CCENT/CCNA ICND1 Exam Guide: Introduction to Wireless LANs

Wireless LAN Security

All networks today need good security, but WLANs have some unique security requirements. This section examines some of the security needs for WLANs and the progression and maturation of the WLAN security options. It also discusses how to configure the security features.

WLAN Security Issues

WLANs introduce a number of vulnerabilities that do not exist for wired Ethernet LANs. Some of these vulnerabilities give hackers an opportunity to cause harm by stealing information, accessing hosts in the wired part of the network, or preventing service through a denial-of-service (DoS) attack. Other vulnerabilities may be caused by a well-meaning but uninformed employee who installs an AP without the IT department's approval, with no security. This would allow anyone to gain access to the rest of the Enterprise's network.

The Cisco-authorized CCNA-related courses suggest several categories of threats:

• War drivers: The attacker often just wants to gain Internet access for free. This person drives around, trying to find APs that have no security or weak security. The attacker can use easily downloaded tools and high-gain directional antennas (easily purchased and installed).
• Hackers: The motivation for hackers is to either find information or deny services. Interestingly, the end goal may be to compromise the hosts inside the wired network, using the wireless network as a way to access the Enterprise network without having to go through Internet connections that have firewalls.
• Employees: Employees can unwittingly help hackers gain access to the Enterprise network in several ways. An employee could go to an office supply store and buy an AP for less than $100, install the AP in his office, using default settings of no security, and create a small wireless LAN. This would allow a hacker to gain access to the rest of the Enterprise from the coffee shop across the street. Additionally, if the client does not use encryption, company data going between the legitimate employee client PC and the Enterprise network can be easily copied and understood by attackers outside the building.
• Rogue AP: The attacker captures packets in the existing wireless LAN, finding the SSID and cracking security keys (if they are used). Then the attacker can set up her own AP, with the same settings, and get the Enterprise's clients to use it. In turn, this can cause the individuals to enter their usernames and passwords, aiding in the next phase of the attacker's plan.

To reduce the risk of such attacks, three main types of tools can be used on a WLAN:

• Mutual authentication
• Encryption
• Intrusion tools

Mutual authentication should be used between the client and AP. The authentication process uses a secret password, called a key, on both the client and the AP. By using some sophisticated mathematical algorithms, the AP can confirm that the client does indeed know the right key value. Likewise, the client can confirm that the AP also has the right key value. The process never sends the key through the air, so even if the attacker is using a network analysis tool to copy every frame inside the WLAN, the attacker cannot learn the key value. Also, note that by allowing mutual authentication, the client can confirm that the AP knows the right key, thereby preventing a connection to a rogue AP.

The second tool is encryption. Encryption uses a secret key and a mathematical formula to scramble the contents of the WLAN frame. The receiving device then uses another formula to decrypt the data. Again, without the secret encryption key, an attacker may be able to intercept the frame, but he or she cannot read the contents.

The third class of tools includes many options, but this class generally can be called intrusion tools. These tools include Intrusion Detection Systems (IDS) and Intrusion Prevention Systems (IPS), as well as WLAN-specific tools. Cisco defines the Structured Wireless-Aware Network (SWAN) architecture. It includes many tools, some of which specifically address the issue of detecting and identifying rogue APs, and whether they represent threats. Table 11-8 lists the key vulnerabilities, along with the general solution.

Table 11-8. WLAN Vulnerabilities and Solutions

The Progression of WLAN Security Standards

WLAN standards have progressed over the years in response to a growing need for stronger security and because of some problems in the earliest WLAN security standard. This section examines four significant sets of WLAN security standards in chronological order, describing their problems and solutions.

The initial security standard for WLANs, called Wired Equivalent Privacy (WEP), had many problems. The other three standards covered here represent a progression of standards whose goal in part was to fix the problems created by WEP. In chronological order, Cisco first addressed the problem with some proprietary solutions. Then the Wi-Fi Alliance, an industry association, helped fix the problem by defining an industry-wide standard. Finally, the IEEE completed work on an official public standard, 802.11i. Table 11-9 lists these four major WLAN security standards.

Table 11-9. WLAN Security Standards

The word standard is used quite loosely in this chapter when referring to WLAN security. Some of the standards are true open standards from a standards body—namely, the IEEE. Some of the standards flow from the Wi-Fi Alliance, making them de facto industry standards. Additionally, Cisco created several proprietary interim solutions for its products, making the use of the word more of a stretch. However, all of these standards helped improve the original WEP security, so the text will take a closer look at each standard.

Wired Equivalent Privacy (WEP)

WEP was the original 802.11 security standard, providing authentication and encryption services. As it turns out, WEP provided only weak authentication and encryption, to the point that its authentication and encryption can be cracked by a hacker today, using easily downloaded tools. The main problems were as follows:

• Static Preshared Keys (PSK): The key value had to be configured on each client and each AP, with no dynamic way to exchange the keys without human intervention.

  As a result, many people did not bother to change the keys on a regular basis, especially in Enterprises with a large number of wireless clients.

• Easily cracked keys: The key values were short (64 bits, of which only 40 were the actual unique key). This made it easier to predict the key's value based on the frames copied from the WLAN. Additionally, the fact that the key typically never changed meant that the hacker could gather lots of sample authentication attempts, making it easier to find the key.

Because of the problems with WEP, and the fact that the later standards include much better security features, WEP should not be used today.

SSID Cloaking and MAC Filtering

Because of WEP's problems, many vendors included a couple of security-related features that are not part of WEP. However, many people associated these features with WEP just because of the timing with which the features were announced. Neither feature provides much real security, and they are not part of any standard, but it is worth discussing the concepts in case you see them mentioned elsewhere.

The first feature, SSID cloaking, changes the process by which clients associate with an AP. Before a client can communicate with the AP, it must know something about the AP—in particular, the AP's SSID. Normally, the association process occurs like this:

• Step 1 The AP sends a periodic Beacon frame (the default is every 100 ms) that lists the AP's SSID and other configuration information.
• Step 2 The client listens for Beacons on all channels, learning about all APs in range.
• Step 3 The client associates with the AP with the strongest signal (the default), or with the AP with the strongest signal for the currently preferred SSID.
• Step 4 The authentication process occurs as soon as the client has associated with the AP.

Essentially, the client learns about each AP and its associated SSIDs via the Beacon process. This process aids in the roaming process, allowing the client to move around and reassociate with a new AP when the old AP's signal gets weaker. However, the Beacons allow an attacker to easily and quickly find out information about the APs to begin trying to associate and gain access to the network.

SSID cloaking is an AP feature that tells the AP to stop sending periodic Beacon frames. This seems to solve the problem with attackers easily and quickly finding all APs. However, clients still need to be able to find the APs. Therefore, if the client has been configured with a null SSID, the client sends a Probe message, which causes each AP to respond with its SSID. In short, it is simple to cause all the APs to announce their SSIDs, even with cloaking enabled on the APs, so attackers can still find all the APs.

The second extra feature often implemented along with WEP is MAC address filtering. The AP can be configured with a list of allowed WLAN MAC addresses, filtering frames sent by WLAN clients whose MAC address is not in the list. As with SSID cloaking, MAC address filtering may prevent curious onlookers from accessing the WLAN, but it does not stop a real attack. The attacker can use a WLAN adapter that allows its MAC address to be changed, copy legitimate frames out of the air, set its own MAC address to one of the legitimate MAC addresses, and circumvent the MAC address filter.

The Cisco Interim Solution Between WEP and 802.11i

Because of the problems with WEP, vendors such as Cisco, and the Wi-Fi Alliance industry association, looked to solve the problem with their own standards, concurrent with the typically slower IEEE standardization process. The Cisco answer included some proprietary improvements for encryption, along with the IEEE 802.1x standard for end-user authentication. The main features of Cisco enhancements included the following:

• Dynamic key exchange (instead of static preshared keys)
• User authentication using 802.1x
• A new encryption key for each packet

The use of a dynamic key exchange process helps because the clients and AP can then change keys more often, without human intervention. As a result, if the key is discovered, the exposure can be short-lived. Also, when key information is exchanged dynamically, a new key can be delivered for each packet, allowing encryption to use a different key each time. That way, even if an attacker managed to discover a key used for a particular packet, he or she could decrypt only that one packet, minimizing the exposure.

Cisco created several features based on the then-to-date known progress on the IEEE 802.11i WLAN security standard. However, Cisco also added user authentication to its suite of security features. User authentication means that instead of authenticating the device by checking to see if the device knows a correct key, the user must supply a username and password. This extra authentication step adds another layer of security. That way, even if the keys are temporarily compromised, the attacker must also know a person's username and password to gain access to the WLAN.

Wi-Fi Protected Access (WPA)

The Cisco solution to the difficulties of WEP included proprietary protocols as well as IEEE standard 802.1x. After Cisco integrated its proprietary WLAN security standards into Cisco APs, the Wi-Fi Alliance created a multivendor WLAN security standard. At the same time, the IEEE was working on the future official IEEE WLAN security standard, 802.11i, but the WLAN industry needed a quicker solution than waiting on the IEEE standard. So, the Wi-Fi alliance took the current work-in-progress on the 802.11i committee, made some assumptions and predictions, and defined a de facto industry standard. The Wi-Fi Alliance then performed its normal task of certifying vendors' products as to whether they met this new industry standard, calling it Wi-Fi Protected Access (WPA).

WPA essentially performed the same functions as the Cisco proprietary interim solution, but with different details. WPA includes the option to use dynamic key exchange, using the Temporal Key Integrity Protocol (TKIP). (Cisco used a proprietary version of TKIP.) WPA allows for the use of either IEEE 802.1X user authentication or simple device authentication using preshared keys. And the encryption algorithm uses the Message Integrity Check (MIC) algorithm, again similar to the process used in the Cisco-proprietary solution.

WPA had two great benefits. First, it improved security greatly compared to WEP. Second, the Wi-Fi Alliance's certification program had already enjoyed great success when WPA came out, so vendors had great incentive to support WPA and have their products become WPA-certified by the Wi-Fi Alliance. As a result, PC manufacturers could choose from many wireless NICs, and customers could buy APs from many different vendors, with confidence that WPA security would work well.

IEEE 802.11i and WPA-2

The IEEE ratified the 802.11i standard in 2005; additional related specifications arrived later. Like the Cisco-proprietary solution, and the Wi-Fi Alliance's WPA industry standard, 802.11i includes dynamic key exchange, much stronger encryption, and user authentication. However, the details differ enough so that 802.11i is not backward-compatible with either WPA or the Cisco-proprietary protocols.

One particularly important improvement over the interim Cisco and WPA standards is the inclusion of the Advanced Encryption Standard (AES) in 802.11i. AES provides even better encryption than the interim Cisco and WEP standards, with longer keys and much more secure encryption algorithms.

The Wi-Fi Alliance continues its product certification role for 802.11i, but with a twist on the names used for the standard. Because of the success of the WPA industry standard and the popularity of the term "WPA," the Wi-Fi Alliance calls 802.11i WPA2, meaning the second version of WPA. So, when buying and configuring products, you will more likely see references to WPA2 rather than 802.11i.

Table 11-10 summarizes the key features of the various WLAN security standards.

Table 11-10. Comparisons of WLAN Security Features