Discovered: July 11, 1994
Updated: February 13, 2007 11:34:07 AM
Also Known As: Lenart
Type: Virus


AntiCMOS is a simple virus that infects master boot records (MBRs) and DOS boot sectors (DBSs). AntiCMOS spreads only when you attempt to boot a system from an infected floppy disk. There is little difference between the .A and .B strains of AntiCMOS. Other than the triggered event, they are identical.




Area of infection: Floppy boot sectors, master boot records
Likelihood: Common
Region reported: Europe, Hong Kong, U.S.A.
Characteristics: Wild, memory resident
Target platform: DOS

Antivirus Protection Dates

  • Initial Rapid Release version July 11, 1994
  • Latest Rapid Release version August 20, 2008 revision 017
  • Initial Daily Certified version July 11, 1994
  • Latest Daily Certified version August 20, 2008 revision 016

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

Discovered: July 11, 1994
Updated: February 13, 2007 11:34:07 AM
Also Known As: Lenart
Type: Virus


During the start of the boot process, AntiCMOS first reduces the total amount of conventional memory by 2K (CHKDSK reports 653,312 on infected systems), loads itself into memory, redirects the BIOS Disk I/O Services Interrupt 13h, and returns control to the system for further processing of the boot strap. With AntiCMOS now active in memory, all disk reads of exactly one sector using the BIOS Disk I/O services are filtered out. Upon request of such a service, the AntiCMOS first checks to see if the trigger requirements of its payload have been met.

AntiCMOS.A

If the trigger conditions hold true, AntiCMOS.A makes modifications to the system's CMOS data. (A bug within the program all but guarantees the trigger routine will never be executed.) However, if the trigger condition is not met, the MBR (when dealing with the hard drives) or the DBS (when dealing with floppy disks) is read into memory, infected, and then written back to the drive.

AntiCMOS.B

If the trigger conditions hold true, AntiCMOS.B generates sounds from the PC speaker. (A bug within the program all but guarantees the trigger routine will never be executed.) However, if the trigger condition is not met, the MBR (when dealing with the hard drives) or the DBS (when dealing with floppy disks) is read into memory, infected, and then written back to the drive.

Recommendations

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.

Discovered: July 11, 1994
Updated: February 13, 2007 11:34:07 AM
Also Known As: Lenart
Type: Virus



Please read all instructions before you begin.

CAUTION:

  • The procedure described in this document could result in the loss of all data on the computer if not done correctly. Unless you are completely familiar with the use of 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.
To remove the virus:
  1. Shut down Windows, and then turn off the power. Do not simply press the reset button.
  2. Wait 30 seconds for memory to be cleared.
  3. Do one of the following:
    • If you have a NAV 2001 (or later) CD, and your computer can boot from the CD-ROM drive:
      1. Insert the NAV CD in the CD-ROM drive.
      2. Restart the computer and follow the prompts.
      3. Choose to repair all infections.
      4. Remove the floppy disk, and then restart the computer.
      5. Start Norton AntiVirus, and 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 infections.
      4. Remove the floppy disk, and then restart the computer.
      5. Start Norton AntiVirus, and run a full system scan.
    • If you do not have a clean Rescue disk or a NAV 2001 (or later) CD:
      1. Insert an uninfected, write-protected, bootable disk that also contains the FDISK program into the floppy disk drive, and 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 run a full system scan.