Advanced Bypass Techniques: ETW & AMSI Evasion Using Trampoline Hooking

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Introduction

Adversary simulation and red teaming often require bypassing security mechanisms like Event Tracing for Windows (ETW) and the Anti-Malware Scan Interface (AMSI). A new proof-of-concept (PoC) technique leveraging trampoline hooks demonstrates how attackers can alter execution flow to evade detection. This article explores the technical details, implications, and mitigation strategies for such advanced bypass methods.

Learning Objectives

  • Understand how trampoline hooks manipulate execution flow to bypass ETW and AMSI.
  • Learn practical steps to test and mitigate this bypass technique.
  • Explore obfuscation methods to make such attacks harder to detect.

You Should Know

1. Trampoline Hooking for ETW Bypass

Command/Code Snippet (C++):

void HookETW() { 
auto NtTraceEvent = GetProcAddress(GetModuleHandle("ntdll.dll"), "EtwEventWrite"); 
DetourTransactionBegin(); 
DetourUpdateThread(GetCurrentThread()); 
DetourAttach(&(PVOID&)NtTraceEvent, HookedEtwEventWrite); 
DetourTransactionCommit(); 
}

Step-by-Step Guide:

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

  1. Use Microsoft Detours or a custom trampoline to redirect execution to a malicious function (HookedEtwEventWrite).
  2. The hooked function can suppress or alter event logs, evading ETW-based detection.

2. AMSI Bypass via Memory Patching

Command/Code Snippet (PowerShell):

$amsiContext = [System.Runtime.InteropServices.Marshal]::AllocHGlobal(0x1000) 
[System.Runtime.InteropServices.Marshal]::WriteInt32($amsiContext, 0x0)

Step-by-Step Guide:

  1. Allocate memory and patch the AMSI context to return `0` (clean result).
  2. Overwrite critical AMSI functions like `AmsiScanBuffer` to neutralize scans.
  3. Combine with trampoline hooks to persist the bypass.

3. Obfuscation Techniques for OPSEC Safety

Command/Code Snippet (Python):

import base64 
encoded_shellcode = base64.b64encode(open("payload.bin", "rb").read())

Step-by-Step Guide:

1. Encode payloads using Base64 or XOR encryption.

  1. Use dynamic API resolution (e.g., GetProcAddress) to avoid static IOCs.
  2. Implement junk code or sleep timers to evade sandbox analysis.

4. Mitigation: Detecting Trampoline Hooks

Command (Windows Defender ATP):

Get-MpThreatDetection | Where-Object { $_.Name -match "Trampoline" }

Step-by-Step Guide:

  1. Monitor for unexpected hooks in `ntdll.dll` or amsi.dll.
  2. Enable kernel-mode callbacks to detect unauthorized function modifications.
  3. Deploy behavioral analysis tools to flag anomalous execution flow.

5. Cloud Hardening Against Bypass Attacks

Command (Azure Sentinel KQL):

SecurityEvent 
| where EventID == 4657 
| where ProcessName contains "amsi"

Step-by-Step Guide:

1. Audit process integrity levels and module loads.

2. Restrict high-privilege access to critical DLLs.

  1. Use Azure Sentinel to correlate ETW/AMSI bypass attempts with other anomalies.

What Undercode Say

  • Key Takeaway 1: Trampoline hooks are a powerful evasion technique but leave forensic artifacts (e.g., detoured functions).
  • Key Takeaway 2: Defense-in-depth strategies, including memory scanning and heuristic analysis, are critical to counter advanced bypasses.

Analysis:

This PoC highlights the cat-and-mouse game between attackers and defenders. While trampoline hooks are effective, they require low-level access and can be detected via kernel callbacks or machine learning models. Enterprises should prioritize endpoint detection and response (EDR) solutions with real-time memory inspection capabilities. Future attacks may leverage hardware-assisted hooks (e.g., Intel CET), making mitigation even more challenging.

Prediction

As EDRs evolve, attackers will increasingly adopt hardware-level evasion techniques. Defenders must invest in hardware-enforced security (e.g., Microsoft Pluton) and AI-driven anomaly detection to stay ahead. The arms race in cybersecurity shows no signs of slowing down.

For the full PoC, visit the GitHub repo.

IT/Security Reporter URL:

Reported By: Arthur Minasyan – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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