The WiFi Protected Setup protocol is vulnerable to a brute force attack that allows an attacker to recover an access point's WPS pin, and subsequently the WPA/WPA2 passphrase, in just a matter of hours.
Reaver is a WPA attack tool developed by Tactical Network Solutions that exploits a protocol design flaw in WiFi Protected Setup (WPS). This vulnerability exposes a side-channel attack against Wi-Fi Protected Access (WPA) versions 1 and 2 allowing the extraction of the Pre-Shared Key (PSK) used to secure the network. With a well-chosen PSK, the WPA and WPA2 security protocols are assumed to be secure by a majority of the 802.11 security community.
Usage is simple just specify the target BSSID and the monitor mode interface to use:
# reaver -i mon0 -b 00:01:02:03:04:05
Reaver has been designed to be a robust and practical attack against WPS, and has been tested against a wide variety of access points and WPS implementations.On average Reaver will recover the target AP's plain text WPA/WPA2 passphrase in 4-10 hours, depending on the AP. In practice, it will generally take half this time to guess the correct WPS pin and recover the passphrase.
WPS allows users to enter an 8 digit PIN to connect to a secured network without having to enter a passphrase. When a user supplies the correct PIN the access point essentially gives the user the WPA/WPA2 PSK that is needed to connect to the network. Reaver will determine an access point's PIN and then extract the PSK and give it to the attacker.
Get open source version of Reaver at Google Code