W95.Marburg.A / W95.Marburg.B

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Discovered: July 23, 1998
Updated: February 13, 2007 11:49:36 AM
Type: Virus

The W95.Marburg virus is a 32-bit Windows virus that is capable of infecting both Windows 95/98 and Windows NT executables. While the virus can infect both Windows NT and 95/98 executable files, it is not capable of spreading under Windows NT. W95.Marburg does not infect Windows 3.x executable files.

Antivirus Protection Dates

  • Initial Rapid Release version July 23, 1998
  • Latest Rapid Release version July 23, 1998
  • Initial Daily Certified version July 23, 1998
  • Latest Daily Certified version July 23, 1998

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

Technical Description

This virus is classified as a direct-action infector. Any time an infected application file is launched, the virus immediately searches for new files to infect. Specifically, the virus searches for .exe and .scr (screen saver) files in the current directory, in the Windows directory, and in the Windows\System directory. Once the virus has made an attempt to infect files in these directories, it terminates and passes control on to its host application program. The virus does not remain installed in memory and cannot infect subsequent applications once it has yielded control to its host.

W95.Marburg avoids infecting any files whose names start with "PAND", "F-PR", or "SCAN"; furthermore, the virus does not infect any files that contain the letter V in their file name. These precautions allow the virus to avoid infecting antivirus software packages, which as a rule perform self-integrity checks and could alert the user to the virus's presence.

W95.Marburg also has "retroviral" features. Retroviruses are computer viruses that are designed to attack antivirus software in addition to infecting files. This virus attempts to locate several different antivirus data files and deletes them to prevent these antivirus products from detecting W95.Marburg infections. Specifically, the virus deletes files "CHKLIST.MS", "AVP.CRC" and "IVB.NTZ" in any directory where it attempts to infect files.

In addition to these retroviral features, W95.Marburg stands out because it is one of the first polymorphic Windows viruses. Polymorphic viruses change their appearance each time they infect a new file; this makes it more difficult for antivirus software to detect and remove infections. The bulk of the virus's program logic is actually encrypted using one of several different encryption schemes to prevent detection by antivirus software.

W95.Marburg infects Windows application files by appending itself to the last section of a host Windows file. (Windows executable files comprise several sections; each section can contain program logic or data.) The virus then modifies other components of the file such that the application will transfer control to the virus every time it is launched. Infected files grow roughly 7,900 bytes in length and the virus always updates infected files' sizes such that they are evenly divisible by 101. This permits the virus to quickly determine if files are infected without examining their contents. This also means that the virus will occasionally fail to infect certain files because their size is coincidentally evenly divisible by 101. W95.Marburg also infects files that have been marked read-only, and does not change the date or time of files in the directory listing when it infects them.

The W95.Marburg contains a visual payload. If an infected application is launched three months after initial infection, the virus displays the standard Windows error icon (a white X on a red circle) all over the Windows 95 screen making the screen look like it is covered with measles.

Currently, there are two known strains of W95.Marburg: W95.Marburg.A and W95.Marburg.B. These strains are virtually identical and have no major differences.


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 this virus:

  1. On an uninfected computer on which Norton AntiVirus is installed, run LiveUpdate to make sure that you have the most recent virus definitions.
  2. Create a set of Rescue disks.
  3. Insert disk 1 of the Rescue disk set in the floppy disk drive of the infected computer and restart the computer.
  4. In the Rescue command line box, change the text to

    navdx /a /cfg:a /repair /doallfiles
  5. Run the scan.
  6. Repeat steps 4 and 5, but this time, in the Rescue command line box, change the text to:

    navdx /a /cfg:a /prompt /doallfiles

    NOTE: Write down the name and location of any files that you delete. They will have to be replaced from a clean backup.

Writeup By: Carey Nachenberg