Wi-Fi Network Security

This page offers some practical security tips for wireless networks based on the IEEE 802.11 Wi-Fi standards.

Executive summary: use only WPA2 security mode, turn off WPS, and pick a strong passphrase--20 characters or more.

The threats

Threat 1. WEP encryption can be broken fairly easily

WEP is the oldest security standard for Wi-Fi. At a 2005 Information Systems Security Association meeting, presenters from the FBI demonstrated breaking a WEP network in three minutes, using the latest attack tools, all available on the Internet. Where possible WEP should be replaced by the newer, stronger WPA (see below).

Here are details on WEP's vulnerability, if you are interested. A 2001 paper by Scott Fluhrer, Itsik Mantin and Adi Shamir described new attacks on the RC4 cipher that is the basis for Airport and 802.11b Wired Equivalent Privacy (WEP) security. Prof. Shamir states in an e-mail accompanying the release:

"Attached you will find a new paper which describes a truly practical direct attack on WEP's cryptography. It is an extremely powerful attack which can be applied even when WEP's RC4 stream cipher uses a 2048 bit secret key (its maximal size) and 128 bit IV modifiers (as proposed in WEP2). The attacker can be a completely passive eavesdropper (i.e., he does not have to inject packets, monitor responses, or use accomplices) and thus his existence is essentially undetectable. It is a pure known-ciphertext attack (i.e., the attacker need not know or choose their corresponding plaintexts). After scanning several hundred thousand packets, the attacker can completely recover the secret key and thus decrypt all the ciphertexts. The running time of the attack grows linearly instead of exponentially with the key size, and thus it is negligible even for 2048 bit keys."

The paper itself, titled "Weaknesses in the Key Scheduling Algorithm of RC4," has been posted at http://www.eyetap.org/~rguerra/toronto2001/rc4_ksaproc.pdf (in PDF format) and at http://www.crypto.com/papers/others/rc4_ksaproc.ps (in Postscript).

A draft paper by Borisov, Goldberg, and Wagner presents a number of practical attacks on 802.11 Wired Equivalent Privacy (WEP). The long term solution to these problems, as the paper points out, is to rework the 802.11 protocol to use better encryption and message authentication algorithms. Unfortunately a huge infrastructure has grown up around 802.11 and large numbers of modems are in use, including the Apple Airport line. We originally thought that since the encryption and authentication is done in firmware, changes to these algorithms would require all new hardware. The good new is that the far stronger WPA technology can be used with older Airport cards, though not with older Airport base stations. PC users should check with their vendors.

Threat 2. Wi-Fi Protected Setup

A feature added to routers starting in 2007, called Wi-fi Protected Setup or WPS, has a serious security flaw in most implementations that allows an attacker to recover your Wi-Fi passphrase in a few hours. WPS uses an 8-digit number or PIN to make it easier to attach new devices to your network. The flaw lets an attacker test each half of the PIN separately, allowing a relatively fast attack. Just as bad, perhaps, the PIN is printed on the label of most Wi-Fi routers and cannot be changed. Anyone who learns the PIN can access your network. The best defense is to turn WPS off. See http://diceware.blogspot.com/ for more information.

Threat 3. Weak passwords

There is another problem with Wi-Fi security: Wi-Fi software generates the network key from a password or passphrase. Since most user's pick passwords in fairly predictable ways, an attacker equipped with a password guessing program could break into Airport much more quickly, perhaps in a matter of minutes. The solution, of course, is strong passphrases. See below.

Threat 4. 802.11 base stations can be used to leak information from inside corporate security

All the talk of breaks to Airport/802.11 encryption may obscure a greater threat. Tranceivers for 802.11 are small, inexpensive and easy to install. While their range is normally limited to a few hundred feet, the distance can be extended to several miles using directional antennas. Given the standard architecture in most corporate and government buildings, it is fairly easy for a malicious individual who has access to the building to install a covert 802.11 base station.

Equipment is available to scan for unauthorized base stations, but it must be used frequently. More sophisticated attackers can use timers to insure that the covert base station is only on, say, between 3 and 4 am. Microwave transverter technology could be used to shift the 802.11 signals to other parts of the microwave spectrum where standard sniffers will never see them.

It may be necessary for institutions concerned about network security to employ virtual private network technology, such as IPSec, internally as well as externally, i.e. even behind the corporate firewall.

Threat 5. Vulnerability to Jamming

Because it uses relatively weak radio signals, all wireless networking is inherently vulnerable to jamming by a malicious person with a radio transmitter operating in the same frequency band. However, someone who mounts such an attack risks being caught by security personnel using direction finding equipment.

WPA has an added vulnerability in this regard. Because of certain design choices made to allow WPA to operate on older wireless cards, it is potentially vulnerable to an attacker sending a large number of bad packets in the hope of finding one that will be accepted. To reduce this vulnerability, the designers of WPA have it shut down for 30 seconds whenever two bad packets are detected. This however makes WPA vulnerable to a jammer who only has to transmit briefly twice a minute -- a procedure that is much more difficult to track.

A revision of the WPA standard, called WPA-2, is now available. WPA-2 eliminates this vulnerability, but will not run on some old network cards and access points.

What you can do now

Here are some very simple and practical measures that can improve Airport security:

Use WPA2

Apple's OS-X 10.4 and later supports Wi-Fi Protected Access 2 (WPA2), the preferred security standard for wireless networks. WPA2 is designed to replace WEP (Wireless Equivalent Privacy), the optional encryption system that was used in  most Wi-Fi networking systems prior to 2003. WPA2 is still stronger and supported on Apple's AirportExtreme products beginning with  Airport 4.2

WPA2 corrects most of the problems with the older WEP standard. Comparing WPA2 is like comparing a good safe to a manila envelope.

To run WPA2 on non-Apple products, you will have to verify that all clients and base stations are WPA2-compatible.

WPA2 operates in two modes. The more complicated mode is for large organizations and uses Radius servers to distribute keying. Individuals and smaller organizations should use the WPA2 Personal mode. This is called "pre-shared key" mode in non-Apple products. Security is achieved by having everyone use the same passphrase.

WPA2 has many other security features, including access control lists. You may want to review them before setting up the network.

Turn off WPS

Many, but not all makes, of Wi-Fi routers let you turn WPS off in the setup screen.

Use strong Passwords

Wireless system security depends on the strength of the network password. This is especially true for WPA and WPA2 in the Personal or Pre-Shared Key (PSK) mode used in most homes and small offices. We strongly recommend that you only use randomized passwords or passphrases. The Diceware.com page has instructions for simple ways to select passphrases at random using ordinary dice. There are also scripts that will do this. Using dice is more secure, but the scripts are probably adequate for this purpose. We suggest you use one of the following formats for selecting your wireless network and administrative passwords (See the DicewareFAQ for a discussion of passphrase length):

The Diceware FAQ has instructions for selecting random letters using dice. You can also use our Passgen applet. Just select the AAAAA AAAAA AAAAA AAAAA template from the drop down list.

To change your password, launch the Airport Admin Utility. Make sure that the Enable Encryption box is checked and then select Change Network Password. Obviously all other users of the Airport network must be informed of this change in advance, preferably by some method other than ordinary email.

Note: Most non-Apple WEP implementations require hexadecimal passwords. These can be generated using the Passgen applet by selecting the HHHH HHHH HHHH HHHH template from the Passgen drop down list. You can also use dice. Here is a link with more information on setting hexadecimal Airport passwords.

Change passwords periodically

WPA2 is much more secure than WEP, however if you are using personal or shared key mode, passwords should still be changed periodically to reduce the impact of leakage via employee turnover, carelessness or social engineering. Passwords should always be changed when an employee who had access leaves or when a laptop is reported lost or stolen.

In the past we recommended that users of WEP change their password once or even twice a day to reduce the chance of break-ins. However if you are concerned about security enough to actually follow this advice, you should switch to WPA immediately, even if you have to run a bake sale to pay for new base stations.

Distribute passwords securely

Hand them out in person or use internal company mail. Never use un-encrypted e-mail to distribute passwords.

Review base station placement

Airport has limited range, so by careful placement of the base stations you may be able to minimize the areas outside your building where an attacker can receive a strong signal. For example you may want to place a base station near the center of an inside wall rather than by a window. However you should consider that more sophisticated attackers can use high gain directional antennas to extend Airport's range.

Alert security personnel

Make your security staff aware of the Airport threat and suggest that they investigate individuals operating laptops in the company parking lot. Make sure they know what high-gain Wi-Fi antennas look like so they can identify them.

If security personnel are using wireless networking in their operations, they should be equipped with alternate means of communication in case wireless links fail. They should also consider loss of connectivity as a possible indicator of malicious activity (deliberate jamming) and increase alert level until network administration confirms a benign explanation.

Place the Airport network outside of your corporate firewall.

The BGW paper suggests placing wireless networks outside of the corporate firewall. This can limit a successful intruder's ability to access corporate databases. It may reduce the protection afforded to the wireless networked computers themselves, however. Another possibility is to have a separate firewall for wireless users.

WPA's increased security reduces the need for this measure, but it is still worth considering.

Develop and disseminate a written policy on wireless networks

If your organization wishes to maintain security, it is vital that only approved wireless installations be permitted. You may wish to scan your facilities periodically to look for unauthorized base stations.

uperencrypt with IPsec

The most effective solution, is to use separate strong encryption programs, such as IPsec, to secure all data moving over the Airport network, and perhaps the entire corporate network. This is the one solution that affords protection against all known Airport attacks. A proper IPsec installation takes considerable care and effort, however.

Sources of IPsec information and software include:


Designing Airport Networks, Apple Computer.

Wi-Fi Protected Access

Airport is a trademark of Apple Computer. Wi-Fi is a trademark of the Wi-Fi Alliance.

Arnold G. Reinhold

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Copyright (c) 2001-2012, Arnold G. Reinhold, Cambridge, Mass. USA.
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