The following is the fifth part of a six-part series on wireless security. Each tip is excerpted from the Cisco Press book, Network Security First-step by Tom M. Thomas. Check back frequently for the next installment, or go to the main series page for all installments.
You might be wondering why someone would want to use a wireless connection with all the insecurities that seem to go along with it. All is not lost, thanks to something known as Wired Equivalent Protocol, or is it Wireless Encryption Protocol—or it might even be Wired Equivalent Privacy. There seems to be some debate over exactly what WEP stands for among "industry experts." Regardless of how you spell or say it, WEP is an encryption algorithm that can be invoked to encrypt the transmissions between the wireless user and his Wireless access point (WAP).
From its inception, the 802.11b standard was not meant to contain a comprehensive set of enterprise level security tools. Still, the standard includes some basic security measures that can be employed to help make a network more secure. With each security feature, the potential exists for making the network either more secure or more open to attack.
Working on the layered defense concept, the following sections look first at how a wireless device connects to an access point and how you can apply security at the first possible point.
Service Set Identifier (SSID)
By default, the access point broadcasts the SSID every few seconds in beacon frames. Although, this makes it easy for authorized users to find the correct network, it also makes it easy for unauthorized users to find the network name. This feature is what allows most wireless network detection software to find networks without having the SSID upfront.
SSID settings on your network should be considered the first level of security and should be treated as such. In its standards-adherent state, SSID might not offer any protection against who gains access to your network, but configuring your SSID to something not easily guessable can make it more difficult for intruders to know what exactly they are seeing.
If you have your SSID configured to be any of the defaults cited in Table 8-1, you should change the SSID immediately.
Table 8-1 Default Wireless SSIDs
|Cisco||Tsunami, WaveLAN Network|
|Intel||101, 195, xlan, intel|
|Zcomax||any, mello, Test|
A complete listing of manufacturers' SSIDs and even other networking equipment default passwords can be found at http://www.cirt.net/. As you can see, the SSIDs are readily available on the Internet, so it is a good idea to turn off SSID broadcasting as your first step.
Device and Access Point Association
Before any other communications take place between a wireless client and a wireless access point, the two must first begin a dialogue. This process is known as associating. When 802.11b was designed, the IEEE added a feature to allow wireless networks to require authentication immediately after a client device associates with the access point, but before the access point transmission occurs. The goal of this requirement was to add another layer of security. This authentication can be set to either shared key authentication or open key authentication.
You need to use open key authentication because shared key is flawed; although that is counter-intuitive, this recommendation is based on the understanding that other encryption will be used.
Wired Equivalent Privacy (WEP)
There is a lot of misconception surrounding WEP, so let's clear that up right away. WEP is not, nor was it ever meant to be, a security algorithm. WEP was never designed to protect your data from script kiddies or from more intelligent attackers who want to discover your secrets. WEP is not designed to repel; it simply makes sure that you are not less secure because you are not keeping your data in a wire. The problem occurs when people see the word "encryption" and make assumptions. WEP is designed to make up for the inherent insecurity in wireless transmission, as compared to wired transmission. WEP makes your data as secure as it would be on an unencrypted, wired Ethernet network. That is all it is designed to do, period; now your misconceptions are gone and you can move on. WEP can be typically configured in three possible modes:
- No encryption mode
- 40-bit encryption
- 128-bit encryption
WEP is an optional, agreed-upon encryption standard that is configured before the wireless user's connection to the WAP. After it is configured on the both the WAP and the user's end, all communications sent through the air are encrypted, thereby providing a secure link that is reasonably difficult to break, although recently developed hacker tools are gaining ground on this front. A side benefit of using WEP is that users wanting to connect to a WAP using WEP must have it enabled previously on their machine and have the "passphrase" or "key" that is shared between the end user and access point.
Wired Equivalent Privacy (WEP) was intended to give wireless users the security equivalent of being on a wired network. With WEP turned on, when each packet is transmitted from one access point to a client device, each packet is first encrypted by taking the packet's data and a secret 40-bit number and passing them both through a encryption algorithm called RC4. The resulting encrypted packet is then transmitted to the client device. When the client device receives the WEP encrypted packet, it uses the same 40-bit number to pass the encrypted data through RC4 algorithm backward, resulting in the client receiving the data. Of course this process occurs in reverse and a client device is transmitting data to an access point. The encryption key used in this example was 40-bit, but 128-bit is also supported and, given the misconceptions and flaws with WEP, it is recommended that you always use the 128-bit encryption because it is better than 40-bit.
WEP Limitations and Weaknesses
WEP protects the wireless traffic by combining the "secret" WEP key with a 24-bit number (Initialization Vector, or IV), randomly generated, to provide encryption services. The 24-bit IV is combined with either the 40-bit or 104-bit WEP pass phrase to give you a possible full 128 bits of encryption strength and protection—or does it? There are a few issues surrounding the flawed current implementation of WEP:
- WEP's first weakness is the straightforward numerical limitation of the 24-bit Initialization Vector (IV), which results in 16,777,216 (224) possible values. This might seem large, but you know from discussions in Chapter 4, "Security Protocols," that this number is deceiving. The problem with this small number is that eventually the values and thus the keys start repeating themselves; this is how attackers can crack the WEP key.
- The second weakness is that of the possible 16 million values, not all of them are good. For example, the number 1 would not be very good. If an attacker can use a tool to find the weak IV values, the WEP can be cracked.
- WEP's third weakness is the difference between the 64-bit and 128-bit encryption. Perception would indicate that the 128-bit should be twice as secure, right? Wrong. Both levels still use the same 24-bit IV, which has inherent weaknesses. Therefore, if you think going to 128-bit is more secure, in reality, you will gain absolutely no increase in the security of your network.
Of course, freely available tools can accomplish all these things and are ready for the attackers to download and use as discussed in the section, "Essentials First: Wireless Hacking Tools," later in the chapter. Using WEP is better than nothing; however, layering the security of any part of your network is the key to safety and security, as has been established in all earlier chapters. Extensible Authentication Protocol (EAP) is the next level of security and is discussed in the correspondingly titled section.
MAC Address Filtering
MAC address filtering is another way people have tried to secure their networks over and above the 802.11b standards. A network card's MAC address is a 12-digit hexadecimal number that is unique to each and every network card in the world. Because each wireless Ethernet card has its own individual MAC address, if you limit access to the AP to only those MAC addresses of authorized devices, you can easily shut out everyone who should not be on your network.
However, MAC Address filtering is not completely secure and, if you solely rely upon it, you will have a false sense of security. Consider the following:
- Someone will have to keep a database of the MAC address of every wireless device in your network. If there are only 10–20 devices, it is not a problem. However, if you must keep track of hundreds of MAC addresses, this will become a nightmare quickly.
- MAC addresses can be changed, so a determined attacker can use a wireless sniffer to figure out a MAC address that is allowed through and set his PC to match it to consider it valid. Note that encryption takes place at about Layer 2, so MAC addresses will still be visible to a packet sniffer.
Extensible Authentication Protocol (EAP)
802.1X is a standard regarding port level security that the IEEE ratified. This ratification was initially intended to standardize security on wired network ports, but it was also found to be applicable to wireless networking. Extensible Authentication Protocol (EAP) is a Layer 2 (MAC address layer) security protocol that exists at the authentication stage of the security process and, coupled with the security measures discussed thus far, provides a third and final layer of security for your wireless network. Using 802.1X, when a device requests access to the AP, the following steps occur with EAP:
- The access point requests authentication information from the client.
- The user then supplies the requested authentication information.
- AP then forwards the client supplied authentication information to a standard RADIUS server for authentication and authorization.
- Upon authorization from the RADIUS server, the client is allowed to connect and transmit data.
The four commonly used EAP methods in use today are
- The access point requests authentication information from the client.
- EAP-Cisco Wireless (also known as LEAP)
The following sections provide a quick overview of each EAP method.
EAP-MD5 relies on an MD5 hash of a username and password to pass authentication information to the RADIUS server. EAP-MD5 offers no key management or dynamic WEP key generation, thus requiring the use of static WEP keys. This version of EAP does have some limitations:
- Because there is no dynamic WEP key generation available, the added use of EAP provides no increased security over WEP. Attackers can still sniff your airborne traffic and decrypt the WEP key.
- EAP-MD5 does not provide for a means for the client device to ensure that it is transmitting to the proper access point. A client could erroneously transmit to a rogue access point.
Because EAP-MD5 offers no other features over the standard 802.1X, EAP-MD5 is considered the least secure of all the common EAP standards.
EAP-Cisco Wireless, or LEAP as it is more commonly known, is a standard developed by Cisco in conjunction with the 802.1X standard and is the basis for much of the ratified version of EAP. Like EAP-MD5, LEAP accepts a username and password from the wireless device and transmits them to the RADIUS server for authentication. Cisco added additional support beyond what the standard required, resulting in several security benefits as follows:
- LEAP authenticates the client; one-time WEP keys are dynamically generated for each client connection. This means that every client on your wireless network is using a different dynamically generated WEP key that no one knows—not even the user.
- LEAP supports a RADIUS feature called session timeouts, which requires clients to log in again every few minutes. Fortunately, this is all handled without the user having to do anything. Couple this feature with dynamic WEP keys, and your WEP keys will change so often that attackers will not be able to determine the key in time.
- LEAP conducts mutual authentication from client-to-access point and access point-to-client; this stops attackers from introducing rogue access points into your network.
There is presently a single known limitation to running LEAP.
MS-CHAPv1 is used for both the client and access point authentication and is known to have vulnerabilities.
Not everyone has a RADIUS server that is ready to utilize LEAP; however, Cisco access points can be configured with a feature called local AAA Authentication on a per user basis. This allows the user database to reside in the AP instead of RADIUS and works well if you have only a limited number of users.
Microsoft developed EAP-TLS, which is outlined in RFC 2716. Instead of username/password combinations, EAP-TLS uses X.509 certificates to handle authentication. EAP-TLS relies on transport layer security to pass PKI information to EAP. Like LEAP, EAP-TLS offers the following:
- Dynamic one-time WEP key generation
- Mutual authentication
The drawbacks of EAP-TLS include the following:
- PKI is required to use EAP-TLS; however, most companies do not deploy PKI.
- Microsoft Active Directory with a certificate server can be used; however, change is difficult in this model.
- If you are using Open LDAP or Novell Directory Services, you need a RADIUS server; again, not everyone has immediate access to one.
- If you have implemented PKI using VeriSign certificates, all the fields required by EAP-TLS are not present.
Unless you are ready to follow the implementation of EAP-TLS exactly as Microsoft has laid it out, you should probably look for another method.
EAP-TTLS Funk Software (http://www.funk.com/) pioneered EAP-TTLS as an alternative to EAP-TLS. The wireless access point still identifies itself to the client with a server certificate, but the users now send their credentials in username/password form. EAP-TTLS then passes the credentials in any number of administrator specified challenge-response mechanisms (PAP, CHAP, MS-CHAPv1, MS-CHAPv2, PAP/Token Card, or EAP). The only challenges to EAP-TTLS are
- The slightly less secure than dual certificates of EAP-TLS
- The upcoming standard developed by Microsoft and Cisco that works exactly the same way—Protected EAP (PEAP)
Increasing Wireless Security
As discussed, there are some possible means of securing your wireless network beyond WEP. It is unlikely, however, that anyone has a RADIUS server ready and waiting to be used; therefore, you need to identify steps you can take immediately to increase the security of your wireless network. The attention on the pitfalls of wireless LANs has inspired some organizations to ban wireless LANs altogether. However, security-conscious organizations are fortifying their wireless LANs with a layered approach to security that includes the following:
- Putting the wireless network behind its own routed interface so you can shut off access to at a single choke point if necessary
- Discovery of rogue access points and potential associated vulnerabilities
- Physical and logical access point security to ensure that someone cannot walk up to an access point and alter its configuration without your knowledge
- Changing the SSID and then picking a random SSID that gives away nothing about your company or network
- Disabling active SSID broadcasting
- Rotating your broadcast keys every ten minutes or less
- Encryption and authentication, which might include a virtual private network over wireless
- Using 802.1X for key management and authentication
- Looking over the available EAP protocols and deciding which is right for your environment
- Setting the session to time out every ten minutes or less
- Establishing and enforcing wireless network security policies
- Implementing proactive security measures that include intrusion protection
As shown in Figure 8-6, these steps and recommendations can be illustrated as a phased approach, which enforces the concept of first knowing what the vulnerabilities are and moving forward from that point.
Figure 8-6 Stages of Securing Your Wireless Network
Reproduced from the book Network Security First-step, ISBN 1587200996, Copyright 2004, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses. Visit www.ciscopress.com for a detailed description and to learn how to purchase this title.