I first started seeing this problem back in the late 1980s. Hackers would break into a system, set up a few accounts of their own, and then start installing Trojan horses and other kinds of back doors so that they could always return. Trying to contain the damage was like swatting at flies. The system management might remove the bad guy accounts, but they would come back through the back doors and recreate them. Management might reinstall the operating system, but the bad guys would still come back\u2014perhaps through some new service that had been left behind. And since the bad guys invariably had root-level access, there was always a chance that they would mount a revenge attack and wipe out the system.There was really only one way to prevent systems like this from spiraling out of control: save whatever you could, then log out and "nuke from orbit. In other words, a standard operating procedure was to copy critical files and data off the system, then reformat its hard drive and do a clean installation.The "nuke from orbit" approach works, but like real-life atomic weapons, most incidents don't need such drastic measures. It's incredibly wasteful to spend a week or more reinstalling a computer's operating system and applications because of a few modified files. During that time, you are either running a compromised machine or your server is down. What's even worse, nuking occasionally doesn't work. Yes, a complete reinstall would get rid of the compromised log-in program and mail server that the bad guys left behind, but reinstalling the operating system doesn't do much good if the original penetration was due to a stolen password. Into this mess came what should properly be considered one of the computing industry's first intrusion detection systems. Called Tripwire, the program took a snapshot of the computer's operating system, recording the modification date and a cryptographic checksum for every program on a given computer system. This database would then be stored not on the computer itself, but on removable media\u2014usually a floppy disk. If you discovered that your machine was compromised, you could run Tripwire a second time and compare the two databases to find which files the bad guys had modified. Even better, you could run Tripwire on a regular basis to find out if your machine was compromised.A graduate student named Gene Kim and his adviser, Gene Spafford, developed Tripwire at Purdue University. (Professor Spafford and I are coauthors on several computer security books.) Realizing he had a winner, Kim took the research and cofounded a company, also called Tripwire, based in Portland, Ore.The Tripwire of today bears little resemblance to the program developed at Purdue. For starters, the program runs on both Windows and Unix. It has a management console that allows many systems under Tripwire to be centrally administrated. It has a policy language that allows you to specify which files can be changed and which need to remain the same. There is even a reporting engine and an automatic alert notification system. In many ways, these changes mirror the changes that the whole computing industry has undergone during the same time period.But the basic premise of Tripwire is still strong: It's impossible to run a reliable computing system when you don't know which software (and other configuration) files are being changed.Curbing Unauthorized AccessOn the other hand, as Kim told me in a recent interview, he and Spafford didn't understand the extent of what's now called the "change management" problem when they invented Tripwire back in the 1980s. Although the primary motivation for creating Tripwire was to detect intrusions, the real value in the business world, Kim says, has been in detecting unauthorized changes from authorized personnel. Although at times these might be from malicious employees bent on harming the company, more often they are well-intentioned or accidental changes from employees who were simply trying to do their jobs.I have to agree with Kim. I've seen mission-critical servers shut down because a file was accidentally created in a critical directory or because an undocumented configuration file got overwritten by what was supposed to be a "minor" upgrade. And the complexity of change management is only complicated by the inability of most operating systems to track precisely what happens when a patch or upgrade is applied.These are some of the reasons that Tripwire's new marketing focus isn't security against hackers as much as the general problem of operational continuity and change management. Tripwire is increasingly being positioned as a tool both to detect unauthorized changes to configuration files and programs, as well as to verify that the proposed or required changes actually get made.And for these reasons, most Tripwire users have stopped storing their databases on removable floppy disks or CD-ROMs; although, these days you can get nearly the same security by storing the database over a LAN on another computer. The security isn't quite as good, because there is always a chance that the bad guys will break into the computer that stores your Tripwire database. Nevertheless, many system administrators are happy to make this compromise in the interest of convenience. Randy Barr has been running Tripwire on roughly 1,000 mission-critical servers at WebEx Communications for two years. Barr tries to run a tight ship. Every change to the servers has to be proposed, documented and finally approved by a change-control committee. But occasionally things slip through the cracks."There was a user who went in, outside of the change-control process, and made a change," Barr says. "He created a file in a directory to keep notes for himself."You might not think a file with a few notes in it would be a problem\u2014and in most cases it's not. But because the wrong file created in the wrong directory can potentially shut down a server, WebEx's policy is that unauthorized changes such as these should not happen.Further investigation revealed that the employee wasn't supposed to be logged in to the system at all. "The policy states that this is a disciplinable action, so that's how we handled it," says Barr.Tripwire isn't the only way that WebEx could have caught the employee infraction. A unified logging system with alert management would have caught the unauthorized log-in to the server in question. The creation of the file itself could have been caught with the use of C2-level audit logs. (The phrase "C2" comes from the U.S. government's "Orange Book" that defines requirements for different kinds of secure operating systems.) But each of these approaches has problems. A unified log system tends to generate a lot of false positives, since many log-ins are inconsequential. C2 logging, on the other hand, generates a tremendous amount of information and can put a significant drain on system resources.Barak Engel, CSO at InStorecard, says his startup uses Tripwire to comply with Visa's Cardholder Information Security Program (CISP). The program, launched by Visa in June 2001, mandates security standards for merchants and service providers."We needed to have a data integrity solution as part of our Visa CISP," Engel says. He has Tripwire configured to watch, "critical operating system portions and our product directories\u2014things that could actually affect or be affected by any kind of malicious adversary" of the company's Microsoft-based servers. Tripwire automatically e-mails regular reports and notifications whether or not it has detected unauthorized changes.Unfortunately, there are certain attacks that can be very hard for Tripwire to detect. If an attacker breaks into a computer and is able to modify the operating system's kernel, the system can be programmed to fool Tripwire into thinking that files haven't been modified when in fact they have. This kind of attack, commonly built into programs, is called "root kits."In one version of this attack, the attacker makes a copy of each program that he or she wants to modify. The operating system is then modified so that when the computer tries to read the contents of a program, it reads from the original. But when it tries to execute a program, it executes the version that's been modified. In another variant of this attack, the bad guy modifies the Tripwire program itself so that the program stops looking at the files that have been compromised.Both of these attacks can be easily subverted by booting the affected computer from a CD-ROM. If you run with an operating system that's known to be good and a fresh copy of Tripwire, you can rapidly determine where the modifications have been made. Of course, another alternative is to simply evacuate the data that you care about, blast off and nuke from orbit.