as modes or tampering/injection options
  • (optional) a canary value (-cv) – a text value you expect to see in a successful use of the token (e.g. “Welcome, ticarpi”)
    An example request might look like this (using scanning mode for forced-errors):
    $ python3 jwt_tool.py -t https://www.ticarpi.com/ -rc "jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.bsSwqj2c2uI9n7-ajmi3ixVGhPUiY7jO9SUn9dm15Po;anothercookie=test" -rh "Origin: null" -cv "Welcome" -M er

Various responses from the request are displayed:

  • Response code
  • Response size
  • Unique request tracking ID (for use with logging)
  • Mode/options used

Common Workflow

Here is a quick run-through of a basic assessment of a JWT implementation. If no success with these options then dig deeper into other modes and options to hunt for new vulnerabilities (or zero-days!).


Read the token value to get a feel for the claims/values expected in the application:
$ python3 jwt_tool.py eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.aqNCvShlNT9jBFTPBpHDbt2gBB1MyHiisSDdp8SQvgw


Run a Playbook Scan using the provided token directly against the application to hunt for common misconfigurations:
$ python3 jwt_tool.py -t https://www.ticarpi.com/ -rc "jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.bsSwqj2c2uI9n7-ajmi3ixVGhPUiY7jO9SUn9dm15Po;anothercookie=test" -M pb


If any successful vulnerabilities are found change any relevant claims to try to exploit it (here using the Inject JWKS exploit and injecting a new username):
$ python3 jwt_tool.py -t https://www.ticarpi.com/ -rc "jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.bsSwqj2c2uI9n7-ajmi3ixVGhPUiY7jO9SUn9dm15Po;anothercookie=test" -X i -I -pc name -pv admin


Dig deeper by testing for unexpected values and claims to identify unexpected app behaviours, or run attacks on programming logic or token processing:
$ python3 jwt_tool.py -t https://www.ticarpi.com/ -rc "jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.bsSwqj2c2uI9n7-ajmi3ixVGhPUiY7jO9SUn9dm15Po;anothercookie=test" -I -hc kid -hv custom_sqli_vectors.txt


Review any successful exploitation by querying the logs to read more data about the request and :
$ python3 jwt_tool.py -t https://www.ticarpi.com/ -rc "jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.bsSwqj2c2uI9n7-ajmi3ixVGhPUiY7jO9SUn9dm15Po;anothercookie=test" -X i -I -pc name -pv admin

Download: https://github.com/ticarpi/jwt_tool

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