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 <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

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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