Rogue

Challenge

• CTF: HTB Business CTF 2022: Dirty Money
• Name: Rogue
• Category: Forensics
• Difficulty: Easy
• Points: 350
• Description: SecCorp has reached us about a recent cyber security incident. They are confident that a malicious entity has managed to access a shared folder that stores confidential files. Our threat intel informed us about an active dark web forum where disgruntled employees offer to give access to their employer’s internal network for a financial reward. In this forum, one of SecCorp’s employees offers to provide access to a low-privileged domain-joined user for 10K in cryptocurrency. Your task is to find out how they managed to gain access to the folder and what corporate secrets did they steal.

Synopsis

This challenge starts with decrypting SMBv3 traffic given the known user password or NTLM hash.

#!/usr/bin/env python3
"""
This function calculates the random secret key for SMB decryption.
This uses the NTLM hash or plaintext password of the user given a pre-existing packet capture.
"""

# Imports
from Cryptodome.Cipher import ARC4
from Cryptodome.Hash import MD4
import hashlib
import hmac
import argparse
from binascii import hexlify, unhexlify

def generateEncryptedSessionKey(keyExchangeKey, exportedSessionKey):
    cipher = ARC4.new(keyExchangeKey)
    cipher_encrypt = cipher.encrypt
    sessionKey = cipher_encrypt(exportedSessionKey)
    return sessionKey

def swap_bytes(x):
    x = bytearray(unhexlify(x));
    x.reverse();
    return hexlify(x)

# Arguments
parser = argparse.ArgumentParser(
    description="Calculate the Random Session Key based on data from a PCAP (maybe).")
parser.add_argument("-u", "--user", required=True, help="User name")
parser.add_argument("-d", "--domain", required=True, help="Domain name")
parser.add_argument("-p", "--password", required=False,
                    help="Password of User")
parser.add_argument("-ph", "--ntlmpassword", required=False,
                    help="NTLM Password Hash of User")
parser.add_argument("-n", "--ntproofstr", required=True,
                    help="NTProofStr (hex). This can be found in PCAP via ntlmssp.ntlmv2_response.ntproofstr")
parser.add_argument("-k", "--sessionkey", required=True,
                    help="Encrypted Session Key (hex). This can be found in PCAP.")
parser.add_argument("-i", "--sessionid", required=True,
                    help="Session ID (hex). This can be found in PCAP.")
parser.add_argument("-v", "--verbose", action="store_true",
                    help="increase output verbosity")
args = parser.parse_args()

# Upper CaseUsername and Domain
Username = str(args.user).upper().encode('utf-16le')
Domain = str(args.domain).upper().encode('utf-16le')

# Create 'NTLM' Hash of password
if args.ntlmpassword:
    NTLMPassword = unhexlify(args.ntlmpassword)
elif args.password:
    Password = args.password.encode('utf-16le')
    hash1 = MD4.new(Password)
    NTLMPassword = hash1.hexdigest()
    NTLMPassword = unhexlify(NTLMPassword)
else:
    exit("Requires password or NTLM hash")

# Calculate the ResponseNTKey
h = hmac.new(NTLMPassword, digestmod=hashlib.md5)
h.update(Username+Domain)
ResponseNTKey = h.digest()

# Use NTProofSTR and ResponseNTKey to calculate Key Exchange Key
NTProofStr = unhexlify(args.ntproofstr)
h = hmac.new(ResponseNTKey, digestmod=hashlib.md5)
h.update(NTProofStr)
KeyExchKey = h.digest()

# Calculate the Random Session Key by decrypting Encrypted Session Key with Key Exchange Key via RC4
RsessKey = generateEncryptedSessionKey(
    KeyExchKey, unhexlify(args.sessionkey))

if args.verbose:
    print("USER+DOMAIN: " + Username.decode() + "" + Domain.decode())
    print("PASS HASH: " + hexlify(NTLMPassword).decode())
    print("RESP NT:   " + hexlify(ResponseNTKey).decode())
    print("NT PROOF:  " + hexlify(NTProofStr).decode())
    print("KeyExKey:  " + hexlify(KeyExchKey).decode())
print("Session ID: " + swap_bytes(args.sessionid).decode())
print('Random SK:', hexlify(RsessKey).decode())

$ python3 calc_hash.py --verbose --user athomson --ntlmpassword 88d84bad705f61fcdea0d771301c3a7d --domain CORP --ntproofstr d047ccdffaeafb22f222e15e719a34d4 --sessionkey 032c9ca4f6908be613b240062936e2d2 --sessionid 0000a00000000015
USER+DOMAIN: ATHOMSONCORP
PASS HASH: 88d84bad705f61fcdea0d771301c3a7d
RESP NT: 6bc1c5e3a6a4aba16139faad9a3cce6e
NT PROOF: d047ccdffaeafb22f222e15e719a34d4
KeyExKey: 4765b4b66d2d5de5b323708a33d33318
Session ID: 1500000000a00000
Random SK: 9ae0af5c19ba0de2ddbe70881d4263ac
#Decrypt #2
1500000000a00000 9ae0af5c19ba0de2ddbe70881d4263ac
HTB{n0th1ng_c4n_st4y_un3ncrypt3d_f0r3v3r}

Flag: HTB{n0th1ng_c4n_st4y_un3ncrypt3d_f0r3v3r}