WORM_DROPPER.AKO

 Analysis by: Karl Dominguez

 ALIASES:

TrojanDownloader:Win32/Small.gen!I (Microsoft) ; W32.SillyDC (Symantec) ; Trojan.Win32.Pincav.bft (Kaspersky)

 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

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

  • Threat Type: Worm

  • Destructiveness: No

  • Encrypted: Yes

  • In the wild: Yes

  OVERVIEW


If a process named AVP.EXE exists, this malware drops a system driver component as %User Temp%\atapi.sys. The said system file is used by the malware to avoid detection and be able to load into memory. This is detected by Trend Micro as TROJ_AGENT.AGIO.

If a process named SHSTAT.EXE exists, it copies itself as %System Root%\McScript_InUse.exe, terminates itself, and deletes the initially executed copy.

This worm arrives via removable drives. It arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

It injects its dropped file/component to specific processes.

It drops an AUTORUN.INF file to automatically execute the copies it drops when a user accesses the drives of an affected system.

  TECHNICAL DETAILS

File Size:

17,985 bytes

File Type:

PE

File Compression:

FSG

Memory Resident:

Yes

Initial Samples Received Date:

25 Nov 2009

Arrival Details

This worm arrives via removable drives.

It 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 worm drops the following copies of itself into the affected system:

  • %Windows%\Downloaded Program Files\USBKey.exe

(Note: %Windows% is the Windows folder, which is usually C:\Windows or C:\WINNT.)

It drops the following file(s)/component(s):

  • {drive letter}:\RECYCLER\Desktop.ini - used to trick the users that the created folder is a legitimate Recycle Bin directory
  • %User Temp%\j7hgf.bat - batch file used to delete the initiailly executed copy of the malware. This will also be deleted once the execution is done.
  • %System%\MsnShell32.dll - also detected as WORM_DROPPER.AKO

(Note: %User Temp% is the current user's Temp folder, which is usually C:\Documents and Settings\{user name}\Local Settings\Temp on Windows 2000, XP, and Server 2003.. %System% is the Windows system folder, which is usually C:\Windows\System on Windows 98 and ME, C:\WINNT\System32 on Windows NT and 2000, or C:\Windows\System32 on Windows XP and Server 2003.)

It drops and executes the following files:

  • %User Temp%\ushi32.exe - TROJ_DLOADR.AKT

(Note: %User Temp% is the current user's Temp folder, which is usually C:\Documents and Settings\{user name}\Local Settings\Temp on Windows 2000, XP, and Server 2003.)

It injects its dropped file/component to the following processes:

  • Explorer.exe

Autostart Technique

This worm adds the following registry entries to enable its automatic execution at every system startup:

HKEY_CLASSES_ROOT\CLSID\{35CEC8A3-2BE6-11D2-8773-92E220524250}
(default) = "MsnShell32"

HKEY_CLASSES_ROOT\CLSID\{35CEC8A3-2BE6-11D2-8773-92E220524250}\
InprocServer32
(default) = "%System%\MsnShell32.dll"

HKEY_CLASSES_ROOT\CLSID\{35CEC8A3-2BE6-11D2-8773-92E220524250}\
InprocServer32
ThreadingModel = "Both"

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\ShellServiceObjectDelayLoad
MsnShell32 = "{35CEC8A3-2BE6-11D2-8773-92E220524250}"

Other System Modifications

This worm adds the following registry keys:

HKEY_CLASSES_ROOT\CLSID\{35CEC8A3-2BE6-11D2-8773-92E220524250}

Propagation

This worm creates the following folders in all removable drives:

  • {drive letter}:\RECYCLER

It drops the following copy(ies) of itself in all removable drives:

  • {drive letter}:\RECYCLER\MsnHelp.exe

It drops an AUTORUN.INF file to automatically execute the copies it drops when a user accesses the drives of an affected system.

The said .INF file contains the following strings:

[AutoRun]
shell\open=¦=+¬(&O)
shell\open\Command=RECYCLER\MsnHelp.exe
shell\open\Default=1
shell\explore=+-+¦¦_+f¦˜(&X)
shell\explore\command=RECYCLER\MsnHelp.exe
opEn=

Process Termination

This worm terminates the following services if found on the affected system:

  • pccguide.exe

NOTES:

If a process named AVP.EXE exists, it drops a system driver component as %User Temp%\atapi.sys. The said system file is used by the malware to avoid detection and be able to load into memory. This is detected by Trend Micro as TROJ_AGENT.AGIO.

If a process named SHSTAT.EXE exists, it copies itself as %System Root%\McScript_InUse.exe, terminates itself, and deletes the initially executed copy.

  SOLUTION

Minimum Scan Engine:

8.900

FIRST VSAPI PATTERN FILE:

6.651.00

FIRST VSAPI PATTERN DATE:

25 Nov 2009

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 malware/grayware files dropped/downloaded by WORM_DROPPER.AKO

Step 3

Restart in Safe Mode

[ Learn More ]

Step 4

Delete this 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\CurrentVersion\ShellServiceObjectDelayLoad
    • MsnShell32 = {35CEC8A3-2BE6-11D2-8773-92E220524250}

Step 5

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_CLASSES_ROOT\CLSID
    • {35CEC8A3-2BE6-11D2-8773-92E220524250}

Step 6

Search and delete this folder

[ Learn More ]
Please make sure you check the Search Hidden Files and Folders checkbox in the More advanced options option to include all hidden folders in the search result.
  • {drive letter}:\RECYCLER

Step 7

Search and delete AUTORUN.INF files created by WORM_DROPPER.AKO that contain these strings

[ Learn More ]


[AutoRun]
shell\open=¦=+¬(&O)
shell\open\Command=RECYCLER\MsnHelp.exe
shell\open\Default=1
shell\explore=+-+¦¦_+f¦˜(&X)
shell\explore\command=RECYCLER\MsnHelp.exe
opEn=

Step 8

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