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Polymorphic Worm Detection by Instruction Distribution

Polymorphic Worm Detection by Instruction Distribution. Kihun Lee HPC Lab., Postech 2 nd Joint Workshop between Security Research Labs in Korea and Japan 2006. 2. 20. Contents. Introduction Background Polymorphic Worm Related Works Polygraph Using a Control Flow Graph

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Polymorphic Worm Detection by Instruction Distribution

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  1. Polymorphic Worm Detection by Instruction Distribution Kihun Lee HPC Lab., Postech 2nd Joint Workshop between Security Research Labs in Korea and Japan 2006. 2. 20

  2. Contents • Introduction • Background • Polymorphic Worm • Related Works • Polygraph • Using a Control Flow Graph • Problem Definition • Proposal Idea • Conclusions and Future Works

  3. Introduction • Toward defending against Internet worms, NIDSs have been proposed by the security community. • IDS searches inbound traffic for known patterns, or “signature”. • Unfortunately, the worms became more sophisticated! • Substantially changes its payload.

  4. Background Polymorphic Worm (1/2) • IDSs search for similar byte sequence • Author of worm have to prevent this: • ciphering techniques • obfuscating the decryption routine • Can’t find a sufficiently specific sequence. <Figure 1.> Typical polymorphic worm structure

  5. Polymorphic Worm (2/2) Background <Figure 2.> Polymorphic worm cycle

  6. Polygraph Related Works • The system that proposed to defense the polymorphic worms • Idea : use the combination of “short invariant contents” • Assumption : combination of many general contents is sufficiently specific. • Problems : • Even though combine all of them, an outcome can be remain too general. • Decision time is too late. • Token of the signature can be located after a long garbage sequence.

  7. Using a Control Flow Graph Related Works • A complementary approach to reach the same goal with Polygraph • Idea : using structural information of executables • Assumption : at least some parts of a worm contain executable machine code. • Decryptor part of polymorphic worm • Problems : • Because of huge performance overhead, it cannot operate on-line. • generating a graph, coloring the graph • Manufacturing the control flow is not difficult technique.

  8. Problem Definition • Scope of problem: • The worm of which propagation mechanism is using a vulnerability of a server application. • Assumption : • At least some parts of a worm contain executable machine code. • Linear disassemble has a little overhead so that can operate on-line. • Problem definition • Make a decision whether the inbound packet has an executable code or not. • Make a decision whether the executable code is a polymorphic exploit code or a legitimate code.

  9. Motivated Experiment • If disassemble the packet, • Case 1 : executable code tend to • Kinds of instruction : Ü • Number of each instruction : Û • Case 2 : non-executable code tent to • Kinds of instruction : Û • Number of each instruction : Ü • Decoding error (invalid instruction) : Û <Figure 3.> number of each instruction ( sorted by decreasing order)

  10. Threshold Find Executable Code Proposal Idea • Let K = “kinds of instruction” , T = “total number of instructions”, E = “the number of decoding error”. • Calculate the expression : • Non-executable code : tend to very small value. • Executable code : tend to relatively large value. <Figure 4.> Distinguish between executable code and non-executable code

  11. Distinguish Legitimate Code (1/2) Proposal Idea • Use the “verifying instruction” • For example, “call”, “ret”, “int”, etc. • Typically, normal executable code has a lot of “call” instructions. • One “call” instruction per 10~15 instructions. • NOP sled cannot include any “call” instruction. • Decryptor is a very simple routine so that it rarely has a “call” instruction. • Moreover, decryptor can’t know the address of the function of dynamic linking library.

  12. Threshold Distinguish Legitimate Code (2/2) Proposal Idea • Let V = “the number of verifying instruction” • Calculate the expression : • Polymorphic exploit code : is relatively small value. • Legitimate code : is relatively large value. <Figure 4.> Distinguish between exploit code and legitimate code

  13. Conclusions and Future Works • Conclusions • Proposed idea can identify and isolate the polymorphic worm. • It is based on static analysis; so it can runs in real-time. • It can discover the worm traffic by not flow level but packet level examination. • Future Works • Refine the idea. • Investigate more samples to get a generality. • How to extract a signature?

  14. References • J. Newsome, B. Karp, and D. Song. Polygraph: Automatically Generating Signatures for Polymorphic Worms. In IEEE Symposium on Security and Privacy, 2005. • C. Kruegel, E. Kirda, D. Mutz, W. Robertson, and G. Vigna. Polymorphic Worm Detection Using Structural Information of Executables. In RAID 2005. • O. Kolesnikov, and W. Lee. Advanced Polymorphic Worms: Evading IDS by Blending in with Normal Traffic. In 12th ACM conference on Computer and communications security. • P. Akritidis, E.P. Markatos, M. Polychronakis, and K. Anagnostakis. STRIDE: Polymorphic Sled Detection Through Instruction Sequence Analysis. 12th ACM conference on Computer and communications security. • T. DeTristan, T. Ulenspiegel, Y. Malcom, and M. von Underduk. Polymorphic Shellcode Engine Using Spectrum Analysis. http://www.phrack.org/show.php?p=61&a=9 • Etc.

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