A vulnerability exists in hardware implementations of the IEEE802.11 wireless protocol that allows for a trivial but effective attack against the availability of wireless local area network (WLAN) devices.
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 and localization of the attacker difficult.
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), which minimizes 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 (AP), to defer transmission of data for the duration of the attack. When under attack, the device behaves as if the channel is always busy, preventing the transmission of any data over the wireless network.
Previously, attacks against the availability of IEEE 802.11 networks have required specialized hardware and relied on the ability to saturate the wireless frequency with high-power radiation, an avenue not open to discreet attack. This vulnerability makes a successful, low cost attack against a wireless network feasible for a semi-skilled attacker.
Although the use of WLAN technology in the areas of critical infrastructure and systems is still relatively nascent, uptake of wireless applications is demonstrating exponential growth. The potential impact of any effective attack, therefore, can only increase over time.
Wireless hardware devices that implement IEEE 802.11 using a DSSS physical layer. Includes IEEE 802.11, 802.11b and low-speed (below 20Mbps) 802.11g wireless devices. Excludes IEEE 802.11a and high-speed (above 20Mbps) 802.11g wireless devices.
Devices within range of the attacking device will be affected. If an AP is within range, all devices associated with that AP are denied service; if an AP is not within range, only those devices within range of the attacking device are denied service.
Minimum threat characteristics:
* An attack can be mounted using commodity hardware and drivers - no dedicated or high-power wireless hardware is required.
* An attack consumes limited resources on attacking device, so is inexpensive to mount
* Vulnerability will not be mitigated by emerging MAC layer security enhancements ie IEEE 802.11 TGi
* Independent vendors have confirmed that there is currently no defense against this type of attack for DSSS based WLANs
The range of a successful attack can be greatly improved by an increase in the transmission power of the attacking device, and the use of high-gain antennae.
At this time a comprehensive solution, in the form of software or firmware upgrade, is not available for retrofit to existing devices. Fundamentally, the issue is inherent in the protocol implementation of IEEE 802.11 DSSS.
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.
The model of a shared communications channel is a fundamental factor in the effectiveness of an attack on this vulnerability. For this reason, it is likely that devices based on the newer IEEE 802.11a standard will not be affected by this attack where the physical layer uses Orthogonal Frequency Division Multiplexing (OFDM).
It is recognized that the 2.4G Hz band suffers from radio interference problems, and it is expected that operators of the technology will already have in place measures to shield their networks as well as a reduced reliance on this technology for critical applications.
The effect of the DoS on WLANs is not persistent - once the jamming transmission terminates, network recovery is essentially immediate.
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.
At this time, AusCERT continues to recommend that the application of wireless technology should be precluded from use in safety, critical infrastructure and/or other environments where availability is a primary requirement. Operators of wireless LANs should be aware of the increased potential for undesirable activity directed at their networks.