Didier Stevens

Last year I found great x64 shellcode for Windows on McDermott’s site. Not only is it dynamic (lookup API addresses), but it even handles forwarded functions.

But it’s written for MASM, and I prefer to use NASM. Hence I translated it, but also normalized it to adhere to the x64 calling convention and fixed a bug in the error handling.

And I modularized it so you can use it like my 32-bit shellcode.

Here’s the classic MessageBox example:

; x64 shellcode to display a "Hello from injected shell code!" MessageBox, then return to caller
; Written for NASM assembler (http://www.nasm.us) by Didier Stevens
; Source code put in public domain by Didier Stevens, no Copyright
; https://DidierStevens.com
; Use at your own risk
;
; History:
;   2011/12/27: Refactored functions to include file sc-x64-api-functions.asm

%include "sc-x64-macros.asm"

INDEX_KERNEL32_LOADLIBRARYA        equ 0 * POINTERSIZE + STACKSPACE
INDEX_MESSAGEBOXA                            equ 1 * POINTERSIZE + STACKSPACE
APIFUNCTIONCOUNT                            equ 2

segment .text

; Setup environment
sub rsp, STACKSPACE + ROUND_EVEN(APIFUNCTIONCOUNT) * POINTERSIZE        ;reserve stack space for called functions and for API addresses

LOOKUP_API KERNEL32DLL, KERNEL32_LOADLIBRARYA, INDEX_KERNEL32_LOADLIBRARYA

lea rcx, [rel USER32DLL]
call [rsp + INDEX_KERNEL32_LOADLIBRARYA]

LOOKUP_API USER32DLL, USER32_MESSAGEBOXA, INDEX_MESSAGEBOXA, INDEX_KERNEL32_LOADLIBRARYA

; Display MessageBox
xor r9, r9
lea r8, [rel TITLE]
lea rdx, [rel HELLO]
xor rcx, rcx
call [rsp + INDEX_MESSAGEBOXA]

add rsp, STACKSPACE + ROUND_EVEN(APIFUNCTIONCOUNT) * POINTERSIZE
ret

%include "sc-x64-api-functions.asm"

KERNEL32DLL                            db    "KERNEL32.DLL", 0
KERNEL32_LOADLIBRARYA        db    "LoadLibraryA", 0

USER32DLL                                db    "USER32.DLL", 0
USER32_MESSAGEBOXA            db    "MessageBoxA", 0

HELLO                                        db    "Hello from injected shell code!", 0
TITLE                                        db    "Message", 0

Here’s what I changed exactly from the original MASM code:
1) non-volatile registers are preserved (by storing them on the stack)
2) building the DLL name for forwarded functions is done with a variable on the stack frame of lookup_api, and not of the caller
3) the address of LoadLibraryA is passed via r9, and no longer r15
4) lookup_api not only returns the function address in rax, but also stores it in memory at an address provided in r8
5) fixed the error handling bug (stack restoration)
6) added some EQUs to make it easier to use this code as a “library” (include)

You can get the code from my shellcode page. Look for filenames starting with sc-x64 in the zip file.