Threat Explorer

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10 November 1998
13 February 2007
Also Known As:
Systems Affected:

The VBS.Prepend virus is a Windows script virus that replicates by appending the Visual Basic Script to other HTML files.

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

Antivirus Protection Dates

  • Initial Rapid Release version 14 December 1998
  • Latest Rapid Release version 28 September 2010 revision 054
  • Initial Daily Certified version 14 December 1998
  • Latest Daily Certified version 28 September 2010 revision 036
  • Initial Weekly Certified release date 14 December 1998
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

This virus is not the first HTML virus, but the third known HTML virus, all of which are written by the same author. In order for the virus to infect, the virus requires Internet Explorer 4.0 or greater, or a Visual Basic Script (VBS)-capable browser.

The virus targets any file with a .HTM or .HTML extension in the current or parent directories.

Note : This virus cannot infect a computer by browsing an infected Web page via the Internet. The infected file must be locally viewed.

This function requires a user to download or save an infected HTML file onto their local machine, and then load that infected file into a VBS-capable browser with the appropriate security settings disabled.

The virus will only infect one in six times and only infect files on the local machine. Internet Explorer 4.0, with default settings, prompts a user with a Security Alert before allowing the virus to infect. The virus does not infect if it is in the root directory.

When opening a locally infected HTML file with a VBS-capable browser, the virus code is executed. A check is made to verify that this particular file is a local HTML file (URL begins with file://). Then, the infection only continues with a one in six probability.

The virus:
  • Attempts to infect each file in the same or parent directories that have the extension HTM or HTML (case-insensitive).
  • Checks to see whether the files have been infected already. If the files have not yet been infected, the virus will make a temporary copy of the host file.
  • Overwrites the host file with viral code.
  • After overwriting the host file, the virus then appends the original host data from the temporary file. This successfully prepends the viral code. The temporary file is then deleted.
  • Finally, the status bar on the browser will be set to "HTML.Prepend /1nternal."


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.
Writeup By: Eric Chien