💾 Archived View for gmi.noulin.net › rfc › rfc5418.gmi captured on 2022-04-29 at 01:25:40. Gemini links have been rewritten to link to archived content
⬅️ Previous capture (2022-01-08)
-=-=-=-=-=-=-
Keywords: WLAN, security
Network Working Group S. Kelly
Request for Comments: 5418 Aruba Networks
Category: Informational T. Clancy
LTS
March 2009
Control And Provisioning of Wireless Access Points (CAPWAP)
Threat Analysis for IEEE 802.11 Deployments
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Kelly & Clancy Informational [Page 1]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
Abstract
Early Wireless Local Area Network (WLAN) deployments feature a "fat"
Access Point (AP), which serves as a stand-alone interface between
the wired and wireless network segments. However, this model raises
scaling, mobility, and manageability issues, and the Control and
Provisioning of Wireless Access Points (CAPWAP) protocol is meant to
address these issues. CAPWAP effectively splits the fat AP
functionality into two network elements, and the communication
channel between these components may traverse potentially hostile
hops. This document analyzes the security exposure resulting from
the introduction of CAPWAP and summarizes the associated security
considerations for IEEE 802.11-based CAPWAP implementations and
deployments.
Table of Contents
1. Introduction ....................................................4
1.1. Rationale for Limiting Analysis Scope to IEEE 802.11 .......5
1.2. Notations ..................................................6
2. Abbreviations and Definitions ...................................7
3. CAPWAP Security Goals for IEEE 802.11 Deployments ...............8
4. Overview of IEEE 802.11 and AAA Security ........................8
4.1. IEEE 802.11 Authentication and AAA .........................9
4.2. IEEE 802.11 Link Security .................................11
4.3. AAA Security ..............................................11
4.4. Cryptographic Bindings ....................................12
5. Structure of the Analysis ......................................13
6. Representative CAPWAP Deployment Scenarios .....................14
6.1. Preliminary Definitions ...................................14
6.1.1. Split MAC ..........................................15
6.1.2. Local MAC ..........................................15
6.1.3. Remote MAC .........................................15
6.1.4. Data Tunneling .....................................16
6.2. Example Scenarios .........................................16
6.2.1. Localized Modular Deployment .......................16
6.2.2. Internet Hotspot or Temporary Network ..............17
6.2.3. Distributed Deployments ............................18
6.2.3.1. Large Physically Contained Organization ...18
6.2.3.2. Campus Deployments ........................18
6.2.3.3. Branch Offices ............................18
6.2.3.4. Telecommuter or Remote Office .............19
7. General Adversary Capabilities .................................19
7.1. Passive versus Active Adversaries .........................20
8. Vulnerabilities Introduced by CAPWAP ...........................22
8.1. The Session Establishment Phase ...........................22
8.1.1. The Discovery Phase ................................22
8.1.2. Forming an Association (Joining) ...................23
Kelly & Clancy Informational [Page 2]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
8.2. The Connected Phase .......................................23
9. Adversary Goals in CAPWAP ......................................24
9.1. Eavesdrop on AC-WTP Traffic ...............................24
9.2. WTP Impersonation and/or Rootkit Installation .............25
9.3. AC Impersonation and/or Rootkit Installation ..............26
9.4. Other Goals or Sub-Goals ..................................26
10. Countermeasures and Their Effects .............................27
10.1. Communications Security Elements .........................27
10.1.1. Mutual Authentication .............................27
10.1.1.1. Authorization ............................27
10.1.2. Data Origin Authentication ........................29
10.1.3. Data Integrity Verification .......................29
10.1.4. Anti-Replay .......................................29
10.1.5. Confidentiality ...................................29
10.2. Putting the Elements Together ............................30
10.2.1. Control Channel Security ..........................30
10.2.2. Data Channel Security .............................30
11. Countermeasures Provided by DTLS ..............................30
12. Issues Not Addressed By DTLS ..................................31
12.1. DoS Attacks ..............................................31
12.2. Passive Monitoring (Sniffing) ............................32
12.3. Traffic Analysis .........................................32
12.4. Active MitM Interference .................................32
12.5. Other Active Attacks .....................................32
13. Security Considerations .......................................32
14. Acknowledgements ..............................................32
15. References ....................................................33
15.1. Normative References .....................................33
15.2. Informative References ...................................33
Kelly & Clancy Informational [Page 3]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
1. Introduction
Wireless Local Area Network (WLAN) deployments are increasingly
common as the technology matures and wireless interface chips become
standard equipment in laptops and various hand-held 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: IEEE 802.11 Deployment Using RSN
In this architecture, the AP serves as a gatekeeper, facilitating
client access to the network. Typically, the client must somehow
authenticate prior to being granted access, and in enterprise
deployments, this is frequently accomplished using [8021X]. When
using IEEE 802.11, this mode is called a Robust Security Network
(RSN) [80211I]. 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 more trusted
than the wireless LAN, 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. Common AAA protocols are RADIUS [RFC3579] and
Diameter [RFC4072].
The CAPWAP protocol [RFC5415] 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].
Kelly & Clancy Informational [Page 4]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
+------+ +==========================+
|client| | +---+ | | +------+
|(STA) | | +-----+ / L3 \ +----+ | | /optional\ +-----+
+------+ ) )|)| WTP |-( cloud )-| AC |-|---|--( L3 )--| AAA |
/ / | +-----+ \ / +----+ | | \ cloud / +-----+
+------+ | +---+ | | +------+
+==========================+
AP equivalence boundary
Figure 2: WLAN Deployment utilizing CAPWAP
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 that 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.
1.1. Rationale for Limiting Analysis Scope to IEEE 802.11
Although CAPWAP derives from protocols that were designed explicitly
for management of IEEE 802.11 wireless LANs, it may also turn out to
be useful for managing other types of network device deployments,
wireless and otherwise. This might lead one to conclude that a
single security analysis, except for minor per-binding variations,
might be sufficient. However, based on a limited number of
additional related scenarios that have been described as likely
candidates for CAPWAP thus far, e.g., use with Radio Frequency
Identification (RFID) sensors, this does not seem to be a simple,
binary decision.
Contrasting RFID and IEEE 802.11 deployment scenarios, IEEE 802.11
users typically authenticate to some a back-end AAA server, and as a
result of that exchange, derive cryptographic keys that are used by
the STA and WTP to encrypt and authenticate over-air communications.
That is, the threat environment is such that the following typically
holds:
o The user is not immediately trusted, and therefore must
authenticate.
o The WTP is not immediately trusted, and therefore must indirectly
authenticate to the user via the AAA server.
o The AAA server is not necessarily trusted, and therefore must
authenticate.
Kelly & Clancy Informational [Page 5]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
o The medium is not trusted, and therefore communications must be
secured.
RFID tags, on the other hand, typically do not have the same
authentication, confidentiality, and integrity requirements as the
principals in a WLAN deployment, and are not, generally speaking,
well suited to environments in which similar threats exist. As a
result of the combination of WLAN security requirements and the
Medium Access Control (MAC)-splitting behavior that epitomizes the
IEEE 802.11 binding for CAPWAP, there are definite security-related
considerations in the WLAN case that simply do not exist for an RFID-
based adaptation of CAPWAP.
Now, there certainly are similarities and overlapping security
considerations that will apply to any CAPWAP deployment scenario.
For example, authentication of the AC for purposes of WTP device
management operations is probably always important. Even so, the
threats to RFID are different enough to suggest the need for a
separate security analysis in that case. For example, since RFID
tags commonly deployed today implement no over-air authentication,
integrity, or confidentiality mechanisms, the need for similar
mechanisms between the WTP and AC for RFID tag data communications is
not clearly indicated -- that is, the threats are different.
We have limited visibility into the varied ways in which CAPWAP might
be adapted in the future, although we may observe that it seems to
provide the basis for a generalized scalable provisioning protocol.
Given our currently limited view of the technologies for which it
might be used, it seems prudent to recognize that our current view is
colored by the IEEE 802.11 roots of the protocol, and make that
recognition explicit in our analysis. If newly added bindings turn
out to be substantially similar to IEEE 802.11, then the associated
binding documents can simply reference this document in their
security considerations, while calling out any substantive
differences. However, it does appear, based on our current limited
visibility, that per-binding threat analyses will be required.
1.2. Notations
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Kelly & Clancy Informational [Page 6]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
2. Abbreviations and Definitions
o AAA - Authentication Authorization and Accounting
o AC - Access Controller
o AES-CCMP - Advanced Encryption Standard - Counter-mode CBC MAC
Protocol
o AP - (wireless) Access Point
o CAPWAP - Control And Provisioning of Wireless Access Points
o Cert - X509v3 Certificate
o DIAMETER - proposed successor to RADIUS protocol (see below)
o DoS - Denial of Service (attack)
o DTLS - Datagram Transport Layer Security
o EAP - Extensible Authentication Protocol
o MitM - Man in the Middle
o PMK - Pairwise Master Key
o PSK - Pre-Shared Key
o RADIUS - Remote Authentication Dial-In User Service
o STA - (wireless) STAtion
o TK - Transient Key
o TKIP - Temporal Key Integrity Protocol
o WEP - Wired Equivalent Privacy
o WLAN - Wireless Local Area Network
o WTP - Wireless Termination Point
Kelly & Clancy Informational [Page 7]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
3. CAPWAP Security Goals for IEEE 802.11 Deployments
When deployed for use with IEEE 802.11, CAPWAP should avoid
introducing any system security degradation as compared to the fat AP
scenario. However, by splitting the AP functions between the WTP and
AC, CAPWAP places potentially sensitive protocol interactions that
were previously internal to the AP onto the Layer 3 (L3) network
between the AC and WTP. Therefore, the security properties of this
new system are dependent on the security properties of the
intervening network, as well as on the details of the split.
Since CAPWAP does not directly interact with over-air or AAA
protocols, these are mostly out of scope for this analysis. That is,
we do not analyze the basic AAA or IEEE 802.11 security protocols
directly here, as CAPWAP does not modify these protocols in any way,
nor do they directly interact with CAPWAP. However, by splitting AP
functionality, CAPWAP may expose security elements of these protocols
that would not otherwise be exposed. In such cases, CAPWAP should
explicitly avoid degrading the security of these protocols in any way
when compared to the fat AP scenario.
4. Overview of IEEE 802.11 and AAA Security
While this document is not directly concerned with IEEE 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:
Kelly & Clancy Informational [Page 8]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
+-----+ RADIUS/Diameter
| AAA |<==============\\
+-----+ ||
| ||
+-----------+------------+ ||
| | ||
+-----+ +----+ ||
| AC | | AC |<==//
+-----+ +----+
+---| |---+ +---| |---+
| | | |
| | | CAPWAP |
+-----+ +-----+ +-----+ +-----+
| WTP | | WTP | | WTP | | WTP |
+-----+ +-----+ +-----+ +-----+
^ ^^^
^^ ^^^ 802.11i,
^^ ^^ 802.1X, WPA,
+-----+ +-----+ WEP
| STA | | STA |
+-----+ +-----+
/ / / /
+-----+ +-----+
Figure 3: CAPWAP Protocol Hierarchy
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 IEEE 802.11 and AAA
security below. It is very important to note that we only cover
those topics that are relevant to our discussion, so what follows is
not by any means exhaustive. For more detailed coverage of IEEE
802.11-related security topics, see e.g., [80211SEC].
4.1. IEEE 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 IEEE 802.11 link security is to be provided, then some
sort of authentication is required in order to bootstrap the trust
relationship that 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).
Kelly & Clancy Informational [Page 9]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
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 IEEE 802.11
security mechanisms.
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 4: Trust Relationships
The following describes each of the relationships:
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
Kelly & Clancy Informational [Page 10]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
4.2. IEEE 802.11 Link Security
The current CAPWAP binding for IEEE 802.11 primarily supports the use
of IEEE 802.11i [80211I] security on the wireless link. However,
since IEEE 802.11i does not prohibit the use of WEP for link
security, neither does CAPWAP. Nonetheless, use of WEP with CAPWAP
is NOT RECOMMENDED.
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 of
any length after enough packets have been collected [WEPSEC]. As of
late 2008, such attacks are trivial, and take mere seconds to
accomplish.
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 4-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 [RFC5416].
4.3. 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 Diameter [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
Kelly & Clancy Informational [Page 11]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
confidentiality and integrity. The AAA protocols and keys used
SHOULD be consistent with the guidance in [RFC4962].
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.
4.4. Cryptographic 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
variety of security associations when an IEEE 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 an STA trusts
a AAA server who trusts an AC who trusts a WTP doesn't necessarily
mean that the STA should trust the WTP. 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 an STA.
Protection against attacks like these can be very difficult, while
maintaining scalable trust relationships with other entities in the
network. Since multiple protocols are being used, multi-party keying
to derive end-to-end trust relationships is infeasible.
Since CAPWAP is a collection of pairwise trust relationships, in
order to claim CAPWAP is secure, we must believe in the transitivity
of these trust relationships. Its hierarchal nature mitigates the
domino effect, but any compromised device in the hierarchy can affect
those below it in the hierarchy.
Kelly & Clancy Informational [Page 12]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
5. 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?
* 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 exploit
versus 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 that are not (or cannot be) completely
addressed, and we give recommendations for mitigating the resulting
exposure.
Kelly & Clancy Informational [Page 13]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
6. Representative CAPWAP Deployment Scenarios
In this section, we describe some representative CAPWAP deployment
scenarios, and in particular, those scenarios that 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 that span
multiple scenarios. Following this is a sampling of individual
deployment scenarios.
6.1. Preliminary Definitions
The IEEE 802.11 standard describes several frame types, and
variations in WLAN architectures dictate where these frame types
originate and/or terminate (i.e., at the WTP or AC). There are three
basic IEEE 802.11 frame types currently defined:
o Control: 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: 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., IEEE 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. We briefly describe various forms of MAC-splitting
below. For further detail, see [RFC5415] and [RFC5416].
Kelly & Clancy Informational [Page 14]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
6.1.1. Split MAC
Split MAC scenarios are characterized by the fact that some IEEE
802.11 MAC messages are processed by the WTP, while some are
processed by the AC. For example, a Split MAC implementation might
divide IEEE 802.11 frame processing as follows:
WTP
* Beacons
* Probes
* RTS, CTS, ACK, PS-POLL, CF-POLL,CF-END, CF-ACK
AC
* Association/Reassociation/Disassociation
* Authentication/Deauthentication
* Key Management
* IEEE 802.11 Link Security (WEP, TKIP, IEEE 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 IEEE 802.11 functionality.
6.1.2. Local MAC
Local MAC scenarios are characterized by the fact that most IEEE
802.11 MAC messages are processed by the WTP. Some IEE 802.11 MAC
frames must be forwarded to the AC (i.e., IEEE 802.11 Association
Request frames), but in general, the WTP manages the MAC
interactions. Data frames may also be forwarded to the AC, but are
generally bridged locally.
6.1.3. Remote MAC
Remote MAC scenarios are characterized by the fact that all IEEE
802.11 MAC messages are forwarded to the AC. The WTP does not
process any of these locally. This model supports very lightweight
WTPs that need be little more than smart antennas. While Remote MAC
scenarios are not addressed by the current IEEE 802.11 protocol
binding for CAPWAP, the description is included here for
completeness.
Kelly & Clancy Informational [Page 15]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
6.1.4. 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 that
rigidly define these terms and associated handling. Data tunneling
is further discussed for each scenario below.
6.2. 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.
6.2.1. Localized Modular Deployment
The localized modular model describes a WLAN that 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 Virtual LAN). 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
Kelly & Clancy Informational [Page 16]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
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.
6.2.2. 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:
o Primary function is to provide Internet access
o Authentication may be very weak
o There frequently is no IEEE 802.11 link security
Sometimes, the Local MAC model is used. In such cases, the AC may be
"in the clouds" (out on the Internet somewhere), and user data
traffic will typically be locally bridged, rather than tunneled back
to the AC. Some IEEE 802.11 management traffic may be tunneled back
to the AC, but frequently the authentication consists in simply
knowing the Service Set Identifier (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 that
requires verification with some back-end infrastructure prior to
access. In these cases, IEEE 802.11 management frames are tunneled
back to the AC (e.g., user authentication), and stronger IEEE 802.11
link security may be provided (e.g., RSN), 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 scenario, 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.
Kelly & Clancy Informational [Page 17]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
6.2.3. Distributed Deployments
The distributed deployment model describes a more complex WLAN
topology that 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.
6.2.3.1. Large Physically Contained Organization
One distributed deployment example involves a single large
organization that 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, 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.
6.2.3.2. Campus Deployments
Some large organizations have networks that 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
Metropolitan Area Network (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 the AC, as this
facilitates mobility and centralized policy enforcement.
6.2.3.3. 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
Kelly & Clancy Informational [Page 18]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
Internet. For connections crossing potentially hostile hops (e.g.,
the Internet), some sort of Virtual Private Network (VPN) is
typically employed as a protective measure.
In some simple branch office scenarios, there are only WTPs 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.
6.2.3.4. 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
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
stand-alone 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 stand-alone fat APs, 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 supported (i.e., only data traffic destined for the
headquarters is tunneled back to the controller, with Internet-bound
traffic locally bridged).
7. 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
Kelly & Clancy Informational [Page 19]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
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
accordingly.
7.1. Passive versus 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 real-time. 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 Address Resolution Protocol
(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
Kelly & Clancy Informational [Page 20]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
* Can modify packets
* Can delay packets
A passive adversary could be located anywhere along the path between
the AC and WTP, and is characterized by the fact that it only
listens:
+------+
|client| +--------+ |
|(STA) | | WTP | | +------+
+------+ ) ) ) ) | |-----| / \ +------+
/ / +--------+ |-x-( optional )---| AC |
+------+ | | \ cloud / +------+
| | +------+
|
| +-----------+
+--| pass-by |
| listener |
+-----------+
Figure 5: Passive Attacker
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 / +------+
| +------+
Figure 6: Active Man-in-the-Middle Attacker
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
an MitM attack is not so difficult as it might first appear.
Kelly & Clancy Informational [Page 21]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
8. Vulnerabilities Introduced by CAPWAP
In this section, we discuss vulnerabilities that 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 or AC, we do consider the potential effects of
such compromises with respect to CAPWAP and the operational changes
it introduces when compared to stand-alone AP deployments.
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 vulnerabilities assuming that the CAPWAP
communications lack security of any kind. Later, we will evaluate
the protocol given the security mechanisms currently planned for
CAPWAP.
8.1. 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.
8.1.1. 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
Kelly & Clancy Informational [Page 22]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
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).
8.1.2. 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.
8.2. 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:
Kelly & Clancy Informational [Page 23]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
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
off-line, 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.
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 WTPs configuration, including
potentially loading tainted WTP firmware.
9. Adversary Goals in CAPWAP
This section gives an sampling of potential adversary goals. It is
not exhaustive, and makes no judgment 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.
9.1. 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?
Kelly & Clancy Informational [Page 24]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
o What version of firmware do they run?
o What cryptographic capabilities do they have?
Similarly, snooping on tunneled data traffic might potentially reveal
a great deal about the network, including answers to the following:
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?
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.
9.2. 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)
o to facilitate Denial-of-Service attacks against WLAN users or the
network
Kelly & Clancy Informational [Page 25]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
9.3. AC Impersonation and/or Rootkit Installation
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 attacks against WLANs
o to facilitate MitM attacks against WLAN users
o to intercept user mobility context from another AC (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 IEEE 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.
9.4. Other Goals or Sub-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 undesirable
consequences. Here are some examples:
o cause CAPWAP de-association of WTP/AC
o cause IEEE 802.11 de-association of authorized user
o inject/modify/delete tunneled IEEE 802.11 user traffic
* to interact with a specific application
* to launch various attacks on WLAN user systems
Kelly & Clancy Informational [Page 26]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
* to launch protocol fuzzing or other attacks on the AC that
bridges between IEEE 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
approach if we are to provide reasonably good security across the
board.
10. Countermeasures and Their Effects
In the previous sections, we described numerous vulnerabilities that
result from splitting the AP function in two, and also discussed a
number of adversary goals that 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.
10.1. Communications Security Elements
10.1.1. Mutual Authentication
Mutual authentication consists in each side proving its identity to
the other. There are numerous authentication protocols that
accomplish this, typically using either a shared secret (e.g., a pre-
shared 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.
10.1.1.1. Authorization
While authentication consists in proving the identity of an entity,
authorization consists in determining whether that entity should have
access to some resource. The current IEEE 802.11i CAPWAP protocol
binding takes a rather simplistic approach to authorization,
Kelly & Clancy Informational [Page 27]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
depending on the authentication method chosen. If pre-shared keys
are used, authorization is broad and coarse: if the device knows the
pre-shared key, the device is "trusted", meaning the that it is
believed to be what it claims to be ( AC versus WTP), and it is
granted all privilege/access associated with that device class.
When using pre-shared keys, some granularity may be achieved by
creating classes, each with their own pre-shared key, but this still
has drawbacks. For example, while possession of the key may suffice
to identify the device as a member of a given group or class, it
cannot be used to prove a device is either a WTP or an AC. This
means the key can be abused, and those possessing the key can claim
to be either type of device.
When X.509v3 certificates are used for authentication, much more
granular authorization policies are possible. Nonetheless, the
current IEEE 802.11i protocol binding remains simplistic in its
approach (though this may be addressed in future revisions). As
currently defined, the X.509v3 certificates facilitate the following
authorization checks:
o CommonName (CN): the CN contains the MAC address of the device; if
the relying party (AC or WTP) has a method for determining
"acceptability" of a given MAC address, this helps prevent AC/WTP
impersonation, MitM attacks, and may help in limiting DoS attacks
o Extended Key Usage (EKU) key purpose ID bits: CAPWAP uses specific
key purpose ID bits (see [RFC5415] for more information) to
explicitly differentiate between an AC and a WTP. If use of these
bits is strictly enforced, this segregates devices into AC versus
WTP classes, and assists in preventing AC/WTP impersonation, MitM
attacks, and may also help in limiting DoS attacks. However, if
the id-kp-anyExtendedKeyUsage keyPurposeID is supported, this
mechanism may be on a par with pre-shared keys, as a rogue device
has the ability to claim it is either a WTP or AC, unless CN-based
access controls are employed in tandem.
It should be noted that certificate-based authorization and zero
configuration are not fully compatible. Even if the WTPs and the ACs
are shipped with manufacturer-provided certificates, the WTPs need a
way to identify the correct AC is in this deployment (as opposed to
other ACs from the same vendor, purchased and controlled by an
adversary), and the AC needs to know which WTPs are part of this
deployment (as opposed to WTPs purchased and controlled by an
adversary).
Kelly & Clancy Informational [Page 28]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
The threat analysis in this document assumes that WTPs can identify
the correct AC, and the AC can identify the correct WTPs. Analysis
of situations where either of these do not hold is beyond the scope
of this document.
10.1.2. 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
o impersonation
10.1.3. 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.
10.1.4. Anti-Replay
Anti-replay 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.
10.1.5. Confidentiality
Data confidentiality prevents eavesdropping by protecting data as it
passes over the network. This is typically accomplished using
encryption.
Kelly & Clancy Informational [Page 29]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
10.2. Putting the Elements Together
Above we described various security elements and their properties.
Below, we discuss combinations of these elements in terms of CAPWAP.
10.2.1. 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 that mitigates almost all of
the vulnerabilities described above.
10.2.2. Data Channel Security
The CAPWAP data channel contains some IEEE 802.11 management traffic
including association/disassociation, reassociation, and
deauthentication. 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
prevent an adversary from injecting and/or modifying transit data, or
impersonating a valid AC or WTP. Adding confidentiality would
prevent eavesdropping.
11. Countermeasures Provided by DTLS
Datagram TLS (DTLS) is the currently proposed security solution for
CAPWAP. DTLS supports the following security functionality:
o Mutual Authentication (pre-shared secrets or X.509 Certificates)
o Mutual Authorization (pre-shared secrets or X.509 Certificates)
o Data Origin Authentication
o Data Integrity Verification
o Anti-replay
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.
Kelly & Clancy Informational [Page 30]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
12. Issues Not Addressed By DTLS
Unfortunately, DTLS does not solve all of our CAPWAP security
concerns. There are some things it just cannot prevent. These are
enumerated below.
12.1. 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.
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 through 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 real-time, although it may be
detected if exceeding a lost packet threshold triggers an alert.
However, this depends on the MitM not being in a position to delete
the alert before it reaches its appropriate destination.
Kelly & Clancy Informational [Page 31]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
12.2. Passive Monitoring (Sniffing)
CAPWAP protocol security cannot prevent (or detect) passive
monitoring. The best we can do is provide a confidentiality
mechanism.
12.3. 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.
12.4. 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
that 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.
12.5. 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
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.
13. Security Considerations
This document outlines a threat analysis for CAPWAP in the context of
IEEE 802.11 deployments, and as such, no additional CAPWAP-related
security considerations are described here. However, in some cases
additional management channels (e.g., Simple Network Management
Protocol (SNMP)) may be introduced into CAPWAP deployments. Whenever
this occurs, related security considerations MUST be described in
detail in those documents.
14. Acknowledgements
The authors gratefully acknowledge the reviews and helpful comments
of Dan Romascanu, Joe Salowey, Sam Hartman, Mahalingham Mani, and
Pasi Eronen.
Kelly & Clancy Informational [Page 32]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
15. References
15.1. Normative 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-2004: Port-based Network Access
Control", December 2004.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4118] Yang, L., Zerfos, P., and E. Sadot, "Architecture
Taxonomy for Control and Provisioning of Wireless Access
Points (CAPWAP)", RFC 4118, June 2005.
[RFC5415] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
Ed., "Control And Provisioning of Wireless Access Points
(CAPWAP) Protocol Specification", RFC 5415, March 2009.
[RFC5416] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
Ed., "Control And Provisioning of Wireless Access Points
(CAPWAP) Protocol Binding for IEEE 802.11", RFC 5416,
March 2009.
15.2. Informative References
[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.
[RFC4072] Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application", RFC 4072,
August 2005.
[RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication,
Authorization, and Accounting (AAA) Key Management",
BCP 132, RFC 4962, July 2007.
Kelly & Clancy Informational [Page 33]
RFC 5418 CAPWAP 802.11 Threat Analysis March 2009
[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
Kelly & Clancy Informational [Page 34]