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A newly-discovered vulnerability in the 802.11 wireless standard allows attackers to jam wireless networks within a radius of one kilometre using off-the-shelf equipment.

Affecting various hardware implementations of the IEEE 802.11 wireless networking standard - including widely used 802.11b devices - the flaw was found in the collision avoidance routines used to prevent multiple devices from transmitting at the same moment.

"When under attack, the device behaves as if the channel is always busy, preventing the transmission of any data over the wireless network," a security advisory released by AusCERT reads.

The weakness allows miscreants to take down networks within five seconds, according to researchers at Australia's Queensland University of Technology's Information Security Research Centre (ISRC), which discovered the vulnerability.

ISRC's leader of network and systems security research, Associate Professor Mark Looi, whose PhD students, Christian Wullems, Kevin Tham and Jason Smith discovered the flaw, said any organization that relies heavily on wireless infrastructure should take the threat seriously.

"Anyone who's relying on the availability of a wireless network should really consider that their wireless network can be knocked offline at any time," said Looi. "They need to very seriously evaluate that network and decide if it's possible to move away from wireless technology."

While previous denial of service attacks against wireless networks have required specialised hardware and relied on high-power antennas, the new attack will make knocking a wireless network off the air an option for a "semi-skilled" attacker using standard hardware.

"An attacker using a low-powered, portable device such as an electronic PDA and a commonly available wireless networking card may cause significant disruption to all WLAN traffic within range, in a manner that makes identification... of the attacker difficult," The AusCERT advisory read.

Because the flaw is in the 802.11 protocol itself, the vulnerability cannot be mitigated through the use of software or encryption schemes. Replacing wireless devices with those not affected by the flaw seems the only option, said Looi.

"Mitigation strategies are few and far between," Looi said "Organisations could deploy wireless networks that don't use this technology, [but] it will be a very expensive exercise."

The flaw is only present in devices using a Direct Sequence Spread Spectrum (DSSS) physical layer, including IEEE 802.11, 802.11b and 802.11g wireless devices operating at low speed. 802.11a and 802.11g wireless devices configured to operate at speeds above 20Mbps are not affected by the glitch,

AusCERT senior security analyst Jamie Gillespie does not anticipate the wide exploitation of the vulnerability.

"For the average corporate user, we're not expecting to see ongoing denial of service attacks. However, if you have remote equipment that is only connected through wireless it is possible that the connection could be disrupted," Gillespie said. "Some critical infrastructure providers may not deploy wireless... but if any do then they should be looking at mitigation strategies."

The lack of a "measurable result" during an attack is likely to render the average attacker bored, Gillespie added.

Unlike flaws discovered in the WEP encryption scheme, the newly-disclosed vulnerability will not allow an attacker to snoop on network communications.

The ISRC findings will be presented to the Institute of Electrical and Electronic Engineers (IEEE) Wireless Telecommunication Symposium in California on Friday.

Copyright © 2004, SecurityFocus logo

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