From: martin@cs.ualberta.ca (Tim Martin; FSO; Soil Sciences)
Newsgroups: comp.virus
Subject: Re: Monkey [Mon] and Multi-2 [M12] viruses (PC)
Date: 15 Jan 93 23:18:38 GMT

LIBBIE@pucc.PRINCETON.EDU (Libbie Counselman) writes:

>We have been using and supporting F-Prot 2.06a on campus, but have
>discovered 2 viruses that are not detected and not disinfected by
>F-Prot.

>The first one discovered was the Monkey [Mon] virus.  It affects the
>File Allocation Table.  SCAN discovers it, but does not disinfect, but
>Norton Disk Doctor will recover the clean FAT.

I've posted a lot of notes in the past, about Monkey, so you may
want to check the archives.  Monkey doesn't infect the FAT; nothing
does.  It infects the main boot record (MBR) of a hard disk, and
the boot sector of a floppy.  The main twists Monkey has that make
it different from Stoned are that 1. it doesn't keep the partition
table in place in the MBR, and 2. it encodes the MBR before it hides
it (in this case in sector 3 on the hard disk.)  Because the partition
table isn't in place, when you boot from a floppy you can't see the
hard drive partitions, so any attempt to change to drive C: for 
example results in a "invalid drive specification" error.  Because 
the MBR is encoded before it is hidden, general scanner/disinfector
packages can't find the proper partition table values to recover the
MBR properly. F-prot will find it on floppies, and call it "a new
variant of stoned", but will not clean it. 

It has other twists, such as stealth, as well. 

I would like to know what you mean when you say that Norton Disk Doctor
will recover the clean FAT.  The FAT was clean all along.  The problem
is not to recover the FAT but to find appropriate values for the 
partition table.  If NDD manages to do this, from a Monkey-infected
hard disk, I am impressed.  Actually one can take advantage of Monkey's
stealth to get the proper partition table values.  If you know a hard
disk is infected with Monkey, you can boot the computer from the 
hard disk, so the virus is active, and use Norton Utilities or some
such to see what the proper partition table values are.  While the
virus is active, a request to read the MBR sector (sector 1 of side 0,
cylinder 0) will return the MBR (sector 3), properly decoded, instead
of the virus sector.  Copy down the proper table values, reboot from
a clean floppy, use fdisk /mbr (with DOS 5.0) to reinstall the 
MBR executable code portion, and use Norton Utilities or whatever
to type in the correct partition table data.

>The second is known as Multi-2 [M12].  It has a predecessor called
>Multi [M-123], also not recognized by F-Prot.  This one infects .COM
>files, .EXE files, overlays and becomes memory resident.  CLEAN
>apparently disinfects it.

I don't know this one.

>Does anyone know any non-commercial packages (i.e. shareware or freeware)
>that can combat these viruses?

I have written a program called Killmonk that should get rid
of the Monkey virus.  Killmonk is at several of the ftp
sites, but let me know if you can't find it.

Tim.

 -------------------------------------------------------------
  Tim Martin                   *
  Spatial Information Systems  *   These opinions are my own:
  University of Alberta        *      My employer has none!
  martin@cs.ualberta.ca        *
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