1. Autograph Toward Automated, Distributed Worm Signature Detection (Hyang-Ah Kim, Brad Karp) Yunhai & Justin

2. 2 Agenda Autograph Overview Introduction & Motivation Design Goals Autograph System Design  Selecting Suspicious Traffic  Content-Based Signature Generation Evaluation  Local Signature Detection  Distributed Signature Detection Attacks & Limitations Conclusion & Future Work Questions & Answers

3. 3 Autograph Overview What is Autograph  A system that automatically generates signatures for novel Internet worms that propagate using TCP trans port How does it work  Generates signatures by analyzing the prevalence of portions of flow payloads What is the main feature  Designed to produce signatures that exhibit high sensitivity and high specificity

4. 4 Introduction Self-replicating Worm  Remotely exploits a software vulnerability  Severity goes far beyond mere inconvenience High cost in lost productivity  Code Red: $2.6 billion Researches  Worm Quarantine  Worm Origin Identification Using Random Moonwalks Assumes attack flows do not use spoofed source IP address Assumes worm propagation occurs in a tree-like structure

5. 5 Methods for Containing Internet Worm Quarantine Techniques  Destination port blocking Discovering ports on which worms appear to be spreading, and filtering all traffic destined for those ports  Infected source host IP blocking Discovering source addresses of infected hosts and filtering all traffic from those source addresses  Content-based blocking Discovering the payload content string that a worm uses in its infection attempts, and filtering all flows contain it

6. 6 Motivation  How should one obtain worm content signatures for use in content-based filtering accurately and quickly? Worm Signature 05:45:31.912454 90.196.22.196.1716 > 209.78.235.128.80:. 0:1460(1460) ack 1 win 8760 (DF) 0x0000 4500 05dc 84af 4000 6f06 5315 5ac4 16c4E.....@.o.S.Z... 0x0010 d14e eb80 06b4 0050 5e86 fe57 440b 7c3b.N.....P^..WD.|; 0x0020 5010 2238 6c8f 0000 4745 5420 2f64 6566P."8l...GET./def 0x0030 6175 6c74 2e69 6461 3f58 5858 5858 5858ault.ida?XXXXXXX 0x0040 5858 5858 5858 5858 5858 5858 5858 5858XXXXXXXXXXXXXXXX..... 0x00e0 5858 5858 5858 5858 5858 5858 5858 5858XXXXXXXXXXXXXXXX 0x00f0 5858 5858 5858 5858 5858 5858 5858 5858XXXXXXXXXXXXXXXX 0x0100 5858 5858 5858 5858 5858 5858 5858 5858XXXXXXXXXXXXXXXX 0x0110 5858 5858 5858 5858 5825 7539 3039 3025XXXXXXXXX%u9090% 0x01a0 303d 6120 4854 5450 2f31 2e30 0d0a 436f0=a.HTTP/1.0..Co. Signature for CodeRed II Signature : A Payload Content String Specific To A Worm

7. 7 Content-based Blocking Our network X Traffic Filtering Internet Signature for CodeRed II  Can be used by Bro, Snort, Cisco’s NBAR,...

8. 8 Signature derivation is too slow Current Signature Derivation Process  New worm outbreak  Report of anomalies from people via phone/email/newsgroup  Worm trace is captured  Manual analysis by security experts  Signature generation  Labor-intensive, Human-mediated

9. 9 Design Assumptions Focus on TCP worms that propagate via scanning Actually, any transport  in which spoofed sources cannot communicate successfully  in which transport framing is known to monitor Worm’s payloads share a common substring  Vulnerability exploit part is not easily mutable Not polymorphic

10. 10 Agenda Autograph Overview Introduction & Motivation Design Goals Autograph System Design  Selecting Suspicious Traffic  Content-Based Signature Generation Evaluation  Local Signature Detection  Distributed Signature Detection Attacks & Limitations Conclusion & Future Work Questions & Answers

11. 11 Design Goals Automation: Minimal manual intervention Signature quality: Sensitive & specific  Sensitive: match all worms  low false negative rate  Specific: match only worms  low false positive rate Timeliness: Early detection Application neutrality  Broad applicability Bandwidth Efficiency Signature Quantity & Length

12. 12 Agenda Autograph Overview Introduction & Motivation Design Goals Autograph System Design  Selecting Suspicious Traffic  Content-Based Signature Generation Evaluation  Local Signature Detection  Distributed Signature Detection Attacks & Limitations Conclusion & Future Work Questions & Answers

13. 13 Automated Signature Generation Step 1: Select suspicious flows using heuristics Step 2: Generate signature using content- prevalence analysis Our network Traffic Filtering Internet Autograph Monitor Signature X

14. 14 Heuristic: Flows from scanners are suspicious  Focus on the successful flows from IPs who made unsuccessful connections to more than s destinations for last 24hours  Suitable heuristic for TCP worm that scans network Suspicious Flow Pool  Holds reassembled, suspicious flows captured during the last time period t  Triggers signature generation if there are more than  flows S1: Suspicious Flow Selection Reduce the work by filtering out vast amount of innocuous flows Autograph (s = 2) Non-existent  This flow will be selected

15. 15 S1: Suspicious Flow Selection Heuristic: Flows from scanners are suspicious  Focus on the successful flows from IPs who made unsuccessful connections to more than s destinations for last 24hours  Suitable heuristic for TCP worm that scans network Suspicious Flow Pool  Holds reassembled, suspicious flows captured during the last time period t  Triggers signature generation if there are more than  flows Reduce the work by filtering out vast amount of innocuous flows

16. 16 S2: Signature Generation All instances of a worm have a common byte pattern specific to the worm Rationale  Worms propagate by duplicating themselves  Worms propagate using vulnerability of a service Use the most frequent byte sequences across suspicious flows as signatures How to find the most frequent byte sequences?

17. 17 Divide Payloads into Content Block Use the entire payload  Brittle to byte insertion, deletion, reordering GARBAGEEABCDEFGHIJKABCDXXXX Flow 1 Flow 2 GARBAGEABCDEFGHIJKABCDXXXXX

18. 18 Divide Payloads into Content Block Partition flows into non-overlapping small blocks and count the number of occurrences Fixed-length Partition  Still brittle to byte insertion, deletion, reordering GARBAGEEABCDEFGHIJKABCDXXXX Flow 1 Flow 2 GARBAGEABCDEFGHIJKABCDXXXXX

19. 19 Divide Payloads into Content Block Content-based Payload Partitioning (COPP)  Partition if Rabin fingerprint of a sliding window matches Breakmark  Configurable parameters: content block size (minimum, average, maximum), breakmark, sliding window  Content Blocks Breakmark = last 8 bits of fingerprint ( ABCD ) GARBAGEEABCDEFGHIJKABCDXXXX Flow 1 Flow 2 GARBAGEABCDEFGHIJKABCDXXXXX

20. 20 Definition Prevalence  The number of suspicious flows in which each content block occurs

21. 21 Why Prevalence? Worm flows dominate in the suspicious flow pool Content-blocks from worms are highly ranked Nimda CodeRed2 Nimda (16 different payloads) WebDAV exploit Innocuous, misclassified Prevalence Distribution in Suspicious Flow Pool - From 24-hr http traffic trace

22. 22 Select Most Frequent Content Block ABD ABE ACE AD CF CDG B f0 f1 f2 f3 f4 f5 HIJ f6 IHJ f7 GIJ f8

23. 23 A A A E E A F C C C D D DB B B H H G G I I I J J J Select Most Frequent Content Block D C E E A A A A D F C C DG B B B H H G I I I J J J f0 f1 f2 f3 f4 f5 f6 f7 f8 f0 C F f1 C D G f2 A B D f3 A C E f4 A B E f5 A B D f6 H I J f7 I H J f8 G I J

24. 24 Select Most Frequent Content Block A B D A BE A C E A D C F C D G B H I J I H J GI J f0 C F f1 C D G f2 A B D f3 A C E f4 A B E f5 A B D f6 H I J f7 I H J f8 G I J P≥3P≥3 W ≥ 90% Signature: W: target coverage in suspicious flow pool P: minimum occurrence to be selected

25. 25 Signature: A Select Most Frequent Content Block A B D A BE A C E A D C F C D G B H I J I H J GI J f0 C F f1 C D G f2 A B D f3 A C E f4 A B E f5 A B D f6 H I J f7 I H J f8 G I J P≥3P≥3 W ≥ 90% W: target coverage in suspicious flow pool P: minimum occurrence to be selected

26. 26 Select Most Frequent Content Block B DB A A A C E E A D F C C D G B H I J I H J GI J P≥3P≥3 W ≥ 90% Signature: A f0 C F f1 C D G f2 A B D f3 A C E f4 A B E f5 A B D f6 H I J f7 I H J f8 G I J W: target coverage in suspicious flow pool P: minimum occurrence to be selected

27. 27 Select Most Frequent Content Block F C C D G H I J I H J GI J P≥3P≥3 W ≥ 90% Signature: A f0 C F f1 C D G f2 A B D f3 A C E f4 A B E f5 A B D f6 H I J f7 I H J f8 G I J I W: target coverage in suspicious flow pool P: minimum occurrence to be selected

28. 28 Select Most Frequent Content Block F C C DG P≥3P≥3 W ≥ 90% Signature: A f0 C F f1 C D G f2 A B D f3 A C E f4 A B E f5 A B D f6 H I J f7 I H J f8 G I J I Signature: W: target coverage in suspicious flow pool P: minimum occurrence to be selected

29. 29 Agenda Autograph Overview Introduction & Motivation Design Goals Autograph System Design  Selecting Suspicious Traffic  Content-Based Signature Generation Evaluation  Local Signature Detection  Distributed Signature Detection Attacks & Limitations Conclusion & Future Work Questions & Answers

30. 30 Evaluation - Local Objectives  Effect of COPP parameters on signature quality Metrics  Sensitivity = # of true alarms / total # of worm flows  false negatives  Efficiency = # of true alarms / # of alarms  false positives Trace  Contains 24-hour http traffic  Includes 17 different types of worm payloads

31. 31 Signature Quality Larger block sizes generate more specific signatures A range of w (90-95%, workload dependent) produces a good signature

32. 32 Number of Signatures Smaller block sizes generate small # of signatures

33. 33 Agenda Autograph Overview Introduction & Motivation Design Goals Autograph System Design  Selecting Suspicious Traffic  Content-Based Signature Generation Evaluation  Local Signature Detection  Distributed Signature Detection Attacks & Limitations Conclusion & Future Work Questions & Answers

34. 34 Signature Generation Speed Bounded by worm payload accumulation speed  Aggressiveness of scanner detection heuristic s: # of failed connection peers to detect a scanner  # of payloads enough for content analysis  : suspicious flow pool size to trigger signature generation Single Autograph  Worm payload accumulation is slow Internet AAAAAAA tattler Distributed Autograph  Share scanner IP list  Tattler: limit bandwidth consumption within a predefined cap

35. 35 Benefit from tattler Worm payload accumulation (time to catch 5 worms) Signature generation  More aggressive scanner detection (s) and signature generation trigger (  )  faster signature generation, more false positives  With s=2 and  =15, Autograph generates the good worm signature before < 2% hosts get infected Info Sharing Autograph Monitor Fraction of Infected Hosts Aggressive (s = 1) Conservative (s = 4) None Luckiest2%60% Median25% -- TattlerAll<1%15% Many innocuous misclassified flows

36. 36 tattler A modified RTCP (RTP Control Protocol) Limit the total bandwidth of announcements sent to the group within a predetermined cap

37. 37 Agenda Autograph Overview Introduction & Motivation Design Goals Autograph System Design  Selecting Suspicious Traffic  Content-Based Signature Generation Evaluation  Local Signature Detection  Distributed Signature Detection Attacks & Limitations Conclusion & Future Work Questions & Answers

38. 38 Attacks & Limitation Overload due to flow reassembly Solutions  Multiple instances of Autograph on separate HW (port-disjoint)  Suspicious flow sampling under heavy load Abuse Autograph for DoS: pollute suspicious flow pool  Port scan and then send innocuous traffic Solution  Distributed verification of signatures at many monitors  Source-address-spoofed port scan Solution  Reply with SYN/ACK on behalf of non-existent hosts/services

39. 39 Agenda Autograph Overview Introduction & Motivation Design Goals Autograph System Design  Selecting Suspicious Traffic  Content-Based Signature Generation Evaluation  Local Signature Detection  Distributed Signature Detection Attacks & Limitations Conclusion & Future Work Questions & Answers

40. 40 Future Work Attacks Online evaluation with diverse traces & deployment on distributed sites Broader set of suspicious flow selection heuristics  Non-scanning worms (ex. hit-list worms, topological worms, email worms)  UDP worms Egress detection Distributed agreement for signature quality testing  Trusted aggregation

41. 41 Conclusion Stopping spread of novel worms requires early generation of signatures Autograph: automated signature detection system  Automated suspicious flow selection → Automated content prevalence analysis  COPP: robustness against payload variability  Distributed monitoring: faster signature generation Autograph finds sensitive & specific signatures early in real network traces

42. 42 Questions & Answers