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Advisory Number: AV04-023 Vulnerability in IEEE 802.11
12 May 2004


This advisory brings attention to a denial of service vulnerability that exists in hardware implementations of the IEEE 802.11 wireless protocol.


The vulnerability is related to the medium access control (MAC) function of the IEEE 802.11 protocol. WLAN devices perform Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to minimise the likelihood of two devices transmitting simultaneously. Fundamental to the functioning of CSMA/CA is the Clear Channel Assessment (CCA) procedure, used in all standards-compliant hardware and performed by a Direct Sequence Spread Spectrum (DSSS) physical (PHY) layer.

An attack against this vulnerability exploits the CCA function at the physical layer and causes all WLAN nodes within range, both clients and access points, to permanently defer transmission of data. When under attack, the device behaves as if the channel is always busy, preventing the transmission of any data over the wireless network.

Affected devices include those that implement IEEE 802.11 using a DSSS physical layer (i.e. IEEE 802.11, 802.11b and low-speed -below 20Mbps, 802.11g wireless devices.

Not afftected: IEEE 802.11a and high-speed (above 20Mbps) 802.11g wireless devices.

The results of a successful DoS attack will not be directly discernable to an attacker, so an attack of this type may be generally less attractive to mount. The effect of the DoS on WLANs is not persistent - once the jamming transmission terminates, network recovery is essentially immediate.

IEEE 802.11 device transmissions are of low energy and short range, so the range of this attack is limited by the signal strength of the attacking device, which is typically low. Well shielded WLANs such as those for internal infrastructures should be relatively immune, however individual devices within range of the attacker may still be affected. Public access points will remain particularly vulnerable.

Suggested Action

At this time no software or firmware upgrades are available for existing devices, as the issue is inherent in the protocol implementation of IEEE 802.11 DSSS.

PSEPC recommends that environments in which network availability is a primary importance should evaluate and take necessary steps to ensure their susceptability to this vulnerability is limited. For more information please see: http://www.auscert.org.au/render.html?it=4091

Note to Readers

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