DoS Vulnerability Threatens Wireless Networks

By Larry Seltzer  |  Posted 2004-05-14 Print this article Print

UPDATED: Australia's computer emergency response team has identified a vulnerability in the 802.11 wireless specification that could open wireless networks to denial-of-service attacks. But attackers who use it

AusCERT, Australias national computer emergency response team, released an advisory Thursday 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 in conjunction with the IEEE. A spokesperson for the Wi-Fi Alliance was not immediately available for comment on the AusCERT report.
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. Eugene Chang, vice president of strategic business development of Funk Software, a wireless LAN security vendor, said network administrators should be aware that, in the event of an attack, the network will quickly return to normal and that any attack would be limited to "the range of the attackers transmit power." If an attacker used commercial equipment to stage an attack, the range would be similar to that of an access point, he added. "This means that in a large deployment," said Chang, "this will not shut down the entire network, only devices within range of the attackers signal." Chang described the AusCERT report as "fair and clear as to how the attack works" but added that anyone using the signal to stage an attack would put themselves at risk because the source of the attack could be traced. "It should be noted that the attacker must expose their presence by transmitting a signal to use this attack," he said. "Anyone with rogue detection equipment would easily identify the presence and location of the attacker." Chang pointed out that, although the signal uses a standard protocol, using the signal to shut down others equipment is prohibited by law. " The 2.4GHz band is regulated by law even if it is unlicensed," he said. He characterized Wi-Fi DoS attacks as active attacks" that require the attacker to send a signal or packet which betrays the presence of the attacker. "This risk to the attackers is one major characteristic that will help protect many Wi-Fi networks from attack," said Chang. "Further, as the AusCERT advisory indicates, these attacks are DoS attacks and do not compromise any data. This severely limits the benefit (or joy) of mounting the attack. If DoS is the motive, it would be a lot less risk to the attacker to attack the power line entering the business—much more effective, potentially much harder to resume service, and less risk of being apprehended." Editors Note: This story was updated to include the comments of Eugene Chang, vice president of strategic business development at Funk Software. Check out eWEEKs Mobile & Wireless Center at for the latest news, reviews and analysis.
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Larry Seltzer has been writing software for and English about computers ever since—,much to his own amazement—,he graduated from the University of Pennsylvania in 1983.

He was one of the authors of NPL and NPL-R, fourth-generation languages for microcomputers by the now-defunct DeskTop Software Corporation. (Larry is sad to find absolutely no hits on any of these +products on Google.) His work at Desktop Software included programming the UCSD p-System, a virtual machine-based operating system with portable binaries that pre-dated Java by more than 10 years.

For several years, he wrote corporate software for Mathematica Policy Research (they're still in business!) and Chase Econometrics (not so lucky) before being forcibly thrown into the consulting market. He bummed around the Philadelphia consulting and contract-programming scenes for a year or two before taking a job at NSTL (National Software Testing Labs) developing product tests and managing contract testing for the computer industry, governments and publication.

In 1991 Larry moved to Massachusetts to become Technical Director of PC Week Labs (now eWeek Labs). He moved within Ziff Davis to New York in 1994 to run testing at Windows Sources. In 1995, he became Technical Director for Internet product testing at PC Magazine and stayed there till 1998.

Since then, he has been writing for numerous other publications, including Fortune Small Business, Windows 2000 Magazine (now Windows and .NET Magazine), ZDNet and Sam Whitmore's Media Survey.

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