Security Issues in Wireless LANs

Dr. John A. CopelandCommunications Systems Center

Georgia Tech Electrical & Computer Engineeringwww.csc.gatech.edu

copeland@ece.gatech.edu

ECE6612 - Slide Set 14

4/11/2011

Modems Internet VPNs Wireless Hubs

Network Tunnels

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http://www.telecommagazine.com/default.asp?journalid=3&func=articles&page=0203t11&year=2002&month=3

Network Tunnels

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IEEE 802.11g/n Wireless USB Adapter - $ 33

Wireless AP with 5-Port Ethernet Switch - $ 76

Anyone can convert their cube or office Ethernet jack into a Wireless Hub for their Laptop(and a public entry point into the Network)

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Network Tunnels

QuickTime™ and a decompressor

are needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.

[ The Atlanta Journal-Constitution: 3/31/02 ]

Wireless systems are simple to hack Terrorists could use techniques to attack airliners

By DON PLUMMER Atlanta Journal-Constitution Staff Writer

(Bill Corbitt) sits in his car in the short-term parking lot at an airport eating a sandwich, a Pringles can balanced on the dashboard.

After Corbitt left Hartsfield Airport, he located more than 100 wireless networks in an hour of driving around Atlanta. He did not actually break into any of them but later demonstrated how he could by intercepting signals from several wireless systems in use at The Atlanta Journal-Constitution.

"Some of these people are even transmitting their names and the exact locations of the wireless portals,” Corbitt said, pointing out the identifiers as they appeared on the screen of his laptop. Of the 120 wireless systems located, only 32 had activated the encryption protection included with the wireless software.

Bill Corbitt, Bulwarkz Defensive Solutionshttp://www.accessatlanta.com/ajc/news/0302/31wireless.html

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“Network Stumbler” - shows 802.11 Networks

WEP ON

Screen of laptop with Wireless LAN card

No

No

“AiroPeek” maps out who’s talking to who

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Data sniffed off the air from non-WEP session.

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http://airsnort.sourceforge.net

AirSnort is a wireless LAN (WLAN) tool which recovers encryption keys. AirSnort operates by passively monitoring transmissions, computing the encryption key when enough packets have been gathered.

802.11b, using the Wired Equivalent Protocol (WEP), is crippled with numerous security flaws. Most damning of these is the weakness described in " Weaknesses in the Key Scheduling Algorithm of RC4 " by Scott Fluhrer, Itsik Mantin and Adi Shamir. Adam Stubblefield was the first to implement this attack, but he has not made his software public. AirSnort, along with WEPCrack, which was released about the same time as AirSnort, are the first publicly available implementations of this attack.

AirSnort requires approximately 5-10 million encrypted packets to be gathered. Once enough packets have been gathered, AirSnort can guess the encryption password in under a second.

AirSnort - a tool for Cracking WEP Messages

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How Does “Wired Equivalent Protocol (WEP) Work”

What’s wrong with it?

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“The Insecurity of 802.11, an analysis of the Wired Equivalent Privacy protocol”, Black Hat Briefings, 11 July, 2001, Ian Goldberg, Zero-Knowledge Systems, ian@zeroknowledge.com with Nikita Borisov and David Wagner, UC Berkeley).

http://www.cypherpunks.ca/bh2001/index.html

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Wired Equivalent Protocol

V, Key

V varies, but V is sent in the clear. Frames with the same V will have the same "Keystream"

The One-Time Pad

The most secure encryption technique is a “One-Time Pad,” if the Pad is truly random (in WEP it is not).

WEP uses the RC4 encryption algorithm (with a 40 or 112 bit key) to generate a stream of random looking bits, Pi. These are XORed with the message bits, Mi, to produce the cyphertext bits, Ci

Ci = Mi (+) Pi

The receiver has the same key, and can generate a duplicate “pad”, Pi, and recover the original plaintext message, Mi.

Ci (+) Pi = Mi (+) Pi (+) Pi = Mi

since Pi (+) Pi = 0 and Mi (+) 0 = Mi

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“Two-Time” Pad Problem

The least secure encryption technique is a “One-Time Pad” - used twice (or repeatedly).

Consider two intercepted messages (C and D are encrypted M and N) encrypted with the same pad:

C = M (+) P and D = N (+) P

The cracker can XOR these together to get M (+) N:

C (+) D = M (+) P (+) N (+) P = M (+) N

Whenever a byte in M (+) N is zero, M & N have the same byte. One of six bytes in English text is “space”. so one in 36 bytes in M+N is zero because both text strings have spaces there. Given several thousand characters of M+N, the messages can be deciphered.

Then the key sequence can be found: P = N (+) D.13

The pad, or keystream, for WEP is RC4(v,k) which depends only on v and k. “k” is a shared secret that changes rarely, if ever (in most systems (like GTwireless) every user uses the same value of k). So the keystream depends only on the 24-bit value of v.

Since v is transmitted in the clear, the Cracker collects messages until he has two with the same value of v, then he is “in.”

There are 2^24 = 16 million different values of v, but because of the “Birthday Effect,” he is likely to have two that match after collecting only about 6,000 messages.

Number of different pairs = 6000 x 5999 / 2 = 18 million

Even worse, Goldberg reports that all the 802.11 cards observed reset their random number generator for v each time they are activated, meaning they reuse the same sequence of v values (2002).

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“Two-Time” Pad Problem

Defense

These help WEP, but are not totally dependable:

Use the 112-bit key mode, rather than no key or 40-bit key.

Use an access list of MAC (Ethernet) addresses at the hub.

Use a new hub that has WPA (22+ random-character passphrase), or IEEE 802.11i, and only network cards that are fully compatible.

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Wi-Fi Protected Access(WPA upgrades WEP)

IEEE 802.1x authentication is required.

TKIP encryption in WPA-Personal has unique unicast key (weak)

8-bit “Michael” Message Integrity Check (MIC)

Frame counter prevents replay attacks

AES was optional (pre 2011). Some card CPU’s can not manage.

Can support older WEP cards (but no dynamic keys)

802.11 Beacon Frames contain a WEP info element.

WPA has been supported by Windows XP and Apple OSX (2006).

Gtwireless started offering a WPA2 network in 2010 (PEAP, MSCHAPv2).

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WPA vs. WEP

1. Minimum key length increased from 40 to 256 bits

(four keys made from 8 to 63 character passcode).

2. IV (Initial Vector) length was doubled.

3. IV resequencing enforced.

4. Key rotation embedded automatically.

5. Mutual authentication required.

6. MIC to prevent packet tampering.

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Attack code("coWEPtty") can crack passcode "abc123abc123" in less than a minute.* Longer (>22 characters) and more random codes are reasonable secure.**

*H. Berghel, J. Uecker, WiFi Attack Vectors", Comm. ACM, pp 21-28, Aug. 2005.** R. Moskowitz, http://wifinetnews.com/archives/002452.html

WPA2 (IEEE 802.11i)WPA2 implements the mandatory elements of 802.11i

(a, b, g and n define bit rates, operating frequency, ...)

It introduces a new AES-based algorithm, CCMP, that is considered fully secure. RC4 is no longer allowed.After March 13, 2006, WPA2 certification is mandatory for all new devices wishing to be "WiFi" certified.

But if compatibility with older WiFi interfaces is necessary, WPA2, or even WPA, can not be turned on. That's why GTwireless is still available (can be sniffed with Wireshark).

"WiFi Protected Setup" (WPS, 2007-2012) should be turned off. It may be on by default. An attacker can discover the 7-digit PIN and thus the WPA/WPA2 pre-shared key in a few hours.

Note: 7 digits is equivalent to (10/3)*7 = 23 bits

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802.11x, EAP and LEAP

IEEE 802.11x defines a structure for authenticated layer-2 access to any IEEE 802 network (3rd party, CA or AS).

EAP (extensible authentication protocol) allows supplicant (client) to exchange credentials with the AS (authentication server).

LEAP (lightweight EAP) available on Cisco and some Linksys access points. Most popular EAP, but flawed by use of MS-CHAPv2 (NT hashing does not use salt). Attack code "asleap" can discover LEAP passwords using an offline dictionary attack.*

CHAP - Challenge Authentication Protocol - hashes a challenge nonce with password and returns.

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* H. Berghel, J. Uecker, "WiFi Attack Vectors", Comm. ACM, pp 21-28, Aug. 2005

Application

Transport Layer(TCP,UDP)

Network Layer (IP)

802.11Link Layer

Phys. Layer

Network Layer

802.11Link Layer

802.11

Network Layer

Process Process

RouterBuffers Packets thatneed to be forwarded(based on IP address).

Application

Transport Layer(TCP,UDP)

Network Layer (IP)

Data-Link Layer

Phys. Layer

Data Link Layer

Phys. Layer

IPsecIPsec

SSL,TLS SSL,TLS

802.11

Ethernet

EthernetEthernet

EthernetWEP,WPA

Phys. Layer

WEP,WPA

Defense - Higher LevelSecure Protocols

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Defense - War DrivingPatrol the Premises Frequently

looking for Rogue Wireless Signals

Purchase only 802.11 Hubs and PC Cards that have the improved Security standards, or flash memory and can be field upgraded.

Treat all wireless communication channels as "public." Use only secure protocols: VPN, Radius, SSH, SSL, … .

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Wireless Hub

Wireless Subnets should be treated like attachments to the Internet, isolated by Firewalls and Intrusion

Detection Systems

Wireless Sub-Nets can Safely be partof Corporate Networks - if Properly Isolated

IDS

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"Hot Spots" (e.g.Starbucks, airports)

They are dangerous because no encryption is used.

Anything sent in plaintext is viewable by anyone nearby, or across the street with a directional antenna.

Plaintext protocols are: FTP, HTTP (HTML), TELNET, and email without TLS (SNMP, POP, IMAP).

Encrypted protocols are: HTTPS, SSH, SCP, IPsec, and other VPNs, and email with TLS.

Firesheep - HTTP session hijacking

It's extremely common for Web sites to protect your password by encrypting the initial login, but surprisingly uncommon for Web sites to encrypt everything else. This leaves the cookie (and the user) vulnerable. After installing the Firesheep extension you'll see a new sidebar. Connect to any busy open WiFi network and click the big "Start Capturing" button. Then wait. As soon as anyone on the network visits an insecure Web site known to Firesheep, their name and photo will be displayed.

24http://codebutler.com/firesheep

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GTwireless

Uses 40-bit WEP and the key is an open secret - so all traffic is viewable.

IP addresses are assigned by DHCP (to a wireless MAC address) and are blocked until authentication through an HTTPS Web site. (March 2010 - GT requires MAC addresses to be registered)

-but IP address - Mac/IP address pairs are viewable and can be spoofed after a legitimate user goes off the network (voluntarily or DoS'ed).

"Evil Twin" attacks are possible if the victim does not notice the absence of the authentication step, or that the authentication page is HTTP (rather than HTTPS).

Evil Twin Attack

AuthenticationServer---------------Router

GT Network Internet ISP

Hacker’sAuthenticationSpoofer &Man-in-Middle

SSID: G

Twireles

s

WEP: a2b3…SSID: GTwirelessWEP: a2b3…

Geo. P. Burdell

Hacker can see all data, do a Man-in-the-Middle Attack