Windows XP is flawed in the way it searches for wireless access points if a registered access point isn't are available.

Windows XP repeatedly sends packets to access points previously registered, this allows an attacker to sniff valid SSIDs, when an access points is reached with a valid SSID XP will start transmitting packets encrypted with WEP.

WEP is known to be easy to decrypt.

Solution
While this is an inappropriate behavior in XP, part of the problem is due to the weak encryption in wireless LAN technology. The XP SSID broadcast problem can easily be solved by using a third party driver rather than the one supplied by Microsoft.
Further details available to Secunia VIM customers

Provided and/or discovered by
Nobuo Miwa

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