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BKDR_RUSTOCK.XAR

December 16, 2014
 Analysis by: Francis Xavier Antazo
 ALIASES:

Win32/Agent.QGS trojan (NOD32), W32/Agent.DEPO!tr.bdr (FORTINET)

 PLATFORM:

Windows

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

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted:

  • In the wild: Yes

  OVERVIEW


This backdoor arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

It executes then deletes itself afterward.

  TECHNICAL DETAILS

File Size:

235,520 bytes

File Type:

EXE

Memory Resident:

Yes

Arrival Details

This backdoor arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

Installation

This backdoor drops the following files:

  • %User Temp%\RKJ3467F.tmp
  • %Windows%\QosSatup.log
  • %System%\MFI7188L.dll

(Note: %User Temp% is the user's temporary folder, where it usually is C:\Documents and Settings\{user name}\Local Settings\Temp on Windows 2000, Windows Server 2003, and Windows XP (32- and 64-bit); C:\Users\{user name}\AppData\Local\Temp on Windows Vista (32- and 64-bit), Windows 7 (32- and 64-bit), Windows 8 (32- and 64-bit), Windows 8.1 (32- and 64-bit), Windows Server 2008, and Windows Server 2012.. %Windows% is the Windows folder, where it usually is C:\Windows on all Windows operating system versions.. %System% is the Windows system folder, where it usually is C:\Windows\System32 on all Windows operating system versions.)

It executes then deletes itself afterward.

Autostart Technique

This backdoor registers its dropped component as a system service to ensure its automatic execution at every system startup. It does this by creating the following registry entries:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr
Type = "10"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr
Start = "2"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr
ErrorControl = "1"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr
ImagePath = "%SystemRoot%\System32\svchost.exe -k netsvcs"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr
DisplayName = "Windows Audio Shell"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr
ObjectName = "LocalSystem"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr
Description = "Windows Audio Shell"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr\Parameters
ServiceDll = "MFI7188L.dll"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr\Security
Security = "(hex values)"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr\Enum
0 = "Root\LEGACY_AUDIOCLR\0000"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr\Enum
Count = "1"

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr\Enum
NextInstance = "1"

It registers its dropped component as a system service to ensure its automatic execution at every system startup. It does this by creating the following registry keys:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr\Parameters

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr\Security

HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Services\Audioclr\Enum

Other System Modifications

This backdoor modifies the following file(s):

  • %System%\drivers\beep.sys
  • %System%\dllcache\beep.sys

(Note: %System% is the Windows system folder, where it usually is C:\Windows\System32 on all Windows operating system versions.)

  SOLUTION

Minimum Scan Engine:

9.700

Step 1

Before doing any scans, Windows XP, Windows Vista, and Windows 7 users must disable System Restore to allow full scanning of their computers.

Step 2

Note that not all files, folders, and registry keys and entries are installed on your computer during this malware's/spyware's/grayware's execution. This may be due to incomplete installation or other operating system conditions. If you do not find the same files/folders/registry information, please proceed to the next step.

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. Before you could do this, you must restart in Safe Mode. For instructions on how to do this, you may refer to this page If the preceding step requires you to restart in safe mode, you may proceed to edit the system registry.

 
  • In HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services
    • Audioclr

Step 5

Search and delete this file

[ Learn More ]
There may be some files that are hidden. Please make sure you check the Search Hidden Files and Folders checkbox in the "More advanced options" option to include all hidden files and folders in the search result.
  • %User Temp%\RKJ3467F.tmp
  • %Windows%\QosSatup.log
  • %System%\MFI7188L.dll

Step 6

Restore this file from backup only Microsoft-related files will be restored. If this malware/grayware also deleted files related to programs that are not from Microsoft, please reinstall those programs on you computer again.

  • %System%\drivers\beep.sys
  • %System%\dllcache\beep.sys

Step 7

Restart in normal mode and scan your computer with your Trend Micro product for files detected as BKDR_RUSTOCK.XAR. 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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