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

VBS.Yolo.gen@mm is a class of mass-mailing worms that spread by email and network shares, and sometimes, by file-sharing networks. A typical email message, which the worm sends, will have the subject, "RE: Importante," and the attachment, ".vbs".

Ostensibly a "worm-generator" program created variants of VBS.Yolo.gen@mm. A typical version will do the following:
  1. Copy itself as:
    • C:\.vbs
    • C:\je.vbs

  2. Modify the Autoexec.bat file on the local and network drives.

  3. Send itself by email with the subject "RE: Importante."

Other variants may spread by file-sharing networks or carry destructive payloads.


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.gen@mm or other variants, such as VBS.Yolo.X@mm, where X could be any letter.
  3. Reverse the changes made to the Autoexec.bat file (Windows 95/98/Me).
  4. Reverse the changes made to the registry.

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.gen@mm, note the filename (for later use when checking the registry). Then click Delete.

3. Reversing the changes made to the Autoexec.bat file
If you are running Windows 95/98/Me, follow these steps:
  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.

4. Reversing the changes made to the registry

This step is necessary for instances of the worm that modify the registry. One known variant modifies the key:


But, other versions may make modifications elsewhere.

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, and then click OK. (The Registry Editor opens.)

  3. Navigate to the key you want to check, for example:

    HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunOnce

  4. In the right pane, look for the values of the form <anything> = <any file name>.vbs. Delete these entries unless the filenames correspond to legitimate software. Always delete the entries containing the filenames detected as infected with VBS.Yolo.gen@mm.

  5. Exit the Registry Editor.

Writeup By: Heather Shannon