Threat Explorer

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03 December 1998
13 February 2007

The VBS.Enel.3787 virus is a Visual Basic (VB) script virus that can infect .html files.

Note: Virus definitions dated prior to February 12, 2004 detect this threat as HTML.Enel.3787.

VBS.Enel.3787 requires Internet Explorer 4.0 or later. The virus targets any file with a .htm, .html, or .htt extension that is in any of the following folders:
  • C:\Windows\Web
  • C:\InetPub\Wwwroot
  • C:\MyDocu~1 (usually My Documents)

This virus can infect your computer when you access a Web page. However, using default Internet Explorer security settings, you must first approve a security alert. If run, the virus first disables the Internet Zone security settings by changing the following registry keys:

CurrentVersion\Internet Settings\Zones\3\1201

CurrentVersion\Internet Settings\Zones\1\1201

CurrentVersion\Internet Settings\Zones\0\1201


Default security settings will not run the code that changes these registry keys. However, using an Internet Explorer buffer overflow exploit, the code is assumed to be on the local computer, thus giving you the option to run the malicious code when you are presented with the security alert. Approving this security alert then turns off future ActiveX object warnings and allows the virus to run malicious VB script contained in remote Web pages. This can result in the infection of local .html files.

The virus does the following:
  • Checks to see whether the files have already been infected before attempting to infect.
  • If the files have not yet been infected, the virus will make a temporary copy of the host file.
  • Then, it overwrites the host file with viral code.
  • After overwriting the host file, the virus appends the original host data from the temporary file. This successfully prepends the viral code.
  • Then, the temporary file is deleted.

The payload includes the status bar being changed to "HTML.Worm v0.2 /1nternal" and with a 1-in-15 chance, redirection to the virus author's Web site.


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.

Run LiveUpdate to make sure that you have the most recent virus definitions.
  1. Start Norton AntiVirus and then run a full system scan. Be sure that Norton AntiVirus is configured to scan all the files.
  2. Delete all the files detected as VBS.Enel.3787.
  3. The deleted files must be restored from backup copies.
  4. Restore your Internet security settings by running Internet Explorer.

Writeup By: Eric Chien