Horse Boot Virus
Virus Name: Horse Boot
Aliases: Horse Boot Dropper
V Status: Rare
Discovered: May, 1991
Symptoms: BSC; decrease in system & available memory; master boot
sector altered; high density diskette corruption
Eff Length: N/A Bytes
Type Code: BRhX - Resident Boot Sector & Master Boot Sector Infector
Detection Method: ViruScan, Sweep, AVTK, NAV, F-Prot,
IBMAV, NAVDX, VAlert, PCScan, ChAV,
NShld, LProt, Sweep/N, AVTK/N, NAV/N, NProt, Innoc 4.0+
Removal Instructions: MDisk/P, or DOS SYS on system diskettes
The Horse Boot virus was submitted in May, 1991 in the form of a
"dropper" program which installs the virus on a 360K diskette.
Horse Boot, once "dropped" by the install program, is a memory
resident infector of diskette boot sectors and the hard disk
master boot sector (partition table). It is based on the Stoned
When a system is booted from a diskette infected by the Horse Boot
virus, the virus will install itself memory resident at the top
of system memory, but below the 640K DOS boundary. Interrupt 12's
return will not be moved. Total system and available free memory
will decrease by 2,048 bytes as measured by the DOS CHKDSK
program. The virus will also access the system hard disk, and
infect the hard disk's master boot sector with a copy of the virus.
Once the boot has completed, the user will find their current
drive is drive C:, and not drive A:.
Once Horse Boot is memory resident, it will infect non-write
protected diskettes which are exposed to the system, similar to
360K low density 5.25" diskettes infected with Horse Boot will have
their original boot sector moved to the last sector on the diskette.
Infected hard disks will have the original master boot sector moved
to cylinder 0, side 0, sector 7.
Horse Boot treats high density diskettes as low density diskettes.
If a high density diskette becomes infected with Horse Boot, data on
the diskette will be damaged.
Known variant(s) of Horse Boot are:
Horse Boot Dropper: A small .COM program which, when executed,
drops the Horse Boot virus.