A REPORT ON THE INTERNET WORM
Bob Page
University of Lowell
Computer Science Department
November 7, 1988
[Because of the many misquotes the media have been giving,
this report is Copyright (c) Bob Page, all rights reserved.
Permission is granted to republish this ONLY if you republish
it in its entirety.]
Here's the scoop on the "Internet Worm". Actually it's not a virus -
a virus is a piece of code that adds itself to other programs,
including operating systems. It cannot run independently, but rather
requires that its "host" program be run to activate it. As such, it
has a clear analog to biologic viruses -- those viruses are not
considered live, but they invade host cells and take them over, making
them produce new viruses.
A worm is a program that can run by itself and can propagate a fully
working version of itself to other machines. As such, what was loosed
on the Internet was clearly a worm.
This data was collected through an emergency mailing list set up by
Gene Spafford at Purdue University, for administrators of major
Internet sites - some of the text is included verbatim from that list.
Mail was heavy since the formation of the list; it continues to be on
Monday afternoon - I get at least 2-3 messages every hour. It's
possible that some of this information is incomplete, but I thought
you'd like to know what I know so far.
The basic object of the worm is to get a shell on another machine so
it can reproduce further. There are three ways it attacks: sendmail,
fingerd, and rsh/rexec.
THE SENDMAIL ATTACK:
In the sendmail attack, the worm opens a TCP connection to another
machine's sendmail (the SMTP port), invokes debug mode, and sends a
RCPT TO that requests its data be piped through a shell. That data, a
shell script (first-stage bootstrap) creates a temporary second-stage
bootstrap file called x$$,l1.c (where '$$' is the current process ID).
This is a small (40-line) C program.
The first-stage bootstrap compiles this program with the local cc and
executes it with arguments giving the Internet hostid/socket/password
of where it just came from. The second-stage bootstrap (the compiled
C program) sucks over two object files, x$$,vax.o and x$$,sun3.o from
the attacking host. It has an array for 20 file names (presumably for
20 different machines), but only two (vax and sun) were compiled in to
this code. It then figures out whether it's running under BSD or
SunOS and links the appropriate file against the C library to produce
an executable program called /usr/tmp/sh - so it looks like the Bourne
shell to anyone who looked there.
THE FINGERD ATTACK:
In the fingerd attack, it tries to infiltrate systems via a bug in
fingerd, the finger daemon. Apparently this is where most of its
success was (not in sendmail, as was originally reported). When
fingerd is connected to, it reads its arguments from a pipe, but
doesn't limit how much it reads. If it reads more than the internal
512-byte buffer allowed, it writes past the end of its stack. After
the stack is a command to be executed ("/usr/ucb/finger") that
actually does the work. On a VAX, the worm knew how much further from
the stack it had to clobber to get to this command, which it replaced
with the command "/bin/sh" (the bourne shell). So instead of the
finger command being executed, a shell was started with no arguments.
Since this is run in the context of the finger daemon, stdin and
stdout are connected to the network socket, and all the files were
sucked over just like the shell that sendmail provided.
THE RSH/REXEC ATTACK:
The third way it tried to get into systems was via the .rhosts and
/etc/hosts.equiv files to determine 'trusted' hosts where it might be
able to migrate to. To use the .rhosts feature, it needed to actually
get into people's accounts - since the worm was not running as root
(it was running as daemon) it had to figure out people's passwords.
To do this, it went through the /etc/passwd file, trying to guess
passwords. It tried combinations of: the username, the last, first,
last+first, nick names (from the GECOS field), and a list of special
"popular" passwords:
aaa cornelius guntis noxious simon
academia couscous hacker nutrition simple
aerobics creation hamlet nyquist singer
airplane creosote handily oceanography single
albany cretin happening ocelot smile
albatross daemon harmony olivetti smiles
albert dancer harold olivia smooch
alex daniel harvey oracle smother
alexander danny hebrides orca snatch
algebra dave heinlein orwell snoopy
aliases december hello osiris soap
alphabet defoe help outlaw socrates
ama deluge herbert oxford sossina
amorphous desperate hiawatha pacific sparrows
analog develop hibernia painless spit
anchor dieter honey pakistan spring
andromache digital horse pam springer
animals discovery horus papers squires
answer disney hutchins password strangle
anthropogenic dog imbroglio patricia stratford
anvils drought imperial penguin stuttgart
anything duncan include peoria subway
aria eager ingres percolate success
ariadne easier inna persimmon summer
arrow edges innocuous persona super
arthur edinburgh irishman pete superstage
athena edwin isis peter support
atmosphere edwina japan philip supported
aztecs egghead jessica phoenix surfer
azure eiderdown jester pierre suzanne
bacchus eileen jixian pizza swearer
bailey einstein johnny plover symmetry
banana elephant joseph plymouth tangerine
bananas elizabeth joshua polynomial tape
bandit ellen judith pondering target
banks emerald juggle pork tarragon
barber engine julia poster taylor
baritone engineer kathleen praise telephone
bass enterprise kermit precious temptation
bassoon enzyme kernel prelude thailand
batman ersatz kirkland prince tiger
beater establish knight princeton toggle
beauty estate ladle protect tomato
beethoven euclid lambda protozoa topography
beloved evelyn lamination pumpkin tortoise
benz extension larkin puneet toyota
beowulf fairway larry puppet trails
berkeley felicia lazarus rabbit trivial
berliner fender lebesgue rachmaninoff trombone
beryl fermat lee rainbow tubas
beverly fidelity leland raindrop tuttle
bicameral finite leroy raleigh umesh
bob fishers lewis random unhappy
brenda flakes light rascal unicorn
brian float lisa really unknown
bridget flower louis rebecca urchin
broadway flowers lynne remote utility
bumbling foolproof macintosh rick vasant
burgess football mack ripple vertigo
campanile foresight maggot robotics vicky
cantor format magic rochester village
cardinal forsythe malcolm rolex virginia
carmen fourier mark romano warren
carolina fred markus ronald water
caroline friend marty rosebud weenie
cascades frighten marvin rosemary whatnot
castle fun master roses whiting
cat fungible maurice ruben whitney
cayuga gabriel mellon rules will
celtics gardner merlin ruth william
cerulean garfield mets sal williamsburg
change gauss michael saxon willie
charles george michelle scamper winston
charming gertrude mike scheme wisconsin
charon ginger minimum scott wizard
chester glacier minsky scotty wombat
cigar gnu moguls secret woodwind
classic golfer moose sensor wormwood
clusters gorgeous morley serenity yaco
coffee gorges mozart sharks yang
coke gosling nancy sharon yellowstone
collins gouge napoleon sheffield yosemite
commrades graham nepenthe sheldon zap
computer gryphon ness shiva zimmerman
condo guest network shivers
cookie guitar newton shuttle
cooper gumption next signature
[I wouldn't have picked some of these as "popular" passwords, but
then again, I'm not a worm writer. What do I know?]
When everything else fails, it opens /usr/dict/words and tries every
word in the dictionary. It is pretty successful in finding passwords,
as most people don't choose them very well. Once it gets into
someone's account, it looks for a .rhosts file and does an 'rsh'
and/or 'rexec' to another host, it sucks over the necessary files into
/usr/tmp and runs /usr/tmp/sh to start all over again.
Between these three methods of attack (sendmail, fingerd, .rhosts)
it was able to spread very quickly.
THE WORM ITSELF:
The 'sh' program is the actual worm. When it starts up it clobbers
its argv array so a 'ps' will not show its name. It opens all its
necessary files, then unlinks (deletes) them so they can't be found
(since it has them open, however, it can still access the contents).
It then tries to infect as many other hosts as possible - when it
sucessfully connects to one host, it forks a child to continue the
infection while the parent keeps on trying new hosts.
One of the things it does before it attacks a host is connect to the
telnet port and immediately close it. Thus, "telnetd: ttloop: peer
died" in /usr/adm/messages means the worm attempted an attack.
The worm's role in life is to reproduce - nothing more. To do that it
needs to find other hosts. It does a 'netstat -r -n' to find local
routes to other hosts & networks, looks in /etc/hosts, and uses the
yellow pages distributed hosts file if it's available. Any time it
finds a host, it tries to infect it through one of the three methods,
see above. Once it finds a local network (like 129.63.nn.nn for
ulowell) it sequentially tries every address in that range.
If the system crashes or is rebooted, most system boot procedures
clear /tmp and /usr/tmp as a matter of course, erasing any evidence.
However, sendmail log files show mail coming in from user /dev/null
for user /bin/sed, which is a tipoff that the worm entered.
Each time the worm is started, there is a 1/15 chance (it calls
random()) that it sends a single byte to ernie.berkeley.edu on some
magic port, apparently to act as some kind of monitoring mechanism.
THE CRACKDOWN:
Three main 'swat' teams from Berkeley, MIT and Purdue found copies of
the VAX code (the .o files had all the symbols intact with somewhat
meaningful names) and disassembled it into about 3000 lines of C. The
BSD development team poked fun at the code, even going so far to point
out bugs in the code and supplying source patches for it! They have
not released the actual source code, however, and refuse to do so.
That could change - there are a number of people who want to see the
code.
Portions of the code appear incomplete, as if the program development
was not yet finished. For example, it knows the offset needed to
break the BSD fingerd, but doesn't know the correct offset for Sun's
fingerd (which causes it to dump core); it also doesn't erase its
tracks as cleverly as it might; and so on.
The worm uses a variable called 'pleasequit' but doesn't correctly
initialize it, so some folks added a module called _worm.o to the C
library, which is produced from:
int pleasequit = -1;
the fact that this value is set to -1 will cause it to exit after one
iteration.
The close scrutiny of the code also turned up comments on the
programmer's style. Verbatim from someone at MIT:
From disassembling the code, it looks like the programmer
is really anally retentive about checking return codes,
and, in addition, prefers to use array indexing instead of
pointers to walk through arrays.
Anyone who looks at the binary will not see any embedded strings -
they are XOR'ed with 81 (hex). That's how the shell commands are
imbedded. The "obvious" passwords are stored with their high bit set.
Although it spreads very fast, it is somewhat slowed down by the fact
that it drives the load average up on the machine - this is due to all
the encryptions going on, and the large number of incoming worms from
other machines.
[Initially, the fastest defense against the worm is is to create a
directory called /usr/tmp/sh. The script that creates /usr/tmp/sh
from one of the .o files checks to see if /usr/tmp/sh exists, but not
to see if it's a directory. This fix is known as 'the condom'.]
NOW WHAT?
None of the ULowell machines were hit by the worm. When BBN staffers
found their systems infected, they cut themselves off from all other
hosts. Since our connection to the Internet is through BBN, we were
cut off as well. Before we were cut off, I received mail about the
sendmail problem and installed a patch to disable the feature the worm
uses to get in through sendmail. I had made local modifications to
fingerd which changed the offsets, so any attempt to scribble over the
stack would probably have ended up in a core dump.
Most Internet systems running 4.3BSD or SunOS have installed the
necessary patches to close the holes and have rejoined the Internet.
As you would expect, there is a renewed interest in system/network
security, finding and plugging holes, and speculation over what
will happen to the worm's creator.
If you haven't read or watched the news, various log files have named
the responsible person as Robert Morris Jr., a 23-year old doctoral
student at Cornell. His father is head of the National Computer
Security Center, the NSA's public effort in computer security, and has
lectured widely on security aspects of UNIX.
Associates of the student claim the worm was a 'mistake' - that he
intended to unleash it but it was not supposed to move so quickly or
spread so much. His goal (from what I understand) was to have a
program 'live' within the Internet. If the reports that he intended
it to spread slowly are true, then it's possible that the bytes sent
to ernie.berkeley.edu were intended to monitor the spread of the
worm. Some news reports mentioned that he panicked when, via some
"monitoring mechanism" he saw how fast it had propagated.
A source inside DEC reports that although the worm didn't make much
progress there, it was sighted on several machines that wouldn't be
on its normal propagation path, i.e. not gateways and not on the same
subnet. These machines are not reachable from the outside. Morris
was a summer intern at DEC in '87. He might have included names or
addresses he remembered as targets for infesting hidden internal
networks. Most of the DEC machines in question belong to the group he
worked in.
The final word has not been written - I don't think the FBI have even
met with this guy yet. It will be interesting to see what happens.