Detecting Malicious Executables Mr. Mehedy Masud (PhD Student) Prof. Latifur Khan Prof. Bhavani Thuraisingham Department of Computer Science The University of Texas at Dallas Lecture#7
Introduction: Detecting Malicious Executables What are malicious executables? Harm computer systems Virus, Exploit, Denial of Service (DoS), Flooder, Sniffer, Spoofer, Trojan etc. Exploits software vulnerability on a victim May remotely infect other victims Incurs great loss. Example: Code Red epidemic cost $2.6 Billion Malicious code detection: Traditional approach Signature based Requires signatures to be generated by human experts So, not effective against “zero day” attacks
State of the Art: Automated Detection Automated detection approaches: Behavioural: analyse behaviours like source, destination address, attachment type, statistical anomaly etc. Content-based: analyse the content of the malicious executable Autograph (H. Ah-Kim – CMU): Based on automated signature generation process N-gram analysis (Maloof, M.A. et .al.): Based on mining features and using machine learning.
New Ideas Content -based approaches consider only machine-codes (byte-codes). Is it possible to consider higher-level source codes for malicious code detection? Yes: Diassemble the binary executable and retrieve the assembly program Extract important features from the assembly program Combine with machine-code features
Feature Extraction Binary n-gram features Assembly n-gram features Sequence of n consecutive bytes of binary executable Assembly n-gram features Sequence of n consecutive assembly instructions System API call features DLL function call information
The Hybrid Feature Retrieval Model Collect training samples of normal and malicious executables. Extract features Train a Classifier and build a model Test the model against test samples
Hybrid Feature Retrieval (HFR) Training
Hybrid Feature Retrieval (HFR) Testing
Feature Extraction Binary n-gram features Example: Problem: Solution: Features are extracted from the byte codes in the form of n-grams, where n = 2,4,6,8,10 and so on. Example: Given a 11-byte sequence: 0123456789abcdef012345, The 2-grams (2-byte sequences) are: 0123, 2345, 4567, 6789, 89ab, abcd, cdef, ef01, 0123, 2345 The 4-grams (4-byte sequences) are: 01234567, 23456789, 456789ab,...,ef012345 and so on.... Problem: Large dataset. Too many features (millions!). Solution: Use secondary memory, efficient data structures Apply feature selection
Feature Extraction Assembly n-gram features Example: Problem: Features are extracted from the assembly programs in the form of n-grams, where n = 2,4,6,8,10 and so on. Example: three instructions “push eax”; “mov eax, dword[0f34]” ; “add ecx, eax”; 2-grams (1) “push eax”; “mov eax, dword[0f34]”; (2) “mov eax, dword[0f34]”; “add ecx, eax”; Problem: Same problem as binary Solution: Same solution
Feature Selection Select Best K features Selection Criteria: Information Gain Gain of an attribute A on a collection of examples S is given by
Experiments Dataset Disassembly Training, Testing Dataset1: 838 Malicious and 597 Benign executables Dataset2: 1082 Malicious and 1370 Benign executables Collected Malicious code from VX Heavens (http://vx.netlux.org) Disassembly Pedisassem ( http://www.geocities.com/~sangcho/index.html ) Training, Testing Support Vector Machine (SVM) C-Support Vector Classifiers with an RBF kernel
Results HFS = Hybrid Feature Set BFS = Binary Feature Set AFS = Assembly Feature Set
Results HFS = Hybrid Feature Set BFS = Binary Feature Set AFS = Assembly Feature Set
Results HFS = Hybrid Feature Set BFS = Binary Feature Set AFS = Assembly Feature Set
Future Plans System call: seems to be very useful. Need to Consider Frequency of call Call sequence pattern (following program path) Actions immediately preceding or after call Detect Malicious code by program slicing requires analysis
Buffer Overflow Attack Detection Mohammad M. Masud, Latifur Khan, Bhavani Thuraisingham Department of Computer Science The University of Texas at Dallas
Introduction Goal Main Contribution Intrusion detection. e.g.: worm attack, buffer overflow attack. Main Contribution 'Worm' code detection by data mining coupled with 'reverse engineering'. Buffer overflow detection by combining data mining with static analysis of assembly code.
Background What is 'buffer overflow'? How does it happen? A situation when a fixed sized buffer is overflown by a larger sized input. How does it happen? example: ........ char buff[100]; gets(buff); memory buff Stack Input string
Background (cont...) Then what? buff Stack ........ char buff[100]; gets(buff); memory buff Stack Return address overwritten Attacker's code memory buff Stack New return address points to this memory location
Background (cont...) So what? It can now How to stop it? Program may crash or The attacker can execute his arbitrary code It can now Execute any system function Communicate with some host and download some 'worm' code and install it! Open a backdoor to take full control of the victim How to stop it?
Background (cont...) Stopping buffer overflow Preventive approaches Detection approaches Finding bugs in source code. Problem: can only work when source code is available. Compiler extension. Same problem. OS/HW modification Capture code running symptoms. Problem: may require long running time. Automatically generating signatures of buffer overflow attacks.
CodeBlocker (Our approach) A detection approach Based on the Observation: Attack messages usually contain code while normal messages contain data. Main Idea Check whether message contains code Problem to solve: Distinguishing code from data
Severity of the problem It is not easy to detect actual instruction sequence from a given string of bits
Our solution Apply data mining. Formulate the problem as a classification problem (code, data) Collect a set of training examples, containing both instances Train the data with a machine learning algorithm, get the model Test this model against a new message
CodeBlocker Model
Feature Extraction
Disassembly We apply SigFree tool implemented by Xinran Wang et al. (PennState)
Feature extraction Features are extracted using N-gram analysis Control flow analysis What is an n-gram? -Sequence of n instructions Traditional approach: -Flow of control is ignored 2-grams are: 02, 24, 46,...,CE Assembly program Corresponding IFG
Feature extraction (cont...) Control-flow Based N-gram analysis What is an n-gram? -Sequence of n instructions Proposed Control-flow based approach -Flow of control is considered 2-grams are: 02, 24, 46,...,CE, E6 Assembly program Corresponding IFG
Feature extraction (cont...) Control Flow analysis. Generated features Invalid Memory Reference (IMR) Undefined Register (UR) Invalid Jump Target (IJT) Checking IMR A memory is referenced using register addressing and the register value is undefined e.g.: mov ax, [dx + 5] Checking UR Check if the register value is set properly Checking IJT Check whether jump target does not violate instruction boundary
Feature extraction (cont...) Why n-gram analysis? Intuition: in general, disassembled executables should have a different pattern of instruction usage than disassembled data. Why control flow analysis? Intuition: there should be no invalid memory references or invalid jump targets.
Putting it together Compute all possible n-grams Select best k of them Compute feature vector (binary vector) for each training example Supply these vectors to the training algorithm
Experiments Dataset Training, Testing Real traces of normal messages Real attack messages Polymorphic shellcodes Training, Testing Support Vector Machine (SVM)
Results CFBn: Control-Flow Based n-gram feature CFF: Control-flow feature
Novelty / contribution We introduce the notion of control flow based n-gram We combine control flow analysis with data mining to detect code / data Significant improvement over other methods (e.g. SigFree)
Advantages 1) Fast testing 2) Signature free operation 3) Low overhead 4) Robust against many obfuscations
Limitations Need samples of attack and normal messages. May not be able to detect a completely new type of attack.
Future Works Find more features Apply dynamic analysis techniques Semantic analysis
Reference / suggested readings X. Wang, C. Pan, P. Liu, and S. Zhu. Sigfree: A signature free buffer overflow attack blocker. In USENIX Security, July 2006. Kolter, J. Z., and Maloof, M. A. Learning to detect malicious executables in the wild Proceedings of the tenth ACM SIGKDD international conference on Knowledge discovery and data mining Seattle, WA, USA Pages: 470 – 478, 2004.
Email Worm Detection (behavioural approach) Outgoing Emails The Model Feature extraction Test data Training data Machine Learning Classifier Clean or Infected ?
Feature Extraction Per email features Per window features Binary valued Features Presence of HTML; script tags/attributes; embedded images; hyperlinks; Presence of binary, text attachments; MIME types of file attachments Continuous-valued Features Number of attachments; Number of words/characters in the subject and body Per window features Number of emails sent; Number of unique email recipients; Number of unique sender addresses; Average number of words/characters per subject, body; average word length:; Variance in number of words/characters per subject, body; Variance in word length Ratio of emails with attachments
Feature Reduction & Selection Principal Component Analysis Reduce higher dimensional data into lower dimension Helps reducing noise, overfitting Decesion Tree Used to Select Best features
Experiments Data Set Training, Testing: Contains instances for both normal and viral emails. Six worm types: bagle.f, bubbleboy, mydoom.m, mydoom.u, netsky.d, sobig.f Collected from UC Berkeley Training, Testing: Decision Tree: C4.5 algorithm (J48) on Weka Systems Support Vector Machine (SVM) and Naïve Bayes (NB).
Results
Conclusion & Future Work Three approaches has been tested Apply classifier directly Apply dimension reduction (PCA) and then classify Apply feature selection (decision tree) and then classify Decision tree has the best performance Future Plans Combine content based with behavioral approaches Offensive Operations Honeypots, Information operations