Hunting for Metamorphic Engines Wing Wong Mark Stamp Hunting for Metamorphic Engines 1

In This Paper…  Analyze metamorphic malware o Hacker-produced metamorphic code  Measure similarity of software o Based on n-gram analysis  Compute scores o Based on n-grams and o Based on HMMs  This paper is baseline for future work Hunting for Metamorphic Engines 2

Motivation  Many virus construction kits available o Many can produce metamorphic code  So anybody can create “new” version of existing malware o Virtually no technical expertise required  How “effective” is the resulting metamorphic code?  Can we detect metamorphic malware? Hunting for Metamorphic Engines 3

Background  Encrypted, polymorphic, metamorphic o Metamorphic == body polymorphic  Metamorphic vs cloned software o Clone is the norm, but metamorphic could offer advantages to the good guy too…  From the theory, we know malware detection is NP-complete o And metamorphic is at least as hard o But what about practical situation? Hunting for Metamorphic Engines 4

Metamorphism  Metamorphic code changes it “shape”  Well-known examples o W95/Regswap o W32/Ghost o W95/Zperm o MetaPHOR Hunting for Metamorphic Engines 5

Metamorphism  General techniques available o Insertion o Substitution o Transposition o Deletion  Some easier than others  Some more effective against certain detection strategies Hunting for Metamorphic Engines 6

Virus Construction Kits  In this paper, we consider o PS-MPC (Phalcon/Skism Mass Produced Code generator) o G2 (Second Generation virus generator) o MPCGEN (Mass Produced Code GENerator) o NGVCK (Next Generation Virus Construction Kit) o VCL32 (Virus Creation Lab for Win32) Hunting for Metamorphic Engines 7

Virus Construction Kits  Did not consider MetaPHOR o Difficult to work with, finicky  All of these claim to be metamorphic  Are they really? o How can we measure “metamorphism”?  If they are highly metamorphic, can we still detect them? Hunting for Metamorphic Engines 8

Brief Review of Malware Detection  First generation o Signature scanning, wildcards OK  Second generation o Approximate signature scanning; e.g., ignore NOP instructions  Code emulation  Heuristic analysis o Static or dynamic, false positives… Hunting for Metamorphic Engines 9

Machine Learning  Consider the following o Data Mining, Neural Networks, HMMs  Data Mining o Malware-related previous work o Generic approach  Neural Networks o Previous work based on byte trigrams o Developed and used at IBM Hunting for Metamorphic Engines 10

Hidden Markov Models  Train HMM on metamorphic family  Then we can score any file to see how “close” it is to the family  What to use to train such an HMM? o Raw bytes in exe? o Disassembled code? o Opcode sequence?  More on this later… Hunting for Metamorphic Engines 11

Software Similarity  How to quantify metamorphism?  In general, how to measure similarity of software?  Given program 1 and program 2..  We develop a score o Score of 0 means “no similarity” o Score of 1 means “virtually identical” Hunting for Metamorphic Engines 12

N-gram Similarity  Given executable files X and Y  Extract opcode sequences from each o Suppose X has n opcodes o Suppose Y has m opcodes  How to compare the sequences?  Many possible ways --- here we use n- gram analysis o That is, we compare subsequences Hunting for Metamorphic Engines 13

N-gram Similarity  Extracted opcode sequences o X=(x 0,x 1,…,x n-1 ) and Y=(y 0,y 1,…,y m-1 )  Compare subsequences of length k o Then x i,x i+1,…,x i+k-1 matches y j,y j+1,…,y j+k-1 if they are the same in any order o For each such match, plot the point (i,j) o Remove any segments less than p points  Then score = (x axis covered + y axis covered) / 2 Hunting for Metamorphic Engines 14

N-gram Similarity Example Hunting for Metamorphic Engines 15

N-gram Similarity  Score is between 0 and 1  If program X identical to program Y o Main diagonal is a solid line o And score = 1  Minimum score is 0  The smaller the score, the less similar are the programs Hunting for Metamorphic Engines 16

Typical N-gram Similarity Hunting for Metamorphic Engines 17  Normal (cygwin utility) files

Typical N-gram Similarity Hunting for Metamorphic Engines 18  NGVCK

Typical N-gram Similarity Hunting for Metamorphic Engines 19  G2

N-gram Similarity  Compare members of a “family” with each other Hunting for Metamorphic Engines 20

N-gram Similarity  In graphical form… Hunting for Metamorphic Engines 21

N-gram Similarity Conclusion?  G2 more similar to each other than expected o So, they are not very metamorphic o Ditto for most of the other generators  But, NGVCK viruses more different from each other than expected o So, they are highly metamorphic  Implication wrt signature detection? Hunting for Metamorphic Engines 22

NGVCK Similarity  Compare NGVCK to other families… Hunting for Metamorphic Engines 23

NGVCK Similarity Conclusion?  NGVCK viruses very different from each other o Implies highly metamorphic… o …so, signature detection will fail  But NGVCK viruses are even more different from normal files o Then what about detection? Hunting for Metamorphic Engines 24

Aside: Similar Similarity Measures to Consider?  Given opcode sequences o Edit distance o Other sequence comparison techniques o Statistical measures  Considering raw bytes o Statistical measures o Entropy and other “structural” measures Hunting for Metamorphic Engines 25

Hidden Markov Models  Generic view of HMM Hunting for Metamorphic Engines 26

HMM Notation Hunting for Metamorphic Engines 27

HMM for Metamorphic Detection  Train HMM o Extract opcodes from family executables o Append opcode sequences o Train a model, i.e., determine matrices  Use trained HMM to score files o Given an file, extract opcode sequence o Score sequence against the model o Compare to predetermined threshold Hunting for Metamorphic Engines 28

HMM Scoring: Fine Points  Score computed as log likelihood of the scored sequence o We mormalize score to “log likelihood per opcode” (LLPO) o Why?  How to quantify effectiveness? o ROC curves are very useful o Specifically, area under ROC curve (AUC) Hunting for Metamorphic Engines 29

Results  HMM scoring for NGVCK family Hunting for Metamorphic Engines 30

HMM Scoring: Bottom Line  Signature detection for metamorphic families, except NGVCK  For NGVCK, we can use HMM o Classification is 100% when compared to normal (benign) files o Some misclassifications of other malware (is that good or bad?)  Should include ROC curves, AUC, … Hunting for Metamorphic Engines 31

HMM States: 3 State Model Hunting for Metamorphic Engines 32

N-gram Score  Can also score files using N-grams  Randomly select NGVCK file o Extract its opcode sequence  Given a file we want to score o Extract its opcode sequence o N-gram similarity to NGVCK sequence o Higher similarity, classify as NGVCK o Lower similarity, classify as “not NGVCK” Hunting for Metamorphic Engines 33

N-gram Score Results?  For NGVCK, obtain ideal separation o There exists a threshold for which… o …we can separate NGVCK from normal  Surprisingly strong results o For such a simple similarity score  Why does this work? o We come back to this at the end… Hunting for Metamorphic Engines 34

Compare to Commercial AV  Tested following on our virus sets o eTrust, avast!, AVG  These scanners detected most of the viruses from weak families o That is, G2, VCL32, etc.  But none of the NGVCK viruses detected by any of the 3 scanners Hunting for Metamorphic Engines 35

Conclusion  HMM effective at detecting a highly metamorphic NGVCK malware family  N-gram similarity also effective  NGVCK not detected by commercial AV  So, this detection improves the state of the art  Practical considerations? Hunting for Metamorphic Engines 36

Lessons Learned?  Why can we detect NGVCK family?  In spite of high metamorphism, code is statistically different from normal  “Improved” metamorphic malware?  Metamorphism must be sufficient to evade signature detection  But, metamorphic family must be statistically similar to normal Hunting for Metamorphic Engines 37

Future Work  Build a better metamorphic generator o Some progress here, but still detectable using other detection methods o Still need better generators…  Develop and test other detection strategies o Lots of work done here too o But lots more to do Hunting for Metamorphic Engines 38

References  W. Wong and M. Stamp, Hunting for metamorphic engines, Journal in Computer Virology 2(3): , 2006Hunting for metamorphic engines  M. Stamp, A revealing introduction to hidden Markov modelsA revealing introduction to hidden Markov models Hunting for Metamorphic Engines 39