An astronomer-turned-sleuth traces a German trespasser on our military networks, who slipped through operating system security holes and browsed through sensitive databases. Was it espionage?
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 activities 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, [R] VMS, [R] VM-TSO, [R] EMBOS, [R] 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  or were little more than rumors. For lack of substantive documentation, system designers and managers have not addressed important problems in securing computers. Some efforts to tighten 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. (1)
We first suspected 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  to obtain system-manager privileges. At first we suspected that the culprit was a student prankster at the nearby 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.
ORGANIZING OUR EFFORTS
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 records 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. Knowing 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.
MONITORS, ALARMS, AND TRAFFIC ANALYSIS
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 was complete in that virtually all of these sessions 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 system consumes computing resources and may slow down other processes. In addition, such a 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 …