Data Mining Approaches for Intrusion Detection Wenke Lee and Salvatore J. Stolfo Computer Science Department Columbia University
Overview • Intrusion detection and computer security • Current intrusion detection approaches • Our proposed approach • Data mining • Classification models for intrusion detection • Mining patterns from audit data • System architecture • Current status • Research plans
Overview • Current intrusion detection approaches and problems • Our proposed approach • Data mining • Classification models for intrusion detection • Mining patterns from audit data • System architecture • Current status • Research plans
Intrusion Detection and Computer Security • Computer security goals: confidentiality, integrity, and availability • Intrusion is a set of actions aimed to compromise these security goals • Intrusion prevention (authentication, encryption, etc.) alone is not sufficient • Intrusion detection is needed
Intrusion Detection • Primary assumption: user and program activities can be monitored and modeled • Key elements: • Resources to be protected • Models of the “normal” or “legitimate” behavior on the resources • Efficient methods that compare real-time activities against the models and report probably “intrusive” activities.
Audit Records Inductive Learning Engine Learning Agent Audit Data Preprocessor Activity Data Base Detection Agent Rules Detection Models (Base) Detection Engine Evidence from Other Agents Evidence Meta Detection Agent (Meta) Detection Engine Final Assertion Action/Report Decision Table Decision Engine
Connection Records 10:35:41.5 126.96.36.199.30 > 188.8.131.52.80 : . 512:1024(512) ack 1 win 9216 10:35:41.5 184.108.40.206.20 > 220.127.116.11.3241: . ack 1073 win 16384 10:35:41.6 18.104.22.168.2623 > 22.214.171.124.21: . ack 2650 win 16225 tcpdump Learning execve(“/usr/ucb/finger”, … open(“/dev/zero … mmap(… ... truss Profile System call Sequence execve open mmap ... Learning Profile
Intrusion Detection • Two categories of techniques: • Misuse detection: use patterns of well-known attacks to identify intrusions • Anomaly detection: use deviation from normal usage patterns to identify intrusions
Current Intrusion Detection Approaches • Misuse detection: • Record the specific patterns of intrusions • Monitor current audit trails (event sequences) and pattern matching • Report the matched events as intrusions • Representation models: expert rules, Colored Petri Net, and state transition diagrams
Current Intrusion Detection Approaches • Anomaly detection: • Establishing the normal behavior profiles • Observing and comparing current activities with the (normal) profiles • Reporting significant deviations as intrusions • Statistical measures as behavior profiles: ordinal and categorical (binary and linear)
Current Intrusion Detection Approaches • Main problems: manual and ad-hoc • Misuse detection: • Known intrusion patterns have to be hand-coded • Unable to detect any new intrusions (that have no matched patterns recorded in the system) • Anomaly detection: • Selecting the right set of system features to be measured is ad hoc and based on experience • Unable to capture sequential interrelation between events
Our Proposed Approach • A systematic framework to: • Build good models: • select appropriate features of audit data to build intrusion detection models • Build better models: • architect a hierarchical detector system that combines multiple detection models • Build updated models: • dynamically update and deploy new detection system as needed
Our Proposed Approach • Support for the feature selection and model construction process: • Apply data mining algorithms to find consistent inter- and intra- audit record (event) patterns • Use the features and time windows in the discovered patterns to build detection models • A support environment to semi-automate this process
Our Proposed Approach • Combining multiple detection models: • Each (base) detector model monitors one aspect of the system • They can employ different techniques and be independent of each other • The learned (meta) detector combines evidence from a number of base detectors
Our Proposed Approach • An intelligent agent-based architecture: • learning agents: continuously compute (learn) the detection models • detection agents: use the (updated) models to detect intrusions
Data Mining • KDD (Knowledge Discovery in Database): • The process of identifying valid, useful and understandable patterns in data • Steps: understanding the application domain, data preparation, data mining, interpretation, and utilizing the discovered knowledge • Data mining: applying specific algorithms to extract patterns from data
Data Mining • Relevant data mining algorithms: • Classification: maps a data item into one of several pre-defined categories • Link analysis: determines relations between fields in the database • Sequence analysis: models sequence patterns
Data Mining • Why is it applicable to intrusion detection? • Normal and intrusive activities leave evidence in audit data • From the data-centric point view, intrusion detection is a data analysis process • Successful applications in related domains, e.g., fraud detection, fault/alarm management
Building Classifiers for Intrusion Detection • Experiments in constructing classification models for anomaly detection • Two experiments: • sendmail system call data • network tcpdump data • Use meta classifier to combine multiple classification models
Classification Models on sendmail • The data: sequence of system calls made by sendmail. • Classification models (rules): describe the “normal” patterns of the system call sequences. • The rule set is the normal profile of sendmail • Detection: calculate the deviation from the profile • large number/high scores of “violations” to the rules in a new trace suggests an exploit
Classification Models on sendmail • The sendmail data: • Each trace has two columns: the process ids and the system call numbers • Normal traces: sendmail and sendmail daemon • Abnormal traces: sunsendmailcap, syslog-remote, syslog-remote, decode, sm5x and sm56a attacks.
Classification Models on sendmail • Data preprocessing: • Use sliding window to create sequence of consecutive system calls • Label the sequences to create training data:
Classification Models on sendmail • Experiment 1 - learning patterns of normal sequences: • Each record: n consecutive system calls plus a class label, “normal” or “abnormal” • Training data: sequences from 80% of the normal traces plus some of the attack traces • Testing data: traces not used in training • Use RIPPER to learn specific rules for the minority classes
sendmail Experiment 1 • Examples of output RIPPER rules: • if the 2nd system call is vtimes and the 7th is vtrace, then the sequence is “normal” • if the 6th system call is lseek and the 7th is sigvec, then the sequence is “normal” • … • if none of the above, then the sequence is “abnormal”
sendmail Experiment 1 • Using the learned rules to analyze a new trace: • label all sequences according to the rules • define a region as l consecutive sequences • define a “abnormal” region as having more “abnormal” sequences than normal ones • calculate the percentage of “abnormal” regions • the trace is “abnormal” if the percentage is above a threshold
sendmail Experiment 1 • Hypothesis: need specific rules of “normal” sequences to detect “unknown/new” intrusions • Some results using various normal v.s. abnormal distributions: • Experiment A: 46% normal, length 11 • Experiment B: 46% normal, length 7 • Experiment C: 54% normal, length 11 • Experiment D: 54% normal, length 7
sendmail Experiment 1 • All 4 experiments: • Training data includes sequences from intrusion traces in Bold and Italic, and sequences from 80% of the normal sendmail traces • Percentage of abnormal “regions” of each trace (showed in the table) is used as the intrusion indicator • The output rule sets contain ~250 rules, each with 2 or 3 attribute tests. This compares with the total ~1,500 different sequences. • Experiment A and B generate rules that characterize “normal” sequences of length 11 and 7 respectively • Experiment C and D generate rules that characterize “abnormal” sequences of length 11 and 7 respectively
sendmail Experiment 1 3.4 1.9 0.9 0.7 Anomaly detectors A and B performs better then misuse detectors C and D.
Classification Models on sendmail • Experiment 2 - learning to predict normal system call: • Each record: n-1 consecutive system calls plus a class label, the nth or the middle system call • Training data: sequences from 80% of the normal traces (no abnormal traces) • Testing data: traces not used in training • Use RIPPER to learn rules
sendmail Experiment 2 • Examples of output RIPPER rules: • if the 3rd system call is lstat and the 4th is write, then the 7th is stat • if the 1st system call is sigblock and the 4th is bind, then the 7th is setsockopt • … • if none of the above, then the 7th is open
sendmail Experiment 2 • Using the learned rules to analyze a new trace: • predict system calls according to the rules • if a rule is violated, the “violation” score is increased by 100 times the accuracy of the rule • the trace is “abnormal” if the violation score is above a threshold
sendmail Experiment 2 • Some results: • Experiment A: predict the 11th system call • Experiment B: predict the middle system call in a sequence of length 7 • Experiment C: predict the middle system call in a sequence of length 11 • Experiment D: predict the 7th system call
sendmail Experiment 2 • All 4 experiments: • Training data includes only the sequences from 80% of the normal sendmail traces • Output rules predict what should be the “normal” nth or the middle system call • Score of rule “violation” (mismatch) of each trace (showed in the table) is used as the intrusion indicator • The output rule sets contain ~250 rules, each with 2 or 3 attribute tests. This compares with the total ~1,500 different sequences.
sendmail Experiment 2 3.7 3.3 1.2 1.3 The 11th (A) and 4th (B) system call are more predictable
Classification Models on sendmail • Lessons learned: • Normal behavior can be established and used to detect anomalous usage • Need to collect near “complete” normal data in order to build the “normal” model • But how do we know when to stop collecting? • Need tools to guide the audit data gathering process
Classification Models on tcpdump • The tcpdump data (part of a public data visualization contest): • Packets of incoming, out-going, and internal broadcast traffic • One trace of normal network traffic • Three traces of network intrusions
Classification Models on tcpdump • Data preprocessing: • Extract the “connection” level features: • Record connection attempts • Monitor data packets and count: # of bytes in each direction, resent rate, hole rate, etc. • Watch how connection is terminated
Classification Models on tcpdump • Data Preprocessing: • Each record has: • start time and duration • participating hosts and ports (applications) • statistics (e.g., # of bytes) • flag: “normal” or a connection/termination error • protocol: TCP or UDP • Divide connections into 3 types: incoming, out-going, and inter-lan
Classification Models on tcpdump • Building classifier for each type of connections: • Use the destination service (port) as the class label • Training data: 80% of the normal connections • Testing data: 20% of the normal connections and connections in the 3 intrusion traces • Apply RIPPER to learn rules
Classification Models on tcpdump • The output RIPPER rules describe the “normal” characteristics of the destination services. The rule set is the profile of the normal network traffic. • Using the rules to analyze tcpdump traces: • Examine each connection record according to the rules • Calculate the percentage of misclassification (violation of a rule). This percentage is the deviation from the profile.
Classification Models on tcpdump • Results - misclassification rate on each type of connections: This model is not very effective in detecting intrusions
Classification Models on tcpdump • Adding temporal features for better models: • Examine all connections in the past n seconds, and count: • the number of connection errors, all other errors, connections to system services, user applications, and connection to the same service as the current connection • average duration and data bytes of all connections; and the same averages of connections to the same service.
Classification Models on tcpdump • Results of adding the temporal features, the time window is 30 seconds: Adding temporal statistical features improves the effectiveness of the detection models
Effects of time window length on misclassification rate How do we obtain the optimal time window length?
Classification Models on tcpdump • Lessons learned: • Data preprocessing requires extensive domain knowledge • Adding temporal features improves classification accuracy • Need tools to guide (temporal) feature selection
Building Classifiers for Intrusion Detection • Meta classifier that combines evidence from multiple detection models: • Build base classifiers that each model one aspect of the system • The meta learning task: • each record has a collection of evidence from base classifiers, and a class label “normal” or “abnormal” on the state of the system • Apply a learning algorithm to produce the meta classifier
Mining Patterns from Audit Data • Association rules: describe multi-feature (attribute) correlation from a database • X => Y , confidence, support: • X and Y are subsets of the attribute values in a record • support is the percentage of records that contain X and Y • confidence is support(X+Y)/support(X)
Association Rules • Motivations: • Audit data can be easily formatted into a database table • Program executions and user activities have frequent correlation among system features • Incremental updating of the rule set is easy • An example from the .sh_history : • trn => rec.humor, [0.3, 0.1] • Meaning: 30% of the time when using trn, the user is reading rec.humor; and reading this newsgroup constitutes 10% of all sh commands
Mining Patterns from Audit Data • Frequent Episodes: frequent events occurring within a time window • X => Y, confidence, support, window: • X and Y are subsets of the attribute values in a record • support is the percentage of (sliding) windows that contain X and Y • confidence is support(X+Y)/support(X)
Frequent Episodes • Motivation: • Sequence information needs to be included in a detection model • An example from a department’s web log: • home, research => theory, [0.2, 0.05],  • Meaning: 20% of the time, after home and research pages are visited (in that order), the theory is then visited within 30 seconds from when home is visited; and visiting these three pages constitutes 5% of all visits to the web site