Network Working Group T. Clancy
Internet-Draft LTS
Intended status: Standards Track K. Hoeper
Expires: May 7, 2009 NIST
November 3, 2008
Channel Binding Support for EAP Methods
draft-clancy-emu-chbind-04
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Abstract
This document defines how to implement channel bindings for
Extensible Authentication Protocol (EAP) methods to address the lying
NAS problem.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
4. Channel Bindings . . . . . . . . . . . . . . . . . . . . . . . 6
5. Channel Binding Protocol . . . . . . . . . . . . . . . . . . . 9
6. System Requirements . . . . . . . . . . . . . . . . . . . . . 10
7. Lower-Layer Bindings . . . . . . . . . . . . . . . . . . . . . 11
7.1. General Attributes . . . . . . . . . . . . . . . . . . . . 12
7.2. IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . . . 12
7.2.1. IEEE 802.11r . . . . . . . . . . . . . . . . . . . . . 12
7.2.2. IEEE 802.11s . . . . . . . . . . . . . . . . . . . . . 12
7.3. IEEE 802.16 . . . . . . . . . . . . . . . . . . . . . . . 13
7.4. Wired 802.1X . . . . . . . . . . . . . . . . . . . . . . . 13
7.5. Point to Point Protocol (PPP) . . . . . . . . . . . . . . 13
7.6. Internet Key Exchange v2 (IKEv2) . . . . . . . . . . . . . 13
7.7. 3GPP2 . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.8. PANA . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. AAA-Layer Bindings . . . . . . . . . . . . . . . . . . . . . . 13
9. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9.1. Trust Model . . . . . . . . . . . . . . . . . . . . . . . 14
9.2. Consequences of Trust Violation . . . . . . . . . . . . . 15
9.3. Privacy Violations . . . . . . . . . . . . . . . . . . . . 16
10. Operations and Management Considerations . . . . . . . . . . . 16
10.1. System Impact . . . . . . . . . . . . . . . . . . . . . . 16
10.2. Cost-Benefit Analysis . . . . . . . . . . . . . . . . . . 17
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1. Normative References . . . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Attacks Prevented by Channel Bindings . . . . . . . . 18
A.1. Enterprise Subnetwork Masquerading . . . . . . . . . . . . 18
A.2. Forced Roaming . . . . . . . . . . . . . . . . . . . . . . 19
A.3. Downgrading attacks . . . . . . . . . . . . . . . . . . . 19
A.4. Bogus Beacons in IEEE 802.11r . . . . . . . . . . . . . . 20
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A.5. Forcing false authorization in IEEE 802.11i . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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1. Introduction
The so-called "lying NAS" problem is a well-documented problem with
the current Extensible Authentication Protocol (EAP) architecture
[RFC3748] when used in pass-through authenticator mode. Here, a
Network Access Server (NAS), or pass-through authenticator, may
represent one set of information (e.g. network identity,
capabilities, configuration, etc) to the backend Authentication,
Authorization, and Accounting (AAA) infrastructure, while
representing contrary information to EAP clients. Another
possibility is that the same false information could be provided to
both the EAP client and EAP server by the NAS.
A concrete example of this may be an IEEE 802.11 access point with a
security association to a particular AAA server. While there may be
some identity tied to that security association, there's no reason
the access point needs to advertise a consistent identity to clients.
In fact, it may include whatever information in its beacons (e.g.
different SSID or security properties) it desires. This could lead
to situations where, for example, a client joins one network that is
masquerading as another.
Another current limitation of EAP is its minimal ability to perform
authorization. Currently EAP servers can only make authorization
decisions about network access based on information they know about
peers. If the same EAP server controls access to multiple networks,
it has little information about the NAS to which the peer is
connecting, and does not know what information the NAS may be
claiming about the network to the peer. A mechanism is needed that
allows the EAP server to apply more detailed policies to
authorization.
This document defines and implements EAP channel bindings to solve
these two problems, using a process in which the EAP client provides
information about the characteristics of the service provided by the
authenticator to the AAA server protected within the EAP method.
This allows the server to verify the authenticator is providing
information to the peer consistent with the defined network policy,
and that the peer is authorized to access the network in the manner
described by the NAS. "AAA Payloads" defined in
[I-D.clancy-emu-aaapay] proposes a mechanism to carry this
information.
2. Terminology
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. The key
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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 [RFC2119].
3. Problem Statement
In a [RFC4017]-compliant EAP authentication, the EAP client and EAP
server mutually authenticate each other, and derive keying material.
However, when operating in pass-through mode, the EAP server can be
far removed from the authenticator. A malicious or compromised
authenticator may represent incorrect information about the network
to the client in an effort to affect its operation in some way.
Additionally, while an authenticator may not be compromised, other
compromised elements in the network could provide false information
to the authenticator that it could simply be relaying to EAP clients.
Our goal is to ensure that the authenticator is providing correct
information to the EAP client during the initial network discovery,
selection, and authentication.
There are two different types of networks to consider: enterprise
networks and service provider networks. In enterprise networks, we
assume a single administrative domain, making it feasible for an EAP
server to have information about all the authenticators in the
network. In service provider networks, global knowledge is
infeasible due to indirection via roaming. When a client is outside
its home administrative domain, the goal is to ensure that the level
of service received by the client is consistent with the contractual
agreement between the two service providers.
The following are a couple example attacks possible by presenting
false network information to clients.
o Enterprise Network: A corporate network may have multiple virtual
LANs (VLANs) running throughout their campus network, and have
IEEE 802.11 access points connected to each VLAN. Assume one VLAN
connects users to the firewalled corporate network, while the
other connects users to a public guest network. The corporate
network is assumed to be free of adversarial elements, while the
guest network is assumed to possibly have malicious elements.
Access Points on both VLANs are serviced by the same EAP server,
but broadcast different SSIDs to differentiate. A compromised
access point connected to the guest network could advertise the
SSID of the corporate network in an effort to lure clients to
connect to a network with a false sense of security regarding
their traffic. Conditions and further details of this attack can
be found in the Appendix.
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o Service Provider Network: An EAP-enabled mobile phone provider
operating along a geo-political boundary could boost their cell
towers' transmission power and advertise the network identity of
the neighboring country's indigenous provider. This would cause
unknowing handsets to associate with an unintended operator, and
consequently be subject to high roaming fees without realizing
they had roamed off their home provider's network. This scenario
can be considered as "lying provider" problem, because here the
provider tampers with the transmission power and then configures
its NAS to broadcast another network's identity. For the purpose
of channel bindings as defined in this draft, it does not matter
which local entity (or entities) is "lying" in a service provider
network (local NAS, local authentication server and/or local
proxies), because the only information received from the visited
network that is verified by channel bindings is the information
the home authentication server received from the last hop in the
communication chain. In other words, channel bindings enable the
detection of inconsistencies in the information from a visited
network, but cannot determine which entity is lying. Naturally,
channel bindings for EAP methods can only verify the endpoints
and, if desirable, intermediate hops need to be protected by the
employed AAA protocol.
To address these problems, a mechanism is required to validate
unauthenticated information advertised by EAP authenticators.
4. Channel Bindings
EAP channel bindings seek to authenticate previously unauthenticated
information provided by the authenticator to the EAP peer, by
allowing the client and server to compare their perception of network
properties in a secure channel.
It should be noted that the definition of EAP channel bindings
differs somewhat from channel bindings documented in [RFC5056], which
seek to securely bind together the end points of a multi-layer
protocol, allowing lower layers to protect data from higher layers.
Unlike [RFC5056], EAP channel bindings do not ensure the binding
different layers of a session but rather the information advertised
to EAP client by an authenticator acting as pass-through device
during an EAP execution.
There are two main approaches to EAP channel bindings:
o After keys have been derived during an EAP execution, the peer and
server can, in an integrity-protected channel, exchange plaintext
information about the network with each other, and verify
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consistency and correctness.
o Network information can be uniquely encoded into an opaque blob
that can be included directly into the derivation of the EAP
session keys.
Both approaches are only applicable to key deriving EAP methods and
both have advantages and disadvantages. Advantages of exchanging
plaintext information include:
o It allows for policy-based comparisons of network properties,
rather than requiring precise matches for every field, which
achieves a policy-defined consistency, rather than bitwise
equality. This allows network operators to define which
properties are important and even verifiable in their network.
o EAP methods that support extensible, integrity-protected channels
can easily include support for exchanging this network
information. In contrast, direct inclusion into the key
derivation would require revisions to existing EAP methods or a
wrapper EAP method.
o Given it doesn't affect the key derivation, this approach
facilitates debugging, incremental deployment, backward
compatibility and a logging mode in which verification results are
recorded but do not have an affect on the remainder of the EAP
execution. The exact use of the verification results can be
subject to the network policy. Additionally, consistent
information canonicalization and formatting for the key derivation
approach would likely cause significant deployment problems.
The following are advantages of directly including channel binding
information in the key derivation:
o EAP methods not supporting extensible, integrity-protected
channels could still be supported, either by revising their key
derivation, revising EAP, or wrapping them in a universal method
that supports channel binding.
o It can guarantee proper channel information, since subsequent
communication would be impossible if differences in channel
information yielded different session keys on the EAP client and
server.
The scope of EAP channel bindings differs somewhat depending on the
type of deployment in which they are being used. In enterprise
networks, they can be used to authenticate very specific properties
of the authenticator (e.g. MAC address, supported link types and
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data rates, etc), while in service provider networks they can
generally only authenticate broader information about a roaming
partner's network (e.g. network name, roaming information, link
security requirements, etc). The reason for the difference has to do
with the amount of information you expect your home EAP server to
know about the authenticator and/or network to which the peer is
connected. In roaming cases, the home server is likely to only know
information contained in their roaming agreements.
With any multi-hop AAA infrastructure, many of the specific NAS
properties are obscured by the AAA proxy that's decrypting,
reframing, and retransmitting the underlying AAA messages.
Especially service provider networks are affected by this and the
information received from the last hop may not contain much
verifiable information any longer. For example, information such as
the NAS IP address may not be known to the EAP server. This affects
the ability of the EAP server to verify specific NAS properties.
However, often verification of the MAC or IP address of the NAS is
not useful for improving the overall security posture of a network.
More often it is useful to make policy decisions about services being
offered to peers. For example, in an IEEE 802.11 network, the EAP
server may wish to ensure that clients connecting to the corporate
intranet are using secure link- layer encryption, while link-layer
security requirements for clients connecting to the guest network
could be less stringent. These types of policy decisions can be made
without knowing or being able to verify the IP address of the NAS
through which the peer is connecting. Furthermore, as described in
the next section, channel bindings also verify the information
provided by peer and a local policy database, where both pieces of
information are unaffected by the processing of intermediate hops.
Consequently, even if some information got lost in transition and
thus may not be known to the EAP server, the server is still able to
carry out the channel binding verification.
Also, a peer's expectations of a network may also differ. In a
mobile phone network, peers generally don't care what the name of the
network is, as long as they can make their phone call and are charged
the expected amount for the call. However, in an enterprise network
a peer may be more concerned with specifics of where their network
traffic is being routed.
Any deployment of channel bindings should take into consideration
both what information the EAP server is likely to know, and also what
type of network information the peer would want and need
authenticated.
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5. Channel Binding Protocol
This section defines a protocol for verifying channel binding
information during an EAP authentication. The protocol uses the
approach where plaintext data is exchanged, since it allows channel
bindings to be used more flexibly in varied deployment models.
---
-------- ------------- / \ ----------
|EAP peer|<---->|Authenticator|<-->( AAA )<-->|EAP Server|
-------- ------------- \ / ----------
. i1 . --- . | ______
.<-----------------. . | (______)
. . i2 . \--| |
. .-------------------------->. |Policy|
. i1 . | DB |
.--------------------------------------------->. (______)
. isConsistant(i1, i2, Policy) .
.<---------------------------------------------.
Figure 1: Overview of Channel Binding
Channel bindings are always provided between two communication
endpoints, here the EAP client and server, who communicate through an
authenticator in pass-trough mode. During network advertisement,
selection, and authentication, the authenticator presents
unauthenticated information, labeled i1 for convenience, about the
network to the peer. This information, i1, could include an
authenticator identifier and the identity of the network it
represents, in addition to advertised network information such as
offered services and roaming information. As there is no established
trust relationship between the peer and authenticator, there is no
way for the peer to validate this information.
Additionally, during the transaction the authenticator presents a
number of information properties about itself to the AAA
infrastructure which may or may not be valid. We label this
information i2. Note that i2 is the information the EAP server
receives from the last hop in the communication chain which is not
necessarily the authenticator. In those cases i2 may be different
from the original information sent by the authenticator because of en
route processing or malicious modifications. As a result, in the
service provider model, typically the EAP server is able to verify
only the last-hop portion of i2, or values propagated by proxy
servers.
Our goal is to transport i1 from the peer to the server, and allow
the server to verify the consistency of i1 from the peer and i2 from
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the authenticator against the information stored in its local policy
database.
By doing this, we allow the EAP server the opportunity to make
informed decisions about authorization. The EAP server can
authenticate the authenticator via the AAA security association, and
using this channel bindings mechanism it can now authorize the
circumstances under which a peer connects to the authenticator.
This information, i1, could include an authenticator identifier and
the identity of the network it represents, in addition to advertised
network information such as offered services and roaming information.
To prevent attacks by a lying NAS or lying provider, the EAP server
must be able to verify that i1 either matches its knowledge of the
network (enterprise model) or is consistent with the contractual
agreement between itself and the roaming partner network to which the
client is connected (service provider model). Additionally, it
should verify that this information is consistent with i2.
The protocol defined in this document is a single round trip between
the EAP peer and server that can be piggybacked to the EAP method
execution, and formats data elements as Diameter AVPs. We provide
requirements for a transport protocol.
6. System Requirements
The channel binding protocol defined in this document must be
transported after keying material has been derived between the EAP
peer and server, and before the peer would suffer adverse affects
from joining an adversarial network. To satisfy this requirement, it
should occur either during the EAP method execution or during the EAP
lower layer's secure association protocol.
The transport protocol for carrying channel binding information MUST
support end-to-end (i.e. between the EAP peer and server) message
integrity protection to prevent the adversarial NAS or AAA device
from manipulating the transported data. The transport protocol
SHOULD provide confidentiality. The motivation for this that the
channel bindings could contain private information, including peer
identities, which SHOULD be protected.
If transporting data directly within an EAP method, it MUST be able
to carry integrity protected data from the EAP peer to server. EAP
methods SHOULD provide a mechanism to carry protected data from
server to peer. EAP methods MUST export channel binding data to the
AAA subsystem on the EAP server. EAP methods MUST be able to import
channel binding data from the lower layer on the EAP peer.
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One way to transport the single round-trip exchange is as a series of
Diameter AVPs formatted and encapsulated in EAP methods per
[I-D.clancy-emu-aaapay]. For each lower layer, this document defines
the parameters of interest, and the appropriate Diameter AVPs for
transporting them. Additionally, guidance on how to perform
consistency checks on those values will be provided.
In order to minimize data formatting inconsistencies, parameters
useful for channel binding MUST be allocated from the standard RADIUS
space. Two AVPs are considered equivalent for the purpose of channel
binding if they have the same AVP Code, Vendor-Specific Bit, AVP
Length, Vendor-ID (if Vendor-Specific Bit is set), and data.
7. Lower-Layer Bindings
This section discusses AVPs of some EAP-employing lower layer link
protocols that seem appropriate for providing channel bindings (i.e.
data from "i1" in Section Section 5). The discussion is limited to
protocols that establish fresh authentic keying material because such
keying material is necessary to protect the integrity of all AVPs
that are exchanged as part of the channel binding. For each
protocol, a variety of network information that can be encapsulated
in AVPs is of interest for server and peer to ensure channel binding.
The respective appropriate AVPs depend on the lower layer protocol as
well as on the network type (i.e. enterprise network or service
provider network) of an application.
For each EAP lower layer, a variety of AAA properties may be of
interest to the server. These values may already be known by the
server, or may be transported to the server via an existing RADIUS or
Diameter connection.
As part of the channel binding protocol, the EAP peer sends
encapsulated AVPs to the server. The server then validates the
received information by comparing it to information stored in a local
database. If the received information is unsatisfactory given some
validation policy, the server SHOULD respond by halting the EAP
authentication and returning an EAP-Failure.
If validation is successful, the server SHOULD send a message
indicating the success to the client. In addition, the server MAY
respond back to the EAP peer with information encapsulated in AVPs
that can be of use to the peer, and information the peer may not have
any way of otherwise knowing.
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7.1. General Attributes
This section lists AVPs useful to all link-layers.
NAS-Port-Type: Indicates the underlying link-layer technology used
to connect (e.g. IEEE 802.11, PPP, etc), and MUST be included by
the EAP client, and SHOULD be verified against the database and
NAS-Port-Type received from the NAS.
Cost-Information: AVP from the Diameter Credit-Control Application
[RFC4006] to the peer indicating how much peers will be billed for
service and MAY be included by the EAP client and verified against
roaming profiles stored in the database.
7.2. IEEE 802.11
The client SHOULD transmit to the server the following fields,
encapsulated within the appropriate Diameter AVPs:
Called-Station-Id: contains BSSID and SSID and MUST be included by
the EAP client, and SHOULD be verified against the database and
Called-Station-Id received from the NAS
[TODO: Need a way to transport the RSN-IE.]
7.2.1. IEEE 802.11r
In addition to the AVPs for IEEE 802.11, an IEEE 802.11r client
SHOULD transmit the following additional fields:
Mobility-Domain-Id: Identity of the mobility domain and MUST be
included by the EAP client, and SHOULD be verified against the
database and Mobility-Domain-Id received from the NAS
[I-D.aboba-radext-wlan]
7.2.2. IEEE 802.11s
In addition to the AVPs for IEEE 802.11, an IEEE 802.11s client
SHOULD transmit the following additional fields:
Mesh-Key-Distributor-Domain-Id: Identity of the Mesh Key Distributor
Domain and MUST be included by the EAP client, and SHOULD be
verified against the database and Mesh-Key-Distributor-Domain-Id
received from the NAS [I-D.aboba-radext-wlan]
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7.3. IEEE 802.16
TBD
7.4. Wired 802.1X
TBD
7.5. Point to Point Protocol (PPP)
TBD
7.6. Internet Key Exchange v2 (IKEv2)
TBD
7.7. 3GPP2
TBD
7.8. PANA
TBD
8. AAA-Layer Bindings
This section discusses which AAA attributes in RADIUS Accept-Request
messages can and should be validated by a AAA server (i.e. data from
"i2" in Section Section 5). As noted before, this data can be
manipulated by AAA proxies either to enable functionality (e.g.
removing realm information after messages have been proxied) or
maliciously (e.g. in the case of a lying provider). As such, this
data cannot always be easily validated. However as thorough of a
validation as possible should be conducted in an effort to detect
possible attacks.
User-Name: This value should be checked for consistency with the
database and any method-specific user information. If EAP method
identity protection is employed, this value typically contains a
pseudonym or keyword.
NAS-IP-Address: This value is typically the IP address of the
authenticator, but in a proxied connection it likely will not
match the source IP address of an Access-Request. A consistency
check MAY verify the subnet of the IP address was correct based on
the last-hop proxy.
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Called-Station-Id: This is typically the MAC address of the NAS. On
an enterprise network, it MAY be validated against the MAC address
is one that has been provisioned on the network.
Calling-Station-Id: This is typically the MAC address of the EAP
Client, and verification of this is likely difficult, unless EAP
credentials have been provisioned on a per-host basis to specific
L2 addresses. It SHOULD be validated against the database in an
enterprise deployment.
NAS-Identifier: This is an identifier populated by the NAS, and
could be related to the MAC address, and should be validated
similarly to the Called-Station-Id.
NAS-Port-Type: This specifies the underlying link technology. It
SHOULD be validated against the value received from the client in
the information exchange, and against a database of authorized
link-layer technologies.
9. Security Considerations
9.1. Trust Model
We consider a trust model in which the peer and server trust each
other. This is not unreasonable, considering they already have a
trust relationship. In this trust model, client and authentication
server are honest while the authenticator is maliciously sending
false information to client and/or server. The following are the
trust relationships:
o The server trusts that the channel binding information received
from the client is the information that the client received from
the authenticator.
o The client trusts the channel binding result received from the
server.
o The server trusts the information contained within its local
database.
In order to establish the first two trust relationships during an EAP
execution, an EAP method MUST provide the following:
o mutual authentication between client and server
o derivation of keying material including a key for integrity
protection of channel binding messages
o sending i1 from client to server over an integrity-protected
channel
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o sending the result and optionally i2 from server to client over an
integrity-protected channel
9.2. Consequences of Trust Violation
If any of the trust relationships listed in Section 7.1 are violated,
channel binding cannot be provided. In other words, if mutual
authentication with key establishment as part of the EAP method as
well as protected database access are not provided, then achieving
channel binding is not feasible.
Dishonest peers can only manipulate the first message i1 of the
channel binding protocol. In this scenario, a peer sends i1' to the
server. If i1' is invalid, the channel binding validation will fail
and the server will abort the EAP authentication. On the other hand
if i1' passes the validation, either the original i1 was wrong and
i1' corrected the problem or both i1 and i1' constitute valid
information. All cases do not seem to be of any benefit to a peer
and do no pose a security risk.
Dishonest servers can send EAP-Failure messages and abort the EAP
authentication even if the received i1 is valid. However, servers
can always abort any EAP session independent of whether channel
binding is offered or not. On the other hand, dishonest servers can
claim a successful validation even for an invalid i1. This can be
seen as collaboration of authenticator and server. Channel binding
can neither prevent nor detect such attacks. In general such attacks
cannot be prevented by cryptographic means and should be addressed
using policies making servers liable for their provided information
and services.
Additional network entities (such as proxies) might be on the
communication path between peer and server and may attempt to
manipulate the channel binding protocol. If these entities do not
possess the keying material used for integrity protection of the
channel binding messages, the same threat analysis applies as for the
dishonest authenticators. Hence, such entities can neither
manipulate single channel binding messages nor the outcome. On the
other hand, entities with access to the keying material must be
treated like a server in a threat analysis. Hence such entities are
able to manipulate the channel binding protocol without being
detected. However, the required knowledge of keying material is
unlikely since channel binding is executed before the EAP method is
completed, and thus before keying material is typically transported
to other entities.
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9.3. Privacy Violations
While the channel binding information exchanged between EAP peer and
EAP server (i.e. i1 and the optional result message) must always be
integrity-protected it may not be encrypted. In the case that these
messages contain identifiers of peer and/or network entities, the
privacy property of the executed EAP method may be violated. Hence,
in order to maintain the privacy of an EAP method, the exchanged
channel binding information must be encrypted.
10. Operations and Management Considerations
This section analyzes the impact of channel bindings on existing
deployments of EAP.
10.1. System Impact
As with any extension to existing protocols, there will be an impact
on existing systems. Typically the goal is to develop an extension
that minimizes the impact on both development and deployment of the
new system, subject to the system requirements. In this section we
discuss the impact on existing devices that currently utilize EAP,
assuming the channel binding information is transported within the
EAP method execution.
The EAP peer will need an API between the EAP lower layer and the EAP
method that exposes the necessary information from the NAS to be
validated to the EAP peer, which can then feed that information into
the EAP methods for transport. For example, an IEEE 802.11 system
would need to make available the various information elements that
require validation to the EAP peer which would properly format them
and pass them to the EAP method. Additionally, the EAP peer will
require updated EAP methods that support transporting channel binding
information. While most method documents are written modularly to
allow incorporating arbitrary protected information, implementations
of those methods would need to be revised to support these
extensions. Driver updates are also required so methods can access
the required information.
No changes to the pass-through authenticator would be required.
The EAP server would need an API between the database storing NAS
information and the individual EAP server. The EAP methods need to
be able to export received channel binding information to the EAP
server so it can be validated.
Additionally, an interface is necessary for populating the EAP server
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database with the appropriate parameters. In the enterprise case,
when a NAS is provisioned, information about what it should be
advertising to peers needs to be entered into the database. In the
service provider case, there should be a mechanism for entering
contractual information about roaming partners.
To ease operator burden it is highly recommended that there be a
mechanism for automatically populating the EAP server policy
database. Channel bindings could be enabled to allow peers to
transmit the NAS information to the EAP server, but the policy could
be configured to allow all connections. The obtained information
could be used to auto-generate policy information for the database,
assuming there are no adversarial elements in the network during the
auto-population phase.
Channel binding validation can also be implemented incrementally. An
initial database may be empty, and all channel bindings are
automatically approved. Operators can then incrementally add
parameters to the database regarding specific authenticators or
groups of authenticators that must be validated. Additionally, a
network could also self-form this database by putting the network
into a "learning" mode, and could then recognize inconsistencies in
the future.
10.2. Cost-Benefit Analysis
[TBD]
11. IANA Considerations
This document contains no IANA considerations.
12. References
12.1. Normative References
[I-D.aboba-radext-wlan]
Aboba, B., Malinen, J., Congdon, P., and J. Salowey,
"RADIUS Attributes for IEEE 802 Networks",
draft-aboba-radext-wlan-09 (work in progress),
October 2008.
[I-D.ietf-dime-rfc3588bis]
Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
"Diameter Base Protocol", draft-ietf-dime-rfc3588bis-13
(work in progress), November 2008.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
12.2. Informative References
[I-D.clancy-emu-aaapay]
Clancy, T., "EAP Method Support for Transporting AAA
Payloads", Internet Draft draft-clancy-emu-aaapay-01,
July 2008.
[RFC4006] Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and J.
Loughney, "Diameter Credit-Control Application", RFC 4006,
August 2005.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, August 2008.
[HC07] Hoeper, K. and L. Chen, "Where EAP Security Claims Fail",
ICST QShine, August 2007.
Appendix A. Attacks Prevented by Channel Bindings
In the following it is demonstrated how the presented channel
bindings can prevent attacks by malicious authenticators
(representing the lying NAS problem) as well as malicious visited
networks (representing the lying provider problem).
A.1. Enterprise Subnetwork Masquerading
As outlined in Section 3, an enterprise network may have multiple
VLANs providing different levels of security. In an attack, a
malicious NAS connecting to a guest network with lesser security
protection could broadcast the SSID of a subnetwork with higher
protection. This could lead clients to believe that they are
accessing the network over secure connections, and, e.g., transmit
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confidential information that they normally would not send over a
weakly protected connection. This attack works under the conditions
that clients use the same set of credentials to authenticate to the
different kinds of VLANs and that the VLANs support at least one
common EAP method. If these conditions are not met, the EAP server
would not authorize the clients to connect to the guest network,
because the clients used credentials and/or an EAP method that is
associated with the corporate network.
A.2. Forced Roaming
Mobile phone providers boosting their cell tower's transmission power
to get more users to use their networks have occurred in the past.
The increased transmission range combined with a NAS sending a false
network identity lures users to connect to the network without being
aware of that they are roaming.
Channel bindings would detect the bogus network identifier because
the network identifier send to the authentication server in i1 will
neither match information i2 nor the stored data. The verification
fails because the info in i1 claims to come from the peer's home
network while the home authentication server knows that the
connection is through a visited network outside the home domain. In
the same context, channel bindings can be utilized to provide a "home
zone" feature that notifies users every time they are about to
connect to a NAS outside their home domain.
A.3. Downgrading attacks
A malicious authenticator could modify the set of offered EAP methods
in its Beacon to force the peer to choose from only the weakest EAP
method(s) accepted by the authentication server. For instance,
instead of having a choice between EAP-MD5-CHAP, EAP-FAST and some
other methods, the authenticator reduces the choice for the peer to
the weaker EAP-MD5-CHAP method. Assuming that weak EAP methods are
supported by the authentication server, such a downgrading attack can
enable the authenticator to attack the integrity and confidentiality
of the remaining EAP execution and/or break the authentication and
key exchange. The presented channel bindings prevent such
downgrading attacks, because peers submit the offered EAP method
selection that they have received in the beacon as part of i1 to the
authentication server. As a result, the authentication server
recognizes the modification when comparing the information to the
respective information in its policy database.
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A.4. Bogus Beacons in IEEE 802.11r
In IEEE 802.11r, the SSID is bound to the TSK calculations, so that
the TSK needs to be consistent with the SSID advertised in an
authenticator's Beacon. While this prevents outsiders from spoofing
a Beacon it does not stop a "lying NAS" from sending a bogus Beacon
and calculating the TSK accordingly.
By implementing channel bindings, as described in this draft, in IEEE
802.11r, the verification by the authentication server would detect
the inconsistencies between the information the authenticator has
sent to the peer and the information the server received from the
authenticator and stores in the policy database.
A.5. Forcing false authorization in IEEE 802.11i
In IEEE 802.11i a malicious NAS can modify the beacon to make the
client believe it is connected to a network different from the on the
client is actually connected to.
In addition, a malicious NAS can force an authentication server into
authorizing access by sending an incorrect Called-Station-ID that
belongs to an authorized NAS in the network. This could cause the
authentication server to believe it had granted access to a different
network or even provider than the one the client got access to.
Both attacks can be prevented by implementing channel bindings,
because the server can compare the information that was sent to the
client, with information it received from the authenticator during
the AAA communication as well as the information stored in the policy
database.
Authors' Addresses
T. Charles Clancy
Laboratory for Telecommunications Sciences
US Department of Defense
College Park, MD
USA
Email: clancy@ltsnet.net
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Katrin Hoeper
National Institute of Standards and Technology
100 Bureau Drive, mail stop 8930
Gaithersburg, MD 20878
USA
Email: khoeper@nist.gov
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