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Wi-Fi Protected Setup easily unlocked by security flaw

Couple of hours of brute force will crack a network's PIN

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Security researcher Stefan Viehböck has demonstrated a critical flaw in the Wi-Fi Protected standard that opens up routers to attack and has prompted a US-CERT Vulnerability notice.

Wi-Fi Protected Setup (WPS) is used to secure access to wireless networks and requires each router to have a unique eight-digit PIN. One mode of use allows a device to connect by just presenting that PIN, opening the way for a client to just try every available PIN. Worse still, the protocol splits the PIN into two halves which reduces the attack time to a couple of hours.

Eight digits should produce 100,000,000 possible combinations, and testing various routers Viehböck found it took an average of around two seconds to test each combination. So brute forcing should take several years unless the router was particularly responsive.

But the protocol used by Wi-Fi Protected Setup reports back after the first four digits have been entered, and indicates if they are right, which means they can be attacked separately. The last of the eight digits is just a checksum, so having got the first four the attacker only then has to try another 1,000 combinations (identifying the other three digits) and the entire PIN is known.

That combination means that our attacker only has to try 11,000 different combinations to find the right PIN, reducing the attack time to a couple of hours.

In documented tests (PDF, surprisingly understandable) Viehböck found that of all the routers he tried only the one from Netgear had any sensible response to being repeatedly presented with incorrect PINs, slowing its responses to mitigate against the attack, but with only 11,000 combinations to try that only extended the attack time to a day or so.

Most services will start to slow up when incorrect credentials are presented repeatedly, but it seems router manufacturers have relied on the huge number of possible PINs to keep them safe. Hopefully that means a simple software fix, but until then the US-Cert is recommending that WPS be switched off, and going back to the MAC Address white list. ®

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