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AA-2004.02                     AUSCERT Advisory

      Denial of Service Vulnerability in IEEE 802.11 Wireless Devices
                                13 May 2004
Last Revised: --

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1.  Description

  A vulnerability exists in hardware implementations of the IEEE
  802.11 wireless protocol[1] 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 localisation 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
  minimises 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 specialised 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.

2. Platform

  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.

3.  Impact

  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:

    o An attack can be mounted using commodity hardware and
    drivers - no dedicated or high-power wireless hardware is
    required

    o An attack consumes limited resources on attacking device,
    so is inexpensive to mount

    o Vulnerability will not be mitigated by emerging MAC layer
    security enhancements ie IEEE 802.11 TGi

    o Independent vendors have confirmed that there is
    currently no defence 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.

3.  Workarounds/Mitigation

  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 recognised 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.

REFERENCES:

[1] IEEE-SA Standards Board, "IEEE Std IEEE 802.11-1999 Information
    Technology - Telecommunications and Information Exchange Between
    Systems-Local and Metropolitan Area Networks - Specific Requirements
    - Part 11: Wireless LAN Medium Access Control (MAC) And Physical Layer
    (PHY) Specifications," IEEE 1999.
    http://standards.ieee.org/getieee802/download/802.11-1999.pdf

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AusCERT would like to thank the Queensland University of Technology (QUT)
Information Security Research Centre (ISRC) for the information contained
in this advisory. AusCERT would like to thank all vendors that participated
in this process and provided recommendations for mitigation and/or
confirmed details of the vulnerability.
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AusCERT has made every effort to ensure that the information contained
in this document is accurate.  However, the decision to use the information
described is the responsibility of each user or organisation. The decision to
follow or act on information or advice contained in this security bulletin is
the responsibility of each user or organisation, and should be considered in
accordance with your organisation's site policies and procedures. AusCERT
takes no responsibility for consequences which may arise from following or
acting on information or advice contained in this security bulletin.

If you believe that your computer system has been compromised or attacked in 
any way, we encourage you to let us know by completing the secure National IT 
Incident Reporting Form at:

        http://www.auscert.org.au/render.html?it=3192

AusCERT also maintains a World Wide Web service which is found on:
http://www.auscert.org.au.

Internet Email: auscert@auscert.org.au
Facsimile:      (07) 3365 7031
Telephone:      (07) 3365 4417 (International: +61 7 3365 4417)
                AusCERT personnel answer during Queensland business
                hours which are GMT+10:00 (AEST).  On call after hours
                for member emergencies only.

Postal:
Australian Computer Emergency Response Team
The University of Queensland
Brisbane
Qld  4072
AUSTRALIA


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Revision History


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