Risk Level 2: Low

June 1, 1998
April 25, 2002 2:39:44 PM
Systems Affected:
CIH is a virus that infects the 32-bit Windows 95/98/NT executable files, but can function only under Windows 95/98 and ME. It does not function under Windows NT or Windows 2000. When an infected program is run under Windows 95/98/ME, the virus becomes resident in memory. To remove the virus, do one of the following:

  • Recommended method: Use the Symantec Security Response CIH Removal Tool, which removes the virus from memory and prevents the need to reboot from a clean system disk.
  • Reboot the computer from a Rescue Disk.
  • Reboot the computer from the Norton AntiVirus (NAV) 2001/2002 CD, if your computer allows this option.

If this is not done, the virus will infect every file scanned with Norton AntiVirus or with any antivirus program.

Although Windows NT system files can be infected, the virus cannot become resident or infect files on a computer running Windows NT or Windows 2000. The virus does not function under DOS, Windows 3.1, or on Macintosh computers. Once the virus is resident, the CIH virus infects other files when accessed.

The files infected by CIH may have the same size as the original files, due to the unique infection mode of CIH. The virus searches for empty, unused spaces in the file. Next, it breaks itself up into smaller pieces and inserts its code into these unused spaces. When NAV repairs a file infected by CIH, it looks for these small viral pieces and removes them from the file.

As of April, 1999, three known, similar variants of this virus exist. CIH versions 1.2 and 1.3 have a payload that will trigger on April 26, commemorating Chernobyl, the Soviet nuclear disaster, which occurred on April 26, 1986. CIH version 1.4 has a payload that will trigger on the 26th of any month. The payloads of all the versions of CIH are the same.

The first payload overwrites the hard disk with random data, starting at the beginning of the disk (sector 0) using an infinite loop. The overwriting of the sectors does not stop until the system has crashed. As a result, the computer will not boot from the hard disk or floppy disk. Also, the data that has been overwritten on the hard disk will be very difficult or impossible to recover. You must restore the data from backups.

The second payload tries to cause permanent damage to the computer. This payload attacks the Flash BIOS (a part of your computer that initializes and manages the relationships and data flow between the system devices, including the hard drive, serial and parallel ports, and the keyboard) and tries to corrupt the data stored there. As a result, nothing may be displayed when you start the computer. A computer technician would need to fix this.


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: Motoaki Yamamura
Summary| Technical Details| Removal

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