Advanced Fileless Attack Techniques: Bypassing Windows Defender Kernel Callbacks

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Introduction

Fileless attacks and stealthy execution methods are increasingly used by red teams and adversaries to evade traditional security measures. Techniques like sRDI (Shellcode Reflective DLL Injection), thread hijacking, and encryption enable malicious code execution without leaving traces on disk. In this article, we explore how these methods bypass Windows Defender Kernel Callbacks.

Learning Objectives

  • Understand fileless attack techniques like sRDI and thread hijacking.
  • Learn how encryption and section manipulation enhance stealth.
  • Explore methods to bypass Windows Defender Kernel Callbacks.

You Should Know

1. Shellcode Reflective DLL Injection (sRDI)

Command/Code Snippet:

 Convert DLL to position-independent shellcode 
$Shellcode = ConvertTo-Shellcode -File Mimikatz.dll -FunctionName "Coffee"

Step-by-Step Guide:

  1. Use a tool like `sRDI` to convert a DLL into position-independent shellcode.
  2. Inject the shellcode into a process using APIs like `VirtualAllocEx` and CreateRemoteThread.
  3. The shellcode self-loads without touching disk, evading file-based detection.

2. Thread Hijacking for Stealthy Execution

Command/Code Snippet:

HANDLE hThread = OpenThread(THREAD_ALL_ACCESS, FALSE, targetThreadId); 
QueueUserAPC((PAPCFUNC)shellcodeAddr, hThread, NULL);

Step-by-Step Guide:

  1. Identify a target thread in a legitimate process (e.g., explorer.exe).
  2. Allocate memory and write shellcode into the target process.
  3. Use `QueueUserAPC` to force the thread to execute the shellcode.

3. Bypassing Windows Defender Kernel Callbacks

Command/Code Snippet:

// Disable kernel callbacks via undocumented NTAPI 
NtSetInformationProcess(GetCurrentProcess(), ProcessCallbackFlags, &flags, sizeof(flags));

Step-by-Step Guide:

  1. Locate and modify kernel callback structures in memory.
  2. Use direct syscalls or undocumented APIs to disable monitoring.
  3. Validate evasion by testing against Defender’s behavioral detection.

4. Encryption & Section Manipulation

Command/Code Snippet:

 XOR-encrypt shellcode 
encrypted_sc = bytes([b ^ 0xAA for b in shellcode])

Step-by-Step Guide:

  1. Encrypt payloads with XOR or AES to avoid signature detection.

2. Modify PE headers/sections to hide malicious code.

  1. Use tools like `PE-Bear` to inspect and alter section permissions.

5. API Unhooking for Evasion

Command/Code Snippet:

 Unhook NTDLL.dll 
Invoke-Unhook -ModuleName "ntdll.dll" -CleanMemory

Step-by-Step Guide:

1. Identify hooked API functions in loaded DLLs.

  1. Overwrite hooked functions with clean copies from disk.

3. Verify unhooking via memory comparison tools.

What Undercode Say

  • Key Takeaway 1: Fileless attacks leverage memory-based execution to evade EDR/AV solutions.
  • Key Takeaway 2: Kernel callback manipulation is a critical technique for bypassing modern defenses.

Analysis:

The rise of fileless malware underscores the need for behavioral detection over signature-based approaches. While techniques like sRDI and thread hijacking are powerful, defenders can mitigate them by monitoring abnormal process behavior, API calls, and memory anomalies. Future security solutions will likely integrate machine learning to detect such stealthy attacks proactively.

Prediction

As EDR solutions improve, attackers will shift toward more advanced kernel-level obfuscation and hardware-based evasion (e.g., leveraging Intel CET). The cat-and-mouse game between offensive and defensive security will intensify, requiring continuous adaptation from both sides.

IT/Security Reporter URL:

Reported By: Saad Ahla – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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