The U.S. Computer Emergency Readiness Team warned this week of a security flaw in a popular tool intended to make it easier to add additional devices to a secure Wi-Fi network.

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On Tuesday, the organization, known as US-CERT, cited findings from security researcher Stefan Viehbock, who uncovered the security hole in the so-called Wi-Fi Protected Set-up, or WPS, protocol, which is often bundled into Wi-Fi routers. The WPS protocol is designed to allow unskilled home users to set up secure networks using WPA encryption without much hassle. Users are then able to type in a shortened PIN instead of a long pass-phrase when adding a new device to the secure network.

That method, however, also makes it much easier for hackers to break into a secure Wi-Fi network, US-CERT says. The security threat could affect millions of consumers, since the WPS protocol is enabled on most Wi-Fi routers sold today.

"A few weeks ago I decided to take a look at the Wi-Fi Protected Setup (WPS) technology," Viehbock said in a blog post. "I noticed a few really bad design decisions which enable an efficient brute force attack, thus effectively breaking the security of pretty much all WPS-enabled Wi-Fi routers. As all of the more recent router models come with WPS enabled by default, this affects millions of devices worldwide."

The basic problem is that the security of the 8-digit PIN falls dramatically with more attempts to key in the password. When an attempt fails, the hacker can figure out if the first four digits of the code are correct. From there it can then narrow down the possibilities on the remaining digits until the code is cracked. Viehbock said a hacker can get into a secure Wi-Fi hotspot in about two-hours using this method to exploit a vulnerability.

Here's how US-CERT describes the flaw:

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.

US-CERT said in its warning that there is no known fix to the security problem. Instead, the group recommends that users disable the WPS function on their routers. The warning lists several wireless router vendors as selling devices that are affected by the security hole: Buffalo, D-Link, Cisco Linksys, Netgear, Technicolor, TP-Link, and ZyXEL.

US-CERT indicated in its warning that it notified router vendors that are affected by the security issue in early December, but so far the vendors have not offered a response nor have any of them issued statements.

CNET also contacted the vendors listed by US-CERT, but has not yet received a response from any of them.