Because they use radio waves, wireless LANs are open to hackers trying to access sensitive information or spoil the operation of the network. In fact, most wireless LANs don't implement any form of reliable security, enabling access to just about anyone. I've proven that by driving around several large cities recently and using 802.11 packet sniffing tools to detect wireless LANs. I found that many major corporations, retail stores, airports, and homes are wide open.

After deploying a wireless LAN, you need to implement a security assessment, which ensures that the WLAN complies with effective security policies. For most situations, this is necessary whether or not the network implements effective security mechanisms. Don't put too much trust in the design of a system. It's best to run tests to be certain that the network is hardened enough to guard against unauthorized persons attacking company resources.

802.11 Wired Equivalent Privacy (WEP) doesn't provide enough security for most enterprise wireless LAN applications. Because of static key usage, it's fairly easy to crack WEP with off-the-shelf tools. This motivates IT managers to use stronger dynamic forms of WEP. 

Mutual authentication is a critical component of any network, especially a wireless LAN. The user should authenticate with the network, and the edge device, such as an access point or wireless router, should also authenticate with the user. This establishes trust among everyone and avoids the perils of man-in-the-middle attacks.