Security '03 Paper
[Security '03 Technical Program]
802.11 Denial-of-Service Attacks:
Real Vulnerabilities and Practical Solutions
John Bellardo and Stefan Savage
Department of Computer Science and Engineering
University of California at San Diego
The convenience of 802.11-based wireless access networks has led to
widespread deployment in the consumer, industrial and military sectors.
However, this use is predicated on an implicit assumption of
confidentiality and availability. While the security flaws in 802.11's
basic confidentially mechanisms have been widely publicized,
the threats to network availability are far less widely
appreciated. In fact, it has been suggested that 802.11 is highly
susceptible to malicious denial-of-service (DoS) attacks targeting its
management and media access protocols. This paper provides an
experimental analysis of such 802.11-specific attacks - their
practicality, their efficacy and potential low-overhead implementation
changes to mitigate the underlying vulnerabilities.
The combination of free spectrum, efficient channel coding and cheap
interface hardware have made 802.11-based access networks extremely
popular. For a couple hundred dollars a user can buy an 802.11 access
point that seamlessly extends their existing network connectivity for
almost 100 meters. As a result, the market for 802.11-based LANs
exceeded $1 Billion in 2001 and includes widespread use in the home,
enterprise and government/military sectors, as well as an emerging
market in public area wireless networks. However, this
same widespread deployment makes 802.11-based networks an attractive
target for potential attackers. Indeed, recent research has
demonstrated basic flaws in 802.11's encryption
mechanisms [FMS01,BGW01] and
authentication protocols [ASJZ01] - ultimately leading
to the creation of a series of protocol extensions and replacements
(e.g., WPA, 802.11i, 802.1X) to address these problems. However, most
of this work has focused primarily on the requirements of access
control and confidentiality, rather than availability.
In contrast, this paper focuses on the threats posed by denial-of-service
(DoS) attacks against 802.11's MAC protocol. Such attacks, which
prevent legitimate users from accessing the network, are a vexing
problem in all networks, but they are particularly threatening in the
wireless context. Without a physical infrastructure, an attacker is
afforded considerable flexibility in deciding where and when to
attack, as well as enhanced anonymity due to the difficulty in
locating the source of individual wireless transmissions. Moreover,
the relative immaturity of 802.11-based network management tools makes
it unlikely that a well-planned attack will be quickly diagnosed.
Finally, as we will show, vulnerabilities in the 802.11 MAC protocol
allow an attacker to selectively or completely disrupt service to the
network using relatively few packets and low power consumption.
This paper makes four principal contributions. First, we provide a
description of vulnerabilities in the 802.11 management and media
access services that are vulnerable to attack. Second, we demonstrate
that all such attacks are practical to implement by circumventing the
normal operation of the firmware in commodity 802.11 devices.
Third, we implement two important classes of denial-of-service attacks
and investigate the range of their practical effectiveness. Finally,
we describe, implement and evaluate non-cryptographic countermeasures
that can be implemented in the firmware of existing MAC hardware.
The rest of this paper is structured as follows: Section 2
describes related security research conducted by others in academia,
as well as unpublished, but contemporaneous, work from the ``blackhat''
security community. Section 3 describes and
categorizes existing denial-of-service vulnerabilities in 802.11's MAC
protocol. In Section 4 we use live experiments and
simulation to analyze the practicality and efficacy of these attacks,
followed by an evaluation of low-overhead countermeasures to mitigate
the underlying vulnerabilities. Finally, we summarize our findings in
A great deal of research has already been focused on 802.11 network
security. Most of this work has focused on weaknesses in the wired
equivalency protocol (WEP) intended to provide data privacy between
802.11 clients and access points. WEP relies on shared secret keys
to support a challenge-response authentication protocol and for
encrypting data packets. In 2001, Fluhrer et al. identified recurring
weak keys in WEP, and showed how to use them to recover the secret
key [FMS01]. Once the key is known, an attacker can both
fully utilize network resources and monitor the traffic of other
network nodes. In a recent paper, Stubblefield et al., demonstrate an
implementation of this attack that was able to recover a 128-bit WEP
key purely through passive monitoring [SIR02]. In
addition, Borisov et al. have identified vulnerabilities that allow
WEP-protected frames to be modified, new frames to be injected,
authentication frames to be spoofed and plain text to be recovered
from encrypted frames - all without knowing the shared secret
While these works comprise the best known body of 802.11 security
research, there has also been some attention focused on
denial-of-service vulnerabilities unique to 802.11. As part of his
PhD thesis, Lough identifies a number of security vulnerabilities in
the 802.11 MAC protocol, including those that lead to the
deauthentication/disassociation and virtual carrier-sense attacks
presented in this paper [Lou01]. However, while Lough's
thesis identifies these vulnerabilities, it does not validate them
empirically. We demonstrate that such validation is critical to
assessing the true threat of such attacks.
In addition to Lough's work, Faria and Cheriton
consider the problems posed by authentication DoS attacks. They
identify those assumption violations that lead to the vulnerabilities
and propose a new authentication framework to address the
problems [FC02]. Unlike their work, this paper focuses on
validating the impact of the attacks and developing light-weight
solutions that do not require significant changes to existing standards
or extensive use of cryptography.
The deauthentication/disassociation attack is fairly straightforward
to implement and while writing this paper we discovered several in the
``black hat'' community who had done so before us. Lacking
publication dates it is difficult to determine the ordering of these
efforts, but we are aware of three implementations to date: one by
Baird and Lynn (AirJack) presented at BlackHat Briefings in July of
2002, another due to Schiffman and presented at the same event
(Omerta), and a tool by Floeter (void11) that appears to be roughly
contemporaenous [LB02,Sch02,Flo02]. As part of his
implementation, Schiffman also discusses a general purpose toolkit,
called Radiate, for injecting raw 802.11 frames into the channel.
However, since this toolkit works through the firmware it is only able
to generate a subset of legitimate 802.11 frames. Compared to this
previous work, our contribution lies in evaluating the impact of the
attack, providing a cheap means to mitigate such attacks and in
providing an infrastructure for mounting a wider class of attacks
(including the virtual carrier-sense attack).
Congestion-based MAC layer denial of service attacks have also been
studied previously. Gupta et al. examined DoS attacks in 802.11 ad hoc
networks and show that traditional wireline-based detection and
prevention approaches do not work, and propose the use of MAC layer
fairness to mitigate the problem [GKF02].
Kyasanur and Vaidya also look at congestion-based MAC DoS attacks, but
from a general 802.11 prospective, not the purely ad hoc
prospective [KV03]. They
propose a straightforward method for detecting such attacks. In
addition they propose and simulate a defense where
uncompromised nodes cooperate to control the frame rate at the
compromised node. Compared to these papers, we focus on attacks on the
802.11 MAC protocol itself rather than pure resource consumption
Finally, to provide a long-term solution to 802.11's security problems, the
802.11 TGi working group has proposed the standard use of the 802.1X
protocol [IEE01] for authentication in future versions of
802.11 products, in addition to both short-term and long-term
modifications to the privacy functions. However, while the working
group is clearly aware of threats from unauthenticated management
frames and spoofed control frames (e.g., [Abo02,Moo02]), to the
best of our knowledge there is no protection against such attacks in
the current drafts under discussion.
The 802.11 MAC layer incorporates functionality uniquely designed to
address problems specific to wireless networks. In particular, this
includes the ability to discover networks, join and leave
networks, and coordinate access to the radio medium. The
vulnerabilities discussed in this section result directly from this
additional functionality and can be broadly placed into two
categories: identity and media-access control.
Identity vulnerabilities arise from the implicit trust 802.11 networks
place in a speaker's source address. As is the case with
wired Ethernet hosts, 802.11 nodes are identified at the MAC layer
with globally unique 12 byte addresses. A field in the MAC frame
holds both the senders and the receivers addresses, as reported by
the sender of the frame. For ``class one'' frames, including most
management and control messages, standard 802.11 networks do not
include any mechanism for verifying the correctness of the self-reported
identity. Consequently, an attacker may ``spoof'' other
nodes and request various MAC-layer services on their behalf. This
leads to several distinct vulnerabilities.
Exemplifying this problem is the deauthentication
attack. After an 802.11 client has selected an access point
to use for communication, it must first authenticate itself to the AP
before further communication may commence. Moreover, part of the
authentication framework is a message that allows clients and access points
to explicitly request deauthentication from one another.
Unfortunately, this message itself is not authenticated using
any keying material. Consequently the attacker may spoof this
message, either pretending to be the access point or the client,
and direct it to the other party (see
Figure 1). In response, the access point or
client will exit the authenticated state and will refuse all
further packets until authentication is reestablished. How long
reestablishment takes is a function of how aggressively the client
will attempt to reauthenticate and any higher-level timeouts or
backoffs that may suppress the demand for communication. By
repeating the attack persistently a client may be kept from
transmitting or receiving data indefinitely.
One of the strengths of this attack is its great flexibility: an
attacker may elect to deny access to individual clients, or even
rate limit their access, in addition to simply denying service to the
entire channel. However, accomplishing these goals efficiently
requires the attacker to promiscuously monitor the channel and
send deauthentication messages only when a new authentication has
successfully taken place (indicated by the client's attempt to
associate with the access point). As well, to prevent a client from
``escaping'' to a neighboring access point, the attacker must
periodically scan all channels to ensure that the client has not
switched to another overlapping access point.
A very similar vulnerability may be found in the association protocol
that follows authentication. Since a client may be authenticated
with multiple access points at once, the 802.11 standard provides a
special association message to allow the client and access point to
agree which access point shall have responsibility for forwarding
packets to and from the wired network on the client's behalf. As
with authentication, association frames are unauthenticated, and
802.11 provides a disassociation message similar to the
deauthentication message described earlier. Exploiting this
vulnerability is functionally identical to the deauthentication
attack. However, it is worth noting that the disassociation
attack is slightly less efficient than the deauthentication attack.
This is because deauthentication forces the victim node to do more
work to return to the associated state than does disassociation,
ultimately requiring less work on the part of the attacker.
Graphical depiction of the deauthentication attack. Note
that the attacker needs only generate one packet for
every six exchanged between the client and access point.
The power conservation functions of 802.11 also present several
identity-based vulnerabilities. To conserve energy,
clients are allowed to enter a sleep state during which they are
unable to transmit or receive. Before entering the sleep state the
client announces its intention so the access point can start buffering
any inbound traffic for the node. Occasionally the client awakens and
polls the access point for any pending traffic. If there is any
buffered data at this time, the access point
delivers it and subsequently discards the contents of its buffer. By
spoofing the polling message on behalf of the client, an attacker cay
cause the access point to discard the clients packets while it is
Along the same vein, it is potentially possible to trick the client node into
thinking there are no buffered packets at the access point when in
fact there are. The presence of buffered packets is indicated in a
periodically broadcast packet called the traffic indication map, or
TIM. If the TIM message itself is spoofed, an attacker may convince a
client that there is no pending data for it and the client will
immediately revert back to the sleep state.
Finally, the power conservation mechanisms rely on time
synchronization between the access point and its clients so clients
know when to awake. Key synchronization information, such as the
period of TIM packets and a timestamp broadcast by the access point,
are sent unauthenticated and in the clear. By forging these management
packets, an attacker can cause a client node to fall out of sync with
the access point and fail to wake up at the appropriate times.
While all of the vulnerabilities in this section could be resolved with
appropriate authentication of all messages, it seems unlikely that such
a capability will emerge soon. With an installed base of over
15 million legacy 802.11 devices, the enormous growth of the public-area
wireless access market and the managerial burden imposed
by the shared key management of 802.1X, it seems unlikely that
there will be universal deployment of mutual authentication
infrastructure any time soon. Moreover, it is not clear whether future
versions of the 802.11 specification will protect management frames such as
deauthentication (while it is clear they are aware of the problem,
the current work of the TGi working group still leaves the
deauthentication operation unprotected).
Graphical depiction of the virtual carrier-sense attack in action.
The gradient portion of the attacker's frame indicates time
reserved by the duration field although no data is actually sent.
Continually sending the attack frames back to back prevents other
nodes from sending legitimate frames.
802.11 networks go through significant effort to avoid transmit
collisions. Due to hidden terminals
perfect collision detection is not possible and a combination of
physical carrier-sense and virtual carrier-sense mechanisms are
employed in tandem to control access to the
channel [BDSZ94]. Both of these mechanisms may be
exploited by an attacker.
First, to prioritize access to the radio medium four time windows are
defined. For the purposes of this discussion
only two are important: the Short Interframe Space (SIFS) and
the longer Distributed Coordination Function Interframe Space (DIFS).
Before any frame can be sent the sending radio must observe a quiet
medium for one of the defined window periods. The SIFS window is used
for frames sent as part of a preexisting frame exchange (for example,
the explicit ACK frame sent in response to a previously transmitted
data frame). The DIFS window is used for nodes wishing to
initiate a new frame exchange. To avoid all nodes transmitting
immediately after the DIFS expires, the time after the DIFS is
subdivided into slots. Each transmitting node randomly and with equal
probability picks a slot in which to start transmitting. If a
collision does occur (indicated implicitly by the lack of an immediate
acknowledgment), the sender uses a random exponential backoff
algorithm before retransmitting.
Since every transmitting node must wait at least an SIFS interval, if not
longer, an attacker may completely monopolize the channel by sending a
short signal before the end of every SIFS period. While this attack would
likely be highly effective, it also requires the attacker to expend
considerable energy. A SIFS period is only 20 microseconds on 802.11b
networks, leading to a duty cycle of 50,000 packets per second in
order to disable all access to the network.
A more serious vulnerability arises from the virtual carrier-sense
mechanism used to mitigate collisions from hidden terminals. Each
802.11 frame carries a Duration field that indicates the number of
microseconds that the channel is reserved. This value, in turn, is
used to program the Network Allocation Vector (NAV) on each
node. Only when a node's NAV reaches 0 is it allowed to transmit.
This feature is principally used by the explicit request to
send (RTS) / clear to send (CTS) handshake that can be used
to synchronize access to the channel when a hidden terminal may be
interfering with transmissions.
During this handshake the sending node first sends a small
RTS frame that includes a duration large enough to complete the RTS/CTS
sequence - including the CTS frame, the data frame, and the
subsequent acknowledgment frame. The destination node replies to the
RTS with a CTS, containing a new duration field updated to account for the time
already elapsed during the sequence. After the CTS is sent, every node
in radio range of either the sending or receiving node
will have updated their NAV and will defer all transmissions for the
duration of the future transaction. While the RTS/CTS feature is
rarely used in practice, respecting the virtual carrier-sense function
indicated by the duration field is mandatory in all 802.11 implementations.
An attacker may exploit this feature by asserting a large duration
field, thereby preventing well-behaved clients from gaining access to
the channel (as shown in Figure 2).
While it is possible to use almost any frame type to control the NAV,
including an ACK, using the RTS has some advantages. Since a
well-behaved node will always respond to RTS with a CTS, an attacker
may co-opt legitimate nodes to propagate the attack further than it
could on its own. Moreover, this approach allows an attacker to
transmit with extremely low power or using a directional antennae,
thereby reducing the probability of being located.
The maximum value for the NAV is 32767, or roughly 32 milliseconds on
802.11b networks, so in principal an attacker need only transmit
approximately 30 times a second to jam all access to the channel.
Finally, it is worth noting that RTS, CTS and ACK frames are not
authenticated in any current or upcoming 802.11 standard. However,
even if they were authenticated, this would only provide
non-repudiation since, by design, the virtual-carrier sense feature
impacts all nodes on the same channel.
While the previous vulnerabilities are severe in principal, understanding
their true threat potential requires evaluating the practicality
of implementing them and how well they perform in practice. In this
section we examine these issues as well as discussing the efficacy
of several low-overhead defense mechanisms.
Practical Attacks and Defenses
From a purely practical perspective, a key engineering
question is, ``Can an attack be generated with commodity
hardware?'' While theoretical vulnerabilities are clearly
important, we feel that attacks with software implementations
represent a qualitatively greater threat since they are available to
a dramatically expanded set of potential attackers.
At first glance this appears to be a trivial problem since all 802.11 Network Interface Cards (NIC) are inherently able to generate arbitrary frames. However, in
practice, all 802.11(a,b) devices we are aware of
implement key MAC functions in firmware and moderate access to the
radio through a constrained interface. The implementation of this
firmware, in turn, dictates the limits of how a NIC can be used by an
attacker. Indeed, in reviewing preprints of this paper, several
802.11 experts declared the virtual carrier-sense attack infeasible
in practice due to such limitations.
In testing a wide variety of 802.11 NICs we have found that most allow
the generation of management frames necessary to exploit the identity
attacks described earlier - typically using semi-documented or
undocumented modes of operation, such as HostAP and HostBSS mode in
Intersil firmware. However, these same devices do not typically allow the
generation of any control frames, permit other key fields (such as
Duration and FCS) to be specified by the host, or allow reserved or
illegal field values to be transmitted. Instead, the firmware
overwrites these fields with appropriate values after the host
requests that queued data be transmitted. While it might be possible
to reverse-engineer the firmware to remove this limitation, we
believe the effort to do so would be considerable. Instead, we have
developed an alternative mechanism to sidestep the limitations imposed
by the firmware interface. To understand our approach it is first
necessary to understand the architecture of existing 802.11 products.
Most commodity 802.11 devices, including those using Intersil Prism,
Lucent/Agere/Orinoco/Proxim Hermes and Cisco Aironet chipsets are based on an
initial MAC design originated by Choice Microsystems (since acquired
by Intersil). In this architecture, all low-level functions -
including frame transmission, scheduling, acknowledgement, and
fragmentation - are implemented in firmware while the host is simply
responsible for managing data transfer to and from the device. Data
transfer is achieved through a firmware-implemented ``Buffer Access
Path'' (BAP) that shields the driver writer from the details of NIC
memory management and synchronization. While the BAP interface will
typically accept raw 802.11 frames, these packets are then further
interpreted by concurrent firmware processes. As a result, only a
subset of potential frames can be successfully transmitted by the host.
A block diagram depicting how the ``aux port'' can be used to circumvent the limitations imposed by the firmware. By using this raw memory interface, the host can transform ``normal'' packets into arbitrary 802.11 frames as they are transmitted.
However, Choice-based MACs also provide an unbuffered, unsychronized raw
memory access interface for debug purposes - typically called the
``aux port''. By properly configuring the host and NIC, it is
possible to write a frame via the BAP interface, locate it in the
NIC's SRAM, request a transmission, and then modify the packet via the
aux port - after the firmware has processed it, but before it is
actually transmitted. This process is depicted in Figure 3.
To synchronize the host and NIC, a simple
barrier can be implemented by spinning on an 802.11 header field (such
as duration) that is overwritten by the firmware. Alternatively, the
host can continuously overwrite if synchronization is
unnecessary. In practice, this ``data race'' approach, while
undeniably ugly, is both reliable and permits the generation of arbitrary
802.11 MAC frames. Using this method we are able to implement any of
the attacks previously described using off-the-shelf hardware. We
believe we are the first to demonstrate this capability using
Our prototype, called Swat, consists of an iPAQ H3600 Pocket PC, running
Familiar Linux, with a DLink DWL-650 PCMCIA 802.11 interface mounted
in a standard PC Card sleeve. The entire device weighs approximately
375g (a bit over 12 oz) and is easily concealed in a coat pocket. More modern
Pocket PCs, such as the Toshiba e740/e750 and the HP iPAQ 5450,
include integral 802.11 functionally and could accomplish the same
feats with roughly half the size and weight.
iPAQ H3600 with Dlink DWL-650 card, running Swat attack
testing tool. Individual clients and AP's are identified either
using MAC address or by passively monitoring the channel and
extracting destination IP addresses and DNS names.
To experiment with denial-of-service attacks we have built a
demonstration application that passively monitors wireless channels for APs
and clients. Individual clients are identified initially by their MAC
address, but as they generate traffic, a custom DNS resolver and a slightly modified version of
dsniff [Son] is used to isolate better identifiers
(e.g., userids, DNS address of IMAP server, etc). These identifiers
can be used to select individual hosts for attack, or all hosts may be
attacked en masse. The application and the actual device are pictured in
In the remainder of this section, we analyze the impact of the
deauthentication attack and a preliminary defense mechanism, followed
by a similar examination of the virtual carrier-sense attack and defense.
Our implementation of this attack promiscuously monitors all
network activity, including non-data 802.11 frames, and matches the
source and destination MAC address against a list of attack targets.
If a data or association response frame is received from a target, we
issue a spoofed deauthentication frame to the access point on behalf
of the client. To avoid buffer overflow in congested networks on the
attacking machine, deauthentication frames are rate limited to 10
frames per second per client. This limit is reset when an access
point acknowledges receipt of a deauthentication frame.
We tested this implementation in a small 802.11 network composed of 7
machines: 1 attacker, 1 access point, 1 monitoring station, and 4
legitimate clients. The access point was built using the Linux HostAP
driver, which provides an in-kernel software-based access
point. Each of the clients attempted to transfer, via ftp, a large file
through the access point machine - a transfer which exceeded the
testing period. We mounted two attacks on the network. The first,
illustrated by the thin rectangle in Figure 5, was
directed against a single client running MacOS X. This client's
transfer was immediately halted, and even though the attack lasted
less than ten seconds, the client did not resume transmitting at its
previous rate for more than a minute. This amplification was due to
a combination of an extended delay while the client probed for other
access points and the exponential backoff being employed by the ftp
server's TCP implementation.
The second attack, delineated by the wider rectangle in the same
figure, was directed against all four clients. Service is virtually
halted during this period, although the Windows XP client is able to
send a number of packets successfully. This anomaly has two sources.
First, these are not data packets from the ftp session but rather UDP
packets used by Window's DCE RPC service and not subject to TCP's
congestion control procedure. Second, there is a small race condition in
our attack implementation between the time a client receives the
successful association response and the time the attacker sends the
deauthentication frame. The WinXP client used this small
window to send approximately ten UDP packets before the attacking node
shut them down. Modifying the implementation to send the
deauthentication packets after both authentication and association
would mitigate this effect.
Packets sent by each of the 4 client nodes during the
deauthentication attack. The first attack, against the MacOS client,
started at second 15 and lasted 8 seconds. The second attack
against all the clients started at 101 and lasted for 26 seconds. The
attacking node consumes a negligible amount of bandwidth due to the
A number of smaller, directed attacks were performed in addition to
those in Figure 5. The small tests were done using the
extended 802.11 infrastructure found at UCSD with varied victims.
Recent versions of Windows, Linux, and the MacOS all gave up on the
targeted access point and kept trying to find others. Slightly older
versions of the same systems never attempted to switch access points
and were completely disconnected using the less sophisticated version
of the attack. The attack even caused one device, an HP Jornada
Pocket PC, to
The deauthentication vulnerability can be solved directly by
explicitly authenticating management frames and dropping invalid
requests. However, the standardization of such capabilities is still
some ways off and it is clear that legacy MAC designs do not have
sufficient CPU capacity to implement this functionality as a software
upgrade. Therefore, system-level defenses with low-overhead can still
offer significant value. In particular, by delaying the effects of
deauthentication or disassociation requests (e.g., by queuing such
requests for 5-10 seconds) an AP has the opportunity to observe
subsequent packets from the client. If a data packet arrives after a
deauthentication or disassociation request is queued, that request is
discarded - since a legitimate client would never generate packets in
that order. The same approach can be used in reverse to mitigate
forged deauthentication packets sent to the client on behalf of the AP.
This approach has the advantage that it can be implemented
with a simple firmware modification to existing NICs and access
points, without requiring a new management structure.
To test this defense we modified the access point used in our
experiments as described above, using a timeout value of 10 seconds
for each management request. We then executed the previous experiment
again using the ``hardened'' access point. The equivalent results can
be seen in Figure 6. From this graph it is difficult to tell
that the attack is active, and the client nodes continue their
activity oblivious to the misdirection being sent to the access point.
However, our proposed solution is not without drawbacks. In particular, it
opens up a new vulnerability at the moment in which mobile clients
roam between access points. The association message is used to
determine which AP should receive packets destined for the mobile
client. In certain circumstances leaving the old association
established for an additional period of time may prevent the routing
updates necessary to deliver packets through the new access point.
Or, in the case of an adversary, the association could be kept open
indefinitely by spoofing packets from the mobile client to the
spoofed AP - keeping the association current. While both these
situations are possible, we will argue that they are unlikely to
represent a new threat in practice.
Packets sent by each of the 4 client nodes during the
deauthentication attack with an access point modified to defend
against this attack. The first attack, against the MacOS client,
started at second 10 and lasted 12 seconds. The second attack
against all the clients started at 30 and lasted through the end of
the trace. The attacking node consumes a negligible amount of
bandwidth due to the rate limiting.
There are two main infrastructure configurations that support roaming.
For lack of a better name we refer to these as ``intelligent'' and
``dumb''. In the ``intelligent'' configuration the access points have
an explicit means of coordination. This coordination can be used to,
among other things, update routes for and transfer buffered packets
between access points when a mobile node changes associations. Since
there is not currently a standard for this coordination function,
AP's offering such capabilities typically use proprietary
protocols that work only between homogenous devices. In contrast
``dumb'' access points have no explicit means of coordination and
instead rely on the underlying layer-two distribution network
(typically Ethernet) to reroute packets as a mobile client's MAC
address appears at a new AP (and hence a new Ethernet switch port).
Intelligent infrastructures, due to their preexisting coordination,
are easily modified to avoid the aforementioned problems caused by the
deassociation timeout. Since the mobile node must associate with
the new access point before it can transmit data, and since the access
points are coordinated (either directly or through a third party), the
old access point can be informed when the mobile node makes a new
association. Based on this information the old access point can
immediately honor the clients deauthentication request. While an
attacker can spoof packets from the mobile host to create confusion,
this vulnerability exists without the addition of the deferred
deassociation mechanisms we have described.
Dumb infrastructures are slightly more problematic because of their
lack of coordination and reliance on the underlying network topology.
If that underlying topology is a broadcast medium, which is a rarity
these days, there is no problem because all packets are already
delivered to all access points. If the underlying topology is
switched, then a protocol is used (typically a spanning tree
distribution protocol) to distribute which MAC addresses are served by
which ports. Existing switches already gracefully
support moving a MAC address from one port to another, but have problems when
one MAC address is present across multiple ports. In the non-adversarial case
the mobile node will switch access points, proceed to send data using
the new access point, and cease sending data through the old access
point. From the switches perspective this is equivalent to a MAC
switching ports. The mobile node may not receive data packets until
it has sent one - allowing the switch to learn its new port - but that
limitation applies regardless of the deauthentication timeout. In the
adversarial case the attacking node could generate spoofed traffic
designed to confuse the switch. However, this does not represent a
significant new vulnerability - even without the delay on
deauthentication/disassociation an attacker can spoof a packet from an
mobile client in order to create this conflict (including a WEP
protected packet after key recovery).
Motivated by the success of the previous attack, we built an
implementation exploiting the NAV vulnerability. We generated a
variety of packet streams with a range of large duration values -
including continuous runs of RTS frames, CTS frames, and ACK frames
destined for APs, hosts and unallocated addresses. We verified that
packets were being sent as expected using a separate machine to
monitor the channel being targeted. To our surprise, while our
implementation carried out the attacks faithfully, they did not have
the expected impact. We repeated these experiments using both Lucent
WavePoint II and Apple Airport Extreme access points and with a
variety of host NIC cards, all with the same results. After careful
examination of traces collected during these attacks we have come to the
conclusion that most of the devices available to us do not properly
implement the 802.11 MAC specification and are improperly resetting
their NAV. In particular, we have witnessed APs and NICs alike emit
packets within a millisecond after the broadcast of a CTS frame with a
duration of 32767. Figure 7 shows a trace excerpt illustrating this behavior - the initial CTS
frame should keep the channel idle for 32ms, and yet after scarcely a
millisecond has passed the channel is in use by another host.
Such activity should be impossible under the
802.11 standard since nodes receiving the CTS cannot assume that they
will be able to sense the carrier (or even significant radio energy)
since the transmitter may be a hidden terminal. We have not conducted
a thorough survey of 802.11 gear, so these deficiencies may be unique
to the hardware in our environment. However, given the prevalence of
the Choice design we would not be surprised if this bug is prevalent.
Excerpt from a typical virtual carrer-sense attack trace using CTS
frames. MAC address 00:03:93:ea:e7:0f is the access point, and
fc:1d:e7:00:15:01 is an unallocated MAC address.
10.0.1.2 is the uploading client, and 10.0.10.2 is the receiving machine.
The first TCP data frame is sent 1.1 ms after a CTS that reserved the
medium for > 32
ms. In the second CTS sequence the data frame is sent
after 3.4 ms.
|1.294020|| ||fc:1d:e7:00:15:01||32767||802.11 CTS|
Under the assumption that these bugs will be removed in future 802.11
products (since they effectively prevent RTS/CTS from working as well
as the 802.11 Point Coordinator Function and all related
Quality-of-Service services based on 802.11) the remainder of
this section explores the NAV vulnerability in the context of the
popular ns simulator (which implements the protocol faithfully).
We implemented the virtual carrier-sense attack by modifying the
ns [NS] 802.11 MAC layer implementation to allow arbitrary duration
values to be sent
periodically, 30 times a second, by the attacker. The attacker's
frames were sent using the
normal 802.11 access timing restrictions, which was necessary to
prevent the attacker from excessively colliding with other in-flight
frames (and thereby increase the amount of work required of the attacker).
In addition the attacker was modified to ignore all duration values
transmitted from any other node. The network topology was chosen to
mimic many existing 802.11 infrastructure deployments: a
single access point node, through who all traffic was being sent, 18
static client nodes and 1 static attacker node, all within radio
distance of the access point. As with the previous experiments, ftp
was used to generate the long-lived network traffic. We simulated
attacks using ACK frames with large duration values, as well as the RTS/CTS
sequence described earlier. Figure 8 shows the ACK
flavor of the virtual carrier-sense attack in action, but both provided similar results:
the channel is completely blocked for the duration of the attack.
The virtual carrier-sense attack is much harder to defend against in practice than the
One approach to mitigate its effects is to place a limit on the duration
values accepted by nodes. Any packet containing a larger duration value is
simply truncated to the maximum value allowable. Strict adherence to
the required use of the NAV feature indicates two different limits: a
low cap and a high cap. The low cap has a value equal to the
amount of time required to send an ACK frame, plus media access
backoffs for that frame. The low cap is usable when the only packet
that can follow the observed packet is an ACK or CTS. This includes
RTS and all management (association, etc) frames. The high cap,
on the other hand, is used when it is valid for a data packet to
follow the observed frame. The limit in this case needs to include the
time required to send the largest data frame, plus the
media access backoffs for that frame. The high cap must be used in
two places: when observing an ACK (because the ACK my be part of a
MAC level fragmented packet) and when observing a CTS.
Results from the ACK based virtual carrier-sense attack simulation with 18 client nodes. The attack begins at time 40 and ends at time 60. The dark region at the bottom of the graph during the attack is the attacker.
We modified our simulation to add these limits, assuming that a value
of 1500 bytes as the largest packet. While this is not strictly the largest
packet that can be sent in an 802.11 network, it is the largest packet
sent in practice because 802.11 networks are typically bridged to Ethernet, which has a
roughly 1500 byte MTU. Figure 9 shows a simulation of
this defense under the same conditions as the prior simulation. While
there is still significant perturbation, many of the individual
sessions are able to make successful forward progress. However, we
found that simply by increasing the attacker's frequency to 90 packets
per second, the network could still be shut down. This occurs
because the attacker is using ACK frames, whose impact on the NAV is limited by the
To further improve upon this result requires us to abandon portions
of the standard 802.11 MAC functionality. At issue is the
inherent trust that nodes place in the duration value sent by other nodes.
By considering the different frame types that carry duration values we can
define a new interpretation of the duration that allows us to avoid most
possible DoS attacks. The four key frame types that contain duration values
are ACK, data, RTS, and CTS, and we consider each in turn.
Under normal circumstances the only time a ACK frame should carry a
large duration value is when the ACK is part of a fragmented packet
sequence. In this case the ACK is reserving the medium for the next
fragment. If fragmentation is not used then there is no reason to
respect the duration value contained in ACK frames. Since fragmentation is
almost never used (largely due to the fact that default fragmentation
thresholds significantly exceed the Ethernet MTU) removing it from
operation altogether will have minimal impact on existing networks.
Like the ACK frame, the only legitimate occasion a data frame can carry a
large duration value is if it is a subframe in a fragmented packet
exchange. Since we have removed fragmentation from the network, we
can safely ignore the duration values in all data frames.
The third frame type to be concerned with is the RTS frame. The RTS
frame is only valid in an RTS-CTS-data transmission sequence. If an
RTS is seen on the network, it follows that the node seeing the RTS will
also be able to observe the data frame. The 802.11 specification
precisely defines the time a CTS frame, and subsequent data frame,
will be sent. Therefore the duration value in the RTS packet can be
treated speculatively - respected until such time as a data frame
should be sent. If the data frame is not observed at
the correct time, either the sender has moved out of range or the RTS
request was spoofed. In either case it is safe for the other node
to undo the impact of this duration on the NAV. This interpretation
is, in fact, allowed under the existing 802.11 standards.
Results from the ACK based virtual carrier-sense attack simulation with 18
client nodes modified to implement defense. The attack begins at
time 40 and ends at time 60. The dark region at the bottom of the
graph during the attack is the attacker.
The last frame to consider is the CTS frame. If a lone CTS frame is
observed there are two possibilities: the CTS frame was unsolicited or the
observing node is a hidden terminal. These are the only two cases
possible, since if the observing node was not a hidden terminal it
would have heard the original RTS frame and it would be handled
accordingly. If the unsolicited CTS is addressed to a valid,
in-range node, then only the valid node knows the CTS is bogus. It
can prevent this attack by responding to such a CTS with a null
function packet containing a zero duration value - effectively undoing the
attackers channel reservation. However, if an unsolicited CTS is
addressed to a nonexistent node, or a node out of radio range, this
is indistinguishable from a legitimate hidden terminal. In this case,
there is insufficient information for a legitimate node to act. The
node issuing the CTS could be an attacker, or they may simply be
responding to a legitimate RTS request that is beyond the radio range
of the observer.
An imperfect approach to this final situation, is to allow each node
to independently choose to ignore lone CTS packets as the fraction of
time stalled on such requests increases. Since hidden terminals are a
not a significant efficiency problem in most networks (as evidenced
by the fact that RTS/CTS are rarely employed and since the underlying
functionality does not seem to work in many implementations) setting this threshold
at 30 percent, will provide normal operation in most legitimate
environments, but will prevent an attacker from claiming more than a
third of the bandwidth using this attack.
It should also be noted that existing 802.11 implementations use
different receive and carrier-sense thresholds. The different values
are such that, in an open area, the interference radius of a node is
approximately double its transmit radius. In the hidden terminal case
this means that although the hidden terminal can not receive the data
being transmitted, it still detects a busy medium and will not generate
any traffic that would interfere with the data, so the possibility of
an unsolicited CTS followed by an undetectable data packet is very
But ultimately the only foolproof solution to this problem is to extend
explicit authentication to 802.11 control packets. Each
client-generated CTS packet contains an implicit claim that it was
sent in response to a legitimate RTS generated by an access point.
However, to prove this claim, the CTS frame must contain a
fresh and cryptographically signed copy of the originating RTS. If
every client shares keying material with all surrounding access points
it is then possible to authenticate lone CTS requests directly.
However, such a modification is a significant alternation to the
existing 802.11 standard, and it is unclear if it offers sufficient
benefits relative to its costs. In the meantime, the system-level
defenses we have described provide reasonable degrees of protection
with extremely low implementation overhead and no management burden.
Should media-access based denial-of-service attacks become prevalent,
these solutions could be deployed quickly with little effort.
802.11-based networks have seen widespread deployment across many
fields, mainly due to the physical conveniences of radio-based
communication. This deployment, however, was predicated in part on the
user expectation of confidentiality and availability. This paper
addressed the availability aspect of that equation. We examined the
802.11 MAC layer and identified a number of vulnerabilities that could be
exploited to deny service to legitimate users. We described
software infrastructure for generating arbitrary 802.11 frames using
commodity hardware and then used this platform to implement versions
of the deauthentication and virtual carrier-sense
attacks. We found that the former attack was highly effective in
practice, while the latter is only a theoretical vulnerability due to
implementation deficiencies in commodity 802.11 gear. In addition to
demonstrating the attacks, we described and analyzed potential
countermeasures. These countermeasures represent a stopgap measure,
one that can be implemented with low overhead on existing hardware,
but not a long term substitute for appropriate per-packet
authentication mechanisms. Overall, we believe this paper helps to
underscore the care that must be taken when deploying 802.11 networks
in mission critical applications.
We would like to thank the anonymous reviewers for their comments and
in particular their pointers to the ``blackhat'' literature. Our
shepherd, David Wagner, similarly provided input that was very
helpful. We would like to thank Geoffrey Voelker, Anand
Balachandran, and Daniel Faria for providing feedback on earlier drafts of this paper.
Finally, a special thanks goes to the residents of csl-south at UCSD,
who were at times unwitting victims of this research. This work was
funded by DARPA Grant N66001-01-1-8933 and NIST Grant 60NANB1D0118.
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