CrowdStrike Researchers Investigate the Threat of Patchless AMSI Bypass Attacks

By /

Listen to this Post

Featured Image

Introduction

Patchless attacks represent a growing threat in cybersecurity, leveraging legitimate system functionalities to evade detection. CrowdStrike’s latest research highlights how adversaries exploit User Exceptions to bypass the Anti-Malware Scan Interface (AMSI) without modifying system memory. This article explores detection strategies and introduces VEH², an advanced evasion technique demonstrated at Black Hat MEA 2023.

Learning Objectives

  • Understand how Patchless AMSI Bypass exploits User Exceptions.
  • Learn detection methods for standard and advanced (VEH²) techniques.
  • Explore mitigation strategies to harden systems against these attacks.

1. AMSI Bypass via User Exceptions

Command/Code Snippet (PowerShell):

bash
$ExceptionHandler = {
[System.Runtime.InteropServices.Marshal]::WriteInt32(bash0xDEADBEEF, 0x41414141)
}
Add-Type -TypeDefinition @”
using System;
public class AMSIBypass {
public static void TriggerException() {
throw new Exception(“AMSI Bypass”);
}
}
“@
[/bash]

Step-by-Step Guide:

  1. Objective: Abuse exception handling to disrupt AMSI scanning.
  2. Execution: The script triggers a custom exception, which AMSI may fail to scan due to runtime context.
  3. Detection: Monitor for abnormal exception handlers targeting AMSI-related processes (e.g., amsi.dll).

2. VEH²: Silent Hardware Breakpoints

Command/Code Snippet (C++):

bash
PVOID handler = AddVectoredExceptionHandler(1, VEH2_Handler);
// Set hardware breakpoint via DR0-DR7 registers
CONTEXT ctx = { CONTEXT_DEBUG_REGISTERS };
ctx.Dr0 = (DWORD64)targetAddress;
ctx.Dr7 |= (1 << 0); // Enable breakpoint
SetThreadContext(GetCurrentThread(), &ctx);
[/bash]

Step-by-Step Guide:

1. Objective: Evade detection by avoiding memory patches.

2. Execution:

  • Register a Vectored Exception Handler (VEH).
  • Set hardware breakpoints to hijack execution flow silently.

3. Detection: CrowdStrike’s behavioral analysis flags:

  • Unusual VEH registration sequences.
  • Mismatched call stacks during exception handling.
  1. Detecting VEH² with Event Tracing for Windows (ETW)

Command/Code Snippet (PowerShell):

bash
Query ETW providers for VEH activity
Get-WinEvent -ProviderName “Microsoft-Windows-Kernel-EventTracing” |
Where-Object { $.Id -eq 12 -and $.Message -like “VectoredHandler” }
[/bash]

Step-by-Step Guide:

1. Objective: Leverage ETW to trace VEH registration.

  1. Execution: Filter Event ID 12 (kernel exceptions) for VEH artifacts.
  2. Mitigation: Enable ETW logging and audit exception handler modifications.

4. Mitigating Patchless Attacks via Memory Protections

Command/Code Snippet (Windows):

bash
bcdedit /set {current} nx AlwaysOn
bcdedit /set {current} cfg EnforcementLevel=2
[/bash]

Step-by-Step Guide:

1. Objective: Harden systems against code injection.

2. Execution:

  • Enable Data Execution Prevention (DEP) and Control Flow Guard (CFG).

3. Validation: Verify settings via `bcdedit /enum`.

5. Red Team OpSec: Reducing Artifacts

Command/Code Snippet (C):

bash
// Remove VEH after use
RemoveVectoredExceptionHandler(handler);
[/bash]

Step-by-Step Guide:

1. Objective: Minimize forensic traces.

2. Execution:

  • Remove handlers post-exploitation to limit memory artifacts.

3. Detection: Hunt for rapid VEH registration/removal cycles.

What Undercode Say

Key Takeaways:

  1. Stealth Over Complexity: VEH² demonstrates that advanced attackers prioritize stealth over technical novelty, leveraging undocumented OS behaviors.
  2. Detection Gaps: Behavioral analysis remains imperfect against techniques like thread hijacking or spoofed call stacks.

Analysis:

CrowdStrike’s research underscores the cat-and-mouse game in cybersecurity. While VEH² represents a leap in evasion, defensive tools are adapting with deeper kernel introspection. Future threats may exploit hypervisor-level vulnerabilities, necessitating hardware-assisted defenses. Organizations must balance threat hunting with foundational hardening (e.g., DEP, CFG) to mitigate such attacks.

Prediction

Patchless attacks will proliferate as EDR solutions improve memory scanning. Adversaries will shift to firmware/hardware-based evasion (e.g., Intel CET bypasses), requiring closer collaboration between OS vendors and security researchers to close detection gaps.

IT/Security Reporter URL:

Reported By: Activity 7340814753092513792 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

Join Our Cyber World:

💬 Whatsapp | 💬 Telegram

Related Posts: