The JSON Web Token Toolkit v2

jwt_tool.py is a toolkit for validating, forging, scanning and 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)

The preferred usage for jwt_tool is with the official Dockerhub-hosted jwt_tool docker image
The base command for running this is as follows:
Base command for running jwt_tool:
docker run -it --network "host" --rm -v "${PWD}:/tmp" -v "${HOME}/.jwt_tool:/root/.jwt_tool" ticarpi/jwt_tool

By using the above command you can tag on any other arguments as normal.
Note that local files in your current working directory will be mapped into the docker container's /tmp directory, so you can use them using that absolute path in your arguments.
i.e.
/tmp/localfile.txt

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 -r requirements.txt

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.

• 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.

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>
or the Docker base command:
$ docker run -it --network "host" --rm -v "${PWD}:/tmp" -v "${HOME}/.jwt_tool:/root/.jwt_tool" ticarpi/jwt_tool

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

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

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

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

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

For a list of options call the usage function: Some options such as Verifying tokens require additional parameters/files to be provided:
$ python3 jwt_tool.py -h

A more detailed user guide can be found on the wiki page.

Head over to the JWT Attack Playbook for a detailed run-though of what JWTs are, what they do, and a full workflow of how to thoroughly test them for vulnerabilities, common weaknesses and unintended coding errors.

Regex for finding JWTs in Burp Search
(make sure 'Case sensitive' and 'Regex' options are ticked)
[= ]eyJ[A-Za-z0-9_-]*\.[A-Za-z0-9._-]* - url-safe JWT version
[= ]eyJ[A-Za-z0-9_\/+-]*\.[A-Za-z0-9._\/+-]* - all JWT versions (higher possibility of false positives)

• JWT Attack Playbook (https://github.com/ticarpi/jwt_tool/wiki) - for a thorough JWT testing methodology

• A great intro to JWTs - https://jwt.io/introduction/

• A lot of the initial inspiration for this tool comes from the vulnerabilities discovered by Tim McLean.
  Check out his blog on JWT weaknesses here: https://auth0.com/blog/critical-vulnerabilities-in-json-web-token-libraries/

• A whole bunch of exercises for testing JWT vulnerabilities are provided by Pentesterlab (https://www.pentesterlab.com). I'd highly recommend a PRO subscription if you are interested in Web App Pentesting.

  PLEASE NOTE: This toolkit will solve most of the Pentesterlab JWT exercises in a few seconds when used correctly, however I'd strongly encourage you to work through these exercises yourself, working out the structure and the weaknesses. After all, it's all about learning...