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Virus Profile: Parity Boot.A

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Virus Profile information details
Risk Assessment: Home Low | Corporate Low
Date Discovered: 1/1/0001
Date Added: 11/30/1998
Origin: Germany
Length: 512 Bytes
Type: Virus
Subtype: Boot
DAT Required: 4002
Removal Instructions


This is a virus detection. Viruses are programs that self-replicate recursively, meaning that infected systems spread the virus to other systems, which then propagate the virus further. While many viruses contain a destructive payload, it's quite common for viruses to do nothing more than spread from one system to another.

Indication of Infection

From time to time, infected systems hang and display the following message:


Total system and available free memory, as indicated by the DOS CHKDSK, decreases by 1,024 bytes.

Methods of Infection

The only way to infect a computer with an MBR/Boot Sector infector is to attempt to boot from an infected floppy diskette. The boot sector of the diskette has the code to determine if the diskette is bootable, and to display the "Non-system disk or disk error" message. It is this code that harbors the infection. By the time the non-system disk error message comes up, the infection has occurred.

Once the virus is executed, it will infect the hard drive's MBR and may become memory resident. With every subsequent boot, the virus will be loaded into memory and will attempt to infect floppy diskettes accessed by the machine.



Virus Characteristics

Parity_Boot.A is a memory resident, stealth, Master Boot Record(MBR)/Boot Sector infector. This virus is regarded as destructive, as it corrupts data on the partition table and can also cause corruption in the root directory.

Upon infection, Parity_Boot.A becomes memory resident. Interrupt 12's return is moved to 9FC0. The virus copies the MBR Side 0, Cylinder 0, Sector 1 to Side 0, Cylinder 0, Sector 14, then infects the MBR.

Once the Parity_Boot.A virus is memory resident, it infects non-write protected diskettes as they are accessed. On 360K 5.25" diskettes the original boot sector is moved to sector 11. On 1.2M 5.25" diskettes, the original boot sector is moved to sector 28. In both cases, these sectors are the last sector in the root directory and any directory entries originally located in these sector are overwritten.

It is a Stealth virus. It redirects attempts to read the hard disk MBR and diskette boot sectors to the original MBR and boot sectors.


Variants information
Virus Name Type Subtype Differences
Parity_Boot.B Virus Boot
Parity_Boot.C Virus Boot
Parity_Boot.D Virus Boot
Parity_Boot.E Virus Boot

Windows 95/98:
Note for Windows 9x systems - during the boot process a Windows95 created boot disk will access the hard drive for information. Because of this an image of the virus may be in memory but not active.

To remove the virus, follow the following steps:
- If you use the McAfee emergency disk, hit F8 at the starting Windows 95 message, and select Step-by-step Configuration. Say yes to everything except processing the autoexec.bat file.
- At the a:, type

Windows NT/2000:
Shut down the PC and turn the power off. Obtain or create a virus free boot disk and scan disk. After booting, at the A:\ prompt, execute the following command:
BOOTSCAN C: /boot /clean

Once the virus has been removed, remove all floppy diskettes from the computer and reboot from the hard drive.

This will also clean an NTFS Master Boot Record and allow Windows NT to successfully reboot from the hard disk drive. VirusScan for DOS will not be able to read the rest of the NTFS partition. After starting Windows, execute VirusScan or NetShield to detect and clean Windows NT file infections which may exist.