Online Documentation Server
Net technology
Web technology
Data bases
Other docs



Вся предоставленная на этом сервере информация собрана нами из разных источников. Если Вам кажется, что публикация каких-то документов нарушает чьи-либо авторские права, сообщите нам об этом.

Maximum Security:

A Hacker's Guide to Protecting Your Internet Site and Network

Previous chapter Next chapter Contents


Levels of Attack

This chapter examines various levels of attack. An attack is any unauthorized action undertaken with the intent of hindering, damaging, incapacitating, or breaching the security of your server. Such an attack might range from a denial of service to complete compromise and destruction of your server. The level of attack that is successful against your network depends on the security you employ.

When Can an Attack Occur?

An attack can occur any time your network is connected to the Internet. Because most networks are connected 24 hours a day, that means attacks can occur at any time. Nonetheless, there are some conventions that you can expect attackers to follow.

The majority of attacks occur (or at least commence) late at night relative to the position of the server. That is, if you are in Los Angeles and your attacker is in London, the attack will probably occur during the late night-early morning hours Los Angeles time. You might think that crackers would work during the day (relative to the target) because the heavy traffic might obscure their activity. There are several reasons, however, why crackers avoid such times:

  • Practicality--The majority of crackers hold jobs, go to school, or spend time in other environments during the day that may preclude cracking. That is, these characters do more than spend time in front of a machine all day. This differs from the past, when most crackers were kids at home, with nothing to do.

  • Speed--The network is becoming more and more congested. Therefore, it is often better to work during times that offer fast packet transport. These windows depend largely on geographical location. Someone in the southwestern United States who is attacking a machine in London would best conduct their affairs between 10:00 p.m. and 12:00 a.m. local time. Playing the field slightly earlier will catch local traffic (people checking their mail before bed, users viewing late news, and so on). Working much later will catch Netizens of the UK waking up to check their e-mail. Going out through Mae East (the largest and busiest Internet exchange gateway) in the early morning hours may be fast, but once across the Atlantic, speed dies off quickly. Anyone who stays up all night surfing the Net will confirm this. Once you hit the morning e-mail check, the Net grinds to a halt. Try it sometime, even locally. At 4:00 a.m. things are great. By 7:00 a.m., you will be praying for a T3 (or SONET).

  • Stealth--Suppose for a moment that a cracker finds a hole. Suppose further that it is 11:00 a.m. and three system administrators are logged on to the network. Just what type of cracking do you suppose can be done? Very little. Sysads are quick to track down bizarre behavior if they are there to witness it. I once had a system administrator track me down immediately after I grabbed her password file. She was in Canada and I was in Los Angeles. She issued me a talk instruct before I could even cut the line. We had a lovely, albeit short, conversation. This also happened once when I broke into a server in Czechoslovakia. The lady there had a Sun and an SGI. I cracked the SGI. The conversation there was so good, I stayed connected. We discussed her security and she actually gave me an account on an old SPARC at her university. The account probably still exists.

Favorite targets of crackers are machines with no one on them. For a time, I used a workstation in Japan to launch my attacks because no one ever seemed to be logged in. I Telnetted out of that machine, back into the United States. I found a similar situation with a new ISP in Rome. (I can say no more, because they will definitely remember me and my identity will be blown. They actually told me that if I ever came to hack in Italy, I should look them up!)

With such machines, you can temporarily take over, setting things to your particular tastes. Moreover, you have plenty of time to alter the logs. So be advised: Most of this activity happens at night relative to your geographical location.

TIP: If you have been doing heavy logging and you have only limited time and resources to conduct analysis of those logs, I would concentrate more on the late night connection requests. These portions of your logs will undoubtedly produce interesting and bizarre information.

What Operating Systems Do Crackers Use?

Operating systems used by crackers vary. Macintosh is the least likely platform for a cracker; there simply aren't enough tools available for MacOS, and the tools needed are too much trouble to port. UNIX is the most likely platform and of that class, probably FreeBSD or Linux.

The most obvious reason for this is cost. For the price of a $39 book on Linux (with the accompanying CD-ROM), a cracker gets everything he could ever need in the way of tools: C, C++, Smalltalk, Perl, TCP/IP, and much more. Moreover, he gets the full source code to his operating system.

This cost issue is not trivial. Even older workstations can be expensive. Your money will buy more computing power if you stay with an IBM compatible. Today, you can get a 100MHz PC with 8MB of RAM for $300. You can put either FreeBSD or Linux on that machine and suddenly, you have a powerful workstation. Conversely, that same $300 might buy you a 25MHz SPARCstation 1 with a disk, monitor, and keyboard kit. Or perhaps an ELC with an external disk and 16MB of RAM. Compounding this is the problem of software. If you get an old Sun, chances are that you will also be receiving SunOS 4.1.x. If so, a C compiler (cc) comes stock. However, if you buy an RS/6000 with AIX 4.1.x, you get a better deal on the machine but you are forced to get a C compiler. This will probably entail getting GCC from the Internet. As you might guess, a C compiler is imperative. Without it, you cannot build the majority of tools distributed from the void. This is a big consideration and one reason that Linux is becoming much more popular.

NOTE: Compatibility is not really an issue. The majority of good tools are written under the UNIX environment and these can be easily ported to the free UNIX platforms. In fact, in many cases, binaries for Linux and FreeBSD already exist (although I readily admit that this is more prevalent for FreeBSD, as early implementations of Linux had a somewhat eclectic source tree that probably more closely resembled AIX than other traditional flavors, like SunOS). This is somewhat of a cult issue as well. Purists generally prefer BSD.

I should mention that professional crackers (those who get paid for their work) can probably afford any system. You can bet that those forces in American intelligence investigating cyberwar are using some extreme computing power. For these individuals, licensing and cost are not issues.


It is fairly common to see crackers using either SolarisX86 or SCO as a platform. This is because even though these products are licenseware, they can easily be obtained. Typically, crackers using these platforms know students or are students. They can therefore take advantage of the enormous discounts offered to educational institutions and students in general. There is a radical difference between the price paid by a student and the price paid by the average man on the street. The identical product's price could differ by hundreds of dollars. Again, because these operating systems run on PC architecture, they are still more economical alternatives. (SolarisX86 2.4 became enormously popular after support was added for standard IDE drives and CD-ROM devices. Prior to the 2.4 driver update, the system supported only SCSI drives: a slightly more expensive proposition.) And of course, one can always order demo disks from Sun and simply keep the distribution, even though you are in violation of the license.


UNIX platforms are popular because they generally require a low overhead. A machine with Windows 95 and all the trimmings requires a lot of RAM; in contrast, you can run Linux or FreeBSD on a paltry 386 and gain good performance (provided, of course, that you do not use X). This is reasonable, too, because even tools that have been written for use in the X environment usually have a command-line interface as well (for example, you can run SATAN in CLI).


The Microsoft platform supports many legitimate security tools that can be used to attack remote hosts. Of that class, more and more crackers are using Windows NT. It outperforms 95 by a wide margin and has advanced tools for networking as well. Also, Windows NT is a more serious platform in terms of security. It has access control as well, so crackers can safely offer remote services to their buddies. If those "friends" log in and attempt to trash the system, they will be faced with the same controls as they would on a non-cracker-friendly box.

Moreover, NT is becoming more popular because crackers know they must learn this platform. As NT becomes a more popular platform for Internet servers (and it will, with the recent commitments between DEC and Microsoft), crackers will need to know how to crack these machines. Moreover, security professionals will also develop tools to test internal NT security. Thus, you will see a dramatic rise in the use of NT as a cracking platform.

NOTE: Windows 95 tools are also rapidly emerging, which will greatly alter the state of cracking on the Net. Such tools are typically point and click, requiring little skill on the part of the operator. As these tools become more common, you can expect even more security violations on the Net. Nonetheless, I don't think 95 will ever be a major platform for serious crackers.

Origin of Attacks

Years ago, many attacks originated from universities because that is where the Internet access came from. Most crackers were youngsters who had no other easy means of accessing the Internet. This naturally influenced not only the origin of the attack but also the time during which the attack happened. Also, real TCP/IP was not available as an option in the old days (at least not from the comfort of your home, save a shell account).

Today the situation is entirely different. Crackers can crack your network from their home, office, or vehicle. However, there are some constants. For instance, serious crackers do not generally use providers such as America Online, Prodigy, or Microsoft Network. (The obvious exceptions are those crackers who utilize stolen credit-card numbers. In those cases, AOL is an excellent choice.) One reason for this is that these providers will roll over a hacker or cracker to the authorities at the drop of a hat. The suspect may not have even done anything wrong (smaller ISPs may simply cut them loose). Ironically, big providers allow spammers to pummel the Internet with largely unwanted advertising. Go figure. Curiosity is frowned upon, but stone-cold commercialism is A-OK.

Furthermore, these providers do not offer a UNIX shell environment in addition to garden-variety PPP. A shell account can facilitate many actions that are otherwise more difficult to undertake. System tools available that can provide increased functionality include the various shells, Perl, AWK, SED, C, C++, and a handful of system commands (showmount is one; rusers is another).

So the picture of a typical cracker is developing: This is a person who works late at night, who is armed with a UNIX or an NT box and advanced tools, and, with all likelihood, is using a local provider.

What Is the Typical Cracker Like?

The typical cracker can probably be described by at least three qualities in the following profile:

  • Can code in C, C++, or Perl--These are general requirements, because many of the baseline security tools are written in one or more of these languages. At minimum, the cracker must be able to properly interpret, compile, and execute the code. More-advanced crackers can take code not expressly written for a particular platform and port it to their own. Equally, they may develop new modules of code for extensible products such as SATAN and SAFEsuite (these programs allow the integration of new tools written by the user).

  • Has an in-depth knowledge of TCP/IP--No competent cracker can get along without this requirement. At minimum, a cracker must know how the Internet works. This knowledge must necessarily go deeper than just what it takes to connect and network. The modern, competent cracker must know the raw codes within TCP/IP, such as the composition of IP packet headers. This knowledge, however, need not be acquired at school and therefore, a B.S. in Computer Science is not required. Many individuals get this knowledge by networking equipment within their home or at their place of business.

  • Uses the Internet more than 50 hours per month--Crackers are not casual users. To watch a cracker at work is to watch someone who truly knows not only his or her own machine, but the Net. There is no substitute for experience, and crackers must have it. Some crackers are actually habitual users and suffer from insomnia. No joke.

  • Intimately knows at least two operating systems--One of these will undoubtedly be UNIX or VMS.

  • Has (or had) a job using computers--Not every cracker wakes up one morning and decides to devote a major portion of his or her life to cracking. Some have had jobs in system administration or development. These individuals tend to be older and more experienced. In such cases, you are probably dealing with a professional cracker (who probably has had some experience developing client/server applications).

  • Collects old, vintage, or outdated computer hardware or software--This may sound silly, but it isn't. Many older applications and utilities can perform tasks that their modern counterparts cannot. For example, I recently had a hard drive that reported bad sectors. I reformatted it a dozen times and tried various disk utilities to repair it; still, I had problems. After several tries with modern utilities, I turned to a very obscure program called, coded many years ago. It repaired the problem in no time, reformatting the disk clean. Other examples include old utilities that can format disks to different sizes, break up large files for archiving on disks, create odd file systems, and so forth. As a cracker's experience grows, he or she collects such old utilities.

What Is the Typical Target Like?

The typical target is hard to pin down because crackers attack different types of networks for different reasons. Nonetheless, one popular target is the small, private network. Crackers are well aware of organizational behavior and financial realities. Because firewalls are expensive to acquire and maintain, smaller networks are likely to go without or obtain inferior products. Also, few small companies have individuals assigned specifically to anti-cracking detail (think about the Finnish report I mentioned in Chapter 4, "Just Who Can Be Hacked, Anyway?"). Finally, smaller networks are more easily compromised because they fit this profile:

  • The owners are new to the Internet

  • The sysad is experienced with LANs rather than TCP/IP

  • Either the equipment or the software (or both) are old (and perhaps outdated)

NOTE: Seizing such a network is generally easier, as it is maintaining a box there. Crackers refer to this as owning a box, as in "I just cracked this network and I now own a box there." This owning refers to a condition where the cracker has root, supervisor, or administrator privileges on the box. In other words, the cracker has total control of the machine and, at any time, could totally down or otherwise destroy the network.

This profile, however, is not set in stone. Many crackers prefer to run with the bleeding-edge target, seeing whether they can exploit a newly discovered hole before the sysad plugs it. In this instance, the cracker is probably cracking for sport.

Another issue is familiarity. Most crackers know two or more operating systems intimately from a user standpoint, but generally only one from a cracking standpoint. In other words, these folks tend to specialize. Few crackers are aware of how to crack multiple platforms. For example, perhaps one individual knows VAX/VMS very well but knows little about SunOS. He will therefore target VAX stations and ultimately, perhaps through experience, DEC Alphas.

Universities are major targets in part because they possess extreme computing power. For example, a university would be an excellent place to run an extensive password cracking session. The work can be distributed over several workstations and can thus be accomplished much more quickly than by doing it locally. Another reason universities are major targets is that university boxes usually have several hundred users, even in relatively small network segments. Administration of sites that large is a difficult task. There is a strong chance that a cracked account can get lost in the mix.

Other popular targets are government sites. Here, you see the anarchistic element of the cracker personality emerging: the desire to embarrass government agencies. Such an attack, if successful, can bring a cracker great prestige within the subculture. It does not matter if that cracker is later caught; the point is, he or she cracked a supposedly secure site. This telegraphs the news of the cracker's skill to crackers across the Internet.

Why Do They Want to Attack?

There are a number of reasons why crackers might want to attack your system:

  • Spite--Plainly stated, the cracker may dislike you. Perhaps he is a disgruntled employee from your company. Perhaps you flamed him in a Usenet group. One common scenario is for a cracker to crack an ISP with which he once had an account. Perhaps the ISP discovered the cracker was cracking other networks or storing warez on its box. For whatever reason, the ISP terminated the cracker's account, and now the cracker is out for revenge.

  • Sport--Perhaps you have been bragging about the security of your system, telling people it's impenetrable. Or worse, you own a brand-spanking-new system that the cracker has never dealt with before. These are challenges a cracker cannot resist.

  • Profit--Someone pays a cracker to bring you down or to get your proprietary data.

  • Stupidity--Many crackers want to impress their friends, so they purposefully undertake acts that will bring the FBI to their door. These are mostly kids.

  • Curiosity--Many crack purely for sake of curiosity, simple enjoyment of the process, or out of boredom.

  • Politics--A small (but significant) percentage of crackers crack for political reasons. That is, they seek press coverage to highlight a particular issue. This could be animal rights, arms control, free speech, and so forth. This phenomenon is much more common in Europe than in the U.S. Americans fall victim to pride or avarice far more often than they do to ideology.

All of these reasons are vices. These vices become excess when you break the law. With breaking the law comes a certain feeling of excitement; that excitement can negatively influence your reasoning.

About Attacks

At what point can you say you have suffered a network attack? Some insist that it is the moment when crackers either penetrate your network or temporarily disable any portion of it. Certainly, from a legal point of view, this could be a valid place to mark the event called an attack (though, in some jurisdictions, intent and not the successful completion of the act will suffice).

Although the legal definition of an attack suggests that it occurs only after the act is completed and the cracker is inside, it is my opinion that the mere undertaking of actions that will result in a network break-in constitutes an attack. The way I see it, you are under attack the moment a cracker begins working on the target machine.

The problem with that position is that sometimes, partly out of sophistication and partly out of opportunity, a cracker will take some time to actually implement an attack. For example, a series of fishing expeditions may occur over a period of weeks. These probes in themselves could not reasonably be called attacks because they do not occur contiguously. If a cracker knows that logs are being run, he may opt for this "slow boat to China" approach. The level of paranoia in system administrators varies; this is not a quality that a cracker can accurately gauge without undertaking some action (perhaps trying a mock attack from a temporary address and waiting for the response, repercussions, or activity from the sysad). However, the majority of system administrators do not fly off the handle at a single instruction from the void unless that instruction is quite obviously an attack.

An example of an obvious attack is when the log reveals the attempt of an old sendmail exploit. This is where the cracker issues two or three command lines on port 25. These commands invariably attempt to trick the server into mailing a copy of the /etc/passwd file back to the cracker. If a system administrator sees this, he will obviously be concerned. However, contrast that with evidence of a showmount query. A system administrator may well know that a showmount query is an ominous indication, but it cannot be indisputably classed as an attempted intrusion. In fact, it is nothing more than evidence of someone contemplating an intrusion, if that.

These techniques of gradually gaining information about a system have their advantages and their pitfalls. For example, the cracker may come from different addresses at different times, quietly knocking on the doors (and checking the windows) of a network. Sparse logging evidence from disparate addresses may not alarm the average system administrator. In contrast, a shotgun approach (heavy scanning) will immediately alert the sysad to a problem. Unless the cracker is reasonably certain that an exploit hole exists on a machine, he will not conduct an all-out scanning attack (at least, not if he is smart).

If you are just getting started in security, the behavior of crackers is an important element of your education; this element should not be neglected. Security technicians usually downplay this, because they maintain a high level of disdain for the cracker. Nonetheless, even though sites employ sophisticated security technology, crackers continue to breach the security of supposedly solid servers.

Most crackers are not geniuses. They often implement techniques that are tried, true, and well known in the security community. Unless the cracker is writing his own tools, he must rely on available, existing tools. Each tool has limitations peculiar to its particular design. Thus, from the victim's point of view, all attacks using such tools will look basically the same. Attacks by crackers using strobe will probably look identical as long as the target machine is, say, a SPARC with SunOS 4.1.3. Knowing those signatures is an important part of your security education. However, the study of behavior goes a bit deeper.

Most crackers learn their technique (at least the basics) from those who came before them. Although there are pioneers in the field (Kevin Mitnik is one), the majority of crackers simply follow in the footsteps of their predecessors. These techniques have been described extensively in online documents authored by crackers, and such documents are available at thousands of locations on the Internet. In them are extremely detailed examples of how to implement a particular class of attack.

The new cracker typically follows these instructions to the letter, often to his detriment because some attack methods are pathetically outdated (solutions have since been devised and the cracker employing them is wasting his own time). If you examine such an attack in your logs, it may look almost identical to sample logs posted by security professionals in various technical presentations designed with the express purpose of illustrating cracking examples.

TIP: You can create scripts that will extract such attacks from logs. These scripts are really nothing more than powerful regex searches (Perl is most suitable for this) that scan log files for strings that commonly appear during or after such an attack. These output strings generally differ only slightly from platform to platform. The key is, if you have never seen those strings, generate some. Once you know the construct of the output, you will know what to scan for. Likewise, check out some of the tools I reference later in this chapter. These tools are designed for wholesale scanning of large log files.

However, there comes a point within a cracker's experience where he begins to develop specialized methods of implementing attacks. Some of these methods emerge as a result of habit; others emerge because the cracker realizes that a tool can be used for more than its express purpose. These types of attacks, called hybrid attacks, are where one or more techniques are used in concert to produce the ultimate end. (The example given in the preceding paragraphs is where an apparent denial-of-service attack is actually one phase of a spoofing attack.) Incredibly, there may be crackers who still use traditional type-one-command-at-a-time techniques, in which case, you will see all sorts of interesting log messages.

In any event, studying the behavior of crackers in actual cracking situations is instructive. There are documents of this sort on the Internet, and you should obtain at least two or three of them. One of the most extraordinary papers of this class was written by Bill Cheswick, then of AT&T Bell Laboratories. Cheswick begins this classic paper as follows:

On January 7 1991 a cracker, believing he had discovered the famous sendmail DEBUG hole in our Internet gateway machine, attempted to obtain a copy of our password file. I sent him one.

Cheswick forwarded the password file and allowed the cracker to enter a protected environment. There, the cracker was observed as he tried various methods to gain leveraged access and ultimately, to delete all the files. The attack had an apparent originating point at Stanford University, but it was later determined that the cracker was operating from the Netherlands. At the time, such activity was not unlawful in the Netherlands. Therefore, though the calls were ultimately traced and the cracker's identity known, he was reportedly untouchable. At any rate, the cracker proceeded to make a series of clumsy attempts to crack a specific machine. The story that Cheswick relates from there is truly fascinating. Cheswick and his colleagues created a special, protected (chroot) environment in which the cracker was free to crack as he pleased. In this way, the cracker could be observed closely. The paper contains many logs and is a must read.

Cross Reference: Find Cheswick's "An Evening With Berferd In Which a Cracker is Lured, Endured and Studied" online at

NOTE: Tsutomu Shimomura and Weitse Venema were also involved in this case, which spanned a fairly lengthy period of time. Shimomura reportedly assisted in capturing the network traffic, while Venema monitored the cracker (and his associates) in the Netherlands. Also, Cheswick reports that Steve Bellovin constructed a throwaway machine that they intended to use for such cases. They reasoned that such a machine would provide a better environment to observe a cracker at work, because the machine could actually be compromised at a root level (and perhaps even the file system could be destroyed). They would simply locate the machine on a network segment on which a sniffer could also be installed. Thus, if the cracker destroyed the file system of the instant machine, they could still reap the benefit of the logs. This is truly an important paper. It is humorous, entertaining, and enormously instructive.

NOTE: As it happens, Steve Bellovin did provide a dedicated bait machine, which would later become the model for other such machines. In the referenced paper, there is an extensive discussion of how to build a jail like the one the folks at Bell Labs used for the Berferd.

Other such reports exist. A particularly scathing one was authored by Tsutomu Shimomura, who had a cracker who closely resembled the Berferd mentioned above. The individual claimed to be from the Mitnik Liberation Front (the name of this so-called organization says it all). In any event, this individual "compromised" a baited machine, similar to the one that Bellovin reportedly constructed. Shimomura's commentary is interlaced between failed attempts by the cracker to accomplish much. There are logs of the sessions. It is an interesting study.

Cross Reference: Shimomura's paper is located online at

Another engrossing account was authored by Leendert van Dorn, from Vrije University in the Netherlands. It is titled "Computer Break-ins: A Case Study" (January 21, 1993). The paper addresses various types of attacks. These techniques were collected from actual attacks directed against Vrije University. Some of the attacks were quite sophisticated.

Cross Reference: Find van Dorn's account online at

Perhaps a better-known paper is "Security Breaches: Five Recent Incidents at Columbia University." Because I analyze that paper elsewhere in this text, I will refrain from doing so again. However, it is an excellent study (some 23 pages in all) that sheds significant light on the behavior of crackers implementing attacks.

Cross Reference: "Security Breaches: Five Recent Incidents at Columbia University" can be found online at

Gordon R. Meyer wrote a very interesting paper titled "The Social Organization of the Computer Underground" as his master's thesis at Northern Illinois University. In it, Meyer analyzed the computer underground from a sociological point of view and gathered some enlightening information. The paper, although dated, provides excerpts from radio and television interviews, message logs, journals, and other publications. Although Meyer's paper does not reveal specific methods of operation in the same detail as the papers mentioned earlier, it does describe (with considerable detail and clarity) the social aspects of cracking and crackers.

Cross Reference: Meyer's paper, written in August, 1989, is located online at

The Sams Crack Level Index

Figure 26.1 shows six levels, each representing one level of depth into your network. I will refer to these as levels of sensitivity. Points along those levels identify the risks associated with each cracking technique. I will refer to those as states of attack.

FIGURE 26.1.
The Sams crack level index.

Levels of Sensitivity

The levels of sensitivity in all networks are pretty much the same (barring those using secure network operating systems). The common risks can be summed up in a list, which has basically not changed for a decade. The list rarely changes, except with the introduction of new technologies, such as ActiveX, that allow arbitrary execution of binaries over the Net.

The majority of crackers capitalize on the holes we hear about daily in security newsgroups. If you have frequented these groups (or a security mailing list) you will have read these words a thousand times:

  • "Oh, they had test.cgi still installed in their cgi-bin directory."

  • "It was a Linux box and apparently, they installed sudo and some of the demo users."

  • "It was the phf script that did them in."

Level One

Attacks classified in the level-one category are basically irrelevant. Level-one attacks include denial-of-service attacks and mail bombing. At best, these techniques require 30 minutes of your time to correct. This is because these attacks are instituted with the express purpose of nuisance. In most instances, you can halt these problems by applying an exclusionary scheme, as discussed in Computer Security Advisory 95-13 (SATAN Update), issued by the University of Pittsburgh:

Denial-of-service attacks are always possible: The best way to deal with this is to react to intrusions by adding intruder source hosts/networks into the DENY listings in the inetd.sec. There is no proactive way to avoid this without disabling networking altogether.

TIP: If you uncover evidence of a denial-of-service attack, you should look elsewhere on the system for possible intrusions. Flooding and denial-of-service attacks are often precursors (or even integral portions) of a spoofing attack. If you see a comprehensive flooding of a given port on one machine, take note of the port and what it does. Examine what service is bound to it. If that service is an integral part of your internal system--where other machines use it and the communication relies on address authentication--be wary. What looks like a denial-of-service attack could in fact be the beginning of a breach of network security, though generally, denial-of-service attacks that last for long periods of time are just what they appear to be: nuisances.

There are some instances in which a denial-of-service attack can be more serious. Certain, obscure configurations of your network could foster more threatening conditions. Christopher Klaus of Internet Security Systems defined several such configurations in a post concerning denial-of-service attacks. In that posting, Klaus wrote:

By sending a UDP packet with incorrect information in the header, some Sun-OS 4.1.3 UNIX boxes will panic and then reboot. This is a problem found frequently on many firewalls that are on top of a Sun-OS machine. This could be high risk vulnerability if your firewall keeps going down.

Klaus also addressed other denial-of-service attacks in that post. I would recommend reviewing it. Klaus provides information on vulnerabilities for NT, Novell, Linux, and UNIX generally.

Cross Reference: Klaus's posting can be found online at

If the attack is a syn_flood attack, there are some measures you can take to identify the cracking party. Currently, four major syn_flooding utilities are floating around on the Internet. At least two of these tools have a fundamental flaw within them that reveals the identity of the attacker, even if indirectly. These tools have provisions within their code for a series of PING instructions. These PING instructs carry with them the IP address of the machine issuing them. Therefore, if the cracker is using one of these two utilities, he is telegraphing his IP address to you for each PING. Although this will not give you the e-mail address of the party, you can, through methods described earlier in this book, trace it to its ultimate source. (As noted, traceroute will reveal the actual network the cracker is coming from. This is generally the second-to-last entry on the reverse traceroute lookup.) The problem with this, however, is that you must log heavily enough to capture all the traffic between you and the cracking party. To find that IP address, you will have to dig for it. At any rate, you have a 50 percent chance of the cracker using such a flawed utility.

NOTE: The remaining two utilities for syn_flooding do not have this PING flaw. The developers of these tools were a bit more sophisticated. They added a provision to randomize the purported IP address. This naturally presents a much more difficult situation to the victim. Even low-level analysis of the received packets is a waste of time. However, to the inexperienced system administrator, this could be a bit confusing. Tricky, right?

Most denial-of-service attacks represent a relatively low-level risk. Even those attacks that can force a reboot (of over-utilization of a processor) are only temporary problems. These types of attacks are vastly different from attacks where someone gains control of your network. The only truly irritating thing about denial-of-service attacks is that in the same way that they are low-level risks, they are also high-level possibilities. A cracker implementing a denial-of-service attack need have only very limited experience and expertise. These attacks are therefore common, though not nearly as common as mail bombings.

As for mail bombings, the perpetrators are usually easily tracked. Furthermore, bozo files (kill files) and exclusionary schemes basically render these attacks utterly harmless (they ultimately bring more sorrow to the perpetrator than anyone). The only real exception to this is where the bombing is so consistent and in such volume that it cripples a mail server.

Other level-one intrusions consist of knuckleheads initiating Telnet sessions to your mail or news server, trying to ascertain shared out directories and whatnot. As long as you have properly secured your network, these activities are harmless. If you haven't properly configured shares, or if you are running the r services (or other things you shouldn't), some of these garden- variety level-one techniques can expand into real trouble.

Levels Two and Three

Levels two and three involve things like local users gaining read or write access to files (or directories) they shouldn't. This can be a problem, depending largely on the character of the file(s). Certainly, any instance of a local user being able to access the /tmp directory can be critical. This could potentially pave a pathway to level-three issues (the next stage) where a user could conceivably gain write access as well (and thus progress to a level-four environment). This is an issue primarily for UNIX administrators or NT administrators.

NOTE: Microsoft Windows 95 does not have granular access control and therefore, barring installation of some third-party, access-control device, Windows 95 networks are completely insecure. Because of this, level-two attacks are critical and can easily progress to levels three, four, five, and six in seconds. If you run such a network, immediately get an access-control device of some sort. If you do not, anyone (at any time) can delete one or more critical files. Many programs in the Windows 95 environment rely on file dependencies. As long as you run a Windows 95 network connected to the Internet (without access control or closing the holes in Internet Explorer), it is only a question of how long before someone mangles your network. By deleting just a few files on a Windows 95 network, a cracker can incapacitate it permanently. If you have the ability to do so, monitor all traffic to ports 137-139, where the sharing process occurs. Furthermore, I would strictly prohibit users within that network from installing Web or FTP servers. If you are running the Microsoft platform and want to provide servers open to the outside world (an idea that I would furiously argue against), get NT.

Local attacks are a bit different. The term local user is, I realize, a relative one. In networks, local user refers to literally anyone currently logged to any machine within the network. Perhaps a better way to define this is to say that a local user is anyone who has a password to a machine within your local network and therefore has a directory on one of your drives (regardless of what purpose that directory serves: a Web site, a local hard disk drive on one of the workstations, and so forth).

The threat from local users correlates directly to what type of network you are maintaining. If you are an ISP, your local users could be anyone; you have probably never met or spoken to 90 percent of your local users. As long as their credit card charges ring true each month, you probably have little contact with these folks even by e-mail (barring the distribution of monthly access or maintenance reports; this interaction doesn't really count as contact, though). There is no reason to assume that these faceless persons are not crackers. Everyone but your immediate staff should be suspect.

An attack initiated by a local user can be either pathetic or extremely sophisticated. Nevertheless, no matter what level of expertise is behind these attacks, will almost invariably originate over Telnet. I have indicated before that if you are an ISP, it is an excellent idea to isolate all shell accounts to a single machine. That is, logins should only be accepted on the one or more machines that you have allocated for shell access. This makes it much easier to manage logs, access controls, loose protocols, and other potential security issues.

TIP: In general, you should also segregate any system boxes that are going to house user-created CGI.

These machines should be blocked into their own networked segment. That is, they should be surrounded by either routers or hubs, depending on how your network is configured. The topology should ensure that bizarre forms of hardware address spoofing cannot leak beyond that particular segment. This brings up some issues of trust, a matter I address later in this book.

There are only two kinds of attack you will encounter. The less serious one is the roving user, a cracker who is new to the subject and therefore looks around for things (oh, they might print the passwd file to SDTOUT, see if they can read any privileged files, and whatnot). Conversely, you may encounter an organized and well-thought-out attack. This is where the attacker already knows your system configuration well. Perhaps he previously assessed it from an account with another provider (if your system gives away information from the outside, this is a definite possibility).

For those using access-control-enabled environments, there are two key issues regarding permissions. Each can affect whether a level-two problem escalates into levels three, four, or five. Those factors are

  • Misconfiguration on your part

  • Holes inherent within software

The first contingency arises when you don't properly understand the permission scheme. This is not a crime. I recognize (though few will admit it) that not every UNIX or NT system administrator is a guru. It takes time to acquire in-depth knowledge of the system. Just because you have earned a B.S. in CS doesn't mean you will know for certain that your system is secure. There are tools to check for common misconfigurations, and I offer quite a few throughout this book. If you have even the slightest suspicion that permissions may be set inaccurately, get these tools and double-check.

TIP: Many security tools come with tutorials about vulnerabilities. SATAN is a great example. The tutorials included with SATAN are of significant value and can be used to understand many weaknesses within the system, even if you do not run UNIX. For example, suppose you are a journalist and want to gain a better understanding of UNIX security. You don't need UNIX to read the HTML tutorials included with SATAN.

The second contingency is more common than you think. In fact, it crops up all the time. For example, according to the CERT advisory titled "Vulnerability in IRIX csetup" (issued in January, 1997):

The CERT Coordination Center has received information about a vulnerability in the csetup program under IRIX versions 5.x, 6.0, 6.0.1, 6.1, and 6.2. csetup is not available under IRIX 6.3 and 6.4. By exploiting this vulnerability, local users can create or overwrite arbitrary files on the system. With this leverage, they can ultimately gain root privileges.

Cross Reference: Find this advisory online at

Take a good look at this advisory. Note the date. This is not some ancient advisory from the 1980s. This appeared very recently. These types of problems are not exclusive to any one company. Holes are routinely found in programs on every manner of operating system, as noted in the CERT advisory titled "Vulnerability in Solaris admintool" (August, 1996):

AUSCERT has received a report of a vulnerability in the Sun Microsystems Solaris 2.x distribution involving the program admintool. This program is used to provide a graphical user interface to numerous system administration tasks. This vulnerability may allow a local user to gain root privileges...In Solaris 2.5, admintool is set-user-id root by default. That is, all file accesses are performed with the effective uid of root. An effect of this is that the vulnerability will allow access to any file on the system. If the vulnerability is exploited to try and create a file that already exists, the contents of that file will be deleted. If the file does not exist, it will be created with root ownership and be world writable.

Cross Reference: Find this advisory online at

It makes no difference what flavor you are running. Bugs are posted for almost all operating systems. Most networked systems see at least one advisory a month of this nature (by this nature, I mean one that can lead to leveraged or even root access). There is no immediate solution to this problem because most of these holes are not apparent at the time the software is shipped. The only solution is that you subscribe to every mailing list germane to bugs, holes, and your system. In this respect, security is a never-ending, learning process.

There are some techniques that you can employ to keep up with the times. First, if you subscribe to several mailing lists, you will be hammered with e-mail. Some lists generate as many as 50 messages a day. On UNIX platforms, this is not much of a problem, because you can control how these messages are written to the disk at their time of arrival (by trapping the incoming address and redirecting the mail to a particular directory and so forth). In a Microsoft Windows environment, however, that volume of mail can be overwhelming for someone busy with other tasks. If you are the system administrator of a network running NT, there are several actions you can take. One is to direct different lists to different accounts. This makes management of incoming mail a bit easier (there are also products on the market for this sort of thing). Nonetheless, no matter what platform you use, you should fashion scripts to analyze those mail messages before you read them. I would install Perl (which is also available for NT) and use it to scan the messages for strings that would likely appear in a post relevant to your specific configuration. With a little effort, you can even create a script that rates these hits by priority.

Level Four

Level-four issues are usually related to outsiders being able to access internal files. This access may vary. They may be able to do no more than verify the existence of certain files, or they may be able to read them. Level-four problems also include those vulnerabilities whereby remote users--without valid accounts--can execute a limited number of commands on your server.

The highest percentage of these holes arise through misconfiguration of your server, bad CGI, and overflow problems.

Levels Five and Six

Levels five and six consist of conditions whereby things are allowed to occur that never should. Any level five or six hole is fatal. At these stages, remote users can read, write, and execute files (usually, they have used a combination of techniques to get to this stage). Fortunately, if you have closed levels two, three, and four, it is almost impossible that you will ever see a level five or six crisis. If you close lesser avenues of entry, a level-six vulnerability is most likely to originate with a vendor's faulty software.

Response Levels

What do you do if you discover an attack in progress? It depends on the situation.

Responding to Level-One Attacks

Level-one attacks can be treated as described previously. Filter the incoming address and contact the attacker's service provider. These are minor inconveniences. Only when the denial-of-service attack appears to be related to some other form of attack (perhaps more sophisticated) or where it continues for some time (as in the case) should you bother to do more than exclude the incoming traffic. However, if you are in a situation identical to Panix, you may want to contact CERT or other authorities.

Responding to Level-Two Attacks

Level-two attacks can be dealt with internally. There is no reason to leak information that local users can access things they shouldn't. Basically, freeze or eliminate the user's account. If there are complaints, let your lawyers sort it out. If you "counsel" the individual, you will see poor results. Within a month, he or she will be at it again. You are not engaged in a game. There is no guarantee that this internal user is just an innocent, curious individual. One last thing: give no warning about freezing the account. This way, you can preserve any evidence that might otherwise be deleted.

NOTE: In cases where you cannot cut the user loose entirely (perhaps the user is an employee), you can give warnings and make the user's position contingent on compliance. Carefully document the incident as well, so that if further problems occur, the user has no case for a wrongful termination action if fired.

Responding to Level-Three, -Four, and -Five Attacks

If you experience any sort of an attack higher than a level two, you have a problem. Your job, then, is to undertake several actions:

  • Isolate the network segment so that the activity can only occur in a small area

  • Allow the activity to continue

  • Log all activity heavily

  • Make every effort (using a different portion of the network) to identify the source or sources of the attacks

You are dealing with a criminal. Under state and federal statutes, this type of access is a crime. If you are to capture that criminal, you will need evidence. Generating that evidence will take time.

The standards of evidence in an Internet criminal case are not exactly settled. Certainly, the mere act of someone trying to retrieve your /etc/passwd file by sendmail will not support a criminal case. Nor will evidence of a handful of showmount requests. In short, to build an iron-clad case against an intruder, you must have some tangible evidence that the intruder was within your network or, alternatively, some tangible evidence identifying the intruder as the one who downed your server in a denial-of-service attack. To do this, you must endure the brunt of the attack (although you can institute come safeguards to ensure that this attack does not harm your network).

My advice in such a situation would be to call in not only some law enforcement but also at least one qualified security firm to assist in snagging the offender. The most important features of such an operation are logs and, of course, locating the perpetrator. You can provide the logs on your own. However, as far as tracing the individual, you can only go so far. You might start with a simple traceroute and, before you're finished, you may have implemented a dozen different techniques only to find that the network from which the perpetrator is hailing is either also a victim (that is, the cracker is island hopping), a rogue site, or even worse, located in a country beyond the reach of the U.S. Justice Department. In such cases, little can be done besides shoring up your network and getting on with your business. Taking any other course of action might be very costly and largely a waste of time.


In this chapter, you learned about levels of attack. These levels of attack are defined numerically (level one being the least harmful, level six being the most harmful). This chapter discusses how to combat attacks of various levels, and informs you of tools you can use to wage a successful battle.


UNIX Incident Guide How to Detect an Intrusion.

Securing Internet Information Servers. CIAC-2308.

Threat Assessment of Malicious Code and Human Computer Threats. L.E. Bassham and T.W. Polk. National Institute of Standards and Technology. Report to the U.S. Army Vulnerability/Survivability Study Team, NISTIR 4939. October, 1992.

Hackers in the Mist. R. Blake. Northwestern University, Independent study in anthropology. December 2, 1994.

Computer Break-ins: A Case Study. Leendert van Dorn. Vrije University. January 21, 1993.

Concerning Hackers Who Break into Computer Systems. Presented at the 13th National Computer Security Conference, October 1, 1990.

Selling Security: Security Policies Are Key to a Strong Defense, But Top Management Must First Be Brought on Board. C. Waltner. InfoWorld.

The United States vs. Craig Neidorf: A Debate on Electronic Publishing Constitutional Rights and Hacking. D.E. Denning. Communications of the ACM, March, 1991.

An Evening With Berferd In Which a Cracker is Lured, Endured and Studied. B. Cheswick. AT&T Bell Labs.

Recombinant Culture: Crime in the Digital Network. C. E. A. Karnow. Presented at Defcon II, July 1994.

The Baudy World of the Byte Bandit: A Postmodernist Interpretation of the Computer Underground. G. Meyer and J. Thomas. Department of Sociology, Northern Illinois University. March 5, 1990.

Intrusion Detection

An Introduction to Intrusion Detection. Aurobindo Sundaram.

Intrusion Detection for Network Infrastructures. S. Cheung, K.N. Levitt, and C. Ko. 1995 IEEE Symposium on Security and Privacy, Oakland, CA, May 1995.

Fraud and Intrusion Detection in Financial Information Systems. S. Stolfo, P. Chan, D. Wei, W. Lee, and A. Prodromidis. 4th ACM Computer and Communications Security Conference, 1997.

Detecting Unusual Program Behavior Using the Statistical Component of the Next- Generation Intrusion Detection Expert System (NIDES). Debra Anderson, Teresa F. Lunt, Harold Javitz, Ann Tamaru, and Alfonso Valdes. SRI-CSL-95-06, May 1995. (Available in hard copy only.)

Intrusion Detection Systems (IDS): A Survey of Existing Systems and A Proposed Distributed IDS Architecture. S.R. Snapp, J. Brentano, G.V. Dias, T.L. Goan, T. Grance, L.T. Heberlein, C. Ho, K.N. Levitt, B. Mukherjee, D.L. Mansur, K.L. Pon, and S.E. Smaha. Technical Report CSE-91-7, Division of Computer Science, University of California, Davis, February 1991.

A Methodology for Testing Intrusion Detection Systems. N. F. Puketza, K. Zhang, M. Chung, B. Mukherjee, and R. A. Olsson. IEEE Transactions on Software Engineering, Vol.22, No.10, October 1996.

GrIDS--A Graph-Based Intrusion Detection System for Large Networks. S. Staniford-Chen, S. Cheung, R. Crawford, M. Dilger, J. Frank, J. Hoagland, K. Levitt, C. Wee, R. Yip, and D. Zerkle. The 19th National Information Systems Security Conference.

NetKuang--A Multi-Host Configuration Vulnerability Checker. D. Zerkle and K. Levitt, Proceedings of the 6th Usenix Security Symposium. San Jose, California. 1996.

Simulating Concurrent Intrusions for Testing Intrusion Detection Systems: Parallelizing Intrusions. M. Chung, N. Puketza, R.A. Olsson, and B. Mukherjee. Proceedings of the 1995 National Information Systems Security Conference. Baltimore, Maryland. 1995.

Holding Intruders Accountable on the Internet. S. Staniford-Chen and L.T. Heberlein. Proceedings of the 1995 IEEE Symposium on Security and Privacy, Oakland, CA, 8-10 May 1995.

Machine Learning and Intrusion Detection: Current and Future Directions. J. Frank. Proceedings of the 17th National Computer Security Conference, October 1994.

Another Intrusion Detection Bibliography.

Intrusion Detection Bibliography.

Bibliography on Intrusion Detection. The Collection of Computer Science Bibliographies.

Previous chapter Next chapter Contents

© Copyright, Macmillan Computer Publishing. All rights reserved.

With any suggestions or questions please feel free to contact us