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BKDR_CHOSZ.DLL

October 09, 2012
 Analysis by: Abraham Latimer Camba
 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

 OVERALL RISK RATING:
 DAMAGE POTENTIAL:
 DISTRIBUTION POTENTIAL:
 REPORTED INFECTION:

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted: Yes

  • In the wild: Yes

  OVERVIEW

Infection Channel:

Dropped by other malware


This backdoor may be dropped by other malware.

It executes commands from a remote malicious user, effectively compromising the affected system.

It deletes the initially executed copy of itself.

  TECHNICAL DETAILS

File Size:

131,072 bytes

File Type:

EXE, DLL

Memory Resident:

Yes

Initial Samples Received Date:

19 Sep 2012

Payload:

Connects to URLs/IPs, Steals information, Compromises system security

Arrival Details

This backdoor may be dropped by the following malware:

Installation

This backdoor drops the following copies of itself into the affected system and executes them:

  • All Users' %User Profile%\Application Data\mspool.dll

(Note: %User Profile% is the current user's profile folder, which is usually C:\Windows\Profiles\{user name} on Windows 98 and ME, C:\WINNT\Profiles\{user name} on Windows NT, and C:\Documents and Settings\{user name} on Windows 2000, XP, and Server 2003.)

Autostart Technique

This backdoor registers itself as a system service to ensure its automatic execution at every system startup by adding the following registry entries:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
Description = "Windows mspool service."

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
DisplayName = "mspool"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
ErrorControl = "1"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
ImagePath = "%SystemRoot%\system32\svchost.exe -k LocalService"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
ObjectName = "LocalSystem"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
Start = "2"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
Type = "10"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Parameters
ServiceDll = "All Users' %User Profile%\Application Data\mspool.dll\Application Data\mspool.DLL"

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Parameters
ServiceMain = BofMrf

Other System Modifications

This backdoor adds the following registry keys:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Enum

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Parameters

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Security

HKEY_LOCAL_MACHINE\Classes\SS

It modifies the following registry entries:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows NT\CurrentVersion\SvcHost
LocalService = "{Default values} mspool"

(Note: The default value data of the said registry entry is {Default values}.)

Backdoor Routine

This backdoor executes the following commands from a remote malicious user:

  • Download files
  • Get drive information
  • Get system information
  • Log keystrokes and active window
  • Manage files and folders
  • Manage processes
  • Manage registry
  • Manage services
  • Perform remote shell

It connects to the following URL(s) to send and receive commands from a remote malicious user:

  • http://{BLOCKED}n.{BLOCKED}1.us

Other Details

This backdoor deletes the initially executed copy of itself

  SOLUTION

Minimum Scan Engine:

9.200

Step 1

For Windows XP and Windows Server 2003 users, before doing any scans, please make sure you disable System Restore to allow full scanning of your computer.

Step 2

Remove the malware/grayware file that dropped/downloaded BKDR_CHOSZ.DLL

Step 3

Restart in Safe Mode

[ Learn More ]

Step 4

Delete this registry key

[ Learn More ]

Important: Editing the Windows Registry incorrectly can lead to irreversible system malfunction. Please do this step only if you know how or you can ask assistance from your system administrator. Else, check this Microsoft article first before modifying your computer's registry.

  • In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
    • mspool
  • In HKEY_LOCAL_MACHINE\SOFTWARE\Classes 
    • SS
 

Step 5

Restore this modified registry value

[ Learn More ]

Important: Editing the Windows Registry incorrectly can lead to irreversible system malfunction. Please do this step only if you know how or you can ask assistance from your system administrator. Else, check this Microsoft article first before modifying your computer's registry.

  • In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SvcHost
    • From: LocalService = "{Default values} mspool"
      To: LocalService = {Default values}

Step 6

Restart in normal mode and scan your computer with your Trend Micro product for files detected as BKDR_CHOSZ.DLL. If the detected files have already been cleaned, deleted, or quarantined by your Trend Micro product, no further step is required. You may opt to simply delete the quarantined files. Please check this Knowledge Base page for more information.


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