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A Framework for Constructing
Features and Models for Intrusion
Detection Systems
Authors: Wenke Lee & Salvatore J.Stolfo
Published in ACM Transactions on Information and System
Security, Volume 3, Number 4, 2000.
Presented By Suchandra Goswami
Contributions of this work
• Development of automated Intrusion Detection
Systems (IDSs) rather than pure knowledge
encoding and engineering approaches
• Provides a novel framework called MADAM ID,
for Mining Audit Data for Automated Models for
Intrusion Detection
• First work applying data mining and machine
learning algorithms for IDSs
Introduction
• Intrusion Detection (ID) is the art of detecting
inappropriate, incorrect, or anomalous activity.
• ID systems that operate on a host to detect
malicious activity on that host are called hostbased ID systems
• ID systems that operate on network data flows
are called network-based ID systems.
• statistical anomaly detection and patternmatching most commonly used for ID
• Two main ID techniques: misuse detection and
anomaly detection
• Misuse detection – Use patterns of well-known
attacks or weak spots of the system to match
and identify known intrusions
• E.g, signature rule for “guessing password
attack” will be “there are more than 4 failed login
attempts within 2 minutes”
• Anomaly detection – Flag observed activities
that deviate significantly from the established
normal usage profiles.
• This research takes a data centric point of view
and ID is considered to be a data analysis
process
• Data mining programs used to compute models
that accurately capture actual behavior
(patterns)
• Eliminates the need to manually analyze and
encode intrusion patterns
• Validated with large amounts of audit data
Some Data Mining techniques and
its application to IDSs
• Classification – maps data items into one of
several pre-defined categories. Algorithms
generally output classifiers like decision trees or
rules.
• E.g, gather sufficient “normal” and “abnormal”
audit data. Apply classification algorithm to learn
a classifier that can label unseen audit data
normal or abnormal
• Link Analysis – determines relation/correlation
between fields in the database records. E.g,
“emacs” may be highly associated with “C” files
• Sequence analysis – models sequential
patterns. Algorithms discover what time based
sequence of audit events are frequently
occurring together.
• Frequent event patterns provide guidelines for
incorporating temporal statistical measures into
intrusion detection models
• E.g, patterns from audit data containing network
based DoS attacks suggest that several per-host
and per-service measures should be included
models
features
Patterns
evaluation
feedback
Connection/session
records
Packets/events
(ASCII)
Raw
audit
data
Data Mining process of building ID models
Data Mining techniques used in
MADAM ID
• Audit data consist of pre-processed timestamped audit records with a number of
features/fields
• ID is considered to be a classification problem
• Given a set of records, where one of the
features is a class label, classification algorithms
compute a model that uses the most
discriminating feature values to describe a
concept
Telnet Records
label
service
flag
hot
Failed_logins
compromised
Root_
shell
su
duration
….
normal
telnet
SF
0
0
0
0
0
10.2
…
normal
telnet
SF
0
0
0
3
1
2.1
…
guess
telnet
SF
0
6
0
0
0
26.2
…
normal
telnet
SF
0
0
0
0
0
126.2
…
overflow
telnet
SF
3
0
2
1
0
92.5
…
normal
telnet
SF
0
0
0
0
0
2.1
…
guess
telnet
SF
0
5
0
0
0
13.9
…
overflow
telnet
SF
3
0
2
1
0
92.5
…
normal
telnet
SF
0
0
0
0
0
1248
…
…
….
…
…
…
…
…
…
…
…
Rule Learning
• RIPPER is a classification rule
learning program that generate
rules
• Accuracy of classification
model depends on the set of
features provided in the
training set
• Classification algorithm looks
for features with large
information gain
• Adding per-host and perservice temporal service
reulted in significant
improvement in accuracy
RIPPER RULE
Meaning
Guess :- failed_logins ≥ 4
If number of failed logins
is at least 4, then this
telnet connection is
“guess”, a guessing
password attack
Overflow :- hot ≥ 3,
compromised ≥ 2,
root_shell = 1
If the number of hot
indicators is at least 3, the
number of compromised
conditions is at least 2,
and a root shell is
obtained, then this telnet
connection is a buffer
overflow attack
…..
…..
Normal :- true
If none of the above, then
this connection is
“normal”
Meta-classification
• Meta-learning is a mechanism for inductively
learning the correlation of predictions made by a
number of base classifiers
• Each record in training data has the true class
label and the predictions made by the base
classifiers
• Meta-classifier combines the base models to
make a final prediction
• IDS should consist of multiple cooperative
lightweight subsystems that monitors separate
parts of the network environment
Association Rules
• Program executions and user activities exhibit
frequent correlations among system features
• Goal of mining association rules - derive multifeature correlations from database table
• Support(X) - % of records that contain item set X
where each record is a set of items
• Association rule – an expression of the form
X → Y, [c,s] where X and Y are itemsets,
X ∩ Y = Φ, s = support(X U Y)
c = support(X U Y) / support(X) is the confidence
time
hostname
command
arg1
arg2
am
pascal
mkdir
dir1
…
am
pascal
cd
dir1
…
am
pascal
vi
text
…
am
pascal
tex
vi
…
am
pascal
subject
progress
…
am
pascal
vi
text
…
am
pascal
vi
text
…
am
pascal
subject
progress
…
am
pascal
vi
text
…
vi → time = am, hostname = pascal, arg1 = text, [1.0, 44.4]
Support(vi) = 44.4%
When using vi to edit a file, the user is always (i.e, 100% of the time) editing
a text file, in the morning, and at host pascal; and 44.4% of the command data
matches this pattern
Frequent Episodes
• A frequent episode rule is the expression
X,Y
→ Z,[c,s,w] where w is the width of the time
interval [t1 , t2 ] during which the episode occurs
• Mined frequent episodes from audit data contain
association among features used to construct
temporal statistical features for building
classifiers
Network Connection Records
timestamp
duration
service
Src_host
Dst_host
Src_
bytes
Dst_
bytes
flag
1.1
0
http
Spoofed_1
victim
0
0
S0
1.1
0
http
Spoofed_2
Victim
0
0
S0
1.1
0
http
Spoofed_3
Victim
0
0
S0
1.1
0
http
Spoofed_4
Victim
0
0
S0
1.1
0
http
Spoofed_5
Victim
0
0
S0
1.1
0
http
Spoofed_6
Victim
0
0
S0
1.1
0
http
Spoofed_7
Victim
0
0
S0
…….
…….
…..
…..
……..
…….
…..
…..
10.1
2
ftp
A
B
200
300
SF
Flag = S0, service = http, dst_host =‘victim’ used to describe the SYN flood attack
Victim → service = ‘http’, src_byte = 0, dst_byte = 0, flag = ‘S0’ [1.0, 0.7, 0]
Constructing features from
intrusion patterns
• Parse frequent episodes and use three operators, count,
percent, and average to construct statistical features
Procedure
– E.g, assume F0 say dst_host is a reference feature and the width of the
episode is w seconds
– Add the following features that examine only the connections in the past
w seconds that share the same value in dst_host as the current
connection
– Add a feature that computes “ the count of these connections”
– Let F1 be service, src_host or dst_host other than F0. If the same value
of F1 is in all item sets of the episode, add a feature that computes “%
of connections having same F1 value as the current connection”
– Let V2 be a value (e.g, S0) of a feature F2 (say flag). If V2 is in all the
itemsets of the episode, add a feature that computes “% of connections
having same V2”; otherwise if F2 is a numerical feature, add a feature
that “computes the average of F2 values”.
Example to illustrate feature
construction
• Suppose record 7 in slide 17 is our current connection ( F0 value =
‘victim’, F1 value = ‘http’)
Assume w = 0, F0 is the feature ‘dst_host’
• Count number of connections in the past w = 0 time units having the
same value for feature F0 i.e, dst_host = ‘victim’ ( = 7 for this e.g)
• Create a new feature count_F0 (containing value 7 in this e.g)
• Assume F1 is the feature ‘service’
• Compute the % of connections having service = ‘http’ in the past w =
0 time units for a given F0 i.e, dst_host = ‘victim’ ( = 100% for this
e.g)
• Create a new feature pcnt_F1_F0 (containing value 100 in this e.g)
• Assume V2 = S0
• Compute the % of connections having V2 = ‘S0’ in the past w = 0
time units for a given F0 i.e, dst_host = ‘victim’
• Create a new feature pcnt_V2_F0 (containing value 100 in this e.g)
Experimentation
• Experiments were conducted on 1998 DARPA
Intrusion Detection Evaluation Program data and
DARPA BSM data
• Algorithms and tools of MADAM ID were used to
process audit data, mine patterns, construct
features and build RIPPER classifiers
• DARPA data – 4 gigabytes of compressed
tcpdump data of 7 weeks of network traffic
• Data was processed into 5 million connection
records of about 100 bytes each
• DoS, R2L, U2R, PROBING attacks in training
data
Model
# of features
in records
# of rules
# of features
used in rules
22
55
11
traffic
20
26
4+9
Host traffic
14
8
1+5
Model Complexities content
User Anomaly Detection
• Goal is to determine whether the behavior of a
user is normal or not
• Difficult to classify a single event by a user as
normal or abnormal
• A user’s actions during a login session needs to
be studied as a whole to determine whether
he/she is behaving normally
• Approach
- Mine the frequent patterns from command data
- Form the normal usage profile of the user
- Analyze a login session by comparing its similarity to the profile
User Descriptions
User Anomaly Description
Strengths
• Paper very well written
• Exhaustive experimentation with real world data
• Developed a simple, intuitive yet powerful
method for feature construction
• First attempt to incorporate data mining
algorithms in IDS
• Experimented with both misuse detection and
user anomaly detection
• Models performed better than the systems built
with knowledge engineering approaches
• Critique their own work
Weaknesses
• Results show that the tools are not effective for
attacks having large variance in behavior (like
DoS and R2L)
• Results depend on quality and quantity of
training data – may lead to overtraining
• Network anomaly detection not implemented to
detect new attacks
• Computationally expensive
Future Improvements
• Develop algorithms to learn network anomaly
detection models
• ID models should be sensitive to cost factors like
development cost, operational cost, i.e, needed
resources, cost of damages of an intrusion, cost
of detecting and responding to potential intrusion
• Algorithms should incorporate user-defined
factors and policies to compute cost-sensitive ID
models