VBS.Prepend

SUMMARY

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

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

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Protection

• Virus Definitions (LiveUpdate™ Weekly) December 14, 1998
• Virus Definitions (Intelligent Updater) December 14, 1998

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: Low
• Threat Containment: Easy
• Removal: Easy

Damage

• Damage Level: Low

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

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.

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

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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."

Recommendations

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

• Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
• If a blended 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 (for example, all Windows-based computers should have the current Service Pack installed.). Additionally, please apply any security updates that are mentioned in this writeup, in trusted Security Bulletins, or on vendor Web sites.
• 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.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate infected computers quickly to prevent further compromising your organization. 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.