Threat Explorer

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30 December 2002
13 February 2007
Systems Affected:

VBS.Fit.A is a .vbs worm that copies itself to open network shares on computers within the IP range to The existence of the file MSNetldr.vbs is the sign of a possible infection.

Antivirus Protection Dates

  • Initial Rapid Release version 31 December 2002
  • Latest Rapid Release version 28 September 2010 revision 054
  • Initial Daily Certified version 31 December 2002
  • Latest Daily Certified version 28 September 2010 revision 036
  • Initial Weekly Certified release date 31 December 2002
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

When the worm is executed, it does the following:
  1. Copies itself to %Windir%\MSNetldr.vbs.
    NOTE: %Windir% is a variable. The worm locates the Windows installation folder (by default, this is C:\Windows or C:\Winnt) and copies itself to that location.
2. Adds the value
    MSNetldr %Windows\MSNetldr.vbs
    to the registry key
    so that the worm runs when you start Windows.

3. Continually generates random IP addresses within the range to and attempts to create a mapped drive (Z:) to an open C drive
network share on the remote computer.

4. If the worm is successful in performing this, it attempts to copy itself as MSNetldr.vbs to the following folders on the remote computer:
    • Windows\Start Menu\Programs\Startup
    • Windows\Menú Inicio\Programas\Inicio
    • Windows\Menu Iniciar\programas\Iniciar
    • Windows\Menu Démarrer\Programmes\Démarrage
    • Windows\Startmenü\Programme\Autostart
    • Windows\menu avvio\programmi\esecuzione automatica
    • Documents and Settings\All Users\Start Menu\Programs\Startup
    • Documents and Settings\All Users\Menú Inicio\Programas\Inicio
    • Documents and Settings\All Users\Menu Iniciar\Programas\Inicializar
    • Dokumente und einstellungen\all users\Startmenü\programme\autostart
    • Documents and settings\All users\Menu avvio\Programmi\Esecuzione automatica


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.

These instructions are for all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.
  1. Update the virus definitions.
  2. Run a full system scan and delete all the files detected as VBS.Fit.A.
  3. Delete the value

    MSNetldr %Windows\MSNetldr.vbs

    from the registry key

For further details, read the following instructions.

Updating the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:
  • Running LiveUpdate, which is the easiest way to obtain the virus definitions. These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate) line at the top of this writeup.
  • Downloading the definitions using the Intelligent Updater. The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater) line at the top of this writeup.

    The Intelligent Updater virus definitions are available here. For detailed instructions on how to download and install the Intelligent Updater virus definitions from the Symantec Security Response Web site, click here.

Scanning for and deleting the infected files
  1. Start your Symantec antivirus program, and make sure that it is configured to scan all files.
  2. Run a full system scan.
  3. If any files are detected as infected with VBS.Fit.A, click Delete.

Deleting the value from the registry

CAUTION : Symantec strongly recommends that you back up the registry before you make any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified keys only. Read the document, "How to make a backup of the Windows registry ," for instructions.
  1. Click Start, and then click Run. (The Run dialog box appears.)
  2. Type regedit, and then click OK. The Registry Editor opens.
  3. Navigate to the key

  4. In the right pane, delete the value

    MSNetldr %Windows\MSNetldr.vbs
  5. Exit the Registry Editor.

Writeup By: Eric Chien