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 WPA and WPA2 security protocols. The vulnerability exposes a side-channel attack against WPA versions 1 and 2 allowing the extraction of the Pre-Shared Key (PSK) used to secure the network.
Source: https://thehackernews.com/2011/12/reaver-brute-force-attack-tool-cracking.html