Security Incidents on the Internet
John D. HOWARD <email@example.com>
This paper presents an analysis of trends in Internet security based on an investigation of 4,299 Internet security-related incidents reported to the CERT Coordination Center (CERT/CC) from 1989 through 1995. Prior to this research, our knowledge of actual Internet security incidents was limited and primarily anecdotal. This research
With the exception of denial-of-service attacks, security incidents were found to be increasing at a rate less than Internet growth. Estimates showed that most, if not all, severe incidents were reported to the CERT/CC, and that more than one out of three above average incidents (in terms of duration and number of sites) were reported. Estimates also indicated that a typical Internet site was involved in, at most, around one incident (of any kind) per year, and a typical Internet host in, at most, around one incident in 45 years. The probability of gaining privileged access was around an order of magnitude less likely. As a result, simple and reasonable security precautions should be sufficient for most Internet users.
This paper presents a summary of trends in Internet security based on an investigation of 4,299 Internet security-related incidents reported to the CERT/CC from 1989 through 1995. The CERT/CC (originally known as the Computer Emergency Response Team Coordination Center) provides the Internet community with a single organization for coordinating responses to security incidents [HoR91:25]. The CERT/CC is located at Carnegie Mellon University's Software Engineering Institute, Pittsburgh, Pennsylvania, USA (http://www.cert.org/).
Prior to this research, the CERT/CC was unable to release specific Internet security incident information. Our knowledge of actual security incidents on the Internet was therefore limited and primarily anecdotal.
Development of agreed-upon terminology and principles of classification (a taxonomy) is a necessary prerequisite to systematic studies in any field of inquiry [McK82:3]. Development of comprehensive taxonomies in the field of computer security has been an intractable problem of increasing interest [Amo94:31]. In this section, I present the taxonomy I developed based on my experience in classifying actual Internet attacks and incidents.
The terms attack and incident are often used interchangeably in computer security literature, which results in considerable confusion. In order to classify actual Internet incidents, a distinction must be made between these terms. An attack I defined to be a single unauthorized access attempt or unauthorized use attempt, regardless of success. An incident, on the other hand, I defined to be a group of attacks that can be distinguished from other incidents because of the distinctiveness of the attackers and the degree of similarity of sites, techniques, and timing.
The taxonomy I developed has two parts. First is a "full" taxonomy for Internet attacks. This attack taxonomy is broader in scope than most previous taxonomies, because I did not attempt to enumerate all computer security flaws or methods of attack, but rather attempted to provide a broad, inclusive framework. My intention was to reorient the focus of the taxonomy toward a process, rather than a single classification category, in order to provide both an adequate classification scheme for actual Internet attacks and a taxonomy that would aid in thinking about computer and network security.
The complete attack taxonomy is shown in Figure 2.1, which depicts a simplification of the path an attacker must take in order to accomplish the attacker's objectives. To be successful, an attacker must find one or more paths that can be connected, perhaps simultaneously. This taxonomy suggests that computer security is preventing attackers from achieving objectives by making any complete connections through the six steps depicted.
People attack computers. They do so through a variety of methods and for a variety of objectives. What I found to distinguish the categories of attackers was a combination of who they are and their objective (what they want to accomplish). I have divided attackers into the following six categories, based on their objectives:
These six categories of attackers, and their four categories of objectives, are shown in the leftmost and rightmost blocks of Figure 2.1. These blocks serve as the two ends of the operational sequence of computer and network attacks. In between are the "tools, vulnerabilities, access, and results" which link attackers to their objectives.
In order to successfully accomplish an objective, an attacker must either obtain unauthorized access to a computer or network, or, if authorized to have access, must use that access in an unauthorized way. This is shown in the left side of the access block of Figure 2.1. The unauthorized access or use is to processes or to files or data in transit through processes. These are depicted in the right side of the access block (Figure 2.1).
In order to reach the desired process, an attacker must take advantage of a computer or network vulnerability, which is a flaw allowing an unauthorized access or use [Amo94:2]. A vulnerability may arise in three ways:
Between obtaining access and the attacker's objectives, I conceptualized the results of attack. At this point in the sequence of an attack, the attacker has access to the desired processes, files, or data in transit. The attacker is now free to exploit this access to alter files, deny service, obtain information, or use the available services. Figure 2.1 depicts these results of attack, which are defined as follows [Amo94:3-4,31; RuG91:9-10; Coh95:55-56]:
The final connection to be made in the operational sequence that leads attackers to their objectives is the tools of attack. This is also the most difficult connection to make because of the wide variety of methods available to exploit vulnerabilities in computers and networks. When authors make lists of methods of attack, often they are actually making lists of tools. The approach taken here was to establish the following categories (see Figure 2.1):
Except for the physical attack, information exchange, and data tap categories, each of the tool categories may contain the other tool categories within them. For example, toolkits contain scripts, programs, and sometimes autonomous agents. So when a toolkit is used, the scripts and programs category is also included. User commands also must be used for the initiation of scripts, programs, autonomous agents, toolkits, and distributed tools. In other words, there is an order to some of the categories in the tools block, from the simple user command category to the more sophisticated distributed tools category. What made these categories mutually exclusive when actually applied to the CERT/CC records was that I classified attacks according to the highest category of tool used.
Data were extracted from each incident after the incidents were reconstructed from the CERT/CC records. These data were used to classify each incident according to reporting date, starting date, ending date, number of sites, number of messages, reporting site names, other site names, level of attack, methods of operation, corrective actions, and CERT tracking numbers. "Level of attack" and "methods of operation" consisted of keywords describing the types of attacks in the incident, based on the attack taxonomy. The level of attack was the highest "level" of access the attacker achieved (the access block of the taxonomy, Figure 2.1).
A total of 4,567 incidents over this 7-year period was reconstructed from the CERT/CC records. This total included 268 false alarms (6%) and 4,299 actual incidents (94%) ranging from log-in attempts to large incidents involving break-ins at the root level. The number of incidents increased each year at a rate between 41% (1991 to 1992) and 62% (1993 to 1994). The exception to this took place between 1994 and 1995 when the number of incidents reported decreased slightly.
As stated in Section 2, the center of the connection between attackers and their objectives is the attacker's requirement for unauthorized access or unauthorized use. Most of the CERT/CC incidents (89%) were unauthorized access incidents: root break-ins (28%), account break-ins (24%), and access attempts (38%). Relative to the growth in Internet hosts, each of these access categories was found to be decreasing over the period of this research, with root-level break-ins at a rate around 19% less than the increase in Internet hosts, account-level break-ins at a rate around 11% less, and access attempts at a rate around 17% less.
Of the 4,299 actual incidents reported to the CERT/CC, 458 (11%) were classified as unauthorized use incidents: denial-of-service attacks (2.4%), corruption of information incidents (3%), and disclosure of information incidents (5%). The growth in total unauthorized use incidents was around 9% per year greater than the growth in Internet hosts.
The 4,299 incidents, which are based on reporting date, are plotted in Figure 3.1, grouped by month.
The number of incidents reported to the CERT/CC, as shown in Figure 3.1, is actually not a good indication of either the activity at the CERT/CC or the state of Internet security because 1) the incidents are presented according to reporting date, which is an inaccurate representation of the incidents in time; 2) the incidents are not comparable due to wide variations in duration, in the number of sites involved, and in the severity or success of the attack; and 3) the number of incidents is not compared to the growth of Internet hosts.
A more informative method of presenting Internet incident information is to use the total average sites per day for each category of attack. This can be derived by summing the average sites per day from each incident in a category using each starting date, ending date, and the total number of sites involved.
Although the absolute number of incidents generally grew, the sites per day data showed there was a steady decline in security activity reported to the CERT/CC, compared to the size of the Internet, since peaking in 1990. The slopes of the growth in all sites per day for all incidents and for root and account-level break-ins (Figure 3.2) were both around 7% less than the growth rate in the number of Internet hosts (a = 1%, R2 = 9.4%).
In order to identify the most severe incidents in the CERT/CC records, a criterion was developed from this research as follows: root break-ins, ³ 79 days' duration, ³ 62 sites, and ³ 87 messages to/from the CERT/CC. This selected 25 incidents with an average of 190 days' duration, 164 sites, and 433 messages in the CERT/CC record, as summarized in Table 3.1.
The distribution of these incidents over time is presented in Figure 3.3, which plots a rectangle representing each incident. The horizontal dimension of each incident corresponds to the duration, and the height corresponds to the average sites per day as listed in Table 3.1.
There were two predominant trends seen in the 25 severe incidents. First, the sophistication of intruder techniques progressed from simple user commands, scripts, and password cracking to the use of tools, such as sniffers (1993) and toolkits (1994), and finally to intricate techniques that fool the basic operation of the Internet Protocol (1995). The second trend was that intruders became increasingly difficult to locate and identify. In the early incidents, the attackers tended to be a few individuals confined to a specific location or group of locations, and as a consequence, tended to be easily identifiable. As intruder tools became more sophisticated and the size of the Internet grew, the severe incidents involved more attackers operating in many different locations. The newest and most sophisticated techniques allowed the attackers to obtain nearly total obscurity.
For these 25 incidents, a three-phase process of attack was consistently used: 1) gain access to an account on the target system; 2) exploit vulnerabilities to gain privileged (root) access on that system; and 3) use this privileged access to attack other systems across the network.
Recording of attack taxonomy categories in the CERT/CC records was not systematic or complete. As a result, this information is incomplete. Some valuable information, however, can be obtained by determining the relative frequency with which various attack taxonomy categories appear in the CERT/CC incident records. In the CERT/CC records, more information was found about Tools, Vulnerabilities, and Access than the other categories of the taxonomy. Very little information was in the records about the beginning and ending categories, Attackers and Objectives.
A total of 778 incidents (18% of all incidents) reported the use of some tool. From these records, the largest category of tools was scripts or programs (15%). These consisted primarily of Trojan horses (11%) and sniffers (6%). The two general categories of toolkits were tools designed to exploit privileged or root access (1.2%) and scanners (2.6%). These tools appeared relatively late in the CERT/CC records. The CERT/CC records contain very few references to autonomous agents such as worms and viruses. There was no mention in any of the CERT/CC records of the use of data taps or distributed tools. Data taps are physical taps and not attacks across the Internet, which makes them much less likely to be reported to the CERT/CC. Distributed tools do not appear in the CERT/CC records until after the period of this research.
Nearly half of the incidents in the CERT/CC records mention specific vulnerabilities (45%). The most frequently recorded vulnerabilities involved various problems with passwords (22%). Most of the password vulnerabilities were in three categories: password files, which indicated that a password file had been copied (14%); password cracking, generally indicating that passwords had been determined by the operation of a password cracking tool (10%); and weak passwords, which could be easily guessed (4%).
The reputation of sendmail and other mail transfer agents for being "plagued with security problems" was confirmed in the CERT/CC incident records, which contain numerous references to sendmail (10%), SMTP (0.4%), and mail (8%). Problems with implementation of trusted hosts (such as the hosts.equiv or .rhosts file) were recorded in a significant number of incidents (6%), as was configuration (6%), TFTP (6%), NIS and YP (4%), FTP (4%), and NFS (3%).
The CERT/CC records contained 419 incidents with some information about the results category of the taxonomy (10%). The largest category of these results was theft of service (7%), which primarily consisted of FTP abuse (6%). Disclosure of information was another large category of results (6%), which consisted primarily of software piracy (5%). FTP abuse, software piracy, and warez are all related, so it makes sense that they were recorded in a similar number of incidents.
There were 170 incidents in the CERT/CC records that gave information about corruption of information (4%), which primarily consisted of modifying or deleting logs (2%) or deleting files (2%).
Little information was found in the CERT/CC records for some of the categories of the attack taxonomy. One likely cause was that only certain categories of information were necessary for the mission of the CERT/CC, such as vulnerabilities and access level. Other possible reasons were that the information was assumed (and therefore not recorded) or that the CERT/CC does not view itself as actually being responsible for all security problems on the Internet. For example, the VIRUS-L moderated mailing list had a focus on computer virus issues, which may explain the lack of virus information in the CERT/CC records.
Since attacks make up incidents, total Internet security activity could be measured by either the total Internet attack activity or the total Internet incident activity. In order to estimate the number of attacks, some sample of Internet activity is required. Vulnerability studies by U.S. Defense Department agencies can be used for such an estimate. A vulnerability analysis by the U.S. Defense Information Systems Agency (DISA) showed that the probability of an individual attack being reported was around 1 out of 140 (0.7%). In a different study, the U.S. Air Force Information Warfare Center (AFIWC) estimated this probability to be 1 out of 8 (12.5%). Table 5.1 summarizes the estimates of total Internet attack activity based on these studies.
These widely varying estimates indicate that currently we cannot accurately estimate the number of attacks. However, the same is not true for estimations of the number of incidents. Estimates of the rate of reporting of attacks and of the number of attacks per incident could be used to estimate the total number of Internet incidents as follows:
where Nt = the total number of Internet incidents, Nr = the number of Internet incidents reported, P(I) = the probability (percentage) that an incident will be reported, P(A) = the probability that an attack will be reported, and a = the number of attacks per incident.
The DISA and AFIWC studies give us a low and a high estimate of the probability of an attack being reported [P(A)]. The number of attacks per incident was estimated to be between 10 and 1,000 when all CERT/CC data were considered together. Better estimates were obtained when the types of incidents were considered separately. Table 5.2 summarizes the estimates of total Internet incident activity made by estimating attacks per incident or from Site A projections. "Site A" is one Internet site that reported all its incidents to the CERT/CC. These estimates are for one year in 1995.
With respect to severe incidents, using the DISA probability of reporting an attack, the probability of any severe incident not being reported to the CERT/CC was between 0% and 4%. Using the AFIWC probability of reporting an attack, the probability of any severe incident not being reported to the CERT/CC was essentially zero. This confirms the impression the reports themselves give: that it is hard to conceive that a severe Internet security incident would not be reported to the CERT/CC.
There were 394 incidents in the CERT/CC records (9.2%) that were above average both in terms of duration (above 16.5 days) and in terms of the number of sites (above 6.5 sites). When these incidents were isolated and analyzed, it showed that if we assume the DISA probability of report, then a minimum of about 1 out of 2.6 of the above average incidents was reported to the CERT/CC (and nearly all of them may have been reported). If we assume the AFIWC probability, then it was estimated that more than 96% of these incidents were reported to the CERT/CC.
Security is a problem on the Internet. The thousands of successful break-ins described in this research are a testimony to that. Numerous authors -- scholars and sensationalists alike -- go even further by describing the Internet as a "dangerous place" in terms of security.
But just how much of a problem is security on the Internet? What this research shows is a mixed message. On the positive side, this research clearly shows that the state of Internet security is not as bad as some authors have proposed. Both in terms of the absolute numbers of incidents and in the growth of these incidents, the numbers are lower than reported in popular literature and in the press. More importantly, response teams and researchers are not as unaware of Internet security activity as some authors have argued. As shown in Section 5, the most serious incidents on the Internet are reported. In addition, none of the incidents were tremendously destructive. In fact, very few instances of destructive attacks were recorded. Most attacks were in the category of a nuisance (although some were a big nuisance) and not something more destructive or harmful.
Nevertheless, on the negative side, security incidents were clearly not dropping to zero. As shown in Section 3, the rate of growth of Internet incidents was less than the growth of Internet hosts by 7%. But, stated another way, this means that the growth of Internet incidents in absolute terms was nearly at the same pace as the growth of the Internet.
To put this in perspective, we can use the estimates of total Internet incident activity in Section 5 to see how likely we are to be involved in an Internet incident. This yields the very rough estimates in Table 6.1.
This table shows that, according to these estimates, a typical Internet site is involved in no more than about one incident per year. In terms of hosts, the estimates of Table 6.1 show that a typical Internet host is involved in no more than about one incident every 45 years. The CERT/CC records show that some sites and hosts are apparently more attractive because they were involved in many incidents each year. This means that for the average, less attractive sites and hosts, the probability of being involved in an incident is even lower.
In addition, as shown by this research, many of the Internet incidents are minor and often do not involve successful break-ins. As such, the rate at which sites and hosts are involved in serious incidents is even lower. For example, at one site that reported all their incidents to the CERT/CC (Site A), only 7% of incidents involved root break-ins. If this were similar throughout the Internet, then the maximum rate that any one site would be involved in a root break-in would be about once in 10 years, and any individual host about once in 540 years.
[Pik97, USB96:Chart Nos. 129, 138, 318, 1263, NYC97a, NYC97b]
These rates of occurrence are similar to other risks for which we take reasonable precautions. Table 6.2 compares several example risks with Internet security risks. The conclusion we can draw from this is that because there is a steady, but relatively small, level of Internet security incidents, Internet users should take reasonable security precautions, just as they would take for other risks in their lives. In addition, Internet suppliers should produce and distribute products that provide users with reasonable security.
The following sections discuss these implications in more detail.
This year, any specific individual will most likely not be the victim of a violent crime, a house robbery, or a car theft. But this person might. Because of this, an individual is likely to take reasonable precautions to protect him- or herself and his or her property.
This research shows the same is true of the Internet. Unlike what some authors have proposed, an individual Internet user is most likely not going to be the victim of an Internet attack this year. But it might happen. Because of this, an individual should take reasonable precautions to protect files on his or her computer and data as they transit the Internet.
Two analogies illustrate this point. Take, for example, convenience stores. They get robbed sometimes. This could clearly be prevented with a "fortress" security system of physical barriers and armed guards. But then how convenient would that store be to shop in? Instead of ensuring no risk of robbery, the convenience store owner typically takes reasonable precautions against robbery in order to reduce that risk, but accepts some risk in order to ensure the store is still convenient to shoppers.
An Internet user can be perfectly secure from Internet attack by simply disconnecting from the Internet. But then, this user would no longer be an Internet user. Instead of taking this "no risk" strategy, it is probably more appropriate to take reasonable precautions and accept a level of risk that depends on that user's individual needs.
A second analogy is the mail system. We view the mail system as generally being secure enough to send a personal letter or to send a check to pay a bill. On the other hand, most people do not send cash or valuables through the mail unless special precautions are taken. We also don't usually send sensitive personal information on a postcard; instead, we enclose it in an envelope. Sometimes a letter is lost, but not often.
In some ways, the Internet is a less secure system than the U.S. Postal Service ("snail mail" in the computer vernacular). E-mail is usually sent across the Internet in clear text that could be read by other users. On the other hand, Internet e-mail is usually a lot quicker and perhaps more convenient and inexpensive. As such, users may be willing to accept the higher security risk when using the Internet in order to have the capabilities.
Prudent users of the Internet, however, should take precautions in two ways. First, they should take reasonable precautions to protect their files stored on their local computer or stored on the network. And second, they should take reasonable precautions to protect their data in transit on the Internet. For each user of the Internet, the reasonable level of precautions may be different, and it depends on that user's needs.
This research shows that the following basic security precautions should be sufficient for most users:
An additional recommendation for commercial Internet users is to conduct some form of risk analysis to determine the cost-effective level of security needed.
There was no indication in this research that these simple precautions would not be effective in preventing most Internet attacks.
There were very few references to viruses in the CERT/CC records. As such, this research did not indicate that virus protection was required. This research did not, however, examine problems within local area networks. Viruses can be a considerable problem within LANs, particularly for LANs with personal computers (PCs and Macintoshes). As such, an additional precaution that users on LANs with PCs and Macintoshes should take is to use virus protection software that is frequently updated.
Internet suppliers include both commercial vendors that supply hardware and software used to access the Internet and organizations such as the Internet Society and its member organizations that help establish standards for Internet protocols. As noted in the previous sections, this research gave no indication that simple precautions would not be effective in preventing most Internet security problems. Suppliers of Internet products should, therefore, ensure that their protocols and products conveniently provide Internet users with the capability of taking these simple precautions as described in the previous section. In addition, the CERT/CC incident records clearly indicate specific problem areas with respect to Internet security that should be corrected by Internet suppliers.
The recommended corrections are as follows:
This research was only a preliminary analysis of the data derived from the CERT/CC incident records during 1988 to 1995. Both the complete report of this preliminary analysis [How97] and the data set itself can be obtained from the CERT/CC [http://www.cert.org/] or from the author [firstname.lastname@example.org]. Possible research opportunities with this data set are as follows:
The findings of this research could be validated or extended through additional data. This could be accomplished as follows:
Experience during this research has also indicated there are important areas of related research that remain largely unexplored. Among these are
The CERT/CC, with the cooperation of the Sandia National Laboratories, is currently 1) developing "common language" to describe Internet incidents and vulnerabilities, 2) entering incident and vulnerability information into a "knowledge" database, and then 3) making this information available to the Internet community. It is hoped that similar methods will be adopted by other incident response teams. The ultimate objective is for the Internet community to have comparable incident and vulnerability information available near real time. For more information, contact the CERT/CC [http://www.cert.org/] or the author [email@example.com].
References in this paper are placed within brackets at the end of the referenced passage. The reference starts with three letters that identify the author(s), followed by a two-digit number for the year, a colon, and specific page numbers.
[ABH96] Derek Atkins, Paul Buis, Chris Hare, Robert Kelley, Carey Nachenberg, Anthony B. Nelson, Paul Phillips, Tim Ritchey, and William Steen, Internet Security Professional Reference, New Riders Publishing, IN, 1996.
[Amo94] Edward G. Amoroso, Fundamentals of Computer Security Technology, Prentice-Hall PTR, Upper Saddle River, NJ, 1994.
[Coh95] Frederick B. Cohen, Protection and Security on the Information Superhighway, John Wiley & Sons, New York, 1995.
[GAO96] . . . . . . . ., Information Security: Computer Attacks at Department of Defense Pose Increasing Risks, GAO/AIMD-96-84, Government Accounting Office, Washington, DC, May, 1996.
[HoR91] P. Holbrook, and J. Reynolds, editors, Site Security Handbook, RFC 1244, available on the Internet from the Internet Engineering Task Force (IETF), and at numerous other sites.
[How97] John D. Howard, An Analysis of Security Incidents on the Internet, 1989-1995, Ph.D. Dissertation, Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA, April, 1997. Available online at http://www.cert.org/.
[McK82] McKelvey, Bill, Organization Systematics: Taxonomy, Evolution, Classification, University of California Press, Berkeley, CA, 1982.
[RuG91] Deborah Russell and G. T. Gangemi, Sr., Computer Security Basics, O'Reilly & Associates, Inc., Sebastopol, CA, 1991.
[USB96] U.S. Bureau of the Census, Statistical Abstract of the United States: 1996 (116th Edition), Washington, DC, 1996.
[WhK96] Richard White and Greg Kincaid, Information Warfare: An Overview of AFIWC Operations, version 2.3, briefing at the USAF Academy, CO, February 1996