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Title:     Stalking the wily hacker. (includes related articles on the
           definition of hackers, Intruder versus Tracker, legal constraints
           and ethics, and computer security resources)

Journal:   Communications of the ACM  May 1988 v31 n5 p484(14)
           * Full Text COPYRIGHT Assn. for Computing Machinery, Inc. 1988.

Author:    Stoll, Clifford.

Summary:   The experience of the Lawrence Berkeley Laboratory in tracking an
           intruder suggests that any operating system is insecure when
           obvious security rules are ignored. How a site should respond to
           an intrusion, whether it is possible to trace an intruder trying
           to evade detection, what can be learned from tracking an intruder,
           what methods the intruder used, and the responsiveness of the
           law-enforcement community are also discussed.


In August 1986 a persistent computer intruder attacked the Lawrence Berkeley
Laboratory (LBL). Instead of trying to keep the intruder out, we took the
novel approach of allowing him access while we printed out his activites and
traced him to his source. This trace back was harder than we expected,
requiring nearly a year of work and the cooperation of many organizations.
This article tells the story of the break-ins and the trace, and sums up what
we learned.

We approached the problem as a short, scientific exercise in discovery,
intending to determine who was breaking into our system and document the
exploited weaknesses. It became apparent, however, that rather than
innocuously playing around, the intruder was using our computer as a hub to
reach many others. His main interest was in computers operated by the
military and by defense contractors. Targets and keywords suggested that he
was attempting espionage by remotely entering sensitive computers and
stealing data; at least he exhibited an unusual interest in a few,
specifically military topics. Although most attacked computers were at
military and defense contractor sites, some were at universities and research
organizations. Over the next 10 months, we watched this individual attack
about 450 computers and successfully enter more than 30.

LBL is a research institute with few military contracts and no classified
research (unlike our sister laboratory, Lawrence Livermore National
Laboratory, which has several classified projects). Our computing
environment is typical of a university: widely distributed, heterogeneous,
and fairly open. Despite this lack of classified computing, LBL's management
decided to take the intrusion seriously and devoted considerable resources to
it, in hopes of gaining understanding and a solution.

The intruder conjured up no new methods for breaking operating systems;
rather he repeatedly applied techniques documented elsewhere. whenever
possible, he used known security holes and subtle bugs in different operating
systems, including UNIX, VMS, VM-TSO, EMBOS, and SAIL-WAITS. Yet it is a
mistake to assume that one operating system is more secure than another: Most
of these break-ins were possible because the intruder exploited common
blunders by vendors, users, and system managers.

Throughout these intrusions we kept our study a closely held secret. We
deliberately remained open to attacks, despite knowing the intruder held
system-manager privileges on our computers. Except for alerting management
at threatened installations, we communicated with only a few trusted sites,
knowing this intruder often read network messages and even accessed computers
at several computer security companies. We remained in close touch with
law-enforcement officials, who maintained a parallel investigation. as this
article goes to press, the U.S. FBI and its German equivalent, the
Bundeskriminalamt (BKA), continue their investigations. Certain details are
therefore necessarily omitted from this article.

Recently, a spate of publicity surrounded computer break-ins around the world
[23, 33, 37]. With a few notable exceptions (e.g., [24, 36]), most were
incompletely reported anecdotes [7] or were little more than rumors. For
lack of substantive documentation, system designers and mangers have not
addressed important problems in securing computers. Some effrts to lighten
security on common systems may even be misdirected. We hope that lessons
learned from our research will help in the design and management of more
secure systems.

How should a site respond to an attack? Is it possible to trace the
connections of someone trying to evade detection? What can be learned by
following such an intruder? Which security holes were taken advantage of?
How responsive was the law-enforcement community? This article addresses
these issues, and avoids such questions as whether there is anything
intrinsically wrong with browsing through other people's files or with
attempting to enter someone else's computer, or why someone would wish to
read military databases. Nonetheless, the author holds strong opinions on
these subjects.


We firt suspented a break-in when one of LBL's computers reported an
accounting error. A new account had been created without a corresponding
billing address. Our locally developed accounting program could not balance
its books, since someone had incorrectly added the account. Soon afterwards,
a message from the National Computer Security Center arrived, reporting that
someone from our laboratory had attempted to break into one of their
computers through a MILNET connection.

We removed the errant account, but the problem remained. We detected someone
acting as a system manager, attempting to modify accounting records.
Realizing that there was an intruder in the system, we installed line
printers and recorders on all incoming ports, and printed out the traffic.
Within a few days, the intruder showed up again. We captured all of his
keystrokes on a printer and saw how he used a subtle bug in the Gnu-Emacs
text editor [40] to obtain system-manager privileges. At first we suspected
that the culprit was a student prankster at the nearly University of
California. We decided to catch him in the act, if possible. Accordingly,
whenever the intruder was present, we began tracing the line, printing out
all of his activity in real time.


Early on, we began keeping a detailed logbook, summarizing the intruder's
traffic, the traces, our suspicions, and interactions with law-enforcement
people. Like a laboratory notebook, our logbook reflected both confusion and
progress, but eventually pointed the way to the solution. Months later, when
we reviewed old logbook notes, buried clues to the intruder's origin rose to
the surface.

Having decided to keep our efforts invisible to the intruder, we needed to
hide our rcords and eliminate our electronic messages about his activity.
Although we did not know the source of our problems, we trusted our own staff
and wished to inform whoever needed to know. We held meetings to reduce
rumors, since our work would be lost if word leaked out. Knwing the
sensitivity of this matter, our staff kept it out of digital networks,
bulletin boards, and, especially, electronic mail. Since the intruder
searched our electronic mail, we exchanged messages about security by
telephone. Several false electronic-mail messages made the intruder feel
more secure when he illicitly read them.


We needed alarms to instantly notify us when the intruder entered our system.
At first, not knowing from which port our system was being hit, we set
printers on all lines leading to the attacked computer. After finding that
the intruder entered via X.25 ports, we recorded bidirectional traffic
through that set of lines. These printouts proved essential to our
understanding of events; we had records of his every keystroke, giving his
targets, keywords, chosen passwords, and methodologies. The recording wa
complete in that virtually all of these session were captured, either by
printer or on the floppy disk of a nearby computer. These monitors also
uncovered several other attempted intrusions, unrelated to those of the
individual we were following.

Off-line monitors have several advantages over monitors embedded in an
operating system. They are invisible even to an intruder with system
privileges. Moreover, they gave printouts of the intruder's activities on
our local area network (LAN), letting us see his attempts to enter other
closely linked computers. A monitor that records keystrokes within an
operating systm consumes computing resources and may slow down other
processes. In addition, such an monitor must use highly privileged software
and may introduce new security holes into the system. Besides taking up
resources, on-line monitors would have warned the intruder that he was being
tracked. Since printers and personal computers are ubiquitous, and because
RS-232 serial lines can easily be sent to multiple receivers, we used this
type of off-line monitor and avoided tampering with our operating systems.

The alarms themselves were crude, yet effective in protecting our system as
well as others under attack. We knew of researchers developing expert
systems that watch fror abnormal activity [4, 35], but we found our methods
simpler, cheaper, and perhaps more reliable. Backing up these alarms, a
computer loosely coupled into our LAN periodically looked at every process.
Since we knew from the printouts which accounts had been compromised, we only
had to watch for the use of these stolen accounts. We chose to place alarms
on the incoming lines, where serial line analyzers and personal computers
watched all traffic for the use of stolen account names. If triggered, a
sequence of events culminated in a modem calling the operator's pocket pager.
The operator watched the intruder on the monitors. If the intruder began to
delete files or damage a system, he could be immediately disconnected, or the
command could be disabled. When he appeared to be entering sensitive
computers or} downloading sensitive files, line noise, which appeared to be
network glitches, could be inserted into the communications link.

In general, we contacted the system managers of the attacked computers,
though in some cases the FBI or military authorities made the contact.
Occasionally, they cooperated by leaving their systems open. More often,
they immediately disabled the intruder or denied him access. From the
intruder's viewpoint, almost everyone except LBL detected his activity. In
reality, almost nobody except LBL detected him.

Throughout this time, the printouts showed his interests, techniques,
successes, and failures. Initially, we were interested in how the intruder
obtained system-manager privileges. Within a few weeks, we noticed him
exploring our network connections--using ARPANET and MILNET quite handily,
but frequently needing help with lesser known networks. Later, the monitors
showed him leapfrogging through our computers, connecting to several military
bases in the United States and abroad. Eventually, we observed him attacking
many sites over Internet, guessing passwords and accounts names.

By studying the printouts, we developed an understanding of what the intruder
was looking for. We also compared activity on different dates in order to
watch him learn a new system, and inferred sites he entered through pathways
we could not monitor. We observed the intruder's familiarity with various
operating systems and became familiar with his programming style. Buried in
this chatter were clues to the intruder's location and persona, but we needed
to temper inferences based on traffic analysis. Only a complete trace back
would identify the culprit.


Tracing the activity was challenging because the intruder crossed many
networks, seldom connected for more than a few minutes at a time, and might
be active at any time. We needed fast trace backs on several systems, so we
automated much of the process. Within seconds of a connection, our alarms
notified system managers and network control centers automatically, using
pocket pagers dialed by a local modem [42]. Simultaneously, technicians
started tracing the networks.

Since the intruder's traffic arrived from an X.25 port, it could have come
from anywhere in the world. We initially traced it to a nearby dial-up
Tymnet port, in Oakland, California. With a court order and the telephone
company's cooperation, we then traced the dial-up calls to a dial-out modem
belonging to a defense contractor in McLean, Virginia. In essence, their LAN
allowed any user to dial out from their modem pool and even provided a
last-number-redial capability for those who did not know access codes for
remote systems.

Analyzing the defense contractor's long-distance telephone records allowed us
to determine the extend of these activities. By cross-correlating them with
audit trails at other sites, we determined additional dates, times, and
targets. A histogram of the times when the intruder was active showed most
activity occurring at around noon, Pacific time. These records also
demonstrated the attacks had started many months before detection at LBL.

Curiously, the defense contractor's telephone bills listed hundreds of short
telephone calls all around the United States. The intruder had collected
lists of modem telephone numbers and then called them over these modems.
Once connected, he attempted to log in using common account names and
passwords. These attempts were usually directed at military bases; several
had detected intruders coming in over telephone lines, but had not bothered
to trace them. When we alerted the defense contractor officials of their
problem, they tightened access to their outbound modems and these were no
more short connections.

After losinig access to the defense contractor's modems, the still undeterred
intruder connected to us over different links. Through the outstanding
efforts of Tymnet, the full X.25 calling addresses were obtained within
seconds of an attack. These addresses pointed to sources in Germany:
universities in Bremen and Karlsruhe, and a public dial-up modem in another
German city. When the intruder attacked the university in Bremen, he
acquired system-manager privileges, disabled accounting, and used their X.25
links to connect around the world. Upon recognizing this problem, the
university traced the connections to the other German city. This, in turn,
spurred more tracing efforts, coordinating LBL, Tymnet, the university, and
the German Bundespost.

Most connections were purposely convoluted. Figure 1 summarizes the main
pathways that were traced, but the intruder used other connections as well.
The rich connectivity and redundant circuits demonstrate the intruder's
attempts to cover his tracks, or at least his search for new networks to

Besides physical network traces, there were several other indications of a
foreign origin. When the intruder transferred files, we timed round-trip
packet acknowledgments over the network links. Later, we measured the
empirical delay times to a variety of different sites and estimated average
network delay times as a function of distance. This measurement pointed to
an overseas origin. In addition, the intruder knew his way around UNIX,
using AT&T rather than Berkeley UNIX commands. When stealing accounts, he
sometimes used German passwords. In retrospect, all were clues to his
origin, yet each was baffling given our mind-set that "it must be some
student from the Berkeley campus."


The intruder's brief connections prevented telephone technicians from
determining his location more precisely than to a particular German city. To
narrow the search to an individual telephone, the technicians needed a
relatively long connection. We baited the intruder by creating several files
of fictitious text in an obscure LBL computer. These files appeared to be
memos about how computers were to support research for the Strategic Defense
Initiative (SDI). All the information was invented and steepest in
governmental jargon. The files also contained a mailing list and several
form letters talking about "additional documents available by mail" from a
nonexistent LBL secretary. We protected these bogus files so that no one
except the owner and system manager could read them, and set alarms so that
we would know who read them.

While scavenging our files one day, the intruder detected these bogus files
and then spent more than an hour reading them. During that time the
telephone technicians completed the trace. We celebrated with milk shakes
made with homegrown Berkeley strawberries, but the celebration proved
premature. A few months later, a letter arrived from someone in the United
States, addressed to the nonexistent secretary. The writer asked to be added
to the fictitious SDI mailing list. As it requested certain "classified
information," the letter alone suggested espionage. Moreover, realizing that
the information had traveled from someone in Germany to a contact in the
United States, we concluded we were witnessing attempted espionage. Other
than cheap novels, we have no experience in this arena and so left this part
of the investigation to the FBI.


Printouts of the intruder's activity showed that he used our computers as a
way station; although he could become system manager here, he usually used
LBL as a path to connect to the ARPANET/MILNET. In addition, we watched him
used several other networks, including the Magnetic Fusion Energy network,
the High Energy Physics network, and several LANs at invaded sites.

While connected to MILNET, this intruder attempted to enter about 450
computers, trying to log in using common account names like root, guest,
system, or field. He also tried default and common passwords, and often
found valid account names by querying each system for currently logged-in
accounts, using who or finger. Although this type of attack is the most
primitive, it was dismayingly successful: In about 5 percent of the machines
attempted, default account names and passwords permitted access, sometimes
giving system-manager privileges as well.

When he succeeded in logging into a system, he used standard methods to
leverage his privileges to become system manager. Taking advantage of
well-publicized problems in several operating systems, he was often able to
obtain root or system-manager privileges. In any case, he searched file
structures for keywords like "nuclear," "sdi," "kh-11," and "norad."  After
exhaustively searching for such information, he scanned for plain-text
passwords into other systems. This proved remarkably effect ive: Users often
leave passwords in files [2]. Electronic mail describing log-in sequences
with account names and passwords is commonly saved at foreign nodes, allowing
a file browser to obtain access into a distant system. In this manner he was
able to obtain both passwords and access mechanisms into a Cray

Typical of the security holes he exploited was a bug in the Gnu-Emacs
program. This popular, versatile text editor includes its own mail system,
allowing a user to forward a file to another user [40]. As distributed, the
program uses the UNIX Set-User-ID-to-Root feature; that is, a section of the
program runs with system-manager privileges. This movemail facility allows
the user to change file ownership and move files into another's directory.
Unfortunately, the program did not prevent someone from moving a file into
the systems area. Aware of this hole, the intruder created a shell script
that, when executed at root level, would grant him system privileges. He
used the movemail facility to rename his script to masquerade as a utility
periodically run by the system. When the script was executed by the system,
he gained system-manager privileges.

This intruder was impressively presistent and patient. For example, on one
obscure gateway computer, he created an account with system privileges that
remained untouched until six months later, when he began using it to enter
other networked computers. On another occasion, he created several programs
that gave him system-manager privileges and hid them in system software
libraries. Returning almost a year later, he used the programs to become
system manager, even though the original operating-system hole had been
patched in the meantime.

This intruder cracked encrypted passwords. The UNIX operating system stores
passwords in publicly readable, but encrypted form [26]. We observed him
downloading encrypted password files from compromised systems into his own
computer. Within a week he reconnected to the same computer, logging into
new accounts with correct passwords. The passwords he guessed were English
words, common names, or place-names. We realized that he was decrypting
password filed on his local computer by successively encrypting dictionary
words and comparing the results to password file entries. By noting the
length of time and the decrypted passwords, we could estimate the size of his
dictionary and his computer's speed.

The intruder understood what he was doing and thought that he was not
damaging anything. This, alas, was not entirely true. Prior to being
detected, he entered a computer used in the real-time control of a medical
experiment. Had we not caught him in time, a patient might have been
severely injured.

Throughout this time the intruder tried not to destroy or change user data,
although he did destroy several tasks and unknowingly caused the loss of data
to a physics experiment. Whenever possible, he disabled accounting and audit
trails, so there would be no trace of his presence. He planted Trojan horses
to passively capture passwords and occasionally created new accounts to
guarantee his access into computers. Apparently he thought detection less
likely if he did not create new accounts, for he seemed to prefer stealing
existing, unused accounts.


Was the intruder actually spying? With thousands of military computer
attached, MILNET might seem inviting to spies. After all, espionage over
networks can be cost-efficient, offer nearly immediate results, and target
specific locations. Further, it would seem to be insulated from risks of
internationally embarrassing incidents. Certainly Western countries are at
much greater risk than nations without well-developed computer

Some may argue that it is ludicrous to hunt for classified information over
MILNET because there is none. Regulations [21] prohibit classified computers
from access via MILNET, and any data stored in MILNET systems must be
unclassified. On the other hand, since these computers are not regularly
checked, it is possible that some classified information resides on them. At
least some data stored in these computers can be considered sensitive,
especially when aggregated. Printouts of this intruder's activities seem to
confirm this. Despite his efforts, he uncovered little information not
already in the public domain, but that included abstracts of U.S. Army plans
for nuclear, biological, and chemical warfare for central Europe. These
abstracts were not classified, nor was their database.

The intruder was extraordinarily careful to watch for anyone watching him.
He always checked who was logged onto a system, and if a system manager was
on, he quickly disconnected. He regularly scanned electronic mail for any
hints that he had been discovered, looking for mention of his activities or
stolen log-in names (often, by scanning for those words). He often changed
his connection pathways and used a variety of different network user
identifiers. Although arrogant from his successes, he was nevertheless
careful to cover his tracks.

Judging by the intruder's habits and knowledge, he is an experienced
programmer who understands system administration. But he is by no means a
"brilliant wizard," as might be popularly imagined. We did not see him plant
viruses [18] or modify kernel code, nor did he find all existing security
weaknesses in our systeM. He tried, however, to exploit problems in the
UNIX/usr/spool/at [36], as well as a hole in the vi editor. These problems
had been patched at our site long before, but they still exist in many other

Did the intruder cause damage? To his credit, he tried not to erase files
and killed only a few processes. If we only count measurable losses and time
as damage, he was fairly benign [41]. He only wasted systems staff time,
computing resources, and network connection time, and racked up long-distance
telephone tolls and international network charges. His liability under
California law [6], for the costs of the computing and network time, and of
tracking him, is over $100,000.

But this is a narrow view of the damage. If we include intangible losses,
the harm he caused was serious and deliberate. At the least, he was
trespassing, invading others' property and privacy; at worst, he was
conducting espionage. He broke into dozens of computers, extracted
confidential information, read personal mail, and modified system software.
He risked injuring a medical patient and violated the trust of our network
community. Money and time can be paid back. Once trust is broken, the open,
cooperative character of our networks may be lost forever.


Following successful traces, the FPI assured us teh intruder would not try to
enter our system again. We began picking up the pieces and tightening our
sytem. The only way to guarantee a clean system was to rebuild all systems
from source code, change all passwords overnight, and recertify each user.
With over a thousand users and dozens of computers, this was impractical,
especially since we strive to supply our users with uninterrupted computing
services. On the other hand, simply patching known holes or instituting a
quick fix for stolen passwords [27] was not enough.

We settled on instituting password expiration, deleting all expired accounts,
eliminating shared accounts, continued monitoring of incoming traffic,
setting alarms in certain places, and educating our users. Where necessary,
system utilities were compared to fresh versions, and new utilities built.
We changed network-access passwords and educated users about choosing
nondictionary passwords. We did not institute random password assignment,
having seen that users often store such passwords in command files or write
them on their terminals.

To further test the security of our system, we hired a summer student to
probe it [2]. He discovered several elusive, site-specific security holes,
as well as demonstrated more general problems, such as file scavenging. We
would like to imagine that intruder problems have ended for us; sadly, they
have not, forcing us to continue our watch.


Should we have remained open? A reasonable response to the detection of this
attack might have been to disable the security hole and change all passwords.
This would presumably have insulated us from the intruder and prevented him
from using our computers to attack other internet sites. By remaining open,
were we not a party to his attacks elsewhere, possibly incurring legal
responsibility for damage?

Had we closed up shop, we would not have risked embarrassment and could have
resumed our usual activities. Closing up and keeping silent might have
reduced adverse publicity, but would have done nothing to counter the serious
problem of suspicious (and possibly malicious) offenders. Although many view
the trace back and prosecution of intruders as a community service to network
neighbors, this view is not universal [22].

Finally, had we closed up, how could we have been certain that we had
eliminated the intruder? With hundreds of networked computers at LBL, it is
nearly impossible to change all passwords on all computers. Perhaps he had
planted subtle bugs or logic bombs in places we did not know about.
Eliminating him from LBL would hardly have cut his access to MILNET. And, by
disabling his access into our system, we would close our eyes to his
activities; we could neither monitor him nor trace his connections in
real-time. Tracing, catching, and prosecuting intruders are, unfortunately,
necessary to discourage these vandals.


Several laws explicitly prohibit unauthorized entry into computers. Few
states lack specific codes, but occasionally the crimes are to broadly
defined to permit conviction [38]. Federal and California laws have tight
criminal statutes covering such entries, even if no damage is done [47]. In
addition, civil law permits recovery not only of damages, but also of the
costs to trace the culprit [6]. In practice, we found police agencies
relatively uninterested until monetary loss could be quantified and damages
demonstrated. Although not a substitute for competent legal advice, spending
several days in law libraries researching both the statutes and precedents
set in case law proved helpful.

Since this case was international in scope, it was necessary to work closely
with law-enforcement organizations in California, the FBI in the United
States, and the BKA in Germany. Cooperation between system managers,
communications technicians, and network operators was excellent. It proved
more difficult to get bureaucratic organizations to communicate with one
another as effectively. With many organizational boundaries crossed,
including state, national, commercial, university, and military, there was
confusion as to responsibility: Most organizations recognized the seriousness
of these break-ins, yet no one agency had clear responsibility to solve it.
A common response was, "That's an interesting problem, but it's not our

Overcomimng this bureaucratic indifference was a continual problem. Our
laboratory notebook proved useful in motivating organizations: When
individuals saw the extent of the break-ins, they were able to explain them
to their colleagues and take action. In addition, new criminal laws were
enacted that more tightly defined what constituted a prosecutable offense [6,
38, 47]. As these new laws took effect, the FBI became much more interested
in this case, finding statutory grounds for prosecution.

The FBI and BKA maintained active investigations. Some subjects have been
apprehended, but as yet the author does not know the extent to which they
have been prosecuted. With recent laws and more skilled personnel, we can
expect faster and more effective responses from law-enforcement agencies.


In retrospect, we can point to many errors we made before and during these
intrusions. Like other academic organizations, we had given little thought
to securing our system, believing that standard vendor provisions were
sufficient because nobody would be interested in us. Our scientists'
research is entirely in the public domain, and many felt that security
measures would only hinder their productivity. With increased connectivity,
we had not examined our networks for crosslinks where an intruder might hide.
These problems were exacerbated on our UNIX systems, which are used almost
exclusively for mail and text processing, rather than for heavy computation.

Password security under Berkeley UNIX is not optimal; it lacks password
aging, expiration, and exclusion of passwords found in dictionaries.
Moreover, UNIX password integrity depends solely on encryption; the password
file is publicly readable. Other operating systems protect the password file
with encryption, access controls, and alarms.

We had not paid much attention to choosing good passwords (fully 20 percent
of our users' passwords fell to a dictionary-based password cracker).
Indeed, we had allowed our Tymnet password to become public, foolishly
believing that the system log-in password should be our only line of defense.

Once we detected the intruder, the first few days were confused, since nobody
knew what our response ought to be. Our accounting files were misleading
since the system clocks had been allowed to drift several minutes. Although
our LAN's connections had been saved, nobody knew the file format, and it was
frustrating to find that its clock had drifted by several hours. In short,
we were unprepared to trace our LAN and had to learn quickly.

We did not know who to contact in the law-enforcement community. At first,
assuming that the intruder was local, our district attorney obtained the
necessary warrants. Later, as we learned that the intruder was out of state,
we experienced frustration in getting federal law-enforcement support.
Finally, after tracing the intruder abroad, we encountered a whole new set of
ill-defined interfaces between organizations. The investigation stretched
out far beyond our expectations. Naively expecting the problem to be solved
by a series of phone traces, we were disappointed when the pathway proved to
be a tangle of digital and analog connections. Without funding to carry out
an investigation of this length, we were constantly tempted to drop it

A number of minor problems bubbled up, which we were able to handle along the
way. For a while this intruder's activity appeared similar to that of
someone breaking into Stanford University; this confused our investigation
for a short time. Keeping our work out of the news was difficult, especially
because our staff is active in the computing world. Fortunately, it was
possible to recover from the few leaks that occurred. At first, we were
confused by not realizing the depth or extent of the penetrations. Our
initial confusion gave way to an organized response as we made the proper
contacts and began tracing the intruder. As pointed out by others [25, 36],
advance preparations make all the difference.


As a case study, this investigation demonstrates several well-known points
that lead to some knotty questions. Throughout this we are reminded that
security is a human problem that cannot be solved by technical solutions
alone [48].

The almost obsessive persistence of serious penetrators is astonishing. Once
networked, our computers can be accessed via a tangle of connections from
places we had never thought of. An intruder, limited only by patience, can
attack from a variety of directions, searching for the weakest entry point.
How can we analyze our systems' vulnerability in this environment? Who is
responsible for network security? The network builder? The managers of the
end nodes? The network users?

The security weaknesses of both systems and networks, particularly the
needless vulnerability due to sloppy systems management and administration,
result in a surprising success rate for unsophisticated attacks. How are we
to educate our users, system managers, and administrators?

Social, ethical, and legal problems abound. How do we measure the harm done
by these penetrators? By files deleted or by time wasted? By information
copied? If no files are corrupted, but information is copied, what damage
has been done? What constitutes unreasonable behavior on a network?
Attempting to illicitly log in to a foreign computer? Inquiring who is
currently logged in there? Exporting a file mistakenly made world readable?
Exploiting an unpatched hole in another's system?

Closing out an intruder upon discovery may be a premature reflex.
Determining the extent of the damage and cooperating with investigations
argue for leaving the system open. How do we balance the possible benefits
of tracking an intruder against the risks of damage or embarrassment?

Our technique of catching an intruder by providing bait and then watching
what got nibbled is little more than catching flies with honey. It can be
easily extended to determine intruders' interests by presenting them with a
variety of possible subjects (games, financial data, academic gossip,
military news). Setting up alarmed files is straightforward, so this
mechanism offers a method to both detect and classify intruders. It should
not be used indiscriminately, however.

Files with plaintext passwords are common in remote job entry computers, yet
these systems often are not protected since they have little computational
capability. Such systems are usually widely networked, allowing entry from
many sources. These computers are fertile grounds for password theft through
file scavenging since the passwords are left in easily read command
procedures. These files also contain instructions to make the network
connection. Random character passwords make this problem worse, since users
not wishing to memorize them are more likely to write such passwords into
files. How can we make secure remote procedure calls and remote batch job

Passwords are at the heart of computer security. Requirements for a quality
password are few: Passwords must be nonguessable, not in a dictionary,
changed every few months, and easily remembered. User-generated passwords
usually fail to meet the first three criteria, and machine-generated
passwords fail the last. Several compromises exist: forcing "pass phrases"
or any password that contains a special character. There are many other
possibilities, but none are implemented widely. The Department of Defense
recommends pronounceable machine-generated words or pass phrases [5].
Despite such obvious rules, we (and the intruder) found that poor-quality
passwords pervaded our networked communities. How can we make users choose
good passwords? Should we?

Vendors usually distribute weakly protected systems software, relying on the
installer to enable protections and disable default accounts. Installers
often do not care, and system managers inherit these weak systems. Today,
the majority of computer users are naive; they install systems the way the
manufacturer suggests or simply unpackage systems without checking. Vendors
distribute systems with default accounts and backdoor entryways left over
from software development. Since many customers buy computers based on
capability rather than security, vendors seldom distribute secure software.
It is easy to write procedures that warn of obvious insecurities, yet vendors
are not supplying them. Capable, aware system managers with plenty of time
do not need these tools--the tools are for novices who are likely to overlook
obvious holes. When vendors do not see security as a selling point, how can
we encourage them to distribute more secure systems?

Patches to operating-system security holes are poorly publicized and spottily
distributed. This seems to be due to the paranoia surrounding these
discoveries, the thousands of systems without systems administrators, and the
lack of channels to spread the news. Also, many security problems are
specific to a single version of an operating system or require systems
experience to understand. Together, these promote ignorance of problems,
threats, and solutions. We need a central clearinghouse to receive reports
of problems, analyze their importance, and disseminate trustworthy solutions.
How can we inform people wearing white hats about security problems, while
preventing evil people from learning or exploiting these holes? Perhaps
zero-knowledge proofs [20] can play a part in this.

Operating systems can record unsuccessful log ins. Of the hundreds of
attempted log ins into computers attached to internet, only five sites (or
1-2 percent) contacted us when they detected an attempted break-in. Clearly,
system managers are not watching for intruders, who might appear as
neighbors, trying to sneak into their computers. Our networks are like
communities or neighborhoods, and so we are surprised when we find
unneighborly behavior.

Does security interfere with operational demands? Some security measures,
like random passwords or strict isolation, are indeed onerous and can be
self-defeating. But many measures neither interfere with legitimate users
nor reduce the system's capabilities. For example, expiring unused accounts
hurts no one and is likely to free up disk space. Well thought out
management techniques and effective security measures do not bother ordinary
users, yet they shut out or detect intruders.


The intruder's successes and failures provide a reasonable snapshot of
overall security in the more than 20,000 computers connected to Internet. A
more detailed analysis of these attacks is to be published in the Proceedings
of the 11th National Computer Security Conference [43]. Of the 450 attacked
computers, half were unavailable when the intruder tried to connect to them.
He tried to log into the 220 available computers with obvious account names
and trivial passwords. Of these 220 attempted log ins, listed in increasing

* 5 percent were refused by a distant computer (set to reject LBL connects),

* 82 percent failed on incorrect user name/passwords,

* 8 percent gave information about the system status (who, sysstat, etc.),

* 1 percent achieved limited access to databases or electronic-mail shells,

* 2 percent yielded normal user privileges and a programming environment, and

* 2 percent reached system-manager privileges.

Most attempts were into MILNET computers (Defense Data Network address groups
26.i.j.k). Assuming the population is representative of nonmilitary
computers and the last three categories represent successful penetrations, we
find that about 5 percent of Internet computers are grossly insecure against
trivial attacks. This figure is only a lower limit of vulnerability, since
military computers may be expected to be more secure than civilian systems.
Further, cleverer tactics for entering computers could well lead to many more

Whereas the commercial sector is more concerned with data integrity, the
military worries about control of disclosure [8]. With this in mind, we
expect greater success for the browser or data thief in the commercial world.

In a different set of penetrations [37], NASA experienced about 130 break-ins
into its nonclassified, academic computers on the SPAN networks. Both the
NASA break-in and our set of intrusions originated in West Germany, using
similar communications links and searching for "secret" information. Pending
completion of law enforcement and prosecution, the author does not make
conjectures as to the relationships between these different break-ins.

Between 700 and 3000 computers are reachable on the SPAN network (exact
figures depend on whether LANs are counted). In that incident the break-in
success rate was between 4 and 20 percent. Considering the SPAN break-ins
with the present study, we find that, depending on the methods chosen,
break-in success rates of 3-20 percent may be expected in typical network


Perhaps no computer or network can be totally secure. This study suggests
that any operating system will be insecure when obvious security rules are
ignored. From the intruder's widespread success, it appears that users,
managers, and vendors routinely fail to use sound security practices. These
problems are not limited to our site or the few dozen systems that we saw
penetrated, but are networkwide. Lax system management makes patching
utility software or tightening a few systems ineffective.

We found this intruder to be a competent, patient programmer, experienced in
several operating systems. Alas, some system managers violate their
positions of trust and confidence. Our worldwide community of digital
networks requires a sense of responsibility. Unfortunately, this is missing
in some technically competent people.

Some speak of a "hacker ethic" of not changing data [37]. It is astounding
that intruders blithely tamper with someone else's operating system, never
thinking they may destroy months of work by systems people, or may cause
unforeseen system instabilities or crashes. Sadly, few realize the delicacy
of the systems they fool with or the amount of systems staff time they waste.

The foreign origin of the source, the military computers entered, and the
keywords searched suggest international espionage. This author does not
speculate as to whether this actually was espionage, but does not doubt that
someone took the opportunity to try.

Break-ins from abroad seem to be increasing. Probably this individual's
intrusions are different from others only in that his efforts were noticed,
monitored, and documented. LBL has detected other attempted intrusions from
several European countries, as well as from the Orient. Individuals in
Germany [37] have claimed responsibility for breaking into foreign computers.
Such braggadocio may impress an unenlightened public; it has a different
effect on administrators trying to maintain and expand networks. Indeed,
funding agencies have already eliminated some international links due to
these concerns. Break-ins ultimately destroy the network connectivity they
exploit. If this is the object of such groups as the German Chaos Club, Data
Travellers, Network Rangers, or various contributors to 2600 Magazine, it
reflects the self-destructive folly of their apparent cleverness.

Tracking down espionage attempts over the digital networks may be the most
dramatic aspect of this work. But it is more useful to realize that analytic
research methods can be fruitfully applied to problems as bizarre as computer

It seems that everyone wants to hear stories about someone else's troubles,
but few are willing to write about their own. We hope that in publishing
this report we will encourage sound administrative practices. Vandals and
other criminals reading this article will find a way to rationalize breaking
into computers. This article cannot teach these people ethics; we can only
hope to reach those who are unaware of these miscreants.

An enterprising programmer can enter many computers, just as a capable
burglar can break into many homes. It is an understandable response to lock
the door, sever connections, and put up elaborate barriers. Perhaps this is
necessary, but it saddens the author, who would rather see future networks
and computer communities built on honesty and trust.

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