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WORM_VB.FAT

October 09, 2012
 Analysis by: Mark Joseph Manahan
 ALIASES:

Worm:Win32/Backterra.F (Microsoft), W32/Bactera.worm!p2p (McAfee), Win32/VB.NIS worm (Eset), W32.Bactera (Symantec)

 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

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

  • Threat Type: Worm

  • Destructiveness: No

  • Encrypted:

  • In the wild: Yes

  OVERVIEW

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

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:

10,240 bytes

File Type:

EXE

Memory Resident:

Yes

Initial Samples Received Date:

15 May 2010

Arrival Details

This worm 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 then creates the following non-malicious file(s):

  • %System%\scvhost.ini

(Note: %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 the following copies of itself into the affected system:

  • %System%\scvhost\svchost.exe
  • %System%\scvhost\wuauserv.exe

(Note: %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 creates the following folders:

  • %System%\scvhost

(Note: %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.)

Autostart Technique

This worm modifies the following registry entry(ies) to enable its automatic execution at every system startup:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows NT\CurrentVersion\Winlogon
Userinit = "Userinit.exe, %System%\scvhost\svchost.exe, wuauserv.exe"

(Note: The default value data of the said registry entry is "Userinit.exe".)

Other System Modifications

This worm modifies the following registry key(s)/entry(ies) as part of its installation routine:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Explorer\
Advanced\Folder\Hidden\
SHOWALL
CheckedValue = "0"

(Note: The default value data of the said registry entry is "1".)

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Explorer\
Advanced
Hidden = "2"

(Note: The default value data of the said registry entry is "1".)

Propagation

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

  • {removable drive}:\desktop.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=verb1
shell\verb1\command=desktop.exe
shell=Auto

  SOLUTION

Minimum Scan Engine:

9.200

VSAPI OPR PATTERN File:

6.851.00

VSAPI OPR PATTERN Date:

15 May 2010

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

Restart in Safe Mode

[ Learn More ]

Step 3

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\Winlogon
    • From: Userinit = "Userinit.exe, %System%\scvhost\svchost.exe, wuauserv.exe"
      To: Userinit = "Userinit.exe"
  • In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Advanced\Folder\Hidden\SHOWALL
    • From: CheckedValue = "0"
      To: CheckedValue = "1"
  • In HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
    • From: Hidden = "2"
      To: Hidden = "1"

Step 4

Search and delete these folders

[ 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.
  • %System%\scvhost

Step 5

Search and delete this file

[ Learn More ]
There may be some component 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.
  • {removable drive}:\autorun.inf
  • %System%\scvhost.ini

Step 6

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