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Cisco Network Security Fundamentals: Wireless Security

Chapter Description

This chapter covers wireless security—what it is, how it works, how it is configured, what threatens it, and what policies can be designed to secure it.

Risks of Open Wireless Ports

As indicated earlier in the chapter, the use of wireless components in the network infrastructure raises big security issues. You want to keep intruders away from accessing your network, reading and modifying network traffic, and so on. In chronological order, the following techniques were developed to resolve these issues: the SSID, Open Authentication protocol, and the WEP protocol. WEP was designed to tackle these issues and provide some level of security on WLANs as on a physical wire.

SSID Vulnerabilities

The SSID is advertised in plain text in the access point beacon messages. Although beacon messages are transparent to users, an eavesdropper can easily determine the SSID with the use of an 802.11 WLAN packet analyzer such as Sniffer Pro, NetStumbler, and Kismet. Some access-point vendors, including Cisco, offer the option of disabling SSID broadcasts in the beacon messages. But this still leaves the option open for an eavesdropper to find out what the SSID is set to by sniffing the probe response frames from an access point. Using only the SSID as a mode of security is not advisable.

Open Authentication Vulnerabilities

Wireless networks with open authentication create major network vulnerabilities. The access point has no means to determine whether a client is valid. For public WLAN deployments, it might not be possible to implement strong authentication; higher-layer authentication might be required.

Shared Key Authentication Vulnerabilities

Before delving into the main vulnerability in WEP, you need to understand the shared key authentication process in more detail.

WEP Protocol Overview

The WEP protocol is intended to implement three main security goals:

  • Confidentiality

  • Access control

  • Data integrity

Achieving these goals should help you, as network administrator, prevent unauthorized individuals from using your wireless infrastructure or learning the content of your wireless traffic. The shared key authentication process requires that the client configure a static WEP key. Figure 14-7 describes the shared key authentication process, and the steps that follow describe the steps shown in the figure.

Figure 7Figure 14-7 Wireless Station Authentication Using WEP

 

 

Step 1

The client sends an authentication request to the access point requesting shared key authentication.

Step 2

The access point uses the WEP algorithm to generate a random number used in the authentication response containing a challenge text.

Step 3

The client uses its locally configured WEP key to encrypt the challenge text and reply with a subsequent authentication request.

Step 4

If the access point can decrypt the authentication request and retrieve the original challenge text, it responds with an authentication response that grants the client access.


WEP Protocol Vulnerabilities

As you can see in Figure 14-7, the process of exchanging the challenge text occurs over the wireless link and is vulnerable to a man-in-the-middle attack. A cracker can capture both the plain text (challenge text) and the encrypted challenge response.

NOTE

For the attack to work, the man-in-the-middle has to decrypt the challenge response to identify the WEP key. Before 2001, programs such as WEPCrack and Airsnort could identify weak WEP keys and challenges, thus making the job of the cracker easy and fast. Vendors have corrected the firmware that creates keys and challenges, so this is no longer the problem that it once was. The phrase "15 minutes to crack WEP via man-in-the-middle attack" was once true but became invalid more than two years ago.

Figure 14-8 illustrates the attack.

Figure 08Figure 14-8 WEP Vulnerability

WEP encryption is done by performing an exclusive OR (XOR) function on the plain text with the key stream to produce the encrypted challenge.

NOTE

The XOR function can be stated as, "either A or B, but not both." The XOR function produces logic 1 output only if its two inputs are different. If the inputs are the same, the output is logic 0. This function is often referred to as "add without carry."

It is important to note that if the XOR function is performed on the plain text and on the encrypted challenge, the result is the key stream. Therefore, a cracker can easily derive the key stream just by sniffing the shared key authentication process with a protocol analyzer. Lots of other attacks, such as message modification, message injection, and IP redirection, can be based on the same basic intrusion technique.

It looks as if WEP has not met any of the security goals it was intended to address. As a network administrator, you should assume that WEP is not secure. Treat your wireless network as a public network. Put the wireless network outside your firewall and implement additional authentication methods. Virtual private network (VPN), IP Security (IPSec), and secure shell (SSH) are other pieces of higher layer software that encrypt all data from the client application to the server application to make the transaction secure, even across an unencrypted 802.11 link.

Cisco has recognized the vulnerabilities in 802.11 authentication and data privacy. Therefore, to give network administrators a secure WLAN solution that is scalable and manageable, a proprietary Cisco Wireless Security Suite was developed. This suite of security enhancements augments the wireless LAN security by implementing enhancements to 802.11 authentication and encryption.

Countermeasures to WEP Protocol Vulnerabilities

Now that it is clear that many 802.11 networks employ the standard WEP protocol, which is known to have major faults, some 802.11 vendors have come up with proprietary solutions. Before the official IEEE 802.11i was released, Cisco created proprietary solutions to address WEP protocol vulnerabilities. The WEP protocol contains three components:

  • Authentication framework

  • Authentication algorithm

  • Data privacy or encryption algorithm

The Cisco Wireless Security Suite contains an enhancement that exceeds the WEP functionality for each of the components in the previous list.

The IEEE 802.1x standard provides a framework for authentication. A new user-based authentication algorithm with the ability to generate dynamic WEP keys has been developed. This algorithm is called the Extensible Authentication Protocol (EAP). Cisco Light Extensible Authentication Protocol (LEAP) is a proprietary Cisco authentication protocol designed for use in IEEE 802.11 WLAN environments. LEAP's main focuses are on mutual authentication between the network infrastructure and the user, secure derivation of random and user-specific cryptographic session keys, and most importantly, compatibility with existing and widespread network authentication mechanisms (for example, RADIUS).

Additionally, Cisco has developed the Temporal Key Integration Protocol (TKIP) to improve WEP privacy and encryption.

EAP Protocol and the 802.11i Standard

The 802.1x authentication framework is included in the draft for 802.11 MAC layer security enhancements in the IEEE 802.11i specification. The 802.1x framework provides the link layer with extensible authentication normally seen in higher layers. One of the higher layers is EAP, which is also Cisco proprietary. EAP allows negotiation of an authentication protocol for authenticating its peer before allowing network layer protocols to transmit over the link. Figure 14-9 illustrates the relationship between these sublayers.

Figure 9Figure 14-9 802.1x Authentication Framework

EAP is defined in RFC 2284 and was developed to provide strong, easy-to-deploy, and easy-to-administer wireless security. Cisco offers third-party NIC support and RADIUS support to allow customers to use their existing investments in wireless clients as well as existing RADIUS servers. Figure 14-10 illustrates the message flow for the EAP protocol with RADIUS as the authentication method.

Figure 10Figure 14-10 Authentication Framework with RADIUS

As you can see in Figure 14-10, the authentication framework process consists of multiple steps:

Step 1

The station determines 802.11i support from a beacon that is transmitted from the access point.

Step 2

The station starts the session with an EAP frame.

Step 3

The access point sends an EAP identity request message back to the station.

Step 4

The station sends an EAP response (including the station's ID).

Step 5

The access point forwards the packet to the RADIUS server.

Step 6

The RADIUS server sends a response back to the access point including a challenge (EAP authentication type).

Step 7

The access point forwards the challenge to the station.

Step 8

The station sends a challenge response message back (EAP type set to RADIUS).

Step 9

The access point forwards the response to the RADIUS server.

Step 10

The RADIUS server sends an accept message to the access point.

Step 11

The access point forwards an EAP success message to the station.

Step 12

The station is ready to send data.


At this point in time, VPN, IPSec, and SSH, which encrypt all data from the client applications to server applications, make the transaction more secure than only EAP. They are therefore recommended as an additional implemented security layer.

Network administrators should be aware that WLAN deployments should be made as secure as possible, knowing that security is weak in the 802.11 standard. Adding the Cisco Wireless Security Suite can increment security and help to create secure WLANs. The following link describes the Cisco Wireless Security Suite: http://www.cisco.com/en/US/netsol/ns340/ns394/ns348/
ns386/networking_solutions_white_paper09186a00800b3d27.shtml
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