DoS Vulnerability Threatens Wireless Networks

AusCERT, Australia’s national computer emergency response team, released an advisory today identifying a vulnerability in the 802.11 wireless specification that could open wireless networks to denial-of-service attacks.

The advisory targets a new cause for concern among wireless network administrators. Until now, concerns about Wi-Fi security have focused on weak encryption and lack of authentication in WEP (Wired Equivalent Privacy), the native security mechanism in the IEEE 802.11 standard that defines wireless networking.

Those concerns were addressed last year with the introduction of Wi-Fi Protected Access, a security standard developed by the Wi-Fi Alliance industry trade association.

The attack that AusCERT describes is, at bottom, a flaw in the 802.11 protocols. One of the 802.11 physical layers uses DSSS (direct sequence spread spectrum) technology. The DSSS layer performs the CCA (clear channel assessment) procedure, which is an essential part of CSMA/CA, a collision-avoidance scheme that is fundamental to most networking technology, including conventional wired Ethernet.

The attack involves using the CCA layer in a legal but abusive way to instruct other devices in the operating range to defer transmission of data. The other devices will continue to defer for the duration of the attack. If the attack were to end, the network should recover very quickly.

The problem only affects Wi-Fi products that operate in the 2.4GHz band and only those operating at lower speeds, below 20M bps. Therefore, 802.11b products are affected, but 802.11a products, which operate in the 5GHz band, are not. 802.11g products are affected if they are operating at lower speeds.

The Wi-Fi Alliance was not immediately available for comment.

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