From: Matthew Ghio <ghio@myriad.alias.net>
Date: Sun, 7 Apr 1996 16:00:47 +0800
Subject: Re: Was Cohen the first?
In-Reply-To: <35960405162553/0005514706DC3EM@MCIMAIL.COM>
Message-ID: <199604070321.TAA02171@myriad>
MIME-Version: 1.0
Content-Type: text/plain


The following concerns the history of computer viruses.  While an
important issue in computer security, this has no direct relevance to
cryptography, so skip this message if you're looking for crypto...

---

> What's up? I asked. The 17-year-old snickered. Doom was ahead for all Apple
>II owners.  "Don't engage in casual disk-copying with strangers," he said.
> "You might catch an operating-system virus."

One interesting fact is that after the release of DOS 3.3 in 1980, the Apple
II operating system was unchanged for several years, until Apple released a
completly rewritten operating system (ProDOS) in 1984.  During this period,
hackers disassembled DOS 3.3 and its internal functions and data structures
became well-known.  The Apple II ROM contained a debugger/disassembler,
which allowed the operating system to be disassembled and experimentally
modified while it was resident in memory.  In addition, several companies,
including Beagle Brothers and Quality Software, published extensive
information on DOS 3.3, which had been obtained through reverse-engineering.

Apple DOS behaved very predictably: it was always loaded at the same
location in memory, and when it formatted a disk, always wrote the operating
system into the same location on the first three tracks of the floppy disk.
This allowed the development of object code patches to the DOS kernel which
would work reliably because almost everyone was running identical copies of
DOS.

It also made it easy to write viruses.  The simplest was to attach a call to
the sector-write subroutine at the end of the catalog command.  This only
took about 16 bytes of code.  (Wouldn't that have made a neat .sig for you
modern-day perl hackers?)  Whenever the user issued the catalog command
(which gave a list of files, like the unix ls command) it would write out
that page of memory onto the dos image on the disk, thus infecting the disk.
This was entirely benign unless you tried to use a disk with a different or
modified operating system, in which case the patch would not apply cleanly,
and would likely make the disk unbootable.

Despite this, the Apple II never became a popular virus-writing platform.
There are several possible reasons for this, but one of the main ones is
that few Apple II users had hard disks.  On the IBM PC, it was easy for a
virus to get on the hard disk, then systematically infect every floppy disk
put into the system.  Apple II users, in contrast, often booted from
floppies, and often rebooted when switching to a different software package,
thus purging the virus from memory.  (Pressing control-reset on the Apple II
keyboard would always pull the reset line on the CPU, so it wasn't possible
to trap the interrupt like it is possible to trap ctrl-alt-del on the PC.)

A couple bits of interesting virus trivia: Joe Dellinger, then an undergrad
at Texas A&M, set out to write the "perfect" virus, that is, one that would
silently replicate without causing harm, just to see how far it would
spread.  The virus added a tag to the end of the boot sector which read:
GENxxxxxxxTAMU, where xxxxxxx was the generation counter.

A destructive virus called "CyberAIDS", appeared in 1988.  I just looked
this one up in an old magazine article, and, when the virus destroyed a
disk, it printed, among other drivel:
             DISTRIBUTED BY
       Worshippers of Pat / [WOP]
          The Kool/Rad Alliance
  The Robert Dole Presidential Campaign
I wonder if Bob Dole appreciated the free advertising.  (Also remember
that this was in 1988!)

Modern operating systems make kernel-patching viruses like the simple
ten-byte hacks effectively impossible since the operating system is less
predictable with respect to its memory usage, people upgrade more
frequently, and many experienced users compile their own kernel.
Modern protected-mode operating systems are also making boot-sector viruses
obsolete as well.  That leaves executable file viruses.  

By the late eighties, hard disks were becoming fairly standard equipment,
and the "CyberAIDS" virus mentioned above attached itself to executable
files.  Filesystem security and read/execute-only memory pages on Unix
systems make writing effective viruses of this type quite difficult.
Unfortunately, Microsoft's lax attitude toward security allows viruses to
persist on their operating systems, and have made Bill Gates very popular
with the virus writing groups.  In addition, the lack of filesystem security
in Windoze makes the shared libraries, and key system files, prime targets
for malicious code.

Still, as the famous login hack demontrates, it's not impossible to write a
unix virus, if you can get control of the compiler/linker.  It's just that
there are other methods of hacking unix systems (ie buffer overruns) which
provide a more immediate return on the investment of time for the hacker.