- Given a Scenario, Implement Common Protocols and Services
- Given a Scenario, Troubleshoot Security Issues Related to Wireless Networking
- What Next?
Given a Scenario, Troubleshoot Security Issues Related to Wireless Networking
- MAC filter
- Disable SSID broadcast
- Antenna placement
- Power-level controls
- Captive portals
- Antenna types
- Site surveys
- VPN (over open wireless)
Wireless security comes in two major varieties: Wired Equivalent Privacy (WEP) and Wi-Fi Protected Access (WPA). Both include methods to encrypt wireless traffic between wireless clients and WAPs. WEP has been included in 802.11–based products for some time and includes a strategy for restricting network access and encrypting network traffic based upon a shared key. The Wi-Fi Protected Access (WPA and WPA2) standards were developed by the Wi-Fi Alliance to replace the WEP protocol. WPA was developed after security flaws were found in WEP. WPA protects networks by incorporating a set of enhanced security features. WPA-protected networks require users to enter a passkey to access a wireless network. There are two different modes of WPA: WPA-PSK (Personal Shared Key) mode and WPA-802.1X mode, which is more often referred to as WPA-RADIUS or WPA-Enterprise. For the PSK mode, a passphrase consisting of 8 to 63 ASCII characters is all that is required. The Enterprise mode requires the use of security certificates. WPA includes many of the functions of the 802.11i protocol but relies on Rivest Cipher 4 (RC4), which is considered vulnerable to keystream attacks.
WPA2 is based on the IEEE 802.11i standard and provides government-grade security by implementing the AES encryption algorithm and 802.1X-based authentication. AES is a block cipher that encrypts 128-bit blocks of data at a time with a 128-bit encryption key. WPA2 incorporates stricter security standards and is configurable in either the PSK or Enterprise mode. There are two versions of WPA2: WPA2-Personal and WPA2-Enterprise. WPA2-Personal protects unauthorized network access via a password. WPA2-Enterprise verifies network users through a server. WPA2 is backward compatible with WPA and supports strong encryption and authentication for both infrastructure and ad hoc networks. In addition, it has support for the CCMP (Counter Mode with Cipher Block Chaining Message Authentication Code Protocol) encryption mechanism based on the Advanced Encryption Standard (AES) cipher as an alternative to the Temporal Key Integrity Protocol (TKIP). TKIP is an encryption protocol included as part of the IEEE 802.11i standard for WLANs. An AES-based encryption mechanism that is stronger than TKIP.
Wired Equivalent Privacy (WEP) is the most basic form of encryption that can be used on 802.11-based wireless networks to provide privacy of data sent between a wireless client and its AP. Originally, many wireless networks were based on the IEEE 802.11 standard, which had serious data transmission security shortcomings. When this standard was put into place, the 802.11 committee adopted an encryption protocol called WEP. To discuss WEP’s shortcomings, we have to understand how it operates. WEP uses a stream cipher for encryption called RC4. RC4 uses a shared secret key to generate a long sequence of bytes from what is called a generator. This stream is then used to produce the encrypted ciphertext. Early 802.11b networks used 40-bit encryption because of government restrictions. Hackers can crack a 40-bit key in a few hours. It is much easier to break RC4 encryption if a second instance of encryption with a single key can be isolated. In other words, the weakness is that the same keys are used repeatedly. Specifications for the WEP standard are detailed within the 802.11b (Wi-Fi) specification. This specification details a method of data encryption and authentication that may be used to establish a more secured wireless connection.
Although using WEP is much better than no encryption at all, it’s important to understand its limitations so that you have an accurate picture of the consequences and what you must do to properly protect your wireless environment.
The 802.1X standard is a means of wireless authentication. The 802.1X authentication standard is an extension of point-to-point protocol (PPP) that relies on the Extensible Authentication Protocol (EAP) for its authentication needs. EAP is a challenge-response protocol that can be run over secured transport mechanisms. It is a flexible authentication technology and can be used with smart cards, one-time passwords, and public key encryption. It also allows for support of public certificates deployed using auto enrollment or smart cards. These security improvements enable access control to Ethernet networks in public places such as malls and airports. EAP-Transport Layer Security (EAP-TLS) uses certificate-based mutual authentication, negotiation of the encryption method, and encrypted key determination between the client and the authenticating server.
EAP messages are encapsulated into 802.1X packets and are marked as EAP over LAN (EAPOL). After the client sends a connection request to a wireless AP, the authenticator marks all initial communication with the client as unauthorized, and only EAPOL messages are accepted while in this mode. All other types of communication are blocked until credentials are verified with an authentication server. Upon receiving an EAPOL request from the client, the wireless AP requests login credentials and passes them on to an authentication server. Remote Authentication Dial-In User Service (RADIUS) is usually employed for authentication purposes; however, 802.1X does not make it mandatory.
Protected EAP (PEAP) was co-developed by Cisco, Microsoft Corporation, and RSA Security, Inc. PEAP provides several additional benefits within TLS, including an encrypted authentication channel, dynamic keying material from TLS, fast reconnect using cached session keys, and server authentication that protects against the setting up of unauthorized access points. PEAP is a means of protecting another EAP method (such as MS-CHAPv2) within a secure channel. The use of PEAP is essential to prevent attacks on password-based EAP methods. As part of the PEAP negotiation, the client establishes a TLS session with the RADIUS server. Using a TLS session as part of PEAP serves a number of purposes:
- It allows the client to authenticate the RADIUS server; this means that the client only establishes the session with a server holding a certificate that is trusted by the client.
- It protects the MS-CHAPv2 authentication protocol against packet snooping.
- The negotiation of the TLS session generates a key that can be used by the client and RADIUS server to establish common master keys. These keys are used to derive the keys used to encrypt the WLAN traffic.
Secured within the PEAP channel, the client authenticates itself to the RADIUS server using the MS-CHAPv2 EAP protocol. During this exchange, the traffic within the TLS tunnel is visible only to the client and RADIUS server and is never exposed to the WAP.
Lightweight Extensible Authentication Protocol (LEAP) combines centralized two-way authentication with dynamically generated wireless equivalent privacy keys or WEP keys. LEAP was developed by Cisco for use on WLANs that use Cisco 802.11 wireless devices. LEAP is a proprietary EAP method because it requires the use of a Cisco AP. It features mutual authentication; secure session key derivation; and dynamic per-user, per-session WEP keys. However, because it uses unencrypted challenges and responses, LEAP is vulnerable to dictionary attacks. Still, when LEAP is combined with a rigorous user password policy, it can offer strong authentication security without the use of certificates. LEAP can only authenticate the user to the WLAN, not the computer. Without computer authentication, machine group policies will not execute correctly.
Most wireless network routers and access points can filter devices based on their Media Access Control (MAC) address. The MAC address is a unique identifier for network adapters. MAC filtering is a security access control method whereby the MAC address is used to determine access to the network. When MAC address filtering is used, only the devices with MAC addresses configured in the wireless router or access point are allowed to connect. MAC filtering permits and denies network access through the use of blacklists and whitelists. A blacklist is a list of MAC addresses that are denied access. A whitelist is a list of MAC addresses that are allowed access. Blacklisting and whitelisting are discussed in further detail in Chapter 8, “Host Security.”
While giving a wireless network some additional protection, it is possible to spoof the MAC address. An attacker could potentially capture details about a MAC address from the network and pretend to be that device in order to connect. MAC filtering can be circumvented by scanning a valid MAC using a tool such as airodumping and then spoofing one’s own MAC into a validated MAC address. After an attacker knows a MAC address that is out of the blacklist or within the whitelist, MAC filtering is almost useless.
Disable SSID Broadcast
A service set identifier (SSID) is used to identify WAPs on a network. The SSID is transmitted so that wireless stations searching for a network connection can find it. By default, SSID broadcast is enabled. This means that it accepts any SSID. When you disable this feature, the SSID configured in the client must match the SSID of the AP; otherwise, the client does not connect to the AP. Having SSID broadcast enabled essentially makes your AP visible to any device searching for a wireless connection.
To improve the security of your network, change the SSIDs on your APs. Using the default SSID poses a security risk even if the AP is not broadcasting it. When changing default SSIDs, do not change the SSID to reflect your company’s main names, divisions, products, or address. This just makes you an easy target for attacks such as war driving and war chalking. War driving is the act of searching for Wi-Fi wireless networks by a person in a moving vehicle, using a portable computer or other mobile device. War chalking is the drawing of symbols in public places to advertise an open Wi-Fi network. Keep in mind that if an SSID name is enticing enough, it might attract hackers.
Turning off SSID broadcast does not effectively protect the network from attacks. Tools such as Kismet enable nonbroadcasting networks to be discovered almost as easily as broadcasting networks. From a security standpoint, it is much better to secure a wireless network using protocols that are designed specifically to address wireless network threats than to disable SSID broadcast.
Temporal Key Integrity Protocol (TKIP) is the security protocol designed to replace WEP and is also known by its later iterations of Wi-Fi Protected Access (WPA) or WPA2. Similar to WEP, TKIP uses the RC4 algorithm and does not require an upgrade to existing hardware, whereas more recent protocols, such as CCMP, which use the AES algorithm, do require an upgrade. TKIP was designed to provide more secure encryption than WEP by using the original WEP programming, but it wraps additional code at the beginning and end to encapsulate and modify it. To increase key strength, TKIP includes four additional algorithms: a cryptographic message integrity check, an IV sequencing mechanism, a per-packet key-mixing function, and a rekeying mechanism.
TKIP is useful for upgrading security on devices originally equipped with WEP, but does not address all security issues and might not be reliable enough for sensitive transmission. AES is a better choice and has become the accepted encryption standard for WLAN security.
Counter Mode with Cipher Block Chaining Message Authentication Code Protocol (CCMP) is an encryption protocol that forms part of the 802.11i standard for WLANs. CCMP offers enhanced security compared with similar technologies such as TKIP. AES is a block cipher that encrypts 128-bit blocks of data at a time with a 128-bit encryption key. The AES cipher suite uses the Counter-Mode Cipher Block Chaining (CBC) Message Authentication Code (MAC) Protocol (CCMP) as defined in RFC 3610. CCMP uses 128-bit keys with a 48-bit IV that reduces vulnerability to replay attacks. To provide for replay protection, a packet number (PN) field is used. CCMP produces a message integrity code (MIC) that provides data origin authentication and data integrity for the packet payload data. The PN is included in the CCMP header and incorporated into the encryption and MIC calculations. Counter mode makes it difficult for an eavesdropper to spot patterns, and the CBC-MAC message integrity method ensures that messages have not been tampered with.
When designing wireless networks, antenna placement and power output should be configured for maximum coverage and minimum interference. Four basic types of antennas are commonly used in 802.11 wireless networking applications: parabolic grid, yagi, dipole, and vertical. APs with factory-default omni antennas cover an area that is roughly circular and are affected by RF obstacles such as walls. When using this type of antenna, it is common to place APs in central locations or divide an office into quadrants. Many APs use multiple-input, multiple-output (MIMO) antennas. This type of antenna takes advantage of multipath signal reflections. Ideally, locate the AP as close as possible to the antennas. The farther the signal has to travel across the cabling between the AP and the antenna, the more signal loss that occurs. Loss is an important factor when deploying a wireless network, especially at higher power levels. Loss occurs as a result of the signal traveling between the wireless base unit and the antenna.
APs that require external antennas need additional consideration. You need to configure the antennas properly, consider what role the AP serves (AP or bridge), and consider where the antennas are placed. When the antenna is mounted on the outside of the building or the interface between the wired network and the transceiver is placed in a corner, it puts the network signal in an area where it is easy to intercept. Antenna placement should not be used as a security mechanism.
Professional site surveys for wireless network installations and proper AP placement are sometimes used to ensure coverage area and security concerns. Up-front planning takes more time and effort but can pay off in the long run, especially for large WLANs.
One of the principle requirements for wireless communication is that the transmitted wave must reach the receiver with ample power to allow the receiver to distinguish the wave from the background noise. An antenna that is too strong raises security concerns. Strong omnidirectional Wi-Fi signals are radiated to a greater distance into neighboring areas, where the signals can be readily detected and viewed. Minimizing transmission power reduces the chances your data will leak out. Companies such as Cisco and Nortel have implemented dynamic power controls in their products. The system dynamically adjusts the power output of individual access points to accommodate changing network conditions, helping ensure predictable wireless performance and availability.
Transmit power control is a mechanism used to prevent too much unwanted interference between different wireless networks. Adaptive transmit power control in 802.11 WLANs on a per-link basis helps increase network capacity and improves battery life of Wi-Fi-enabled mobile devices.
The captive portal technique enables administrators to block Internet access for users until some action is taken. When a user attempts to access the Internet, the HTTP client is directed to a special web page that usually requires the user to read and accept an acceptable use policy (AUP). By using a captive portal, the web browser is used to provide authentication. Captive portals are widely used in businesses such as hotels and restaurants that offer free Wi-Fi hotspots to Internet users. A captive portal web page can be used to require authentication, require payment for usage, or display some type of policy or agreement. Although captive portals are mainly for Wi-Fi hotspots, you can also use them to control wired access.
Wireless antenna types are either omnidirectional or directional. Omni-directional antennas provide a 360-degree radial pattern to provide the widest possible signal coverage. An example of omnidirectional antennas are the antennas commonly found on APs. Directional antennas concentrate the wireless signal in a specific direction, limiting the coverage area. An example of a directional antenna is a yagi antenna.
The need or use determines the type of antenna required. When an organization wants to connect one building to another building, a directional antenna is used. If an organization is adding Wi-Fi internally to an office building or a warehouse, an omnidirectional antenna is used. If the desire is to install Wi-Fi in an outdoor campus environment, a combination of both antennas would be used.
A site survey is necessary before implementing any WLAN solution, to optimize network layout within each unique location. This is particularly important in distributed wireless network configurations spanning multiple buildings or open natural areas, where imposing structures and tree growth may affect network access in key areas.
A site survey should include a review of the desired physical and logical structure of the network, selection of possible technologies, and several other factors, including the following:
- Federal, state, and local laws and regulations relating to the proposed network solution.
- Potential sources of radio frequency (RF) interference, including local broadcast systems as well as motors, fans, and other types of equipment that generate RF interference. This includes an analysis of potential channel overlap between WAP hardware.
- Available locations for WAP hardware installation and physical network integration connectivity.
- Any special requirements of users, applications, and network equipment that must function over the proposed wireless network solution.
- Whether a point-to-point (ad hoc or wireless bridge) or multipoint wireless solution is required. In most solutions, point-to-multipoint connectivity will be required to support multiple wireless clients from each WAP connected to the physical network.
Data transported over this medium is available to anyone with the proper equipment, and so must be secured through encryption and encapsulation mechanisms no subject to public compromise.
VPN (Over Open Wireless)
VPNs are commonly used to securely connect employees to corporate networks when they are not in the office by using an Internet connection. More organizations are requiring hotspot visitors to VPN into the organizational network because they have no control over the security used in public Wi-Fi hotspots. The same principles that apply to wired VPNs can be applied to VPNs over open wireless networks. The use of a VPN over public Wi-Fi hotspots can increase privacy and provide data protection. VPNs over open wireless are not always immune to man-in-the-middle attacks. They can be susceptible to Wi-Fi-based attacks and VPN-based attacks.
Answer these questions. The answers follow the last question. If you cannot answer these questions correctly, consider reading this section again until you can.
You want to implement non-vendor-specific strong authentication protocols for wireless communications. Which of the following would best meet your requirements? (Select two correct answers.)
- A. EAP
- B. PEAP
- C. LEAP
- D. WEP
Which of the following technologies would be selected when looking to reduce a vulnerability to replay attacks by using 128-bit keys with a 48-bit initialization vector (IV)?
- A. ICMP
- B. WEP
- C. WPA
- D. CCMP
Which of the following technologies would be used by a hotel for guest acceptance of an acceptable use policy?
- A. Site survey
- B. MAC filtering
- C. VPN over wireless
- D. Captive portal
Cram Quiz Answers
- A and B. The IEEE specifies 802.1X and EAP as the standard for secure wireless networking, and PEAP is standards based. PEAP provides mutual authentication and uses a certificate for server authentication by the client, while users have the convenience of entering password-based credentials. Answer C is incorrect because LEAP is a Cisco proprietary protocol. Answer D is incorrect because WEP is the most basic form of encryption that can be used on 802.11-based wireless networks to provide privacy of data sent between a wireless client and its access point.
- D. CCMP uses 128-bit keys with a 48-bit IV that reduces vulnerability to replay attacks. Answer A is incorrect because ICMP is a network troubleshooting protocol. Answer B is incorrect because WEP is the most basic form of encryption that can be used on 802.11-based wireless networks. Answer C is incorrect because WPA protects networks by incorporating a set of enhanced security features. WPA-protected networks require users to enter a passkey in order to access a wireless network.
- D. A captive portal web page can be used to require authentication, require payment for usage, or display some type of policy or agreement. Answer A is incorrect because a site survey is used to optimize network layout within each unique wireless location. Answer B is incorrect because MAC filtering is a security access control method whereby the MAC address is used to determine access to the network. Answer C is incorrect because the use of a VPN over public Wi-Fi hotspots can increase privacy and provide data protection, but is not used to force acceptance of an acceptable use policy.