10 Planning and Implementing Security for Wireless Networks

Wi-Fi Protected Access :
Although WEP with dynamic re-keying is secure enough to meet the needs of most organizations, WEP still has security weaknesses. WEP still uses a separate static key for broadcast packets. An attacker can analyze these broadcast packets to build a map of private IP addresses and computer names. WEP keys have to be renewed frequently, which places an additional burden on RADIUS services.

To address these lingering weaknesses with WEP, the Wi-Fi Alliance, a consortium of the leading wireless network equipment vendors, developed Wi-Fi Protected Access (WPA). WPA can use the same authentication mechanisms and encryption algorithms as WEP. This compatibility allows support for WPA to be added to WAPs with a simple firmware upgrade. However, WPA virtually eliminates WEP’s most exploited vulnerability by using a unique encryption key for each packet.

Other Wireless Security Techniques :
WEP and WPA are the most important wireless network security techniques. However,there are several secondary security techniques that you should be familiar with: media access control (MAC) address filtering, disabling SSID broadcasts, and VPNs.

MAC address filtering
One common technique used to make it more difficult for a casual user to connect to your wireless network is to configure your WAPs to allow only a predefined set of MAC addresses. Just like wired Ethernet cards, every wireless network card is assigned a unique MAC address by the manufacturer.

When a WAP is configured to use MAC address filtering, it will ignore any messages from wireless cards that use a MAC address not on the approved list. While this does improve security, it has significant manageability drawbacks. First, you must manually maintain the list of MAC addresses on your WAP, which would be impossible to do if you managed more than a dozen computers or multiple WAPs. Second, WAPs typically have limited memory and might not be able to store your organization’s complete list of MAC addresses. Third, if an attacker is knowledgeable and determined enough to
circumvent your WEP or WPA encryption, the attacker will also be able to identify and spoof an approved MAC address.

Disabling SSID broadcasts :
WAPs provide the option of disabling SSID broadcasts, but this should not be treated as a security feature. SSID broadcasts allow wireless clients to detect an available wireless network. In fact, Windows XP displays a notification to the user when it first receives a SSID broadcast from a wireless network. This is convenient; if you want users to be actively notified of the presence of the wireless client, you should enable SSID broadcasts.

Disabling SSID broadcasts will prevent the casual computer user from discovering your network, but it does nothing to prevent a skilled attacker from detecting your network. For example, a user with the free Network Stumbler tool installed can quickly identify the SSID of a wireless network that has SSID broadcasts disabled, because 802.11 association/ disassociation messages are always sent unencrypted and contain the SSID that the client wants to associate to or disassociate from.

VPNs :
While a VPN is an excellent solution for securely traversing a public network such as the Internet, VPNs are not the best solution for securing wireless networks. For this kind of application, a VPN is unnecessarily complex and costly. It adds little additional security to dynamic WEP, but it significantly increases costs, reduces usability, and removes important pieces of the functionality.

VPN clients usually require the user to initiate a connection to the VPN server; therefore, the connection will never be as transparent as a wired network connection. Non-Microsoft VPN clients might also prompt for logon credentials, in addition to the standard network or domain logon, when the connection is established. If the VPN disconnects because of a poor wireless signal or because the user is roaming between WAPs, the user has to repeat the connection process.

10 Planning and Implementing Security for Wireless Networks

802.1X authentication :
Although the early implementations of WEP were woefully inadequate, WEP’s vulnerability can be significantly reduced by using 802.1X authentication. 802.1X enables WEP to regularly change the encryption keys, which dramatically reduces the likelihood that an attacker will be able to gather enough data to identify the shared secret.

802.1X employs an Internet Engineering Task Force (IETF) standard protocol called Extensible Authentication Protocol (EAP) to carry the authentication conversation between the client, the WAP, and a Remote Access Dial-In User Server (RADIUS) service. As part of the 802.1X secure authentication process, the EAP method generates an encryption key that is unique to each client. RADIUS forces the client to generate a new encryption key on a regular basis, which makes it more difficult for an attacker to capture enough traffic to identify a key. This allows existing WEP-capable hardware to be used while minimizing WEP’s vulnerabilities.
Specifically, the client must perform the following steps to connect to an 802.1Xauthenticated
wireless network:
1. When the client computer is in range of the WAP, it will try to connect to the Service Set Identifier (SSID) hosted by the WAP. If the client has been configured with shared network authentication, it will authenticate itself to the WAP by using the network key. Because the WAP is configured to allow only 802.1X-authenticated connections, it issues an authentication challenge to the client. The WAP then sets up a restricted channel that allows the client to communicate only with the RADIUS service.
2. The wireless client examines the RADIUS server’s public key certificate to ensure that an attacker is not impersonating the RADIUS server. The client then attempts to authenticate, using 802.1X, to the RADIUS service.
❑ If the client and RADIUS service have been configured to use Protected EAP (PEAP) authentication, the client establishes a Transport Layer Security (TLS) session with the RADIUS service and then transmits credentials using the configured authentication protocol.
❑ If the client and RADIUS service have been configured to use EAP-TLS authentication, the client authenticates by using public key certificates.
3. The RADIUS service checks the client credentials against the directory. If it can authenticate the client’s credentials and the access policy allows the client to connect, it will grant access to the client. The RADIUS service relays the access decision to the WAP. If the client is granted access, the RADIUS service transmits the dynamic shared secret to the WAP. The client and WAP now share common key material that they can use to encrypt and decrypt the traffic that will pass between them.
4. The WAP then bridges the client’s connection to the internal network, completing the 802.1X authentication process. If the client is configured to use Dynamic Host Configuration Protocol (DHCP), it can now request a lease.

PEAP :
PEAP is typically used to authenticate wireless clients by using a user name and password; EAP-TLS is used to authenticate wireless clients by using public key certificates. Although using a user name and password is not as strong as using public key certificates, because passwords can be stolen or guessed, the resulting encryption is still very strong. When PEAP authentication is used with a RADIUS service that forces encryption keys to change regularly, the resulting WEP encryption is not likely to be compromised in a reasonable amount of time. PEAP’s primary advantage over EAP-TLS
is that it is easier to deploy because it does not require you to implement a Public Key Infrastructure (PKI).
The PEAP authentication method has two phases. Phase 1 authenticates the RADIUS server by using the RADIUS server’s public key certificate and then establishes a TLS session to the RADIUS server. Phase 2 requires a second EAP method tunneled inside the PEAP session to authenticate the client to the RADIUS service. This allows PEAP to use a variety of client authentication methods.

EAP-TLS :
EAP-TLS performs the same functions as PEAP by authenticating the client computer and generating keying material that will be used for encrypting the wireless communications. However, EAP-TLS uses public key certificates to authenticate both the client and the RADIUS service. EAP-TLS was designed by Microsoft and is based on an authentication protocol that is nearly identical to the protocol used in the Secure Sockets Layer (SSL) protocol for securing Web transactions. While public key certificates provide strong authentication and encryption, you should only use EAP-TLS if you already have a PKI in place for another application or your organization’s security requirements do not allow simple password authentication.

RADIUS :
RADIUS is a standardized service used primarily to authenticate dial-up users. Windows Server 2003 includes a RADIUS service and proxy named IAS. The traditional use for IAS on Windows networks is to allow an Internet Service Provider (ISP) to authenticate an organization’s users based on the Active Directory domain credentials stored on the organization’s private network.

Because RADIUS is designed to allow network hardware to authenticate against an external user database, WAPs also can use RADIUS to authenticate wireless users as they join the network. Authenticating to a RADIUS service allows user authentication for wireless networks to be centralized, rather than forcing administrators to store user credentials on each WAP.

10 Planning and Implementing Security for Wireless Networks

Lesson 1: Wireless Network Security Fundamentals

Wireless networks have these weaknesses too, but they lack the inherent physical security of wired networks. In fact, most corporate wireless networks can be accessed by people with mobile computers in the business’ parking lot. To make matters worse, attackers have significant motivation to abuse wireless networks. Accessing a wireless network might grant an attacker access to resources on an organization’s internal network. Or it might allow the attacker to access the public Internet while hiding his or her identity, which would allow the intruder to attack hosts on remote networks while disguised with the organization’s IP addresses.

The concerns over the abuse of wireless networks are far from theoretical. Intruders have a wide variety of tools available for detecting, connecting to, and abusing wireless networks. As with most aspects of security, there are technologies available that can help you to limit the vulnerabilities presented by wireless networks. Specifically, you can require wireless communications to be authenticated and encrypted. This provides assurance similar to that offered by the physical security of wired networks. The game between security experts and attackers continues, however, and early wireless authentication and encryption technologies can now be easily defeated by an intruder.

Security Threats :
Because wireless communications are not contained within the physical medium of a wire, wireless networks are more vulnerable to several types of attacks, including:
■ Eavesdropping. Attackers can capture traffic as it is sent between a wireless computer and the WAP. Depending on the type of antenna used and the transmitting power, an attacker might be able to eavesdrop from hundreds or thousands of feet away.
■ Masquerading. Attackers might be able to gain access to restricted network resources by impersonating authorized wireless users.
■ Attacks against wireless clients. Attackers can launch a network-based attack on a wireless computer that is connected to an ad hoc or untrusted wireless network.
■ Denial of service. Attackers can jam the wireless frequencies by using a transmitter,preventing legitimate users from successfully communicating with a WAP.
■ Data tampering. Attackers can delete, replay, or modify wireless communications with a man-in-the-middle attack.

WEP :
WEP is a wireless security protocol that helps protect your information by using a security setting, called a shared secret or a shared key, to encrypt network traffic before transmitting it over the airwaves. This helps prevent unauthorized users from accessing the data as it is being transmitted.

Unfortunately, some smart cryptographers found several theoretical ways to discover WEP’s shared secret by analyzing captured traffic. These theoretical weaknesses were quickly implemented in freely available software. The combination of free tools for cracking WEP encryption, the ease of capturing wireless traffic, and the dense proliferation of wireless networks have led WEP to become the most frequently cracked network encryption protocol today.

Besides weak cryptography, another factor contributing to WEP’s vulnerability is that WEP is difficult to manage because it doesn’t provide any mechanism for changing the shared secret. On wireless networks with hundreds of hosts configured to use a WAP, it is practically impossible to regularly change the shared secret on all hosts. As a result, the WEP shared secret tends to stay the same indefinitely. This gives attackers sufficient opportunity to crack the shared secret and all the time they need to abuse their ill-gotten network access.