CWNA Guide to Wireless LANs, Second Edition

Chapter EightWireless LAN Security and Vulnerabilities

Objectives

• Define information security

• Explain the basic security protections for IEEE 802.11 WLANs

• List the vulnerabilities of the IEEE 802.11 standard

• Describe the types of wireless attacks that can be launched against a wireless network

Security Principles: What is Information Security?

• Information security: Task of guarding digital information– Ensures protective measures properly implemented– Protects confidentiality, integrity, and availability

(CIA) on the devices that store, manipulate, and transmit the information through products, people, and procedures

Security Principles: What is Information Security? (continued)

Information security components

Security Principles: Challenges of Securing Information

• Trends influencing increasing difficultly in information security:– Speed of attacks– Sophistication of attacks– Faster detection of weaknesses

• Day zero attacks

– Distributed attacks• The “many against one” approach

• Impossible to stop attack by trying to identify and block source

Security Principles: Categories of Attackers

• Six categories of attackers:– Hackers

• Not malicious; expose security flaws

– Crackers– Script kiddies– Spies– Employees– Cyberterrorists

Security Principles: Categories of Attackers (continued)

Attacker profiles

Security Principles: Security Organizations

• Many security organizations exist to provide security information, assistance, and training– Computer Emergency Response Team Coordination

Center (CERT/CC)– Forum of Incident Response and Security Teams

(FIRST)– InfraGard– Information Systems Security Association (ISSA)– National Security Institute (NSI)– SysAdmin, Audit, Network, Security (SANS) Institute

Basic IEEE 802.11 Security Protections

• Data transmitted by a WLAN could be intercepted and viewed by an attacker– Important that basic wireless security protections be

built into WLANs

• Three categories of WLAN protections:– Access control– Wired equivalent privacy (WEP)– Authentication

• Some protections specified by IEEE, while others left to vendors

Access Control

• Intended to guard availability of information• Wireless access control: Limit user’s admission to

AP– Filtering

• Media Access Control (MAC) address filtering: Based on a node’s unique MAC address

MAC address

Access Control (continued)

MAC address filtering

Access Control (continued)

• MAC address filtering considered to be a basic means of controlling access– Requires pre-approved authentication– Difficult to provide temporary access for “guest”

devices

Wired Equivalent Privacy (WEP)

• Guard the confidentiality of information– Ensure only authorized parties can view it

• Used in IEEE 802.11 to encrypt wireless transmissions– “Scrambling”

WEP: Cryptography

• Cryptography: Science of transforming information so that it is secure while being transmitted or stored– scrambles” data

• Encryption: Transforming plaintext to ciphertext

• Decryption: Transforming ciphertext to plaintext

• Cipher: An encryption algorithm– Given a key that is used to encrypt and decrypt

messages– Weak keys: Keys that are easily discovered

WEP: Cryptography (continued)

Cryptography

WEP: Implementation

• IEEE 802.11 cryptography objectives:– Efficient– Exportable– Optional– Reasonably strong– Self-synchronizing

• WEP relies on secret key “shared” between a wireless device and the AP– Same key installed on device and AP– Private key cryptography or symmetric

encryption

WEP: Implementation (continued)

Symmetric encryption

WEP: Implementation (continued)

• WEP shared secret keys must be at least 40 bits– Most vendors use 104 bits

• Options for creating WEP keys:– 40-bit WEP shared secret key (5 ASCII characters or

10 hexadecimal characters)– 104-bit WEP shared secret key (13 ASCII characters

or 16 hexadecimal characters)– Passphrase (16 ASCII characters)

• APs and wireless devices can store up to four shared secret keys– Default key used for all encryption

WEP: Implementation (continued)

Default WEP keys

WEP: Implementation (continued)

WEP encryption process

WEP: Implementation (continued)

• When encrypted frame arrives at destination:– Receiving device separates IV from ciphertext – Combines IV with appropriate secret key

• Create a keystream

– Keystream used to extract text and ICV– Text run through CRC

• Ensure ICVs match and nothing lost in transmission

• Generating keystream using the PRNG is based on the RC4 cipher algorithm– Stream Cipher

WEP: Implementation (continued)

Stream cipher

Authentication

• IEEE 802.11 authentication: Process in which AP accepts or rejects a wireless device

• Open system authentication: – Wireless device sends association request frame to

AP• Carries info about supported data rates and service

set identifier (SSID)

– AP compares received SSID with the network SSID• If they match, wireless device authenticated

Authentication (continued)

• Shared key authentication: Uses WEP keys– AP sends the wireless device the challenge text– Wireless device encrypts challenge text with its WEP

key and returns it to the AP– AP decrypts returned result and compares to original

challenge text• If they match, device accepted into network

Vulnerabilities of IEEE 802.11 Security

• IEEE 802.11 standard’s security mechanisms for wireless networks have fallen short of their goal

• Vulnerabilities exist in:– Authentication– Address filtering– WEP

Open System Authentication Vulnerabilities

• Inherently weak– Based only on match of SSIDs– SSID beaconed from AP during passive scanning

• Easy to discover

• Vulnerabilities:– Beaconing SSID is default mode in all APs– Not all APs allow beaconing to be turned off

• Or manufacturer recommends against it

– SSID initially transmitted in plaintext (unencrypted)

Open System Authentication Vulnerabilities (continued)

• Vulnerabilities (continued):– If an attacker cannot capture an initial negotiation

process, can force one to occur– SSID can be retrieved from an authenticated device– Many users do not change default SSID

• Several wireless tools freely available that allow users with no advanced knowledge of wireless networks to capture SSIDs

Open System Authentication Vulnerabilities (continued)

Forcing the renegotiation process

Shared Secret Key Authentication Vulnerabilities

• Attackers can view key on an approved wireless device (i.e., steal it), and then use on own wireless devices

• Brute force attack: Attacker attempts to create every possible key combination until correct key found

• Dictionary attack: Takes each word from a dictionary and encodes it in same way as passphrase– Compare encoded dictionary words against

encrypted frame

Shared Secret Key Authentication Vulnerabilities (continued)

• AP sends challenge text in plaintext– Attacker can capture challenge text and device’s

response (encrypted text and IV) • Mathematically derive keystream

Shared Secret Key Authentication Vulnerabilities (continued)

Table 8-2: Authentication attacks

Address Filtering Vulnerabilities

MAC address attacks

WEP Vulnerabilities

• Uses 40 or 104 bit keys– Shorter keys easier to crack

• WEP implementation violates cardinal rule of cryptography– Creates detectable pattern for attackers– APs end up repeating IVs

• Collision: Two packets derived from same IV– Attacker can use info from collisions to initiate a

keystream attack

WEP Vulnerabilities (continued)

8-13: XOR operations

WEP Vulnerabilities (continued)

8-14: Capturing packets

WEP Vulnerabilities (continued)

• PRNG does not create true random number – Pseudorandom– First 256 bytes of the RC4 cipher can be determined

by bytes in the key itself

WEP attacks

Other Wireless Attacks: Man-in-the-Middle Attack

• Makes it seem that two computers are communicating with each other– Actually sending and receiving data with computer

between them– Active or passive

Intercepting transmissions

Other Wireless Attacks: Man-in-the-Middle Attack (continued)

Wireless man-in-the-middle attack

Other Wireless Attacks: Denial of Service (DoS) Attack

• Standard DoS attack attempts to make a server or other network device unavailable by flooding it with requests– Attacking computers programmed to request, but not

respond

• Wireless DoS attacks are different:– Jamming: Prevents wireless devices from

transmitting– Forcing a device to continually dissociate and re-

associate with AP