Network Working Group S. Kelly
Internet-Draft Aruba Networks
Intended status: Informational T. Clancy
Expires: August 13, 2007 LTS
February 9, 2007
CAPWAP Threat Analysis for 802.11 Deployments
draft-ietf-capwap-threat-analysis-00
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Abstract
Early Wireless LAN (WLAN) deployments feature a "fat" Access Point
(AP) which serves as a standalone interface between the wired and
wireless network segments. However, this model raises scaling,
mobility, and manageability issues, and the CAPWAP protocol is being
developed in reponse. CAPWAP effectively splits the fat AP
functionality into two network elements, and the communication
channel between these components may traverse potentially hostile
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hops. This document analyzes the security exposure resulting from
the introduction of CAPWAP, and summarizes the associated security
considerations for CAPWAP implementations and deployments.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. CAPWAP Security Goals . . . . . . . . . . . . . . . . . . . . 5
3. Overview of 802.11 and AAA Security . . . . . . . . . . . . . 5
3.1. 802.11 Authentication and AAA . . . . . . . . . . . . . . 6
3.2. 802.11 Link Security . . . . . . . . . . . . . . . . . . . 7
3.3. AAA Security . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Cryptographic Channel Bindings . . . . . . . . . . . . . . 8
4. Structure of the Analysis . . . . . . . . . . . . . . . . . . 9
5. Representative CAPWAP Deployment Scenarios . . . . . . . . . . 10
5.1. Preliminary Definitions . . . . . . . . . . . . . . . . . 10
5.1.1. Split MAC . . . . . . . . . . . . . . . . . . . . . . 11
5.1.2. Local MAC . . . . . . . . . . . . . . . . . . . . . . 12
5.1.3. Remote MAC . . . . . . . . . . . . . . . . . . . . . . 12
5.1.4. Data Tunneling . . . . . . . . . . . . . . . . . . . . 12
5.2. Example Scenarios . . . . . . . . . . . . . . . . . . . . 13
5.2.1. Localized Modular Deployment . . . . . . . . . . . . . 13
5.2.2. Internet Hotspot or Temporary Network . . . . . . . . 13
5.2.3. Distributed Deployments . . . . . . . . . . . . . . . 14
6. General Adversary Capabilities . . . . . . . . . . . . . . . . 16
6.1. Passive vs Active Adversaries . . . . . . . . . . . . . . 17
7. Vulnerabilities Introduced by CAPWAP . . . . . . . . . . . . . 18
7.1. The Session Establishment Phase . . . . . . . . . . . . . 19
7.1.1. The Discovery Phase . . . . . . . . . . . . . . . . . 19
7.1.2. Forming an Association (Joining) . . . . . . . . . . . 20
7.2. The Connected Phase . . . . . . . . . . . . . . . . . . . 20
8. Adversary Goals in CAPWAP . . . . . . . . . . . . . . . . . . 21
8.1. Eavesdrop on AC-WTP Traffic . . . . . . . . . . . . . . . 21
8.2. WTP Impersonation and/or Rootkit Installation . . . . . . 22
8.3. AC Impersonation . . . . . . . . . . . . . . . . . . . . . 22
8.4. Other Goals . . . . . . . . . . . . . . . . . . . . . . . 23
9. Countermeasures and Their Effects . . . . . . . . . . . . . . 24
9.1. Communications Security Elements . . . . . . . . . . . . . 24
9.1.1. Mutual Authentication . . . . . . . . . . . . . . . . 24
9.1.2. Data Origin Authentication . . . . . . . . . . . . . . 24
9.1.3. Data Integrity Verification . . . . . . . . . . . . . 25
9.1.4. Antireplay . . . . . . . . . . . . . . . . . . . . . . 25
9.1.5. Confidentiality . . . . . . . . . . . . . . . . . . . 25
9.2. Putting the Elements Together . . . . . . . . . . . . . . 25
9.2.1. Control Channel Security . . . . . . . . . . . . . . . 25
9.2.2. Data Channel Security . . . . . . . . . . . . . . . . 25
10. Countermeasures Provided By DTLS . . . . . . . . . . . . . . . 26
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11. Issues Not Addressed By DTLS . . . . . . . . . . . . . . . . . 26
11.1. DoS Attacks . . . . . . . . . . . . . . . . . . . . . . . 26
11.2. Passive Monitoring (Sniffing) . . . . . . . . . . . . . . 27
11.3. Traffic Analysis . . . . . . . . . . . . . . . . . . . . . 27
11.4. Active MitM Interference . . . . . . . . . . . . . . . . . 27
11.5. Other Active Attacks . . . . . . . . . . . . . . . . . . . 27
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
12.1. Normative References . . . . . . . . . . . . . . . . . . . 28
12.2. Informative References . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
Intellectual Property and Copyright Statements . . . . . . . . . . 30
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1. Introduction
Wireless LAN (WLAN) deployments are increasingly common as the
technology matures and wireless interface chips become standard
equipment in laptops and various handheld computing devices. In the
simplest deployments, WLAN access is entirely provided by a wireless
Access Point (AP), which bridges the client system (station or "STA")
and the Distribution System (DS) or wired network.
+------+
|client| +--------+ |
|(STA) | | Access | | +------+
+------+ ) ) ) ) | Point |-----| /optional\ +-------+
/ / +--------+ |--( L3 )---| AAA |
+------+ | \ cloud / +-------+
| +------+
figure 1
In this architecture the AP serves as a gatekeeper, facilitating
client access to the wired LAN. Typically, the client must somehow
authenticate prior to being granted access, and in enterprise
deployments this is frequently accomplished using IEEE 802.1X
[8021X]. Here, the client is called the "supplicant", the AP is the
"authenticator", and either the AP or an external Authentication,
Authorization, and Accounting (AAA) server fulfill the role of
"authentication server", depending on the authentication mechanism
used.
From the perspective of the network administrator, the wired LAN to
which the AP is attached is typically considered to be trusted,
either because there is some physical boundary around the wired
segment (i.e. the building walls), or because it is otherwise secured
somehow (perhaps cryptographically). The AAA server may reside
within this same physical administrative domain, or it may be remote.
CAPWAP [I-D.ietf-capwap-protocol-specification] modifies this
architecture by splitting the AP into two pieces, the Wireless
Termination Point (WTP), and the Access Controller (AC), and creating
a communications link between the two new components. In this model,
the WTP implements the WLAN edge functions with respect to the user
(wireless transmit/receive), while the AC implements the wired-side
edge functions. For a complete description of CAPWAP architecture,
see [RFC4118].
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+------+ +=================================+
|client| | +------+ | | +------+
|(STA) | | +-----+ / \ +------+ | | /optional\ +-------+
+------+ ) ) ) )|)| WTP |--( L3 )--| AC |-|---|--( L3 )---| AAA |
/ / | +-----+ \ cloud / +------+ | | \ cloud / +-------+
+------+ | +------+ | | +------+
+=================================+
AP equivalence boundary
figure 2
For our purposes, it is useful to think of the entire CAPWAP system
as a sort of "AP equivalent", and the figure above illustrates this
concept. With this model in mind, our ideal is to ensure that CAPWAP
introduces no vulnerabilities which aren't present in the original
fat AP scenario. As we will see, this is not entirely possible, but
from a security perspective we should nonetheless strive to achieve
this equivalence as nearly as we can.
2. CAPWAP Security Goals
CAPWAP should avoid introducing any system security degradation as
compared to the fat AP scenario. In the ideal case, the simple act
of splitting AP functions between the WTP and AC introduces no new
security considerations beyond those relating to the added complexity
of the split. However, in actuality this depends on the security
properties of the L3 network between the AC and WTP.
Limiting our goals in this way implies that over-the-air security is
largely out of scope for this analysis, as is AC-AAA security. This
is appropriate, as CAPWAP does not directly interact with these
protocols (at least, no more so than a fat AP does). However, it is
important to note that CAPWAP interacts indirectly with these in a
number of ways which could potentially introduce subtle new security
exposures. Hence, before commencing with the threat analysis, we
briely overview the relevant elements of 802.11 and AAA
infrastructure security.
3. Overview of 802.11 and AAA Security
While this document is not directly concerned with 802.11 or AAA
security, there are some subtle interactions introduced by CAPWAP,
and there will be related terminology we must touch on in discussing
these. The following figure illustrates some of the complex
relationships between the various protocols, and illustrates where
CAPWAP sits:
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+-----+ RADIUS/Diameter
| AAA |<==============\\
+-----+ ||
| ||
+-----------+------------+ ||
| | ||
+-----+ +----+ ||
| AC | | AC |<==//
+-----+ +----+
+---| |---+ +---| |---+
| | | |
| | | CAPWAP |
+-----+ +-----+ +-----+ +-----+
| WTP | | WTP | | WTP | | WTP |
+-----+ +-----+ +-----+ +-----+
^ ^^^
^^ ^^^ 802.11i,
^^ ^^ 802.1X, WPA,
+-----+ +-----+ WEP
| STA | | STA |
+-----+ +-----+
/ / / /
+-----+ +-----+
CAPWAP is being inserted between the 802.ll link security mechanism
and the authentication server communication, so to provide supporting
context, we give a very brief overview of 802.11 and AAA security
below. It is very important to note that we only cover those topics
which are relevant to our discussion, so what follows is not by any
means exhaustive. For more detailed coverage of 802.11-related
security, topics see e.g. [80211SEC]
3.1. 802.11 Authentication and AAA
IEEE 802.11 provides for multiple methods of authentication prior to
granting access to the network. In the simplest case, open
authentication is used, and this is equivalent to no authentication.
However, if 802.11 link security is to be provided, then some sort of
authentication is required in order to bootstrap the trust
relationship which underlies the associated key establishment
process.
This authentication can be implemented through use of a shared
secret. In such cases the authentication may be implicitly tied to
the link security mechanism, (e.g. when Wired Equivalent Privacy
(WEP) is used with open authentication), or it may be explicit, e.g.
when Wi-fi Protected Access is used with a Pre-Shared Key (WPA-PSK).
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In other cases, authentication requires an explicit cryptographic
exchange, and this operation bootstraps link security. In most such
cases, IEEE 802.1X is used in conjunction with the Extensible
Authentication Protocol [RFC3748] to authenticate either the client
or both the client and the authentication server. This exchange
produces cryptographic keying material for use with 802.11 security
mechnisms.
The resulting trust relationships are complex, as can be seen from
the following (simplified) figure:
/--------------------------------------------\
| TK (6) | EAP Credentials,
V /--------------\ | PMK (3)
+------+ | PSK/Cert(1) | |
|client| V | V
|(STA) | +--------+ | v | +-----+
+------+ ) ) ) ) | WTP |-----| +----+ |--| AAA |
/ / +--------+ |--| AC |--| +-----+
+------+ ^ | +----+ | ^
^ ^ | ^ ^ (2,4) |
| | PTK (7) | | \----------/
| \----------------/ | Radius PSK,
\-----------------------------------/ PMK
4-Way Handshake (w/PMK) (5)
figure 3
1. WTP and AC establish secure link
2. AC establishes secure link with AAA server
3. STA and AAA server conduct EAP, produce PMK
4. AAA server pushes PMK to AC
5. AC and STA conduct 4-way handshake (producing TK, among other
keys)
6. AC pushes TK to WTP (if decentralized encryption is used)
7. WTP/STA use TK for IEEE 802.11 link security
3.2. 802.11 Link Security
The current CAPWAP binding for IEEE 802.11 only supports the use of
IEEE 802.11i security on the wireless link. However, IEEE 802.11i
does not prohibit the use of WEP for link security.
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If WEP is used with CAPWAP, the CAPWAP system will inherit all the
security problems associated with the use of WEP in any wireless
network. In particular, 40-bit keys can be subject to brute-force
attacks, and statistical attacks can be used to crack session keys
after enough packets have been collected [WEPSEC].
Newer link security mechanisms described in IEEE 802.11i, including
TKIP and AES-CCMP, significantly improve the security of wireless
networks. It is strongly recommended that CAPWAP only be used with
these newer techniques.
The only slight insecurity introduced by CAPWAP when using it with
IEEE 802.11i is the handling of the KeyRSC counter. When performing
decentralized encryption, this value is maintained by the WTP, but
needed by the AC to perform the four-way handshake. The value used
during the handshake initializes the counter on the client. In
CAPWAP, this value is initialized to zero, allowing the possibility
for replay attacks of broadcast traffic in the window between initial
authentication and the client receiving the first valid broadcast
packet from the WTP. This slight window of attack has been
documented in the CAPWAP bindings document.
3.3. AAA Security
CAPWAP has very little control over how AAA security is deployed, as
it's not directly bound to AAA as it is to IEEE 802.11. As a result,
CAPWAP can only provide guidance on how to best secure the AAA
portions of a CAPWAP-enabled wireless network.
The AAA protocol is a term that refers to use of either RADIUS
[RFC3579] or Diamater [RFC4072] to transport EAP communications
between the authenticator and the AAA server. Here the authenticator
is the AC, thus the AAA protocol secures the link between the AC and
AAA server. Use of non-unique or low-entropy long-term credentials
to secure the AC-AAA link can severely impact the overall security of
a CAPWAP deployment, and consequently is not recommended. Each AC
should have a mutually-authenticated link that provides robust data
confidentiality and integrity.
Since CAPWAP does not directly interact with AAA, it does not affect
the overall security of a AAA network. In fact, by decreasing the
number of devices that communicate with the AAA server, we can
actually decrease its exposure and risk of attack.
3.4. Cryptographic Channel Bindings
One key shortcoming of both the EAP and AAA models is that they are
inherently two-party models. In CAPWAP deployments, we rely on a
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variety of security associations when an 802.11 WLAN client session
is established. These include:
o WTP-AC (CAPWAP Authentication)
o AC-AAA (AAA Authentication)
o STA-AAA (EAP Method Execution)
o STA-AC (AAA pushes keys to AC)
o STA-WTP (AC pushes keys to WTP)
Each of these security associations involve a pairwise trust
relationship, and none result from a multi-party key agreement
protocol such as Kerberos. In particular, just because a STA trusts
a AAA server who trusts an AC who trusts a WTP doesn't necessarily
mean that the STA should trust the WTP. In particular, the WTP may
be compromised and using his credentials to maintain a trust
relationship with an AC, while advertising false information about
the network to a STA.
Channel bindings are a technique for validating that every device
involved in the formation of a cryptographic trust relationship has
correctly represented itself to all other devices. It can be
performed either by including device information directly into the
cryptographic key generation, or by exchanging cryptographically
signed or MICed tokens indicating identity.
Since CAPWAP is a collection of two-party security protocols and
currently does not support channel bindings, in order to claim CAPWAP
is secure, we must believe in the transitivity of trust
relationships.
4. Structure of the Analysis
In order to conduct a comprehensive threat analysis, there are some
basic questions we must answer:
o Exactly what are we trying to protect?
o What are the risks?
* What are the capabilities of would-be attackers?
* What might be goals of would-be attackers?
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* What are the vulnerabilities or conditions they might attempt
to exploit?
* For various attackers, what is the likelihood of attempting to
exploit a given vulnerability given the cost of the the exloit
vs. the value of attack?
o What potential mitigation strategies are available to us?
o What kinds of trade-offs do these mitigation strategies offer, and
do they introduce any new risks?
This is a very simplistic overview of what we must accomplish below,
but it should give some insight into the manner in which the
discussion proceeds.
As a preliminary, we describe some representative CAPWAP deployment
scenarios. This helps to frame subsequent discussion, so that we
better understand what we are trying to protect. Following this, we
describe a threat model in terms of adversary capabilities,
vulnerabilities introduced by the CAPWAP functionality split, and a
representative sampling of adversary goals. Note that we do not
spend a lot of time speculating about specific attackers here, as
this is a very general analysis, and there are many different
circumstances under which a WLAN might be deployed.
Following the development of the general threat model, we describe
appropriate countermeasures, and discuss how these are implemented
through various means within the CAPWAP protocol. Finally, we
discuss those issues which are not (or cannot be) completely
addressed, and we give recommendations for mitigating the resulting
exposure.
5. Representative CAPWAP Deployment Scenarios
In this section, we describe some representative CAPWAP deployment
scenarios, and in particular, those scenarios which have been taken
into consideration for the current CAPWAP protocol security design.
For clarity, we first provide some preliminary definitions, along
with descriptions of common deployment configurations which span
multiple scenarios. Following this is a sampling of individual
deployment scenarios.
5.1. Preliminary Definitions
The IEEE 802.11 standard describes several frame types, and
variations in WLAN architectures dictate where these frame types
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originate and/or terminate (i.e. at the WTP or AC). There are three
basic 802.11 frame types currently defined:
o Control - in general, these are for managing the transmission
medium (i.e. the airwaves). Examples include RTS, CTS, ACK, PS-
POLL, CF-POLL, CF-END, and CF-ACK
o Management - in general, these are for managing access to the
logical network, as opposed to the physical medium. Example
functions include association, disassociation, reassociation,
deauthentication, beacons, and probes
o Data - transit data (network packets)
There are a number of other services provided by the WLAN
infrastructure, including these:
o Packet distribution
o Synchronization
o Retransmissions
o Transmission Rate Adaptation
o Privacy/Confidentiality/Integrity (e.g. 802.11i)
The manner in which these (and other) services are divided among the
AC and WTP is collectively referred to in terms of "MAC-splitting"
characteristics. For convenience, we briefly describe various forms
of MAC-splitting below. For further detail, see
[I-D.ietf-capwap-protocol-specification] and
[I-D.ietf-capwap-protocol-binding-ieee80211]
5.1.1. Split MAC
Split-MAC scenarios are characterized by the fact that some 802.11
MAC messages are processed by the WTP, while some are processed by
the AC. For example, a split MAC implementation might divide 802.11
frame processing as follows:
WTP
* Beacons
* Probes
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* RTS, CTS, ACK, PS-POLL, CF-POLL,CF-END, CF-ACK
AC
* Association/Reassociation/Disassociation
* Authentication/Deauthentication
* Key Management
* 802.11 Link Security (WEP, TKIP, 802.11i)
The split MAC model is not confined to any one particular deployment
scenario, as we'll see below, and vendors have considerable leeway in
choosing how to distribute the 802.11 functionality.
5.1.2. Local MAC
Local-MAC scenarios are characterized by the fact that all 802.11 MAC
messages are processed by the WTP. Copies of some may be forwarded
to the AC as a matter of convenience (i.e. when the AC needs to
record some info from the frame, for example when providing
accounting services), but the primary function of CAPWAP in local MAC
scenarios is to provide centralized control and management for the
WTPs, and STA packets are generally bridged locally.
5.1.3. Remote MAC
Remote MAC scenarios are characterized by the fact that all 802.11
MAC messages are forwarded to the AC - the WTP does not process any
of these locally. This model supports very light-weight WTP's which
need be little more than smart antennas.
5.1.4. Data Tunneling
Regardless of the approach to MAC-splitting, there is also the matter
of where user data packets are translated between wired and wireless
frame formats, i.e. where the bridging function occurs. In some
cases, user data frames are tunneled back to the AC, and in others,
they are locally bridged by the WTP. While one might expect remote
MAC implementations to always tunnel data packets back to the AC, for
local and split MAC modes the decision is not so clear. Also, it's
important to note that there are no rules or standards in place which
rigidly define these terms and associated handling. Data tunneling
is further discussed for each scenario below.
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5.2. Example Scenarios
In this section we describe a number of example deployment scenarios.
This is not meant to be an exhaustive enumeration; rather, it is
intended to give a general sense of how WLANs currently are or may be
deployed. This will provide important context when we discuss
adversaries and threats in subsequent sections below.
5.2.1. Localized Modular Deployment
The localized modular model describes a WLAN which is deployed within
a single domain of control, typically within either a single building
or some otherwise physically contained area. This would be typical
of a small to medium-sized organization. In general, the LAN enjoys
some sort of physical security (e.g. it's within the confines of a
building for which access is somehow limited), although the actual
level of physical security is often far less than is assumed.
In such deployments, the WLAN is typically an extension of a wired
LAN. However, its interface to the wired LAN can vary. For example,
the interconnection between the WTPs and ACs can have its own wiring,
and its only connection to the LAN is via the AC's Distribution
System (DS) port(s). In such cases, the WLAN frequently occupies its
own distinct addressing partition(s) in order to facilitate routing,
and the AC often acts as a forwarding element.
In other cases, the WTPs may be mingled with the existing LAN
elements, perhaps sharing address space, or perhaps somehow logically
isolated from other wired elements (e.g. by VLAN). Under these
circumstances, it is possible that traffic destined to/from the WLAN
is mixed with traffic to/from the LAN.
Localized deployments such as these could potentially choose any one
of the MAC-splitting scenarios, depending on the size of the
deployment, mobility requirements, and other considerations. In many
cases, any one of the various MAC-splitting approaches would be
sufficient. This implies that user data may be bridged by either the
WTP or the AC.
5.2.2. Internet Hotspot or Temporary Network
The Internet hotspot scenario is representative of a more general
deployment model one might find at cafes, hotels, or airports. It is
also quite similar to temporary WLANs which are created for
conferences, conventions, and the like. Some common characteristics
of these networks include the following:
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o Primary function is to provide Internet access
o Authentication may be very weak
o There frequently is no 802.11 link security
Sometimes, the local-MAC model is used. In such cases the AC may be
"in the cloud" (out on the Internet somewhere), and user data traffic
will typically be locally bridged, rather than tunneled back to the
AC. Some 802.11 management traffic may be tunneled back to the AC,
but frequently the authentication consists in simply knowing the SSID
and perhaps a shared key for use with WEP or WPA-PSK.
In other cases, a split-MAC model may be used. This is more common
in airport and hotel scenarios, where users may have an account which
requires verification with some back-end infrastructure prior to
access. In these cases, 802.11 management frames are tunneled back
to the AC (e.g. user authentication), and stronger 802.11 link
security may be provided (e.g. WPA), but user data is still
typically locally bridged, as the primary goal is to provide Internet
access.
A third variation on this scenario entails tunneling user data
through a local AC in order to apply centralized policy. For
example, in a hotel or airport scenarios, there is no reason to
provide direct access between WLAN users (who typically are within a
private address space), and in fact, allowing such access might
invite various sorts of malicious behavior. In such cases, tunneling
all user data back to the (local) AC provides a security choke point
at which policy may be applied to the traffic.
5.2.3. Distributed Deployments
The distributed deployment model describes a more complex WLAN
topology which features network segments that are typically somehow
spatially separated, although not necessarily so. These segments
might be connected in a physically secure manner, or (if they are
widely separated) they might be connected across potentially hostile
hops. Below we discuss several subgroups of this model.
5.2.3.1. Large Physically-Contained Organization
One distributed deployment example involves a single large
organization which is physically contained, typically within one
large building. The network might feature logically distinct (e.g.
per-department or per-floor) network segments, and in some cases,
there might be firewalls between the segments for access control. In
such deployments, the AC is typically in a centralized datacenter,
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but there might also be a hierarchy of ACs, with a master in the
datacenter, and subordinate ACs distributed among the network
segments. Such deployments typically assume some level of physical
security for the network infrastructure.
5.2.3.2. Campus Deployments
Some large organizations have networks which span multiple buildings.
The interconnections between these buildings might be wired (e.g.
underground cables), or might be wireless (e.g. a point to point MAN
link). The interconnections may be secured, but sometimes they are
not. The organization may be providing outdoor wireless access to
users, in which case some WTPs will typically be located outdoors,
and these may or may not be within physically secure space. For
example, college campuses frequently provide outdoor wireless access,
and the associated WTPs may simply be mounted on a light post.
For such deployments, ACs may be centrally located in a datacenter,
or they may be distributed. User data traffic may be locally
bridged, but more frequently it is tunneled back to AC, as this
facilitates mobility and centralized policy enforcement.
5.2.3.3. Branch Offices
A common deployment model entails an enterprise consisting of a
headquarters and one or more branch offices, with the branches
connected to the central HQ. In some cases, the site-to-site
connection is via a private WAN link, and in others it is across the
Internet. For connections crossing potentially hostile hops (e.g.
the Internet), some sort of VPN is typically employed as a protective
measure.
In some simple branch office scenarios, there are only WTP's at the
remote site, and these are managed by a controller residing at the
central site. In other cases, a branch site may have its own
subordinate controller, with the master controller again residing at
the central site. In the first case, local bridging is often the
best choice for user data, due to latency and link capacity issues.
In the second case, traffic may be locally bridged by the WTPs, or it
may be forwarded to the local subordinate controller for centralized
policy enforcement. The choice depends on many factors, including
local topology and security policy.
5.2.3.4. Telecommuter or Remote Office
It is becoming increasingly common to send WTPs home with employees
for use as a telecommuting solution. While there are not yet any
standards or hard rules for how these work, a fairly typical
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configuration provides split MAC with all user data tunneled back to
the controller in the organization's datacenter so that the WTP is
essentially providing wireless VPN services. These devices may in
some cases provide their own channel security (e.g. IPsec), but
alternative approaches are possible. For example, there may be a
standalone VPN gateway between the WTP and the Internet which
forwards all organization-bound traffic to the VPN gateway.
Similarly, it is becoming increasingly common for travelers to plug a
WTP into a hotel broadband connection. While in many cases, these
WTPs are standalone fat AP's, in some cases they are configured to
create a secure connection to a centralized controller back at
headquarters, essentially forming a VPN connection. In such
scenarios, a split-MAC approach is typical, but split-tunneling may
also be suppported (i.e. only data traffic destined for the
headquarters is tunneled back to the controller, with Internet-bound
traffic locally bridged).
5.2.3.5. Tactical Networks
In some instances, there is a need to quickly establish a temporary
network "in the field". For example, during disaster relief
operations (such as might occur following a natural disaster), it
would be very helpful to be able to quickly establish secure
connectivity between a temporary location in the field and a central
office. In some cases, it may be possible to quickly establish an
insecure uplink of some sort (e.g. DSL/cable modem, satellite
uplink, cellular modem, etc), and then to create a WLAN at the site
for convenient access. This is very similar to telecommuter
scenarios, except that there may be far more people using the WLAN.
A typical configuration for such scenarios might utilize split MAC,
with all user data tunneled back to the controller in the datacenter.
Again, a separate VPN gateway might be placed between the WTP and the
device providing Internet connectivity, or the WTP may be expected to
provide its own channel security.
6. General Adversary Capabilities
This section describes general capabilities we might expect an
adversary to have in various CAPWAP scenarios. Obviously, it is
possible to limit what an adversary can do through various deployment
restrictions (e.g. provide strict physical security for the AC-WTP
link), but such restrictions are simply not always feasible. For
example, it is not possible to provide strict physical security for
various aspects of the telecommuter scenario. Thus, we must consider
what capabilities an adversary might have in the worst case, and plan
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accordingly.
6.1. Passive vs Active Adversaries
One way to classify adversaries is in terms of their ability to
interact with AC/WTP communications. For example, a passive
adversary is one who can observe and perhaps record traffic, but
cannot interact with it. They can "see" the traffic as it goes by,
but they cannot interfere in any way, and they cannot inject traffic
of their own. Note that such an adversary does not necessarily see
all traffic - they may miss portions of a communication e.g. because
some packets traverse a different path, or because the network is so
busy that the adversary can't keep up (and drops packets as a
result).
An active adversary, on the other hand, can directly interact with
the traffic in realtime. There are two modes in which an active
adversary might operate:
Pass-by (or sniffer)
* Can observe/record traffic
* Can inject packets
* Can replay packets
* Can reflect packets (i.e. send duplicate packets to a different
destination, including the to the packet source)
* Can cause redirection (e.g. via ARP/DNS poisoning)
Inline (Man in the Middle, or MitM)
* Can observe/record traffic
* Can inject packets
* Can replay packets
* Can reflect packets (with or without duplication)
* Can delete packets
* Can modify packets
A passive adversary could be located anywere along the path between
the AC and WTP, and is characterized by the fact that it only
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listens:
+------+
|client| +--------+ |
|(STA) | | WTP | | +------+
+------+ ) ) ) ) | |-----| / \ +------+
/ / +--------+ |-x-( optional )---| AC |
+------+ | | \ cloud / +------+
| | +------+
|
| +-----------+
+--| pass-by |
| listener |
+-----------+
An active adversary may attach in the same manner as the passive
adversary (in which case it is in pass-by mode), or it may be lodged
directly in the path between the AC and WTP (inline mode):
+------+
|client| +--------+ |
|(STA) | | WTP | | +------+ +------+
+------+ ) ) ) | |---| |active| / \ +------+
/ / +--------+ |-| MitM |--( optional )---| AC |
+------+ | +------+ \ cloud / +------+
| +------+
If it is not inline, it can only observe and create traffic; if it is
inline, it can do whatever it wishes with the traffic it sees.
It is important to recognize that becoming a MitM does not
necessarily require physical insertion directly into the transmission
path. Alternatively, the adversary can cause traffic to be diverted
to the MitM system, e.g. via ARP or DNS poisoning. Hence, launching
a MitM attack is not so difficult as it might first appear.
7. Vulnerabilities Introduced by CAPWAP
In this section we discuss vulnerabilities which arise as a result of
splitting the AP function across potentially hostile hops. We
primarily consider network-based vulnerabilities, and while in
particular we do not address how an adversary might affect a physical
compromise of the WTP and/or AC, we do consider the potential effects
of such compromises with respect to CAPWAP and the operational
changes it introduces when compared to standalone AP deployments.
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Functionally, we are interested in two general "states of being" with
respect to AC-WTP communications: the session establishment phase or
state, and the "connected" (or session established) state. We
discuss each of these further below. Also, it is important to note
that we first describe vulnerabilites assuming that the CAPWAP
communications lack security of any kind. Later, we will evaluate
the protocol given the security mechanisms currently planned for
CAPWAP.
7.1. The Session Establishment Phase
The session establishment phase consists of two subordinate phases:
discovery, and association or joining. These are treated
individually in the following sections.
7.1.1. The Discovery Phase
Discovery consists of an information exchange between the AC and WTP.
There are several potential areas of exposure:
Information Leakage: During discovery, information about the WTP and
AC hardware and software are exchanged, as well as information
about the AC's current operational state. This could provide an
adversary with valuable insights.
DoS Potential: During Discovery, there are several opportunities for
Denial of Service (DoS), beyond those inherently available to an
inline adversary. For example, an adversary might respond to a
WTP more quickly than a valid AC, causing the WTP to attempt to
join with a non-existent AC, or one which is currently at maximum
load.
Redirection Potential: There are multiple ways in which an adversary
might redirect a WTP during discovery. For example, the adversary
might pretend to be a valid AC, and entice the WTP to connect to
it. Or, the adversary might instead cause the WTP to associate
with the AC of the adversary's choosing, by posing as a DNS or
DHCP server, injecting a spoofed discovery response, or by
modifying valid AC responses.
Misdirection: An adversary might mislead either the WTP or AC by
modifying the discovery request or response in flight. In this
way, the AC and/or WTP will have a false view of the other's
capabilities, and this might cause a change in the way they
interact (e.g. they might not use important features not supported
by earlier versions).
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7.1.2. Forming an Association (Joining)
The association phase begins once the WTP has determined with which
AC it wishes to join. There are several potential areas of exposure
during this phase:
Information Leakage: During association, the adversary could glean
useful information about hardware, software, current
configuration, etc. that could be used in various ways.
DoS Potential: During formation of a WTP-AC association, there are
several opportunities for Denial of Service (DoS), beyond those
inherently available to an inline adversary. For example, the
adversary could flood the AC with connection setup requests. Or,
it could respond to the WTP with invalid connection setup
responses, causing a connection reset. An adversary with MitM
capability could delete critical session establishment packets.
Misdirection: An adversary might mislead either the WTP or AC by
modifying the association request(s) or response(s) in flight. In
this way, the AC and/or WTP will have an inaccurate view of the
other's capabilities, and this might cause a change in the way
they interact.
Some of these types of exposure are extremely difficult to prevent.
However, there are things we can do to mitigate the effects, as we
will see below.
7.2. The Connected Phase
Once the WTP and AC have established an association, the adversary's
attention will turn to the network connection. If we assume a
passive adversary, the primary concern for established connections is
eavesdropping. If we assume an active adversary, there are several
other potential areas of exposure:
Connection Hijacking: An adversary may assume the identity of one
end of the connection and take over the conversation. There are
numerous ways in which the true owner of the identity may be taken
offline, including DoS or MitM attacks.
Eavesdropping: An adversary may glean useful information from
knowledge of the contents of CAPWAP control and/or data traffic.
Modification of User Data: An adversary with MitM capabilities could
modify, delete, or insert user data frames.
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Modification of Control/Monitoring Messages: An adversary with MitM
capability could modify control traffic such as statistics, status
information, etc. to create a false impression at the recipient.
Modification/Control of Configuration An adversary with MitM
capability could modify configuration messages to create
unexpected conditions at the recipient. Likewise, if a WTP is
redirected during discovery (or joining) and connects to an
adversary rather than an authorized AC, the adversary may exert
complete control over the WTP's configuration, including
potentially loading tainted WTP firmware.
8. Adversary Goals in CAPWAP
This section gives an sampling of potential adversary goals. It is
not exhaustive, and makes no judgement as to the relative likelihood
or value of each individual goal. Rather, it is meant to give some
idea of what is possible. It is important to remember that clever
attacks often result when seemingly innocuous flaws or
vulnerabilities are combined in some non-intuitive way. Hence, we
don't rule out some goal that, taken alone, might not seem to make
sense from an adversarial perspective.
8.1. Eavesdrop on AC-WTP Traffic
There are numerous reasons why an adversary might want to eavesdrop
on AC-WTP traffic. For example, it allows for reconnaissance,
providing answers to the following questions:
o Where are the ACs?
o Where are the WTPs?
o Who owns them?
o Who manufactured them?
o What version of firmware do they run?
o What cryptographic capabilities do they have?
o etc...
Similarly, snooping on tunneled data traffic might potentially reveal
a great deal about the network, including answers to the following:
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o Who's using the WLAN?
o When, and for how long?
o What addresses are they using?
o What protocols are they using?
o What over-the-air security are they using?
o Who/what are they talking to?
o etc...
Additionally, access to tunneled user data could allow the attacker
to see confidential information being exchanged by applications (e.g.
financial transactions). An eavesdropper may gain access to valuable
information that either provides the basis for another perhaps more
sophisticated attack, or which has its own intrinsic value.
8.2. WTP Impersonation and/or Rootkit Installation
An adversary might want to impersonate or control an authorized WTP
for many reasons, some of which we might realistically imagine today,
and perhaps others about which we have no clue at this point.
Examples we might reasonably imagine include the following:
o to facilitate MitM attacks against WLAN users
o to leak/inject or otherwise compromise WLAN data
o to give an inaccurate view of the state of the WLAN
o to gain access to a trusted channel to an AC, through which
various protocol attacks might be launched (e.g. hijack client
sessions from other WTPs)
8.3. AC Impersonation
For reasons similar to the WTP impersonation discussed above, an
adversary might want to impersonate an authorized AC for many
reasons. Examples we might reasonably imagine include the following:
o to facilitate DoS attack against WLAN
o to facilitate MitM attacks against WLAN users
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o to intercept user mobility context (possibly including security-
sensitive information such as cryptographic keys)
o to facilitate selective control of one or more WTPs
* modify WTP configuration
* load tainted firmware onto WTP
o to assist in controlling which AC associates with which WTP
o to facilitate 802.11 association of unauthorized WLAN user(s)
o to exploit inter-AC trust in order facilitate attacks on other ACs
In general, AC impersonation opens the door to a large measure of
control over the WLAN, and could be used as the foundation to many
other attacks.
8.4. Other Goals
There are many less concrete goals an adversary might have which,
taken alone, might not seem to have any purpose, but when combined
with other goals/attacks, might have very definite and undesireable
consquences. Here are some examples:
o cause CAPWAP de-association of WTP/AC
o cause 802.11 deassociation of authorized user
o inject/modify/delete tunneled 802.11 user traffic
* to interact with a specific application
* to launch various attacks on WLAN user systems
* to launch protocol fuzzing or other attacks on the AC which
bridges between 802.11 and 802.3 frame formats
o control DNS responses
o control DHCP/BOOTP responses
Anticipating all of the things an adversary might want to do is a
daunting task. Part of the difficulty stems from the fact that we
are analyzing CAPWAP in a very general sense, rather than in a
specific deployment scenario with specific assets and very specific
adversary goals. However, we have no choice but to take this
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approach if we are to provide reasonably good security across the
board.
9. Countermeasures and Their Effects
In the previous sections we described numerous vulnerabilities which
result from splitting the AP function in two, and also discussed a
number of adversary goals which could be realized by exploiting one
or more of those vulnerabilities. In this section, we describe
countermeasures we can apply to mitigate the risks that come with the
introduction of CAPWAP into WLAN deployment scenarios.
9.1. Communications Security Elements
9.1.1. Mutual Authentication
Mutual authentication consists in each side proving its identity to
the other. There are numerous authentication protocols which
accomplish this, typically using either a shared secret (e.g. a
preshared key) or by relying on a trusted third party (e.g. with
digital credentials such as X.509 certificates).
Strong mutual authentication accomplishes the following:
o helps prevent AC/WTP impersonation
o helps prevent MitM attacks
o can be used to limit DoS attacks
9.1.2. Data Origin Authentication
Data origin authentication typically depends on first authenticating
the party at the other end of the channel, and then binding the
identity derived from that authentication process to the data origin
authentication process. Data origin authentication primarily
prevents an attacker from injecting data into the communication
channel and pretending it was originated by a valid endpoint. This
mitigates risk by preventing the following:
o packet injection
o connection hijacking
o modification of control and/or user data
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o impersonation
9.1.3. Data Integrity Verification
Data integrity verification provides assurance that data has not been
altered in transit, and is another link in the trust chain beginning
from mutual authentication, extending to data origin authentication,
and ending with integrity verification. This prevents the adversary
from undetectably modifying otherwise valid data while in transit.
It effectively prevents reflection and modification, and in some
cases may help to prevent re-ordering.
9.1.4. Antireplay
Antireplay is somewhat self-explanatory: it prevents replay of valid
packets at a later time, or to a different recipient. It may also
prevent limited re-ordering of packets. It is typically accomplished
by assigning monotonically increasing sequence numbers to packets.
9.1.5. Confidentiality
Data confidentiality prevents eavesdropping by protecting data as it
passes over the network. This is typically accomplished using
encryption.
9.2. Putting the Elements Together
Above we described various security elements and their properties.
Below, we discuss combinations of these elements in terms of CAPWAP.
9.2.1. Control Channel Security
The CAPWAP control channel is used for forming an association between
a WTP and AC, and subsequently it allows the AC to provision and
monitor the WTP. This channel is critical for the control,
management, and monitoring of the WLAN, and thus requires all of the
security elements described above. With these elements in place, we
can effectively create a secure channel which mitigates almost all of
the vulnerabilities described above.
9.2.2. Data Channel Security
The CAPWAP data channel contains some 802.11 management traffic
including association/disassociation, reassociation, and
deathentication. It also may contain potentially sensitive user
data. If we assume that threats to this channel exist (i.e. it
traverses potentially hostile hops), then providing strong mutual
authentication coupled with data origin/integrity verification would
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prevent an adversary from injecting and/or modifying transit data, or
impersonating a valid AC or WTP. Adding confidentiality would
prevent eavesdropping.
10. Countermeasures Provided By DTLS
Datagram TLS (DTLS) is the currently proposed security solution for
CAPWAP. DTLS supports the following security functionality:
o Mutual Authentication (preshared secrets or X.509 Certificates)
o Data Origin Authentication
o Data Integrity Verification
o Antireplay
o Confidentiality (supports strong cryptographic algorithms)
Using DTLS for both the control and data channels mitigates nearly
all risks resulting from splitting the AP function in two.
11. Issues Not Addressed By DTLS
Unfortunately, DTLS is not a magic elixir that dispenses with all of
our CAPWAP security concerns. There are some things it just cannot
prevent. These are enumerated below.
11.1. DoS Attacks
Even with the security provided by DTLS, CAPWAP is still susceptible
to various types of DoS attack:
o Session Initialization: an adversary could initiate thousands of
DTLS handshakes simultaneously in order to exhaust memory
resources on the AC; DTLS provides a mitigation tool via the
HelloVerifyRequest, which ensures that the initiator can receive
packets at the claimed source address prior to allocating
resources. However, this would not thwart a botnet-style attack.
o Cryptographic DoS: an adversary could flood either the AC or WTP
with properly formed DTLS records containing garbage, causing the
recipient to waste compute cycles decrypting and authenticating
the traffic
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o Session interference: a MitM could either drop important session
establishment packets, or inject bogus connection resets during
session establishment phase. It could also interfere with other
traffic in an established session.
These attacks can be mitigated thorugh various mechanisms, but not
entirely avoided. For example, session initialization can be rate-
limited, and in case of resource exhaustion, some number of
incompletely initialized sessions could be discarded. Also, such
events should be strictly audited.
Likewise, cryptographic DoS attacks are detectable if appropriate
auditing and monitoring controls are implemented. When detected,
these events should (at minimum) trigger an alert. Additional
mitigation might be realized by randomly discarding packets.
Session interference is probably the most difficult of DoS attacks.
It is very difficult to detect in realtime, although it may be
detected if exceeding a lost packet threshhold triggers an alert.
However, this depends on the MitM not being in a position to delete
the alert before it reaches it's appropriate destination.
11.2. Passive Monitoring (Sniffing)
CAPWAP protocol security cannot prevent (or detect) passive
monitoring. The best we can do is provide a confidentiality
mechanism.
11.3. Traffic Analysis
DTLS provides no defense for traffic analysis. If this is a concern,
there are traffic generation and padding techniques designed to
mitigate this threat, but none of these are currently specified for
CAPWAP.
11.4. Active MitM Interference
This was discussed in more limited scope in the section above on DoS
attacks. An active MitM can delete or re-order packets in a manner
which is very difficult to detect, and there is little the CAPWAP
protocol can do in such cases. If packet loss is reported upon
exceeding some threshold, then perhaps detection might be possible,
but this is not guaranteed.
11.5. Other Active Attacks
In addition to the issues raised above, there are other active
attacks that can be mounted if the adversary has access to the wired
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medium. For example, the adversary may be able to impersonate a DNS
or DHCP server, or to poison either a DNS or ARP cache. Such attacks
are beyond the scope of CAPWAP, and point to an underlying LAN
security problem.
12. References
12.1. Normative References
[I-D.ietf-capwap-protocol-binding-ieee80211]
Calhoun, P., "CAPWAP Protocol Binding for IEEE 802.11",
draft-ietf-capwap-protocol-binding-ieee80211-01 (work in
progress), January 2007.
[I-D.ietf-capwap-protocol-specification]
Calhoun, P., "CAPWAP Protocol Specification",
draft-ietf-capwap-protocol-specification-04 (work in
progress), January 2007.
[RFC4118] Yang, L., Zerfos, P., and E. Sadot, "Architecture Taxonomy
for Control and Provisioning of Wireless Access Points
(CAPWAP)", RFC 4118, June 2005.
12.2. Informative References
[80211I] "IEEE Std 802.11i: WLAN Specification for Enhanced
Security", April 2004.
[80211SEC]
Edney, J. and W. Arbaugh, "Real 802.11 Security - Wi-Fi
protected Access and 802.11i", 2004.
[8021X] "IEEE Std 802.1X: Port-based Network Access Control",
June 2001.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
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[RFC4072] Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application", RFC 4072,
August 2005.
[WEPSEC] Petroni, N. and W. Arbaugh, "The Dangers of Mitigating
Security Design Flaws: A Wireless Case Study",
January 2003.
Authors' Addresses
Scott G. Kelly
Aruba Networks
1322 Crossman Ave
Sunnyvale, CA 94089
US
Email: scott@hyperthought.com
T. Charles Clancy
DoD Laboratory for Telecommunications Sciences
8080 Greenmead Drive
College Park, MD 20740
US
Email: clancy@LTSnet.net
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