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.ofrom 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.

Hacker's Wisdom/ A Report On The Internet Worm