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

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

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

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: –1 st synchronous attack against WEP –Example of mitigation problem –Does not rely on knowledge of target network CS577: Advanced Computer Networks

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: –1 st synchronous attack against WEP –Example of mitigation problem –Does not rely on knowledge of target network CS577: Advanced Computer Networks

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

WEP Overview IEEE 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

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

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

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 → 2 24 possible key streams, WLAN MTU 2312 Bytes → ~40 GB Dictionary! CS577: Advanced Computer Networks

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

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

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

The Author’s Attack CS577: Advanced Computer Networks 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

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

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

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

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

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

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