Threat Explorer

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24 April 2003
13 February 2007
Systems Affected:
Microsoft IIS, Windows

VBS.Yolo.B@mm is a mass-mailing worm that spreads by email, file-sharing networks, floppy disks, and network drives. The worm sends mail with the subject, "RE: Importante" and an attachment, called ".vbs."

VBS.Yolo.B@mm also tries to delete several files from your computer. On April 1st, it attempted a Denial of Service (DoS) attack on a particular Web site.

Antivirus Protection Dates

  • Initial Rapid Release version 25 April 2003
  • Latest Rapid Release version 20 August 2008 revision 017
  • Initial Daily Certified version 25 April 2003
  • Latest Daily Certified version 20 August 2008 revision 016
  • Initial Weekly Certified release date 30 April 2003
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

When VBS.Yolo.B@mm executes, it does the following:
  1. Copies itself as:
    • C:\.vbs
    • C:\je.vbs

  2. Adds the values:

    "(Default)" = "c:\.vbs"
    "Loader" = "c:\virii.vbs"

    to the registry key:


    NOTE: The worm does not create the file, virii.vbs, to which the registry value refers.

  3. Copies itself to a floppy disk if one is available, creating the following files:
    • A:\Important.txt.vbs
    • A:\
    • A:\ReadMe.txt.vbs
    • A:\foto\jpg.vbs
    • A:\Galeria de Fotos.vbs
    • A:\ventas.doc.vbs

  4. Attempts to create the file, "Setup.vbs," on all the available network drives. This may fail due to a bug in the worm's code.

  5. Modifies the Autoexec.bat file on each network drive, by adding the line Start C:\.vbs, so that the worm runs when you start Window 95/98/Me.

  6. Tries to delete the files of the following format:
    • C:\Windows\*.log
    • C:\Windows\*.ini
    • C:\*.txt

  7. If the date is April 1, the worm attempts a DoS attack on a certain Web site. The worm does this by creating the file, C:\xDDD.cmd, which contains commands to ping the site. The worm calls the xDDD.cmd file, if the worm is executed on April 1st.

  8. Attempts to spread using file-sharing networks, by making copies of itself in the following folders:
    • KaZaA\My Shared Folder
    • Bearshare\shared
    • Morpheus\My Shared Folder
    • ICQ\shared files

      These paths are hard-coded so that the files will not be copied if the shared folders are in a different location or if they do not exist.
      The filenames are:
        • A*** (name censored)
        • ICQ LITE.vbs
        • ICQ Pro 2003.exe.vbs
        • Shakira-Naked.jpg .vbs
        • Galilea Montijo Sex.jpg.vbs
        • Serial Visual basic.txt.vbs
        • Delphi 7 Trial Crack.vbs
        • Shakira-Suerte.mp3.vbs
        • Grokster.vbs
        • Msn Hack.vbs

  9. Sends itself using Microsoft Outlook or MAPI to all the contacts in the Outlook address list. The message has the form:

    Attachment: [.vbs]
    Subject: RE: Importante
    Adjunto muy importante!!!!!!!!!!


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.

The following instructions pertain to 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.Yolo.B@mm.
  3. Reverse the changes made to the registry.
  4. Repair the system files.

For specific details on each of these steps, read the following instructions.

1. 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 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).
  • 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).

    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.

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

3. Reversing the changes made to the registry

CAUTION : Symantec strongly recommends that you back up the registry before making 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

    Then click OK. (The Registry Editor opens.)

  3. Navigate to the key:


  4. In the right pane, delete the values: 

    "Loader" = "c:\virii.vbs"
    "(Default)" = "c:\.vbs"

  5. Exit the Registry Editor.

4. Repairing the system files
  1. Click Start, and then click Run.

  2. Type the following:

    edit c:\autoexec.bat

    and then click OK. (The MS-DOS Editor opens.)

  3. Delete any lines that look like:

    Start C:\.vbs

  4. Click File, and then click Save.

  5. Click File, and then click Exit.

Writeup By: Heather Shannon