WiFi Protected Setup PIN brute force vulnerability

Overview

The WiFi Protected Setup (WPS) PIN is susceptible to a brute force attack. A design flaw that exists in the WPS specification for the PIN authentication significantly reduces the time required to brute force the entire PIN because it allows an attacker to know when the first half of the 8 digit PIN is correct. The lack of a proper lock out policy after a certain number of failed attempts to guess the PIN on some wireless routers makes this brute force attack that much more feasible.

I. Description

WiFi Protected Setup (WPS) is a computing standard created by the WiFi Alliance to ease the setup and securing of a wireless home network. WPS contains an authentication method called "external registrar" that only requires the router's PIN. By design this method is susceptible to brute force attacks against the PIN.

When the PIN authentication fails the access point will send an EAP-NACK message back to the client. The EAP-NACK messages are sent in a way that an attacker is able to determine if the first half of the PIN is correct. Also, the last digit of the PIN is known because it is a checksum for the PIN. This design greatly reduces the number of attempts needed to brute force the PIN. The number of attempts goes from 10⁸ to 10⁴ + 10³ which is 11,000 attempts in total.

It has been reported that some wireless routers do not implement any kind of lock out policy for brute force attempts. This greatly reduces the time required to perform a successful brute force attack. It has also been reported that some wireless routers resulted in a denial-of-service condition because of the brute force attempt and required a reboot.

II. Impact

An attacker within range of the wireless access point may be able to brute force the WPS PIN and retrieve the password for the wireless network, change the configuration of the access point, or cause a denial of service.

III. Solution

We are currently unaware of a practical solution to this problem.

Workarounds
Disable WPS. 

Although the following will not mitigate this specific vulnerability, best practices also recommend only using WPA2 encryption with a strong password, disabling UPnP, and enabling MAC address filtering so only trusted computers and devices can connect to the wireless network.

Vendor Information

Vendor	Status	Date Notified	Date Updated
Belkin, Inc.	Affected		2011-12-27
Buffalo Inc	Affected		2011-12-27
D-Link Systems, Inc.	Affected	2011-12-05	2011-12-27
Linksys (A division of Cisco Systems)	Affected	2011-12-05	2011-12-27
Netgear, Inc.	Affected	2011-12-05	2011-12-27
Technicolor	Affected		2011-12-27
TP-Link	Affected		2011-12-27
ZyXEL	Affected		2011-12-27

References

http://en.wikipedia.org/wiki/Wi-Fi_Protected_Setup
http://download.microsoft.com/download/a/f/7/af7777e5-7dcd-4800-8a0a-b18336565f5b/WCN-Netspec.doc
http://www.wi-fi.org/wifi-protected-setup/
http://sviehb.wordpress.com/2011/12/27/wi-fi-protected-setup-pin-brute-force-vulnerability/

Credit

Thanks to Stefan Viehböck for reporting this vulnerability.

This document was written by Jared Allar.