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

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The Dangers of Mitigating Security Design
FlawsA Wireless Case Study

• Nick Petroni Jr., William Arbaugh
• University of Maryland

Presented by Abe Murray
CS577 Advanced Computer Networks
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Outline

• Abstract / Intro
• WEP Overview
• Attacks
• Dictionary
• Inductive
• Authors Implementation
• Implementation Results
• Mitigation Angle
• Closing

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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

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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

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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

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Outline

• Abstract / Intro
• WEP Overview
• Attacks
• Dictionary
• Inductive
• Authors Implementation
• Implementation Results
• Mitigation Angle
• Closing

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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

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WEP Overview
Graphic by Petroni and Arbaugh
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Outline

• Abstract / Intro
• WEP Overview
• Attacks
• Dictionary
• Inductive
• Authors Implementation
• Implementation Results
• Mitigation Angle
• Closing

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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) Dont
  need key to participate in network!
• IV size ? 224 possible key streams,
• WLAN MTU 2312 Bytes ? 40 GB Dictionary!

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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

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Step 1 Guess the first byte(s)
Graphic by Petroni and Arbaugh
Table by Petroni and Arbaugh
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Step 2 Guess the next byte
Graphic by Petroni and Arbaugh
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The Authors 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

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Outline

• Abstract / Intro
• WEP Overview
• Attacks
• Dictionary
• Inductive
• Authors Implementation
• Implementation Results
• Mitigation Angle
• Closing

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Implementation Results
Table by Petroni and Arbaugh
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Outline

• Abstract / Intro
• WEP Overview
• Attacks
• Dictionary
• Inductive
• Authors Implementation
• Implementation Results
• Mitigation Angle
• Closing

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Mitigation Angle
Table by Petroni and Arbaugh
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Outline

• Abstract / Intro
• WEP Overview
• Attacks
• Dictionary
• Inductive
• Authors Implementation
• Implementation Results
• Mitigation Angle
• Closing

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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?

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