The Dangers of Mitigating Security Design Flaws: A Wireless Case Study

1. The Dangers of Mitigating Security Design Flaws:A Wireless Case Study Nick Petroni Jr., William Arbaugh University of Maryland Presented by: Abe Murray CS577: Advanced Computer Networks

2. Outline • Abstract / Intro • WEP Overview • Attacks • Dictionary • Inductive • Authors’ Implementation • Implementation Results • “Mitigation” Angle • Closing CS577: Advanced Computer Networks

3. Abstract • Mitigating system flaws is hard to do right • But vendors do this all the time… • Design flaws are hard to patch • Often best approach is to re-architect system… • WLAN Security (WEP) • Shows the FUNDAMENTAL PREMISE that adding security after the fact is near impossible… CS577: Advanced Computer Networks

4. Introduction • The authors present a case study showing: • Mitigating one flaw worsens another flaw • Overall security remains the same • The authors develop an “inductive” attack against WEP: • 1st synchronous attack against WEP • Example of mitigation problem • Does not rely on knowledge of target network CS577: Advanced Computer Networks

5. Introduction • The authors present a case study showing: • Mitigating one flaw worsens another flaw • Overall security remains the same • The authors develop an “inductive” attack against WEP: • 1st synchronous attack against WEP • Example of mitigation problem • Does not rely on knowledge of target network CS577: Advanced Computer Networks

6. Outline • Abstract / Intro • WEP Overview • Attacks • Dictionary • Inductive • Authors’ Implementation • Implementation Results • “Mitigation” Angle • Closing CS577: Advanced Computer Networks

7. WEP Overview • IEEE 802.11 specification calls for “reasonably strong” protection • WEP - “Wired Equivalent Privacy” - fails to deliver • Protects at the Data Link Layer • Symmetric Stream RC4 cipher • Shared secret “k” • Secret used to generate stream of pseudorandom bytes equal in length to target plaintext • Encryption: • Decryption: CS577: Advanced Computer Networks

8. WEP Overview Graphic by Petroni and Arbaugh CS577: Advanced Computer Networks

9. Outline • Abstract / Intro • WEP Overview • Attacks • Dictionary • Inductive • Authors’ Implementation • Implementation Results • “Mitigation” Angle • Closing CS577: Advanced Computer Networks

10. Dictionary Attacks • Definition: Any brute-force attack in which a large table is used or generated • Relevance:RC4 – each key has unique associated pseudorandom stream used for encryption & decryption • Build dictionary of all streams (1 per IV)Don’t need key to participate in network! • IV size → 224 possible key streams, • WLAN MTU 2312 Bytes → ~40 GB Dictionary! CS577: Advanced Computer Networks

11. Inductive Attacks • Approach: Obtain full network access without knowing the key with minimal knowledge of target • HOW?Use known network protocols (redundantly encrypted data) to intelligently guess an initial number of encrypted bytes CS577: Advanced Computer Networks

12. Step 1: Guess the first byte(s): Graphic by Petroni and Arbaugh Table by Petroni and Arbaugh CS577: Advanced Computer Networks

13. Step 2: Guess the next byte: Graphic by Petroni and Arbaugh CS577: Advanced Computer Networks

14. The Author’s Attack • Attack System: • WLAN card operating in promiscuous mode (Intersil Prism 2 chipset) • Ability to directly manipulate transmitted bytes (OpenBSD 3.1 with modified drivers) • Attack Approach: • Choice between ICMP and SNAP/ARP • Choose ARP so at Layer 2, though both work CS577: Advanced Computer Networks

15. Outline • Abstract / Intro • WEP Overview • Attacks • Dictionary • Inductive • Authors’ Implementation • Implementation Results • “Mitigation” Angle • Closing CS577: Advanced Computer Networks

16. Implementation Results Table by Petroni and Arbaugh CS577: Advanced Computer Networks

17. Outline • Abstract / Intro • WEP Overview • Attacks • Dictionary • Inductive • Authors’ Implementation • Implementation Results • “Mitigation” Angle • Closing CS577: Advanced Computer Networks

18. “Mitigation” Angle Table by Petroni and Arbaugh CS577: Advanced Computer Networks

19. Outline • Abstract / Intro • WEP Overview • Attacks • Dictionary • Inductive • Authors’ Implementation • Implementation Results • “Mitigation” Angle • Closing CS577: Advanced Computer Networks

20. Closing Remarks • Authors showed how to mitigate their attack • Stop forwarding packets with bad data • Detect attack activity • Packet Filtering (though effectively cripples network) • Dynamic Rekeying • Neat attack all by itself • Interesting example of how patching bad security rarely works • Questions? CS577: Advanced Computer Networks