Threat Explorer

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17 February 1995
13 February 2007
Also Known As:

NYB is a simple virus that infects Master Boot Records (MBR) and DOS boot sectors (DBS). NYB spreads to a system only when there is an attempt to boot the system from an infected floppy disk.

Antivirus Protection Dates

  • Initial Rapid Release version 17 February 1995
  • Latest Rapid Release version 20 August 2008 revision 017
  • Initial Daily Certified version 17 February 1995
  • Latest Daily Certified version 20 August 2008 revision 016
  • Initial Weekly Certified release date pending
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

During the boot process, NYB loads the MBR into memory and checks for infection. After determining that the MBR is not infected, the NYB stores the uninfected MBR at cylinder 0, side 0, sector 17 on the hard disk. Then, NYB places its virus code into the MBR and rewrites the infected MBR to the hard disk at cylinder 0, side 0, sector 1.

Once the boot process is complete and the NYB virus is active in memory, the virus displays its stealthing capabilities by redirecting any disk reads of the infected MBR or DBS to its clean counterpart. (On floppy disks, the original DBS is stored in the last sector of the root directory.) NYB is highly prolific.


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.

Read all the instructions before you begin.

  • If you do not correctly perform the steps described in this document, data loss could result. Unless you are completely familiar using the DOS FDISK command, we suggest that you obtain the services of a qualified computer consultant.
  • If your computer uses a Dynamic Drive Overlay, or if you are not sure, first follow the instructions in the document, "How to repair a virus when using a dynamic drive overlay or a disk compression utility."
Removing the virus
  1. Shut down Windows, and then turn off the power. Do not simply press the reset button.
  2. Wait 30 seconds for the memory to be cleared.
  3. Do only one of the following:
    • If you have a Norton AntiVirus 2001 (or later) CD, and your computer can start from the CD-ROM drive:
      1. Insert the Norton AntiVirus CD in the CD-ROM drive.
      2. Restart the computer and follow the prompts.
      3. Choose to repair all the infections.
      4. Remove the CD from the CD-ROM drive, and then restart the computer.
      5. Start Norton AntiVirus, and then run a full system scan.

    • If you have a current Rescue Disk set, or can create one on an uninfected computer:
      1. Insert the first, write-protected disk of the Rescue Disk set in the floppy disk drive.
      2. Restart the computer and follow the prompts.
      3. Choose to repair all the infections.
      4. Remove the floppy disk, and then restart the computer.
      5. Start Norton AntiVirus, and then run a full system scan.

    • If you do not have a clean Rescue disk or a Norton AntiVirus 2001 (or later) CD:
      1. Insert an uninfected, write-protected, bootable disk, which also contains the FDISK program, into the floppy disk drive. Then restart the computer.
      2. Type the following, and then press Enter:

        fdisk /mbr

      3. Remove the floppy disk, and then restart the computer.
      4. Start Norton AntiVirus, and then run a full system scan.