Boot virus

Threat description


Category: Malware
Type: Virus
Platform: W32
Aliases: Boot virus, BOO virus, Boot virus, MBR virus, DBR virus


This type of virus infects the Master Boot Record or DOS Boot Record of a hard drive, or the Floppy Boot Record of a floppy drive.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.


More information on scanning and removal options available in your F-Secure product can be found in the Help Center.

You may also refer to the Knowledge Base on the F-Secure Community site for more information.

Contact Support

For further assistance, F-Secure customers can request support online via the Request support or the Chat forms on our Home - Global site.

Technical Details

A boot virus (also known as a boot infector, an MBR virus or DBR virus) targets and infects a specific, physical section of a computer system that contains information crucial to the proper operation of the computer's operating system (OS).

Though boot viruses were common in the early 90s, they became much rarer after most computer motherboard manufacturers added protection against such threats by denying access to the Master Boot Record (the most commonly targeted component) without user permission.

In recent years however, more sophisticated malware have emerged that have found ways to circumvent that protection and retarget the MBR (e.g, Rootkit:W32/Whistler.A).

How a boot virus infects

Boot viruses differ based on whether they target the Master Boot Record (MBR), the DOS Boot Record (DBR) or the Floppy Boot Record (FBR):

  • The MBR is the first sector of a hard drive and is usually located on track 0. It contains the initial loader and information about partition tables on a hard disk.
  • The DBR is usually located a few sectors (62 sectors after on a hard disk with 63 sectors per track) after the MBR, and contains the initial loader for an operating system and logical drive information.
  • The FBR is use for the same purposes as DBR on a hard drive, but it is located on the first track of a diskette.

A boot virus can be further subdivided into either overwriting or relocating:

  • An overwriting boot virus overwrites MBR, DBR or FBR sector with its own code, while preserving the original partition table or logical drive information.
  • A relocating boot virus saves the original MBR, DBR or FBR somewhere on a hard or floppy drive. Sometimes, such an action can destroy certain areas of a hard or floppy drive and make a disk unreadable.

All boot viruses are memory-resident . When an infected computer is started, the boot virus code is loaded in memory. It then traps one of BIOS functions (usually disk interrupt vector Int 13h) to stay resident in memory.

Once resident in memory, a boot virus can monitor disk access and write its code to the boot sectors of other media used on the computer. For example, a boot virus launched from a diskette can infect the computer's hard drive; it can then infect all diskettes that are inserted in the computer's floppy drive.


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