Why Non-broadcast Networks are not a Security Feature

http://technet.microsoft.com/en-us/library/bb726942.aspx

Wireless security consists of two main elements: authentication and encryption. Authentication controls access to the network and encryption ensures that malicious users cannot determine the contents of wireless data frames. Although having users manually configure the SSID of a wireless network in order to connect to it creates the illusion of providing an additional layer of security, it does not substitute for either authentication or encryption.

A non-broadcast network is not undetectable. Non-broadcast networks are advertised in the probe requests sent out by wireless clients and in the responses to the probe requests sent by wireless APs. Unlike broadcast networks, wireless clients running Windows XP with Service Pack 2 or Windows Server® 2003 with Service Pack 1 that are configured to connect to non-broadcast networks are constantly disclosing the SSID of those networks, even when those networks are not in range.

Therefore, using non-broadcast networks compromises the privacy of the wireless network configuration of a Windows XP or Windows Server 2003-based wireless client because it is periodically disclosing its set of preferred non-broadcast wireless networks. When non-broadcast networks are used to hide a vulnerable wireless network—such as one that uses open authentication and Wired Equivalent Privacy—a Windows XP or Windows Server 2003-based wireless client can inadvertently aid malicious users, who can detect the wireless network SSID from the wireless client that is attempting to connect. Software that can be downloaded for free from the Internet leverages these information disclosures and targets non-broadcast networks.

This behavior is worse for enterprise wireless networks because of the number of wireless clients that are periodically advertising the non-broadcast network name. For example, an enterprise wireless network consists of 20 wireless APs and 500 wireless laptops. If the wireless APs are configured to broadcast, each wireless AP would periodically advertise the enterprise’s wireless network name, but only within the range of the wireless APs. If the wireless APs are configured as non-broadcast, each of the 500 Windows XP or Windows Server 2003-based laptops would periodically advertise the enterprise’s wireless network name, regardless of their location (in the office, at a wireless hotspot, or at home).

For these reasons, it is highly recommended that you do not use non-broadcast wireless networks. Instead, configure your wireless networks as broadcast and use the authentication and encryption security features of your wireless network hardware and Windows to protect your wireless network, rather than relying on non-broadcast behavior.