Review of Intrusion and Anomaly Detection Techniques

International
OPEN ACCESS Journal
Of Modern Engineering Research (IJMER)
| IJMER | ISSN: 2249–6645 | www.ijmer.com | Vol. 4 | Iss. 4| Apr. 2014 | 1 |
Review of Intrusion and Anomaly Detection Techniques
Qamar Rayees Khan1
, Muheet Ahmed Butt2
, Majid Zaman3
,
Mohammad Asger4
1, 4
Baba Ghulam Shah Badshah University, Rajouri (J&K), India.
2, 3
University of Kashmir, Srinagar, (J&K), India
I. Introduction
Intrusion detection starts with instrumentation of a computer network for data collection. Pattern-based
software sensors monitor the network traffic and raise alarms when the traffic matches a saved pattern. Security
analysts decide whether these alarms indicate an event serious to warrant a response. A response might be to
shut down a part of a network, to phone the internet service provider associated with suspicious traffic, or to
simple make note of usual traffic for future reference [19].
This paper presents overview in the area of intrusion detection. We have divided the work into three
sections: the first section analyzes anomaly detection systems and their performance. It gives a short overview of
statistical methods for anomaly detection, Predictive pattern generation for anomaly detection and program
based anomaly detection. The second section presents an overview of misuse detection techniques: language
base misuse detection, expert systems and high level state machines for misuse detection. The third section
presents an overview of specification based approaches to intrusion detection: process, behavior monitoring and
process state analysis. Each of the subsections gives a short overview of the techniques of interest and then
presents the data set used.
Background
If the network is small and signatures are kept up to date, the human analyst solution to intrusion
detection works well. But when organizations have a large, complex network the human analyst quickly become
overwhelmed by the number of alarms they need to review [19]. The sensors on the large networks, will
generate over millions alarm per day and that number is increasing. This situation arises from ever increasing
attacks on the commercial tools typically do not provide an enterprise level view of alarms generated by
multiple sensor vendors. Commercial intrusion detection software packages tend to be signature oriented with
little or no state information maintained. These limitations led us to investigate the application of data mining to
this problem.
A secure network must provide the following:
1. Data Confidentiality: Data that are being transferred through the network should be accessible only to
those that have been properly authorized.
Data integrity: Data should maintain their integrity from the moment they are transmitted to
the moment they are actually received. No corruption or data loss is accepted either from random
events or malicious activity.
2. Data availability: The network should be resilient to Denial of Service attacks.
The first threat for a computer network system was realized in 1988 when 23-year old Robert Morris
launched the first worm, which override over 6000 PCs of the ARPANET network. On February 7th, 2000
the first DoS attacks of great volume where launched, targeting the computer systems of large companies
like Yahoo!, eBay, Amazon, CNN, ZDnet and Dadet. More details on these attacks can be found at [20].
Abstract: Intrusion detection is the act of detecting actions that attempt to compromise the
confidentiality, integrity or availability of a resource. With the tremendous growth of network-based
services and sensitive information on networks, network security is getting more and more importance
than ever. Intrusion poses a serious security threat in a huge network environment. The increasing use of
internet has dramatically added to the growing number of threats that inhabit within it. Intrusion
detection does not, in general, include prevention of intrusions. Now a days Network intrusion detection
systems have become a standard component in the area of security infrastructure. This review paper tries
to discusses various techniques which are already being used for intrusion detection.
Keywords: Anomaly, Data Confidentiality, False Positive, Intrusion, Misuse.

The ideal approach to securing the network is to remove all security flaws from individual hosts,
but less expensive and more feasible approaches are used for securing the computer systems. Most
computer systems have some kind of security flaw that may allow intruders or legitimate users to
gain unauthorized access to sensitive information. Moreover, many computer systems have widely
known security flaws that are not fixed due to cost and some other limitations. Even a supposedly
secure system or network can still be vulnerable to insiders misusing their privileges or it can be
compromised by improper operating practices. So the ideal approach to secure a system can be to identify
the process (system) with abnormal behavior and try to fix it.
II. Techniques
The two basic approaches to intrusion detection are misuse intrusion detection and anomaly
intrusion detection. In misuse intrusion detection, know patterns of intrusion (intrusion signatures)
are used to try to indentify 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
1.1 Anomaly Detection
1.1.1 Statistical Methods for Anomaly Detection
In statistical Method for Anomaly detection, the NIDS observes the activity of systems under
study and generates some valuable information that is stored in the form of profiles to represent the
behavior of the system. The profile contains four different sets of measures i.e. activity intensity
measure, audit record distribution measure, categorical measures and ordinal measures. At every point of
time two profiles are maintained for each system a current profile and a stored profile. As the records
are processed the NIDS updates the current profile and calculates the abnormal behavior if any by
comparing the current profile with the stored profile using a function of abnormality for all the measures in
the profile. The stored profile is updated from time to time by merging it with the current profile.
Statistical methods suffers from a major drawback i.e. a skilled attacker can train the method so that it can
accept an abnormal behavior as normal secondly not all the behaviors can be modeled as (Kumar 95).
a) Haystack
Haystack [1] is an example of statistical anomaly-based IDS. It uses both user and group-based
anomaly detection. It models system parameters as independent, Gaussian random variables. Parameters are
considered as abnormal when they fall out of 90% of the data range for the variable. It maintains a database
of user groups and individual profiles. If a user has not previously been detected, a new user profile with
minimal capabilities is created using restrictions based on the user's group membership. It is designed
to detect six types of intrusions: attempted break-ins by unauthorized users, masquerade attacks,
penetration of the security control system, leakage, DoS attacks and malicious use.
b) Bayesian classification
Bayesian classification automatically determines the most probable number of classes for the data,
continuous and discrete measures can be freely mixed. On the other hand, this approach does not
take into account the interdependence of the measures, it is unclear whether the algorithm can perform
classification incrementally as needed for on-line processing, suffers from difficulty to determine
thresholds, susceptibility to be trained by a skillful attacker etc.
1.1.2 Predictive Pattern Generation for Anomaly Detection
Predictive pattern generation for anomaly detection takes into account the relationship and
ordering between events. It assumes that events are not random, but follow a distinguishable pattern. Such
methods learn to predict future activities based on the most recent sequence of events and flag events that
deviate significantly from the predicted.
a) Machine learning
T. Lane and C. Brodley applied the machine learning approach to anomaly detection in [2]. They
built user profiles based on input sequences and compared current input sequences to stored profiles using a
similarity measure. To form a user profile, the approach learns characteristic sequences of actions
generated by users. For learning patterns of actions the system uses the sequence as the unit of comparison. A
sequence is defined as an ordered, fixed-length set of temporally adjacent actions, where actions
consist of UNIX shell commands and their arguments. To characterize user behavior they used only

positive examples. For the purpose of classification, sequences of new actions are classified as
consistent or inconsistent in reference to sequence history using a similarity measure. To classify the
attack, they classify the similarity of each input sequence, yielding a stream of similarity measures. If the
mean value of the current window is greater than the threshold then it is classified as normal.
b). Time-based inductive generalization
Teng, Chen and Lu [3] developed a technique that applies a time-based inductive learning
approach to security audit trail analysis. The system developed sequential rules. The technique uses
time-based inductive engine to generate rule-based patterns that characterize the normal behavior of users.
Each event is one single entry in the audit trail and is described in terms of a number of attributes (event
type, image name, object name, object type, privileges used, status of execution, process ID, etc). The
events in the audit trail are considered to be sequentially related. Rules identify common sequences of events
and the probability of each sequence being followed by other events. Rules with a high level of accuracy and
high confidence are kept, less useful rules are dropped. They define two types of anomaly detection:
deviation detection and detection of unrecognized activities. Low probability events are also flagged,
using a threshold. Anomaly is detected if either deviation or unrecognized activity is detected. This
method can identify the relevant security items as suspicious rather than the entire login session and it
can detect some anomalies that are difficult to detect with traditional approaches. It's highly adaptive to
changes, can detect users who try to train the system, activities can be detected and reported within
seconds. On the other hand, some unrecognized patterns might not be flagged since the initial, predicting
portion of the rule may be matched.
2.1.3. Program-Based Anomaly Detection
a) Program Modeling
The program modeling approach (Garth Barbour Ph.D. thesis [4]) presents an algorithm that
efficiently learns the program behavior. The author claims that the number of false positives never
increases with additional training. If there is a false positive, the representation can learn to accept the run
in the future without negatively impacting the performance of other runs. He also claims that his
technique can detect novel attacks as they occur, without manual specification of program behavior or
attack signatures. His algorithm learns an approximation of the program's behavior. The number of
false positives decreases with training set size. He uses a non-statistical method since statistical
methods would miss minor deviations.Barbour presents an algorithm that learns program behavior, without
any input parameters presented in addition to his algorithm, which would add more reliability to his
results. Input parameters may add additional reliability to detection because in programs with
significant number of system calls, we may have different arguments associated with those system calls.
If a program with limited capabilities is observed, it does not generate too many different system calls and,
hence it is easy to detect any misuse of that program. On the other hand, if we are dealing with a
powerful program, it generates a significant number of system calls and therefore, it is more difficult to
detect misuse of that program.
b) Computational Immunology
Stephanie Forrest is investigating anomaly detection on system processes from the perspective of
immunology [6], [7], [8]. In [6] the authors base their approach on the immunological mechanism of
distinguishing self from non- self and [7] use look ahead pairs. They take short sequences of system calls,
called n-grams that represent samples of normal runs of programs and compare them to sequences of system
calls made by a test program. If any run has a number of mismatches that is higher than a certain number
(percent), it is flagged as anomalous. They extended their work to variable length sequences, based on
random generation of examples of invalid network traffic to detect anomalous behavior. Unlike Barbour
[4], where there is no delay in detecting anomalies, the approach of Forrest has a certain delay due to the
fact that the program computes the percent mismatch to be able to flag or not flag the run as anomalous.
Hofmeyr [7] used the rule of r contiguous bits and showed that fixed length sequences give better
discrimination than look-ahead pairs. Christina Warrender [8] modelled normal behavior of data sets
described below using stide, t-stide, RIPPER and HMMs. She compared learning methods for application to
n-grams: enumerating sequences using look-ahead pairs and contiguous sequences, methods based on relative
frequency of different sequences, RIPPER (a rule-learning system) and HMMs. They used RIPPER to learn
rules representing the set of n-grams. Training samples consisted of set of attributes describing the
object to be classified and a target class to which the object belongs. RIPPER takes training samples and forms
a list of rules to describe normal sequences. For each rule a violation score is established. Each set that
violates a high confidence rule is a mismatch. They created HMMs so that there is one state for each n-gram.

The results showed that HMMs performed marginally better than other methods. She showed that HMMs
performed only marginally better than other simple and not computationally expensive methods and
concluded that the choice of data stream (short sequences of system calls) was more important than the
particular method of analysis.
c). Variable-length audit trail patterns
Wespi [9] points out that usage of fixed-length patterns to represent the process model in [6]
has no rule for selecting the optimal pattern length. On the other hand, Forrest et.al. [7], [8] point out that
the pattern length has an influence on the detection capabilities of the IDS. Wespi eta Initially were
not able to prove significant advantage of variable-length patterns in [10]. Later, in [9] they
showed that the previously stated method outperforms the method of Forrest et. Al [7], [8]. They use
functionality verification test (FVT) suites provided by application developers to generate a training set based on
the normal program's specified behaviors. Their system consists of an off-line and on-line part.
In the next step they apply the Teiresias algorithm [11] to generate the list of maximal variable-length
patterns followed by some pruning during their preprocessing. During operation, a recursive pattern
matching algorithm attempts to find the best matches for the test run. The measure of anomaly is the longest
sequence of unmatched events (more than six consecutive unmatched events are treated as an intrusion). Initial work
generated variable-length patterns using a suffix-tree constructor, a pattern pruner, and a pattern selector. ID based on
this technique can detect the attacks but is prone to issue false alarms. Then they used a modified version of the
pattern generator to create variable-length patterns that reduce the number of false alarms substantially without
reducing the ability to detect intrusions. The authors claim that their algorithm performs better than the fixed-
length sequences of Stephanie Forrest, but they compare their algorithm only for the ftpd process and then claim
that their algorithm outperforms algorithms presented in [6],[7]. This is arguable because it might be the case that they
presented one example where their algorithm outperformed the other algorithms and did not present the instances
where their algorithm failed. Hence, we can conclude that on that specific process algorithm of Wespi et. al. [9] did
outperform the algorithm of Forrest et. al. [6], [7] but, on the other hand, that may or may not be true for other
processes.
d) Program Behavior Profiles
The work of Ghosh et. al. [5] is based on the work of the UNM group. The goal of their approach was to
employ machine learning techniques that can generalize from past behavior so that they are able to detect new
attacks and reduce the false positive rate. The first approach they used was equality matching. The database used was the
normal behavior of programs. Instead of using strace, they use BSM data to collect the system call information and
collect them into fixed size windows. They used the fact that the attacks cause anomalous behavior in clusters, like
Kosoresow in [12]. Their decision choice looks for local clustering of the mismatches. The second model was the feed-
forward topology with back propagation learning rules. Database was not on per-user basis like in [2], but they built
profiles of software behavior and malicious software behavior. During training many networks were trained for
each program and the network that performed the best was selected. The training process consisted of exposing
networks to four weeks of labeled data and performing back propagation algorithm to adjust weights. They trained
Artificial Neural Networks (ANN) to recognize whether small, fixed sequences of events are characteristic of the
programs in which they occur. For each sequence, the ANN produces an output value that represents how anomalous
the sequence is the leaky bucket algorithm was used to classify the program behavior and using leaky bucket they made
use of the rule that two close anomalies have higher influence than when they are apart. The final approach was
Elman network, developed by Jeffrey Elman, since they wanted to add information about the prior sequences
(DFA approach lacked flexibility and ANNs have the ability to learn and generalize). In order to generalize they
employ a recurrent ANN topology of an Elman network. They train the network to predict the next sequence that
will occur at any point in time. The Elman network approach had the best results, with 77.3% detection and no false
positives and 100% detection and fewer false positives than in two other cases. They also tried two approaches in
which they used FSMs for their representation. Audit data was condensed into a stream of discrete events, where
events are system calls recorded in data. Separate automata are constructed for different programs whose audit data
are available for training. The training algorithm is presented with a series of n-grams taken from non-intrusive
BSM data for a given program. The audit data is split into sub-sequences of size n + I (n elements define a state and /
elements are used to label a transition coming out of that state). The second FA-like approach, called a string
transducer makes an attempt to detect subtler statistical deviations from normal behavior. It associates a sequence
of input symbols with a series of output symbols. During training they estimate the probability distribution of the
symbols at each state and during testing, deviations fr8m this probability distribution indicate anomalous behavior. They
use FA whose states correspond to n-grams in the BSM data. For each state they also record information about
successor /-grams that are observed in the training data. During training their goal is to gather statistics about
successor /-grams. They estimate the probability of each /-gram by counting.

1.1.3 Anomaly detection using data mining
The ADAM (Audit Data Analysis and Mining) system [13] is an anomaly detection system. It uses a module
that classifies the suspicious events into false alarms or real attacks. It uses data mining to build a customizable profile
of rules of normal behavior and then classifies attacks (by name) or declares false alarms. ADAM is a
real-time system. To discover attacks in TCPdump audit trail, ADAM uses a combination of association
rules, mining and classification. The system builds a repository of normal frequent item sets that hold
during attack-free periods. Then it runs a sliding window online algorithm that finds frequent item sets in
the last D connections and compares them with those stored in the normal item set repository. With
the rest, ADAM uses a classifier which has previously been trained to classify the suspicious
connections as a known type of attack, unknown type or a false alarm.
Association rules are used to gather necessary knowledge about the nature of the audit data. They derive a set
of rules in form X —> Y, where X and Y are sets of attributes. There are two parameters associated with a rule:
support s and confidence c. The definitions of s and c are as follows. The rule x Y has support s in the
transition set T if s% of transactions in T contains X or Y. The rule x 4 Y has confidence c if c% of
transactions in T that contain X also contain Y. If the item set's support surpasses a threshold, that item set is
reported as suspicious. The system annotates suspicious item sets with a vector of parameters. Since the
system knows where the attacks are in the training set, the corresponding suspicious item set along with
their feature vectors are used to train a classifier. The trained classifier will be able to, given a
suspicious item set and a vector of features, classify it as a known attack (and label it with the name of
attack), as an unknown attack or a false alarm.
1.2 Misuse Detection
1.2.1 Language-based Misuse Detection
Language-based Misuse Detection systems accept a description of the intrusions in a formal
language and use this to monitor for the intrusions. Most languages for misuse systems, including the one
used by NIDES, are low-level and have limited expressiveness.
a) ASAX
Habra et. al. define Advanced Security audit trail Analysis for universal audit trail analysis [14]. ASAX
(Advanced Security audit trail Analysis on UNIX) uses RUSSEL, a rule-based language, specifically
appropriate for audit trail analysis. ASAX sequentially analyzes records using a collection of rules that
are applied to each audit record. A subset of rules is active at any time. They define a normalized audit
data format (NADF) as a canonical format of the Operating System's audit trail.
1.2.2 Expert Systems
Expert systems use lists of conditions that, if satisfied, indicate that an intrusion is taking place. The
conditions are rules that are evaluated based on system or network events. These rules are specified by experts
familiar with the intrusions, generally working closely with the developers of the system.
1.2.3 High-level state machines for misuse detection
The State Transition Analysis Tool (STAT) [15] describes computer penetrations as attack scenarios. It
represents attacks as a sequence of actions that cause transitions that lead from a safe state to a compromised
state. A state represents snapshot of the system's security-relevant properties that are characterized by
means of assertions, which are predicates on some aspects of the security state of the system. The initial
state of a transition diagram does not have any assertions. Each state transition diagram starts with a
signature action that triggers monitoring of the intrusion scenario. Transitions between states are labeled
by the actions required to switch from one state to another. These actions do not necessarily correspond
to audit records. The resulting state transition diagram forms the basis of a rule-based intrusion detection
algorithm.
a). USTAT [16] is an implementation of the STAT tool developed for Solaris BSM. It reads specifications of
the state transitions necessary to complete an intrusion and evaluates an audit trail. Two preconditions must be
met to use
USTAT: the intrusion must have a visible effect on the system state and the visible effect must be recognizable
without knowing the attacker's true identity.
b). STATL [17] is a language that allows description of computer penetrations as sequences of actions that an
attacker performs to compromise a computer system. The attack is modeled as a sequence of steps that
bring a system from an initial safe state to a final compromised state. An attack is represented as a set of
states and transitions, where each transition has an associated action. The notion of timers is used to

express attacks in which some events must happen with an interval following some other event (set of
events). Times are declared as variables using the built-in type timer. STAT can detect cooperative attacks
and attacks that span multiple user sessions, can specify rules at higher level than audit records, is easier to
create and maintain than other rule-based methods, can represent a partial ordering among actions, can
represent longer scenarios than other rule-based systems. On the other hand, it has no general-purpose
mechanism to prune partial matches of attacks other than assertion primitives built into the model.
1.2.4 Emerald
EMERALD (Event monitoring enabling responses to anomalous live disturbances) [18] employs
both anomaly and misuse detection. It includes service analysis that covers the misuse of individual
components and network services within a single domain, domain-wide analysis that covers misuse
visible across multiple services and components and enterprise-wide analysis that covers coordinated
misuse across multiple domains. They also introduce the notion of service monitors that provide
localized analysis of infrastructure and services. EMERALD consists of three analysis units: profiler
engines, signature engines and resolver. Profiler engine performs statistical profile-based anomaly detection
given a generalized event stream of an analysis target. Signature engine requires minimal state management
and employs a rule-coding scheme. The profiler and signature engines receive large volumes of event
logs and produce a smaller volume of intrusion/suspicion reports and send that data to the resolver.
The signature analysis subsystem allows administrators to instantiate a rule set customized to detect
known problem activity occurring on the analysis target.
III. Conclusion
This paper elaborates the foundations of the main anomaly based network intrusion detection
technologies. Considering the surveyed literature, it is clear that in order to be able to secure a
network against the novel attacks, the anomaly based intrusion detection is the best way out. However,
due to its immaturity there are still problems with respect to its reliability. These problems will lead to
high false positives in any anomaly-based IDS. In order to solve this problem, usually different anomaly
detection techniques can be used. Method like Haystack can be used to detect a different set of attacks
like break-in's Dos etc. In the same way some authors have used Machine Learning, Program based, and
time based approaches for network intrusion detection. All the approaches that have been surveyed in the
work perform better from one another in some cases but above all the techniques still need some
improvements in terms of false positive rates and true negative rate.
However, there are still many challenges to overcome in the field of intrusion detection, But
the presented information constitutes an important point to start for addressing Research & Development
in the field of IDS. We find that the majority of surveyed works can be used as starting point for
continuing research work in the field of intrusion detection.
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[20] http://www.securitvfocus.com/news/2445 .

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