Enhancing the Security of Corporate Wi-Fi Networks Using DAIR

1. Enhancing the Security of Corporate Wi-Fi Networks Using DAIR Paramvir Bahl, Ranveer Chandra, Jitendra Padhye, Lenin Ravindranath, Manpreet Singh**,Alec Wolman, Brian Zill Microsoft Research **Cornell University

2. Corporations becoming increasingly dependent on WLAN infrastructure Worldwide enterprise WLAN business expected to grow from $1.1 billion this year to $3.5 billion in 2009 Wi-Fi networks are vulnerable to many threats Rogue AP, Denial of Service, Phishing … DefCon 2005 : Wi-Fi Pistol, Wi-Fi Sniper Rifle, Wi-Fi Bouncing, AirSnarf box Motivation

3. Example : Rogue AP • Careless employee brings AP from home and plugs it into corporate Ethernet • Bypasses corporate Wi-Fi security measures • For example: WPA, 802.1X • Permits unauthorized users to connect to corporate network • Malicious user outside the building? • Widespread Problem • Ongoing concern for MS IT department • Surveyed two major US universities, found multiple rogue APs

4. Need for WiFi Monitoring Systems • Preventive measures such as 802.1X do not guarantee full security • In addition, need WiFi monitoring system to detect problems in operational WiFi networks • Detect Rogue AP by overhearing packets containing unknown BSSID

5. Challenges in Building an Enterprise-scale WiFi Monitoring System • Scale of WLAN • Microsoft’s WLAN has over 5000 APs • Need to deploy many monitors • Rapid fading of signal in indoor environment • Multiple orthogonal channels • May need observations from multiple vantage points • Pinpoint location of rogue AP

6. Example Scenario X X X X X Monitors Rogue AP and Client Demonstrates need for dense deployment of monitors

7. State of the Art AP-based monitoring [Aruba, AirDefense ..] Pros: Easy to deploy (APs are under central control) Cons: Single radio APs can not be effective monitors Specialized sensor boxes [Aruba, AirTight, …] Pros: Can provide detailed signal-level analysis Cons: Expensive, so can not deploy densely Monitoring by mobile clients [Adya et. al., MobiCom’04] Pros: Inexpensive, suitable for un-managed environments Cons: Coverage not predictable: mobile, battery-powered clients Only monitor the channel they are connected on

8. Observation Desktop PC’s with good wiredconnectivity are ubiquitous in enterprises Outfitting a desktop PC with 802.11 wireless is inexpensive Wireless USB dongles are cheap As low as $6.99 at online retailers PC motherboards are starting to appear with built-in 802.11 radios + Combine to create a dense deployment of wireless sensors DAIR: Dense Arrayof Inexpensive Radios

9. AirMonitor DAIR Architecture Land Monitor (1 per subnet) AirMonitor Wired Network Other data: SNMP, Configuration Inference Engine Database

10. Monitor Architecture SQL Helper Database Every 30 seconds: Submit list of all unique BSSIDs seen on a given channel Filter Filter Filter Processor Driver Interface Wireless NIC Driver Wired NIC Driver

11. Key Characteristics of DAIR High sensor density at low cost Leverages existing desktop resources Effective monitoring in indoor environments Can tolerate loss of a few sensors Sensors are (mostly) stationary Provides predictable coverage Permits meaningful historical analysis

12. Applications of the DAIR Platform Security applications Detecting attacks on Wi-Fi networks Responding to such attacks Performance management Monitor RF coverage Load balancing Location service to support above applications

13. A Partial List of Threats to Wi-Fi Networks • Rogue AP / Rogue Wireless Networks • Denial of service • Fake Disassociation [Bellardo and Savage 2003] • NAV attack [Bellardo and Savage, 2003] • DIFS attack [Raya, Hubaux and Aad 2004] • Jamming • Phishing • Set up a “fake” AP that advertises well known SSID • Lure unsuspecting users • Acquire passwords

14. Rogue Wireless Networks • An uninformed or careless employee who doesn’t understand (or chooses not to think about) the security implications • Brings AP from home, and attaches it to the corporate network • Configures desktop PC with wireless interface to create a rogue ad-hoc network • Bypasses security measures such as WPA, 802.1X

15. AirMonitor Simple Solution AirMonitor Database Known: Seen: Inference Engine

16. Problem with the Simple Solution • False Positives • Multi-office buildings • False negatives • Malicious attacker fakes authorized SSID / BSSID • DAIR can help reduce both false positives and false negatives • No foolproof way to avoid false positives/negatives completely • DAIR raises bar while generating fewer alarms

17. Reducing False Positives • Detect whether rogue AP is connected to corporate wired network • Series of tests: • Association test • Source/destination address test • Replay test

18. Association Test ? AirMonitor Database Inference Engine Machine inside corporate firewall If AirMonitor can connect to machine inside firewall via AP then AP is connected to corporate wired network

19. Association Test • Test will fail if AP uses WEP or MAC address filtering • People configure home APs with WEP or MAC filtering • Failure means we need additional tests …

20. Source / Destination Address Test ? AirMonitor Land Monitor Database Inference Engine MAC Addrs Of Subnet Routers Subnet Router

21. Source / Destination Address Test 802.11 Data Frame (with encryption): Unencrypted Header Encrypted Payload MAC Addresses: Receiver Transmitter Destination Access Point Client Known Address? If Destination Address belongs to a subnet router, then AP Is connected to corporate wired network Similar test for Source Address

22. Source / Destination Address Test • Test will fail if AP is really a NAT/Router • Many home APs combine AP and NAT/router functionality • Failure means that additional tests are needed

23. Replay Test X 1 2 3 4 ? AirMonitor X ? X X X Inference Engine Land Monitor AirMonitors capture data packets At the same time LandMonitors are alerted to watch for duplicate packets on wired network. One of the AirMonitors replays captured packets Each packet replayed multiple times

24. Replay Test • No need to decrypt packets • Works for NAT/Routers • Even rogue ad-hoc networks • Fails if replay-resistant crypto scheme is used • WPA2

25. Scalability • Load on database server • Load on individual AirMonitors • Additional wired network traffic

26. Load on Database Server 100 80 60 CPU Load (%) 40 20 0 1AM 5AM 9AM 1PM 5PM 9PM 1AM 12 AirMonitors AirMonitors submit summarized data every 2 minutes Database Server: MS-SQL 2005, 1.7GHz P4 with 1GB RAM

27. Machine running AirMonitor 100 75 50 Load (%) 25 0 1AM 5AM 9AM 1PM 5PM 9PM 1AM Machine not running AirMonitor 100 75 Load (%) 50 25 0 1AM 5AM 9AM 1PM 5PM 9PM 1AM Load on Client Machine Additional Network Traffic: 2-5Kbps per AirMonitor

28. Summary • Built a scalable, cost-effective, dense WLAN monitoring platform in a corporate environment • Explored ways to leverage the platform to monitor threats to Wi-Fi networks

29. Related Work • Campus-wide Wi-Fi monitoring system [Kotz and Essin 2005] • Monitoring corporate network for mobility patterns [Balazinska and Castro 2003] • Tools for analysis of packet-level Wi-Fi traces • WIT [Mahajan et. al. 2006] • JigSaw [Cheng et. al. 2006]

30. DAIR ongoing work • Which channels should each AirMonitor listen on? • What scanning strategy to use? [Deshpande et. al. 2006] • Depends on density of AirMonitors, environment • Building an effective location system • Building performance management tools

31. Backup slides

32. Wired Solutions • Monitor CAM tables for unauthorized Ethernet addresses • Not scalable • Easy to fake Ethernet address • Monitor DHCP requests, deny from unauthorized clients • Bypassed using authorized client as forwarder • IPSec • Not widely used: hard to manage in heterogeneous environments • Bypassed using authorized clients acting as forwarders • Many machines on corporate LANs do not use IPSec • Management servers on switches, printers • Gateway machines

33. Reducing False Negatives • Suspect is using an “authorized” SSID / BSSID • If the “real” AP is still active • Packet sequence numbers not monotonic • If real AP is not active • Determine location of suspect • If different than expected, raise alarm

34. Rogue AP and Client Monitors Example: Indoor WLAN Monitoring 0% 0% 26% 0% 0% 0% 97% 1.7% 0% 0% %0 %0 Rapid loss of signal strength in indoor environments Complex, time-varying signal propagation Red: Beacon reception rate Blue: Data packet reception rate

36. Taxonomy of Attacks on Wi-Fi Networks • Eavesdropping • Passive snooping (perhaps with high-gain antennas) • Nearly impossible to detect • Cryptographic techniques generally considered sufficient. • Intrusion • Rogue AP / Rogue Ad-hoc network • Cryptographic techniques not enough, need continuous monitoring • Denial of Service • Fake deauthentication/disassociation, NAV attacks • Need monitoring system. • Phishing

37. Enterprise-scale WLAN Monitoring System Challenges and Design Requirements • Rapid fading in indoor environments • Complex, time-varying signal propagation • Many orthogonal channels • Need information from many monitors • Dense deployment of monitors • Monitors must be self-configuring • Scalable data gathering and processing • Must cope with incomplete data

38. Replay Test • AirMonitors replay packets with suspect BSSID • If suspect is AP, only replay packets with ToDS bit set • No need to decrypt packet • Each packet is replayed multiple times (say 5) • LandMonitors detect if duplicate packets are seen on wired network • Works for rogue ad-hoc networks • Fails if suspect is using WPA2 or other crypto schemes that are robust against replay attacks

39. Monitor Architecture