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Discovered: January 12, 2001
Updated: February 13, 2007 11:51:11 AM
Type: Virus

This is a macro virus that infects Microsoft Word templates, documents, Win.com, and other files in the \Windows and \Windows\System folders. This virus uses an algorithm to generate a series of payloads. It infects different files in different folders, depending on the date of the month.

Antivirus Protection Dates

  • Initial Rapid Release version January 12, 2001
  • Latest Rapid Release version January 12, 2001
  • Initial Daily Certified version January 12, 2001
  • Latest Daily Certified version January 12, 2001

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Technical Description

When opened, W97M.Invert.B checks the Normal.dot global template and Word documents to see if they have already been infected. The virus will infect these files if they have not yet been infected. The virus then opens C:\Windows\System\Ie3dvu.dll and changes one byte in the file. If the number of infected Word documents is fewer than 25, the virus will not continue to infect other files in Windows folder; it will exit. If the number of Word documents is more than 25, it will continue to infect other files depending the payloads:

  • If the date of the month is 5, 10, 15, 20, 25, or 30, the virus infects the following files if they are not already infected:
  • If date of the month is 3, 6, 9, 12, 15, 18, 21, 24, 27, or 30, the virus infects the following files if they are not already infected:
  • If if the date of the month is 6, 12, 18, 24, or 30, the virus infects the following files if they are not already infected:
  • If date of the month is 9, 18, or 27, the virus infects the following files if they are not already infected:
  • If the date of the month is an even number, the virus infects the Win.com file by inserting some bytes into it. Sometimes Win.com becomes unrepairable, depending on the date of the month.


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.


To remove the virus:

  1. Run LiveUpdate to make sure that you have the most recent virus definitions.
  2. Start Norton AntiVirus (NAV), and run a full system scan. Make sure that NAV is set to scan all files.
  3. If any files are found to be infected with W97M.Invert.B, choose Repair.

NOTE: If Win.com is infected and it cannot be repaired, you may see an error message when you restart the computer. In some cases, you may not be able to start Windows. If this happens, you must replace the Win.com file. For instructions on how to do this, see the Microsoft Knowledge Base article Creating a New Win.com File When You Cannot Start Windows , Article ID: Q136630.

Writeup By: JP Duan