JSON Web Token (JWT)

hacking_jwt jwt_introduction jwt_weak_secret jwt_header_injection

JSON Web Token (JWT) are used to represent signed data. They are commonly used in for authentication and authorization.

The format is: <algo>.<data>.<signature>. Each part is base64 encoded, so the final string is URL-safe while it can be easily decoded.

The data is signed using a secret key. If the secret key is compromised, everyone can sign messages 🔏.

They can be transferred and found in:

  • 📚 Headers (Authorization: Bearer <JWT>)
  • 🍪 Cookies
  • 📄 URL Parameters
  • 💼 Request Body
  • ...

To play around with a JWT cookie, you can use jwt.io.

You can also use jwt-tools (4.7k ⭐):

$ cd /opt
$ sudo git clone https://github.com/ticarpi/jwt_tool
$ cd jwt_tool && pip3 install -r requirements.txt
$ sudo chmod +x /opt/jwt_tool/jwt_tool.py
$ sudo ln -s /opt/jwt_tool/jwt_tool.py /usr/bin/jwt_tool
$ jwt_tool -h

Common usages:

$ jwt_tool 'jwt'             # decode
$ jwt_tool 'jwt' -T          # encode
$ jwt_tool 'jwt' -T -X a     # attack 'algo=none'
$ jwt_tool 'jwt' -T -X i     # attack 'jwk header injection'
$ jwt_tool 'jwt' -T -p "key" # use secret key
$ jwt_tool 'jwt' -C -d /tmp/jwt.secrets.list # crack key

📚 A common handy wordlist is jwt-secrets.

👻 To-do 👻

Stuff that I found, but never read/used yet.

  • PortSwigger JWT
  • hashcat -a 0 -m 16500
  • Attach algo=none means signature is ignored.
  • JWT, (Bearer, encrypted token), modern alternative to (apache) HTTP basic auth? (from=35). HTB/170. RFC.