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A Sense of Self for Unix Processes

Stephanie Forrest, Steven Hofmeyr, Anil Somayaji, Thomas Longstaff
In Proceedings of the 1996 IEEE Symposium on Security and Privacy, IEEE Computer Society Press, Los Alamitos, CA, pp. 120–128
1996

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Stephanie Forrest, Steven A. Hofmeyr, Anil Somayaji
Dept. of Computer Science University of New Mexico
Albuquerque, NM 87131-1386 {forrest,steveah,soma}@cs.unm.edu

Thomas A. Longstaff
CERT Coordination Center, Software Engineering Institute
Carnegie-Mellon University Pittsburgh, PA 15213 [email protected]

Abstract

A method for anomaly detection is introduced in which “normal” is defined by short-range correlations in a process’ system calls. Initial experiments suggest that the definition is stable during normal behavior for standard UNIX programs. Further, it is able to detect several common intrusions involving sendmail and lpr. This work is part of a research program aimed at building computer security systems that incorporate the mechanisms and algorithms used by natural immune systems.

1 Introduction

We are interested in developing computer security methods that are based on the way natural immune systems distinguish self from other. Such “artificial immune systems” would have richer notions of identity and protection than those afforded by current operating systems, and they could provide a layer of general-purpose protection to augment current computer security systems. An important prerequisite of such a system is an appropriate definition of self, which is the subject of this paper. We view the use of immune system inspired methods in computer security as complementary to more traditional cryptographic and deterministic approaches. By analogy, the specific immune response is a secondary mechanism that sits behind passive barriers (e.g., the skin and mucus membranes) and other innate responses (e.g., generalized inflammatory mechanisms). In related work, we studied a number of immune system models based on these secondary mechanisms [10, 13, 11] which provide the inspiration for the project described here.

The natural immune systemhas several propertiesthat we believe are important for robust computer security. These include the following: (1) detection is distributed and each copy of the detection system is unique, (2) detection is probabilistic and on-line, and (3) detectors are designed to recognize virtually any foreign particle, not just those that have been previously seen. These properties and their significance are discussed in [11].

Previously, we developed a computer virus detection method based on these principles [11]. The method was implemented at the file-authentication level, and self was defined statically in terms of files containing programs or other protected data. However, if we want to build a general-purpose protective capability we will need a more flexible sense of self. One problem with this is that what we mean by self in a computer system seems at first to be more dynamic than in the case of natural immune systems. For example, computer users routinely load updated software, edit files, run new programs, or change their personal work habits. New users and new machines are routinely added to computer networks. In each of these cases, the normal behavior of the system is changed, sometimes dramatically, and a successful definition of self will need to accommodate these legitimate activities. An additional requirement is to identify self in such a way that the definition is sensitive to dangerous foreign activities. Immunologically, this is known as the ability to distinguish between self and other. Too narrow a definition will result in many false positives, while too broad a definition of self will be tolerant of some unacceptable activities (false negatives).

This paper reports preliminary results aimed at establishing such a definition of self for Unix processes, one in which self is treated synonymously with normal behavior. Our experiments show that short sequences of system calls in running processes generate a stable signature for normal behavior. The signature has low variance over a wide range of normal operating conditions and is specific to each different kind of process, providing clear separation between different kinds of programs. Further, the signature has a high probability of being perturbed when abnormal activities, such as attacks or attack attempts, occur. These results are significant because most prior published work on intrusion detection has relied on either a much more complex definition of normal behavior or on prior knowledge about the specific form of intrusions. We suggest that a simpler approach, such as the one described in this paper, can be effective in providing partial protection from intrusions. One advantage of a simple definition for normal behavior is the potential for implementing an on-line monitoring system that runs in real-time.

2 Related Work

There are two basic approaches to intrusion detection [16, 15]: misuse intrusion detection and anomaly intrusion detection. In misuse intrusion detection, known patterns of intrusion (intrusion signatures) are used to try to identify intrusions when they happen. In anomaly intrusion detection, it is assumed that the nature of the intrusion is unknown, but that the intrusion will result in behavior different from that normally seen in the system. Many detection systems combine both approaches, a good example being IDES [18, 4, 8]. In this paper we are concerned only with anomaly intrusion detection.

Most previous work on anomaly intrusion detection has determined profiles for user behavior. Intrusions are detected when a user behaves out of character. These anomalies are detected by using statistical profiles, as in IDES [18, 4, 8], inductive pattern generation, as in TIM [19], or neural networks [12]. Generation of user profiles by such methods requires an audit trail of actions for each user. These are typically slowly adaptive, changing profiles gradually to accommodate changing user behavior. Abrupt changes in behavior are flagged as irregular and identified with intrusions.

An alternative approach is taken by Fink, Levitt and Ko [9, 14]. Instead of trying to build up normal user profiles, they focus on determining normal behavior for privileged processes, those that run as root. They define normal behavior using a program specification language, in which the allowed operations (system calls and their parameters) of a process are formally specified. Our approach is similar to theirs, in that we consider processes that run as root. However, it differs in that we use a much simpler representation of normal behavior. We rely on examples of normal runs rather than formal specification of a program’s expected behavior, and we ignore parameter values. An advantage of our approach is that we do not have to determine a behavioral specification from the program code; we simply accumulate it by tracing normal runs of the program.

3 Defining Self

Program code stored on disk is unlikely to cause damage until it runs. System damage is caused by running programs that execute system calls. Thus, we restrict our attention to system calls in running processes. Further, we consider only privileged processes. Monitoring privileged processes has several advantages over monitoring user profiles[14]. Root processes are more dangerous than user processes because they have access to more parts of the computer system. They have a limited range of behavior, and their behavior is relatively stable over time. Also, root processes, especially those that listen to a particular port, constitute a natural boundary with respect to external probes and intrusions. However, there are some limitations. For example, it will be difficult to detect an intruder masquerading as another user (having previously obtained a legal password).

Every program implicitly specifies a set of system call sequences that it can produce. These sequences are determined by the ordering of system calls in the set of the possible execution paths through the program text. During normal execution, some subset of these sequences will be produced. For any nontrivial program, the theoretical sets of system call sequences will be huge, and it is likely that any given execution of a program will produce a complete sequence of calls that has not been observed. However, the local (short range) ordering of system calls appears to be remarkably consistent, and this suggests a simple definition of self, or normal behavior.

We define normal behavior in terms of short sequences of system calls in a running process, currently sequences of lengths 5, 6, and 11. The overallidea is to build up a separate database of normal behaviorfor each processof interest. The database willbe specificto a particular architecture,software version and configuration, local administrative policies, and usage patterns. Given the large variability in how individual systems are currently configured, patched, and used, we conjecture that these individual databases will provide a unique definition of self for most systems. Once a stable database is constructed for a given process, the database can be used to monitor the process’ ongoing behavior. The sequences of system calls form the set of normal patterns for the database, and abnormal sequences indicate anomalies in the running process.

This definition of normal behavior ignores many aspects of process behavior, such as the parameter values passed to system calls, timing information, and instruction sequences between system calls. Certain intrusions might only be detectable by examing other aspects of a process’s behavior, and so we mightneed to consider them later. Our philosophy is to see how far we can go with the simple assumption.

3.1 Details

There are two stages to the proposed algorithm. In the first stage, we scan traces of normal behavior and build up a database of characteristic normal patterns (observed sequences of system calls). Forks are traced individually, and their traces are included as part of normal.1 In the second stage, we scan new traces that might contain abnormal behavior, looking for patterns not present in the normal database. In our current implementation, analysis of traces is performed off-line.

To build up the database, we slide a window of size k+1 across the trace of system calls and record which calls follow which within the sliding window. Suppose we choose k=3 and are given the following sequence of system calls to define normal behavior:

open, read, mmap, mmap, open, getrlimit, mmap, close

As we slide the window across the sequence, we record for each call the call that follows it at position 1, at position 2, and so forth, up to position k. For the first window, from index 1 in the sequence to index 4, the following database is produced:

callposition 1position 2position 3
openreadmmapmmap
readmmapmmap 
mmapmmap  

Whenever a call occurs more than once, it can be followed by several different possible calls. These are all recorded. After sliding the window across the complete sequence, we produce this expanded database:

callposition 1position 2position 3
openread, getrlimitmmapmmap, close
readmmapmmapopen
mmapmmap, open, closeopen, getrlimitgetrlimit, mmap
getrlimitmmapclose 
close   

Once we have the database of normal patterns, we check new traces against it using the same method. We slide a window of size k+1 across the new trace, determining if the sequence of system calls differs from that recorded in the normal database. In our work to date, we simply test for the presence or absence of legal sequences. As an example, suppose that we had constructedthe abovedatabase and were given a new trace of calls, differing at one location from the normal sequence (open replaces mmap as the fourth call in the sequence):

open, read, mmap, open, open, getrlimit, mmap, close

This trace would generate 4 mismatches, because:

We record the number of mismatches as a percentage of the total possible number of mismatches. The maximum number of pairwise mismatches for a sequence of length L with a lookahead of k is:

k(L-k)+(k-1)+(k-2)+\dots+1=k(L-(k+1)/2).

In our example trace, L=8, k=3, and we have 4 mismatches. From the above formula, we get a maximum database size of 18, giving a 22% miss rate. Mismatches are the only observable that we use to distinguish normal from abnormal.

This simple algorithm can be efficiently implemented to run in O(N) time, where Nis the length of the trace (in terms of system calls). For example, our current implementation analyzes traces at an approximate rate of 1250 system calls per second.

4 Experiments

In the previous section we introduced a definition for normal behavior, based on short sequences of system calls. The usefulness of the definition will largely be determined by the answers to the following questions:

This section reports our preliminary answers to these questions. In these experiments, we focus on sendmail although we report some data for lpr. The sendmail program is sufficiently varied and complex to provide a good initial test, and there are several documented attacks against sendmail that can be used for testing. If we are successful with sendmail we conjecture that we will be successful with many other privileged Unix processes. All of our data to date have been generated on Sun SPARC stations running unpatched versions of SunOS 4.1.1 and 4.1.4, using the included sendmail. The strace package, version 3.0, was used to gather information on system calls.

4.1 Building a normal database

Although the idea of collecting traces of normal behavior sounds simple, there are a number of decisions that must be made regarding how much and what kind of normal behavior is appropriate. Specifically, should we generate an artificial set of test messages that exercises all normal modes of sendmail or should we monitor real user mail and hope that it covers the full spectrum of normal (more in the spirit of our approach)? This question is especially relevant for sendmail because its behavior is so varied. If we fail to capture all the sources of legal variations, then it will be easier to detect intrusions and be an unfair test because of false positives. We elected to use a suite of 112 artificially constructed messages, which included as many normal variations as possible. These 112 messages produced a a combined trace length of over 1.5 million system calls. For a window size of 6, the 112 messages produced a database with 6 1500 entries, where one entry corresponds to a single pair of system calls with a lookahead value (e.g., read is a legal successor to open at position 1).

Once the normal database is defined, the next decision is howto measurenew behaviorand determine if it is normal or abnormal. The easiest and most natural measure is simply to count the number of mismatches between a new trace and the database. We report these counts both as a raw number and as a percentage of the total number of matches performed in the trace, which reflects the length of the trace. Ideally, we would like these numbers to be zero for new examples of normal behavior, and for them to jump significantly when abnormalities occur. In a real system, a threshold value would need to be determined, below which a behavior is said to be normal, and above which it is deemed anomalous. In this study, we simply report the numbers, because we are not taking any action or making a binary decision based on them. Because our normal database covers most variations in normal, any mismatches are in principle significant.

Returning to our earlier questions, the size of the normal database is of interest for two reasons. First, if the database is small then it defines a compact signature for the running process that would be practical to check in real-time while the process is active. Conversely, if the database is large then our approach will be too expensive to use for on-line monitoring. Second, the size of the normal database gives an estimate of how much variability there is in the normal behavior of sendmail. This consideration is crucial because too much variability in normal would preclude detecting anomalies. In the worst case, if all possible sequences of length 6 show up as legal normal behavior, then no anomalies could ever be detected. A related question is how much normal behavior should be sampled to provide good coverage of the set of allowable sequences. We used the following procedure to build the normal database:2

  1. Enumerate potential sources of variation for normal sendmail operation.
  2. Generate example mail messages that cause sendmail to exhibit these variations.
  3. Build a normal data base from the sequences produced by step 2.
  4. Continue generating normal mail messages, recording all mismatches and adding them to the normal database as they occur.

We considered variations in message length, number of messages, message content (text, binary, encoded, encrypted), message subject line, sender/receiver and mailers. We also looked at the effects of forwarding, bounced mail and queuing. Lastly, we considered the effects of all these variations in the cases of remote and local delivery. For each test, we generated three databases, one for each different window size (5, 6 and 11). Each database incorporates all of the features described above, producing zero mismatches for mail with any of these features.

Type of Behavior# of msgs.
message length12
number of messages70
message content6
subject2
sender/receiver4
different mailers4
forwarding4
bounced mail4
queuing4
vacation2
total112

Table 1. Types and number of mail messages used to generate the normal database for sendmail.

Table 1 shows how many messages of each type were used to generate the normal databases. We began with message length and tried 12 different message lengths, ranging from 1 line to 300,000 bytes. From this, we selected the shortest length that produced the most varied pattern of system calls (50,000 bytes), and then used that as the standard message length for the remaining test messages. Similarly, with the number of messages in a sendmail run, we first sent 1 message and traced sendmail then we sent 5 messages, tracing sendmail, and so forth, up to 20 messages. This was intended to test sendmail’s response to bursts of messages. We tested message content by sending messages containing ascii text, uuencoded data, gzipped data, and a pgp encrypted file. In each case, a number of variations was tested and a single default was selected before moving on to the next stage. These messages constituted our corpus of normal behavior. We reran this set of standard messages on each different OS and sendmail.cf variant that we tried, thus generating a normal database that was tailored to the exact operating conditions under which sendmail was running. Of the features considered, the following seemed to have little or no effect: the number of messages, message content, subject line, senders/receivers, mailers and queuing. Two more unusual features, forwarded mail and bounced mail, affected remote traces far less than local traces.

Figure 1 shows how new patterns are added to the database over time during a normal sendmail run. The data shown are for 10,000 system calls worth of behavior, but we have also performed runs out to 1.5 million system calls (data not shown), with essentially zero mismatches. Overall, the variability in the behavior of sendmail at the system call level is much smaller than we expected.

Finally, we ask what percentage of the total possible patterns (for sequences of length 6) is covered by the normal database. For example, if the database is completely full (all possible patterns have been recorded as normal) by 3000 system calls, then it would hardly be surprising that no new patterns are seen over time. However, as we discussed earlier, such variability would be useless for identifying anomalous behavior. Consequently, the goal is to find a database that is small with respect to the space of possible patterns. Our initial data here are encouraging. We estimate that the sendmail database described above covers about 5\times 10^{-5}% of the total possible patterns of system calls (that is, sequences built from all possible system calls, about 180 for Unix, not just those invoked by sendmail), an extremely small fraction. This figure is somewhat misleading, however, because it is unlikely that the sendmail program is capable of generating many of these sequences. The most accurate comparison would be against a database that contained all the patterns that sendmail could possibly produce. This would require a detailed analysis of the sendmail source code, an area of future investigation.

Process5611
%#%#%#
sendmail0.000.000.00
ls6.9238.93413.993
ls -l30.023932.130438.0640
ls -a6.7238.33413.493
ps1.2358.328213.0804
ps -ux0.8458.156412.91641
finger4.6214.9275.754
ping13.55614.27015.5131
ftp28.845031.558735.11182
pine25.4152227.6198430.03931
httpd4.33104.84364.7824

Table 2. Distinguishing sendmail from other processes. Each column lists two numbers: the percentage of abnormal sequences (labeled %) and the number of abnormal sequences (labeled #) in one typical trace of each process (when compared against the normal database for sendmail). The columns labeled 5, 6 and 11 refer to the sequence length (window size) used for analysis. The sendmail data show no misses, because sendmail is being compared against its own database.

4.2 Distinguishing Between Processes

To determine how the behavior of sendmail compares with that of other processes, we tested several common processes against the normal sendmail database with 1500 entries. Table 2 compares normal traces of several common processes with those of sendmail. These processes have a significant number of abnormal sequences, approximately, 5–32% for sequences of length 6, because the actions they perform are considerably different from those of sendmail. We also tested the normal database for lpr and achieved similar results (data not shown). lpr shows even more separation than that shown in Figure 2, presumably because it is a smaller program with more limited behavior. These results suggest that the behavior of different processes is easily distinguishable using sequence information alone.

4.3 Anomalous Behavior

We generated traces of three types of behavior that differ from that of normal sendmail: traces of successful sendmail intrusions, traces of sendmail intrusion attempts that failed, and traces of error conditions. In each of these cases, we compared the trace with the normal sendmail database and recorded the number of mismatches. In addition,we tested one successfullpr intrusion and compared its trace with a normal database for lpr. Table 3 shows the results of these comparisons. Each row in the table reports data for one typical trace. In most cases, we have conducted multiple runs of the intrusion with identical or nearly identical results.

Figure 1. Building a normal database. The graph shows how many new patterns are added to the database over time. By running our artificially constructed set of standard messages, a wide variety of normal behavior is seen in the early part of the run (out to about 3000 system calls). After this time, virtually no new patterns are encountered under normal sendmail conditions. These data are a concatenation of the logs used to generate our normal database.

Figure 1. Building a normal database. The graph shows how many new patterns are added to the database over time. By running our artificially constructed set of standard messages, a wide variety of normal behavior is seen in the early part of the run (out to about 3000 system calls). After this time, virtually no new patterns are encountered under normal sendmail conditions. These data are a concatenation of the logs used to generate our normal database.

To date, we have been able to execute and trace four attacks: sunsendmailcp [1], a syslog attack script [2, 7], a decode alias attack, and lprcp [3].

The sunsendmailcp script uses a special command line option to cause sendmail to append an email message to a file. By using this script on a file such as /.rhosts, a local user may obtain root access.

The syslog attack uses the syslog interface to overflow a buffer in sendmail. A message is sent to the sendmail on the victim machine, causing it to log a very long, specially created error message. The log entry overflows a buffer in sendmail, replacing part of the sendmail’s running image with the attacker’s machine code. The new code is then executed,causing the standard I/O of a root-owned shell to be attached to a port. The attacker may then attach to this port at her leisure. This attack can be run either locally or remotely, and we have tested both modes. We also varied the number of commands issued as root after a successful attack.

In older sendmail installations, the alias database contains an entry called “decode,” which resolves to uudecode, a UNIX program that converts a binary file encoded in plain text into its original form and name. uudecode respects absolute filenames, so if a file "bar.uu" says that the original file is "/home/foo/.rhosts," then when uudecode is given “bar.uu,” it will attempt to create foo’s .rhosts file. sendmail will generally run uudecode as the semi-privileged user daemon, so email sent to decode cannot overwrite any file on the system; however, if a file is world-writable, the decode alias entry allows these files to be modified by a remote user.

Anomaly5611
%#%#%#
sunsendmailcp3.8724.1955.2215
syslog:
remote 13.93614.24705.11047
remote 21.41111.51371.7286
local 13.02354.23984.0688
local 24.13073.43095.3883
decode0.3210.3240.357
lprcp1.171.4122.231
sm565a0.4270.4360.689
sm5x1.41101.71572.7453
forward loop1.6431.8582.3138

Table 3. Detecting Anomalies. The table shows the results of tracing sendmail and lpr during various anomalous situations: successful intrusions (sunsendmailcp, syslog, decode, and lprcp), unsuccessful intrusions, (sm565a and sm5x), and abnormal errors (forward loop). The data for the syslogd attack show the results of tracing sendmail (rather than tracing syslogd itself). The % column indicates the percentage of abnormal sequences in one typical intrusion, and the # column indicates the number of abnormal sequences.

The lprcp attack script uses lpr to replace the contents of an arbitrary file with those of another. This exploit uses the fact that older versions of lpr use only 1000 different names for printer queue files, and they do not remove the old queue files before reusing them. The attack consists of getting lpr to place a symbolic link to the victim file in the queue, incrementing lpr’s counter 1000 times, and then printing the new file, overwriting the victim file’s contents.

The results for these four attacks are shown in Table 3. The sunsendmailcp exploit is clearly detected with 5.2% anomalous sequences (for length 11). Likewise, the syslog attack is clearly detected in every run, with the anomalous sequence rate varying between 1.7% and 5.3%, for a sequence window of 6. The decode attack is less detectable at 0.3%, and the lpr attack is detected at 2.2%.

A second source of anomalous behavior comes from unsuccessful intrusion attempts. We tested two remote attack scripts, called sm565a and sm5x [5, 6]. SunOS 4.1.4 has patches that prevent these particular intrusions. The results are shown in Table 3. Overall, the percentage of abnormal sequences is on the low end of the range for successful attacks.

Error conditions provide a third source of anomalous behavior. In general, it would be desirable if error conditions produced less deviation from normal than intrusions but were still detectable. For the one case we have studied, a local forwarding loop, this is what we observed (excluding the decode and lpr exploits). A forwarding loop occurs when a set of $HOME/.forward files form a logical circle. We considered the simplest case, with the following setup:

Email address.forward file
[email protected][email protected]
[email protected][email protected]

Although forwarding loops are not malicious, they can adversely affect machine performance, as hundreds of messages are bounced from machine to machine. Results are reported in Table 3. They differ from normal by a small, yet clear, percentage (2.3%).

5 Discussion

These preliminary experiments suggest that short sequences of system calls define a stable signature that can detect some common sources of anomalous behavior in sendmail and lpr. Because our measure is easy to compute and is relatively modest in storage requirements, it could be plausibly implemented as an on-line system, in which the kernel checked each system call made by processes running as root. Under this scheme, each site would generate its own normal database, based on the local software/hardware configuration and usage patterns. This could be achieved either with a standard set of artificial messages, such as those we use to build our normal database, or it could be completely determined by local usage patterns. It is likely that some combination of the two would be most effective.

The data reported in this paper are preliminary. In addition to testing other processes, especially those that are common routes for intrusion, we would like to extend our sendmail experiments in several ways. These include: testing additional sendmail exploits, conducting systematic runs on common sendmail and sendmail.cf variants, and testing the effect of other system configurations on the normal behavior of sendmail. Another area for further study is the database of normal behavior, for example, how do we choose what behavior to trace? This is especially relevant for sendmail because its behavior is so varied. If we fail to capture all the sources of legal variation, then we will be subject to false positives. On the other hand, if we allow different databases at different sites, then some variation would be desirable both as customizations to local conditions and to satisfy the uniqueness principle stated earlier. Finally, we would like to study the normal behavior of sendmail running on a regularly used mail server. Such real-world data would help confirm the nature of sendmail’s normal behavior in practice, especially when compared with our set of artificial messages.

Our approach is predicated on two important properties: (1) the sequence of system calls executed by a program is locally consistent during normal operation, and (2) some unusual short sequences of system calls will be executed when a security hole in a program is exploited. We believe that there is a good chance that the former condition will be met by many programs, simply because the code of most programs is static, and system calls occur at fixed places within the code. Conditionals and function calls will change the relative orderings of the invoked system calls but not necessarily add variation to short-range correlations. We are also optimistic about the second property. If a program enters an unusual error state during an attempted break-in, and if this error condition executes a sequence of system calls that is not already covered by our normal database, we are likely to notice the attack. Also, if code is replaced inside a running program by an intruder, it would likely execute a sequence of system calls not in the normal database, and we would expect to see some misses. Finally, it is highly likely that a successful intruder will need to fork a new process in order to take advantage of the system. This fork, when it occurs, should be detectable.

However, if an intrusion does not fit into either of these two categories, our method will almost certainly miss it under the current definition of normal. For example, we do not expect to detect race condition attacks. Typically, these types of intrusions involve stealing a resource (such as a file) created by a program running as root, before the program has had a chance to restrict access to the resource. If the root process does not detect an unusual error, a normal set of system calls will be made, defeating our method. This is an important avenue of attack, one that will require a revised definition of “self.” A second kind of intrusion that we will likely miss is the case of an intruder using another user’s account. User profiles can potentially provide coverage for this classof intrusions which are not likely to be detectable in root processes. Although the method we describe here will not provide a cryptographicallystrong or completely reliable discriminator between normal and abnormal behavior, it could potentially provide a lightweight, real-time tool for continuously checking executing code based on frequency of execution.

To achieve reliable discrimination, we need to ensure that our method of flagging sequences as abnormal does not produce too many false negatives or false positives. Currently, we record both the number of absolute misses (of a monitored process against the normal database) as well as the percentage of misses (out of the total number of calls in a trace). Most of the exploits we have studied are very short in terms of the length of time the anomalous behavior takes place. There might be other more appropriate measures than the two we have used, especially in an on-line system, where the length of the trace would be indefinitely long. A related question is the choice of pattern matching rule. We currently monitor only the presence or absence of patterns, not their relative frequency. However, there are many other matching criteria that could be tried. For example, we could represent the possible sequences of legal system calls as a Markov chain and define a criterion based on expected frequencies.

Returning to the larger question of how to build an artificial immune system for a computer, the work reported here constitutes an initial step in this direction. We have identified a signature for self that is stable across a wide variety of normal behavior and sensitive to some common sources of anomalies. Further, the definition provides a unique signature, or identity, for different kinds of processes. A second form of identity is possible because the method used to collect normal traces allows for a unique database at each site. Thus, a successful intrusion at one site would not necessarily be successful at all sites running the same software, and it would increase the chance of at least one site noticing an attack. Networks of computers are currently vulnerable to intrusions at least in part because of homogeneities in the software they run and the methods used to protect them. Our behavioral notion of identity goes well beyond a simple checksum, login, password, or IP address, because it considers dynamic patterns of activity rather than just static components.

However, the current system is a long way from having the capabilities of a natural immune system. We would like to use the definition of self presented here as the basis of future work along these lines. For example, we are not yet using any partial or approximate matching, such as that used by the immune system, and we are not using on-line learning, as in the case of affinity maturation or negative selection. The immune system uses a host of different mechanisms for protection, each specialized to deal with a different type of intruder. A computer immune system could mimic this by incorporating additional mechanisms to provide more comprehensive security. For example, it might be possible to include Kumar’s misuse intrusion detection methods [17, 15] in the form of “memory cells” that store signatures of known intrusions. Finally, we have made no provision for the definition of self to change over time, although the natural immune system is continually replenishing its protective cells and molecules.

6 Conclusions

This paper outlines an approach to intrusion detection that is quite different from other efforts to date, one that appears promising both in its simplicity and its practicality. We have proposed a method for defining self for privileged Unix processes, in terms of normal patterns of short sequences of system calls. We have shown that the definition is compact with respect to the space of possible sequences, that it clearly distinguishes between different kinds of processes, and that it is perturbed by several different classes of anomalous, or foreign, behavior, allowing these anomalies to be detected. The results in this paper are preliminary, and there are attacks that our method is not able to detect. However, it is computationally efficient to monitor short-range orderings of system calls, and this technique could potentially provide the basis for an on-line computer immune system consisting of several detection mechanisms, some inspired by the human immune system, and others derived from cryptographic techniques and more traditional intrusion detection systems.

7 Acknowledgments

The authors thank David Ackley, Patrik D’haeseleer, Andrew Kosoresow, Arthur B. Maccabe, Kevin McCurley, and Nelson Minar for many helpful discussions, ideas, and criticisms. We also thank Jim Herbeck for supporting our neverending need to fiddle with our systems, and Larry Rodgers at the Computer Emergency Response Team (CERT) for helping with the syslog attack. The idea for developing a computer immune system grew out of an ongoing collaboration with Dr. Alan Perelson through the Santa Fe Institute, and some of the experiments were performed using the computer facilities and expertise at CERT. This work is supported by grants from the National Science Foundation (IRI-9157644), Office of Naval Research (N00014-95-1-0364),and Interval Research Corporation.

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1 Due to a limitation of our tracing package, we are not currently following virtual forks.

2 We followed a similar procedure to generate the normal database for lpr and obtained a database of 534 normal patterns.

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