Internet Engineering Task Force (IETF) S. Hartman, Ed.
Request for Comments: 6677 Painless Security
Category: Standards Track T. Clancy
ISSN: 2070-1721 Virginia Tech
K. Hoeper
Motorola Solutions, Inc.
July 2012
Channel-Binding Support
for Extensible Authentication Protocol (EAP) Methods
Abstract
This document defines how to implement channel bindings for
Extensible Authentication Protocol (EAP) methods to address the
"lying Network Access Service (NAS)" problem as well as the "lying
provider" problem.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6677.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
4. Channel Bindings . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Types of EAP Channel Bindings . . . . . . . . . . . . . . 8
4.2. Channel Bindings in the Secure Association Protocol . . . 9
4.3. Channel-Binding Scope . . . . . . . . . . . . . . . . . . 10
5. Channel-Binding Process . . . . . . . . . . . . . . . . . . . 12
5.1. Protocol Operation . . . . . . . . . . . . . . . . . . . . 12
5.2. Channel-Binding Consistency Check . . . . . . . . . . . . 14
5.3. EAP Protocol . . . . . . . . . . . . . . . . . . . . . . . 15
5.3.1. Channel-Binding Codes . . . . . . . . . . . . . . . . 17
5.3.2. Namespace Identifiers . . . . . . . . . . . . . . . . 17
5.3.3. RADIUS Namespace . . . . . . . . . . . . . . . . . . . 18
6. System Requirements . . . . . . . . . . . . . . . . . . . . . 18
6.1. General Transport Protocol Requirements . . . . . . . . . 18
6.2. EAP Method Requirements . . . . . . . . . . . . . . . . . 19
7. Channel-Binding TLV . . . . . . . . . . . . . . . . . . . . . 19
7.1. Requirements for Lower-Layer Bindings . . . . . . . . . . 19
7.2. EAP Lower-Layer Attribute . . . . . . . . . . . . . . . . 20
8. AAA-Layer Bindings . . . . . . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9.1. Trust Model . . . . . . . . . . . . . . . . . . . . . . . 21
9.2. Consequences of Trust Violation . . . . . . . . . . . . . 23
9.3. Bid-Down Attacks . . . . . . . . . . . . . . . . . . . . . 24
9.4. Privacy Violations . . . . . . . . . . . . . . . . . . . . 24
10. Operations and Management Considerations . . . . . . . . . . . 25
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
11.1. EAP Lower Layers Registry . . . . . . . . . . . . . . . . 26
11.2. RADIUS Registration . . . . . . . . . . . . . . . . . . . 26
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
13.1. Normative References . . . . . . . . . . . . . . . . . . . 27
13.2. Informative References . . . . . . . . . . . . . . . . . . 27
Appendix A. Attacks Prevented by Channel Bindings . . . . . . . . 29
A.1. Enterprise Subnetwork Masquerading . . . . . . . . . . . . 29
A.2. Forced Roaming . . . . . . . . . . . . . . . . . . . . . . 29
A.3. Downgrading Attacks . . . . . . . . . . . . . . . . . . . 30
A.4. Bogus Beacons in IEEE 802.11r . . . . . . . . . . . . . . 30
A.5. Forcing False Authorization in IEEE 802.11i . . . . . . . 30
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 peers. Another possibility
is that the same false information could be provided to both the EAP
peer and EAP server by the NAS. A "lying" entity can also be located
anywhere on the AAA path between the NAS and the EAP server.
This problem results when the same credentials are used to access
multiple services that differ in some interesting property. The EAP
server learns which client credentials are in use. The client knows
which EAP credentials are used, but cannot distinguish between
servers that use those credentials. For methods that distinguish
between client and server credentials, either using different server
credentials for access to the different services or having client
credentials with access to a disjoint set of services can potentially
defend against the attack.
As a concrete example, consider an organization with two different
IEEE 802.11 wireless networks. One is a relatively low-security
network for accessing the web, while the other has access to valuable
confidential information. An access point on the web network could
act as a lying NAS, sending the Service Set Identifier (SSID) of the
confidential network in its beacons. This access point could gain an
advantage by doing so if it tricks clients that intend to connect to
the confidential network to connect to it and disclose confidential
information.
A similar problem can be observed in the context of roaming. Here,
the lying entity is located in a visited service provider network,
e.g., attempting to lure peers to connect to the network based on
falsely advertised roaming rates. This is referred to as the "lying
provider" problem in the remainder of this document. The lying
entity's motivation often is financial; the entity may be paid
whenever peers roam to its service. However, a lying entity in a
provider network can also gain access to traffic that it might not
otherwise see.
This document defines and implements EAP channel bindings to solve
the "lying NAS" and the "lying provider" problems, using a process in
which the EAP peer gives 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 that is consistent
with the information received from this authenticator as well as the
information stored about this authenticator. "AAA Payloads" defined
in [AAA-PAY] served as the starting point for the mechanism proposed
in this specification 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
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 an EAP authentication compliant with [RFC4017], the EAP peer 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 both in terms of
network distance and number of entities who need to be trusted in
order to establish trusted communication. A malicious or compromised
authenticator may represent incorrect information about the network
to the peer in an effort to affect its operation in some way.
Additionally, while an authenticator may not be compromised, other
compromised elements in the network (such as proxies) could provide
false information to the authenticator that it could simply be
relaying to EAP peers. Hence, the goal must be to ensure that the
authenticator is providing correct information to the EAP peer 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,
assuming a single administrative domain, it is 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 peer is outside
its home administrative domain, the goal is to ensure that the level
of service received by the peer is consistent with the contractual
agreement between the two service providers. The same EAP server may
need to support both types of networks. For example an enterprise
may have a roaming agreement permitting its users to use the networks
of third-party service providers. In these situations, the EAP
server may authenticate for an enterprise and provider network.
The following are example attacks possible by presenting false
network information to peers.
o Enterprise network: A corporate network may have multiple virtual
LANs (VLANs) available 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 but not the corporate
network could advertise the SSID of the corporate network in an
effort to lure peers 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.
o Enterprise network: The EAP Generic Security Service Application
Program Interface (GSS-API) mechanism [GSS-API-EAP] mechanism
provides a way to use EAP to authenticate to mail servers, instant
messaging servers, and other non-network services. Without EAP
channel binding, an attacker could trick the user into connecting
to a relatively untrusted service instead of a relatively trusted
service. For example, the instant messaging service could
impersonate the mail server.
o Service provider network: An EAP-enabled mobile phone provider
could advertise very competitive flat rates but send per-minute
rates to the home server, thus luring peers to connect to their
network and overcharging them. In more elaborate attacks, peers
can be tricked into roaming without their knowledge. For example,
a mobile phone provider operating along a geopolitical 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. These types of scenarios can be considered as
the "lying provider" problem, because here the provider configures
its NAS to broadcast false information. For the purpose of
channel bindings as defined in this document, 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 enable the determination of which entity is
lying. Naturally, channel bindings for EAP methods can only
verify the endpoints; if desirable, intermediate hops need to be
protected by the employed AAA protocol.
o Enterprise and provider networks: In a situation where an
enterprise has roaming agreements with providers, a compromised
access point in a provider network could masquerade as the
enterprise network in an attempt to gain confidential information.
Today this could potentially be solved by using different
credentials for internal and external access. Depending on the
type of credential, this may introduce usability or man-in-the-
middle security issues.
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 peer 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 endpoints of a multi-layer
protocol, allowing lower layers to protect data from higher layers.
Unlike [RFC5056], EAP channel bindings do not ensure the binding of
different layers of a session; rather, they ensure the accuracy of
the information advertised to an EAP peer by an authenticator acting
as the pass-through device during an EAP execution. The term
"channel bindings" was independently adopted for these two related
concepts; by the time the conflict was discovered, a wide body of
literature existed for each usage. EAP channel bindings could be
used to provide [RFC5056] channel bindings. In particular, an inner
EAP method could be bound to an outer method by including the
[RFC5056] channel-binding data for the outer channel in the inner EAP
method's channel bindings. Doing so would provide a facility similar
to EAP cryptographic binding, except that a man-in-the-middle could
not extract the inner method from the tunnel. This specification
does not weigh the advantages of doing so nor specify how to do so;
the example is provided only to illustrate how EAP channel binding
and [RFC5056] channel binding overlap.
4.1. Types of EAP Channel Bindings
There are two categories of approach 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
consistency and correctness.
o The peer and server can both uniquely encode their respective view
of the network information without exchanging it, resulting into
an opaque blob that can be included directly into the derivation
of EAP session keys.
Both approaches are only applicable to key-deriving EAP methods and
both have advantages and disadvantages. Various hybrid approaches
are also possible. 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 more extensive 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 effect 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 yield different session keys on the EAP peer and
server.
4.2. Channel Bindings in the Secure Association Protocol
This document describes channel bindings performed by transporting
channel-binding information as part of an integrity-protected
exchange within an EAP method. Alternatively, some future document
could specify a mechanism for transporting channel bindings within
the lower layer's secure association protocol. Such a specification
would need to describe how channel bindings are exchanged over the
lower-layer protocol between the peer and authenticator. In
addition, since the EAP exchange concludes before the secure
association protocol begins, a mechanism for transporting the channel
bindings from the authenticator to the EAP server needs to be
specified. A mechanism for transporting a protected result from the
EAP server, through the authenticator, back to the peer needs to be
specified.
The channel bindings MUST be transported with integrity protection
based on a key known only to the peer and EAP server. The channel
bindings SHOULD be confidentiality protected using a key known only
to the peer and EAP server. For the system to function, the EAP
server or AAA server needs access to the channel-binding information
from the peer as well as the AAA attributes and a local database
described later in this document.
The primary advantage of sending channel bindings as part of the
secure association protocol is that EAP methods need not be changed.
The disadvantage is that a new AAA exchange is required, and secure
association protocols need to be changed. As the results of the
secure association protocol change, every NAS needs to be upgraded to
support channel bindings within the secure association protocol.
For many deployments, changing all the NASes is expensive, and adding
channel-binding support to enough EAP methods to meet the goals of
the deployment will be cheaper. However for deployment of new
equipment, or especially deployment of a new lower-layer technology,
changing the NASes may be cheaper than changing EAP methods.
Especially if such a deployment needed to support a large number of
EAP methods, sending channel bindings in the secure association
protocol might make sense. Sending channel bindings in the secure
association protocol can work even with the EAP Re-authentication
Protocol (ERP) [RFC5296] in which previously established EAP key
material is used for the secure association protocol without carrying
out any EAP method during re-authentication.
If channel bindings using a secure association protocol are
specified, semantics as well as the set of information that peers
exchange can be shared with the mechanism described in this document.
4.3. Channel-Binding Scope
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., Medium Access Control (MAC) address,
supported link types and 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 about the
authenticator and/or network to which the peer is connected the home
EAP server is expected to have access to. In roaming cases, the home
server is likely to only have access to information contained in
their roaming agreements.
With any multi-hop AAA infrastructure, many of the NAS-specific AAA
attributes 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
AAA information received from the last hop may not contain much
verifiable information after transformations performed by AAA
proxies. For example, information carried in AAA attributes such as
the NAS IP address may have been lost in transition and thus are not
known to the EAP server. Even worse, information may still be
available but be useless, for example, representing the identity of a
device on a private network or a middlebox. 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, the
best approach is 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 peers connecting to the corporate
intranet are using secure link-layer encryption, while link-layer
security requirements for peers 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.
The properties of the network that the peer wishes to validate depend
on the specific deployment. 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, the administrators of a
peer may be more concerned with specifics of where their network
traffic is being routed and what VLAN is in use. To establish
policies surrounding these requirements, administrators would capture
some attribute such as SSID to describe the properties of the network
they care about. Channel bindings could validate the SSID. The
administrator would need to make sure that the network guarantees
that when an authenticator trusted by the AAA infrastructure to offer
a particular SSID to clients does offer this SSID, that network has
the intended properties. Generally, it is not possible for channel
bindings to detect lying NAS behavior when the NAS is authorized to
claim a particular service. That is, if the same physical
authenticator is permitted to advertise two networks, the AAA
infrastructure is unlikely to be able to determine when this
authenticator lies.
As discussed in the next section, some of the most important
information to verify cannot come from AAA attributes but instead
comes from local configuration. For example, in the mobile phone
case, the expected roaming rate cannot come from the roaming provider
without being verified against the contract between the two
providers. Similarly, in an enterprise, the SSID that a particular
access point is expected to advertise comes from configuration rather
than an AAA exchange (which can be confirmed with channel binding).
The peer and authenticator do not initially have a basis for trust.
The peer has a credential with the EAP server that forms a basis for
trust. The EAP server and authenticator have a potentially indirect
trust path using the AAA infrastructure. Channel binding leverages
the trust between the peer and EAP server to build trust in certain
attributes between the peer and authenticator.
Channel bindings can be important for forming areas of trust,
especially when provider networks are involved, and exact information
is not available to the EAP server. Without channel bindings, all
entities in the system need to be held to the standards of the most
trusted entity that could be accessed using the EAP credential.
Otherwise, a less trusted entity can impersonate a more trusted
entity. However when channel bindings are used, the EAP server can
use information supplied by the peer, AAA protocols and local
database to distinguish less trusted entities from more trusted
entities. One possible deployment involves being able to verify a
number of characteristics about relatively trusted entities while for
other entities simply verifying that they are less trusted.
Any deployment of channel bindings should take into consideration
both what information the EAP server is likely to know or have access
to, and what type of network information the peer would want and need
authenticated.
5. Channel-Binding Process
This section defines the process 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 (see
Section 4.1). In the first subsection, the general communication
infrastructure is outlined, the messages used for channel-binding
verifications are specified, and the protocol flows are defined. The
second subsection explores the difficulties of checking the different
pieces of information that are exchanged during the channel-binding
protocol for consistency. The third subsection describes the
information carried in the EAP exchange.
5.1. Protocol Operation
Channel bindings are always provided between two communication
endpoints (here, the EAP peer and the EAP server), who communicate
through an authenticator typically in pass-through mode.
Specifications treat the AAA server and EAP server as distinct
entities. However, there is no standardized protocol for the AAA
server and EAP server to communicate with each other. For the
channel-binding protocol presented in this document to work, the EAP
server needs to be able to access information from the AAA server
that is utilized during the EAP session (i2 below) and a local
database. For example, the EAP server and the local database can be
co-located with the AAA server, as illustrated in Figure 1. An
alternate architecture would be to provide a mechanism for the EAP
server to inform the AAA server what channel-binding attributes were
supplied and the AAA server to inform the EAP server about what
channel-binding attributes it considered when making its decision.
+ -------------------------+
-------- ------------- | ---------- ______ |
|EAP peer|<---->|Authenticator|<--> | |EAP Server|___(______) |
-------- ------------- | ---------- | DB | |
. . |AAA (______) |
. i1 . +--------------------------+
.<----------------. i2 . .
. .------------> .
. i1 .
.-------------------------------------->.
. CB_success/failure(i1, i2,info) .
.<--------------------------------------.
Figure 1: Overview of Channel-Binding Protocol
During network advertisement, selection, and authentication, the
authenticator presents unauthenticated information, labeled i1, about
the network to the peer. Message 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. Information (such as the type of media in use)
may be communicated implicitly in i1. 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 in the form of AAA attributes about
itself and the current request. These AAA attributes may or may not
contain accurate information. This information is labeled i2.
Message i2 is the information the AAA server receives from the last
hop in the AAA proxy chain which is not necessarily the
authenticator.
AAA hops between the authenticator and AAA server can validate some
of i2. Whether the AAA server will be able to rely on this depends
significantly on the business relationship executed with these
proxies and on the structure of the AAA network.
The local database is perhaps the most important part of this system.
In order for the EAP server or AAA server to know whether i1 and i2
are correct, they need access to trustworthy information, since an
authenticator could include false information in both i1 and i2.
Additional reasons why such a database is necessary for channel
bindings to work are discussed in the next subsection. The
information contained within the database could involve wildcards.
For example, this could be used to check whether IEEE 802.11 access
points on a particular IP subnet all use a specific SSID. The exact
IP address is immaterial, provided it is on the correct subnet.
During an EAP method execution with channel bindings, the peer sends
i1 to the EAP server using the mechanism described in Section 5.3.
The EAP server verifies the consistency of i1 provided by the peer,
i2 provided by the authenticator, and the information in the local
database. Upon the check, the EAP server sends a message to the peer
indicating whether the channel-binding validation check succeeded or
failed and includes the attributes that were used in the check. The
message flow is illustrated in Figure 1.
Above, the EAP server is described as performing the channel-binding
validation. In most deployments, this will be a necessary
implementation constraint. The EAP exchange needs to include an
indication of channel-binding success or failure. Most existing
implementations do not have a way to have an exchange between the EAP
server and another AAA entity during the EAP server's processing of a
single EAP message. However, another AAA entity can provide
information to the EAP server to make its decision.
If the compliance of i1 or i2 information with the authoritative
policy source is mandatory and a consistency check failed, then after
sending a protected indication of failed consistency, the EAP server
MUST send an EAP-Failure message to terminate the session. If the
EAP server is otherwise configured, it MUST allow the EAP session to
complete normally and leave the decision about network access up to
the peer's policy. If i1 or i2 does not comply with policy, the EAP
server MUST NOT list information that failed to comply in the set of
information used to perform channel binding. In this case, the EAP
server SHOULD indicate channel-binding failure; this requirement may
be upgraded to a MUST in the future.
5.2. Channel-Binding Consistency Check
The validation check that is the core of the channel-binding protocol
described in the previous subsection consists of two parts in which
the server checks whether:
1. the authenticator is lying to the peer, i.e., i1 contains false
information, and
2. the authenticator or any entity on the AAA path to the AAA server
provides false information in form of AAA attributes, i.e., i2
contains false information.
These checks enable the EAP server to detect lying NASes or
authenticators in enterprise networks and lying providers in service
provider networks.
Checking the consistency of i1 and i2 is nontrivial, as has been
pointed out already in [HC07]. First, i1 can contain any type of
information propagated by the authenticator, whereas i2 is restricted
to information that can be carried in AAA attributes. Second,
because the authenticator typically communicates over different link
layers with the peer and the AAA infrastructure, different types of
identifiers and addresses may have been presented to both
communication endpoints. Whether these different identifiers and
addresses belong to the same device cannot be directly checked by the
EAP server or AAA server without additional information. Finally, 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 i1 information available to the EAP server can only be verified
against the last-hop portion of i2 or against values propagated by
proxy servers. In addition, checking the consistency of i1 and i2
alone is insufficient because an authenticator could lie to both the
peer and the EAP server, i.e., i1 and i2 may be consistent but both
contain false information.
A local database is required to leverage the above-mentioned
shortcomings and support the consistency and validation checks. In
particular, information stored for each NAS/authenticator (enterprise
scenario) or each roaming partner (service provider scenario) enables
a comparison of any information received in i1 with AAA attributes in
i2 as well as additionally stored AAA attributes that might have been
lost in transition. Furthermore, only such a database enables the
EAP server and AAA server to check the received information against
trusted information about the network including roaming agreements.
Section 7 describes lower-layer-specific properties that can be
exchanged as a part of i1. Section 8 describes specific AAA
attributes that can be included and evaluated in i2. The EAP server
reports back the results from the channel-binding validation check
that compares the consistency of all the values with those in the
local database. The challenges of setting up such a local database
are discussed in Section 10.
5.3. EAP Protocol
EAP methods supporting channel binding consistent with this
specification provide a mechanism for carrying channel-binding data
from the peer to the EAP server and a channel-binding response from
the EAP server to the peer. The specifics of this mechanism are
dependent on the method, although the content of the channel-binding
data and channel-binding response are defined by this section.
Typically the lower layer will communicate a set of attributes to the
EAP implementation on the peer that should be part of channel
binding. The EAP implementation may need to indicate to the lower
layer that channel-binding information cannot be sent. Reasons for
failing to send channel-binding information include an EAP method
that does not support channel binding is selected, or channel-binding
data is too big for the EAP method selected. Peers SHOULD provide
appropriate policy controls to select channel binding or mandate its
success.
The EAP server receives the channel-binding data and performs the
validation. The EAP method provides a way to return a response; the
channel-binding response uses the same basic format as the channel-
binding data.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | NSID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NS-Specific...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | NSID | NS-Specific...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Channel-Binding Encoding
Both the channel-binding data and response use the format illustrated
in Figure 2. The protocol starts with a one-byte code; see
Section 5.3.1. Then, for each type of attribute contained in the
channel-binding data, the following information is encoded:
Length: Two octets of length in network byte order, indicating the
length of the NS-Specific data. The NSID and length octets are
not included.
NSID: Namespace identifier. One octet describing the namespace from
which the attributes are drawn. See Section 5.3.3 for a
description of how to encode RADIUS attributes in channel-binding
data and responses. RADIUS uses a namespace identifier of 1 .
NS-Specific: The encoding of the attributes in a manner specific to
the type of attribute.
A given NSID MUST NOT appear more than once in a channel-binding data
or channel-binding response. Instead, all NS-Specific data for a
particular NSID must occur inside one set of fields (NSID, Length,
and NS-Specific). This set of fields may be repeated if multiple
namespaces are included.
In channel-binding data, the code is set to 1 (channel-binding data),
and the full attributes and values that the peer wishes the EAP
server to validate are included.
In a channel-binding response, the server selects the code; see
Section 5.3.1. For successful channel binding, the server returns
code 2. The set of attributes that the EAP server returns depend on
the code. For success, the server returns the attributes that were
considered by the server in making the determination that channel
bindings are successfully validated; attributes that the server is
unable to check or that failed to validate against what is sent by
the peer MUST NOT be returned in a success response. Generally,
servers will not return a success response if any attributes were
checked and failed to validate those specified by the peer. Special
circumstances such as a new attribute being phased in at a server MAY
require servers to return success when such an attribute fails to
validate. The server returns the value supplied by the peer when
returning an attribute in channel-binding responses.
For channel-binding failure (code 3), the server SHOULD include any
attributes that were successfully validated. This code means that
server policy indicates that the attributes sent by the client do not
accurately describe the authenticator. Servers MAY include no
attributes in this response; for example, if the server checks the
attributes supplied by the peer and they fail to be consistent, it
may send a response without attributes.
Peers MUST treat unknown codes as channel-binding failure. Peers
MUST ignore differences between attribute values sent in the channel-
binding data and those sent in the response. Peers and servers MUST
ignore any attributes contained in a field with an unknown NSID.
Peers MUST ignore any attributes in a response not present in the
channel-binding data.
5.3.1. Channel-Binding Codes
+------+-----------------------------------+
| Code | Meaning |
+------+-----------------------------------+
| 1 | Channel-binding data from client |
| 2 | Channel-binding response: success |
| 3 | Channel-binding response: failure |
+------+-----------------------------------+
5.3.2. Namespace Identifiers
+-----+--------------------------+---------------+
| ID | Namespace | Reference |
+-----+--------------------------+---------------+
| 1 | RADIUS | Section 5.3.3 |
| 255 | Reserved for Private Use | |
+-----+--------------------------+---------------+
5.3.3. RADIUS Namespace
RADIUS attribute-value pairs (AVPs) are encoded with a one-octet
attribute type followed by a one-octet length followed by the value
of the RADIUS attribute being encoded. The length includes the type
and length octets; the minimum legal length is 3. Attributes are
concatenated to form the namespace-specific portion of the packet.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Type | Length | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Figure 3: RADIUS AVP Encoding
The full value of an attribute is included in the channel-binding
data and response.
6. System Requirements
This section defines requirements on components used to implement the
channel-bindings protocol.
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. This document describes a
protocol for performing channel binding within EAP methods. As
discussed in Section 4.2, an alternative approach for meeting this
requirement is to perform channel bindings during the secure
association protocol of the lower layer.
6.1. General Transport Protocol Requirements
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 is that the
channel bindings could contain private information, including peer
identities, which SHOULD be protected. If confidentiality cannot be
provided, private information MUST NOT be sent as part of the
channel-binding information.
Any transport needs to be careful not to exceed the MTU for its
lower-layer medium. In particular, if channel-binding information is
exchanged within protected EAP method channels, these methods may or
may not support fragmentation. In order to work with all methods,
the channel-binding messages must fit within the available payload.
For example, if the EAP MTU is 1020 octets, and EAP - Generalized
Pre-Shared Key (EAP-GPSK) is used as the authentication method, and
maximal-length identities are used, a maximum of 384 octets is
available for conveying channel-binding information. Other methods,
such as EAP Tunneled Transport Layer Security (EAP-TTLS), support
fragmentation and could carry significantly longer payloads.
6.2. EAP Method Requirements
When transporting data directly within an EAP method, the method MUST
be able to carry integrity-protected data from the EAP peer to server
and from EAP server to peer. EAP methods MUST exchange channel-
binding data with the AAA subsystem hosting the EAP server. EAP
methods MUST be able to import channel-binding data from the lower
layer on the EAP peer.
7. Channel-Binding TLV
This section defines some channel-binding TLVs. While message i1 is
not limited to AAA attributes, for the sake of tangible attributes
that are already in place, this section discusses AAA AVPs that are
appropriate for carrying channel bindings (i.e., data from i1 in
Section 5).
For any lower-layer protocol, network information of interest to the
peer and server can be encapsulated in AVPs or other defined payload
containers. The appropriate AVPs depend on the lower-layer protocol
as well as on the network type (i.e., enterprise network or service
provider network) and its application.
7.1. Requirements for Lower-Layer Bindings
Lower-layer protocols MUST support EAP in order to support EAP
channel bindings. These lower layers MUST support EAP methods that
derive keying material, as otherwise no integrity-protected channel
would be available to execute the channel-bindings protocol. Lower-
layer protocols need not support traffic encryption, since this is
independent of the authentication phase.
The data conveyed within the AVP type MUST NOT conflict with the
externally defined usage of the AVP. Additional TLV types MAY be
defined for values that are not communicated within AAA attributes.
In general, lower layers will need to specify what information should
be included in i1. Existing lower layers will probably require new
documents to specify this information. Lower-layer specifications
need to include sufficient information in i1 to uniquely identify
which lower layer is involved. The preferred way to do this is to
include the EAP-Lower-Layer attribute defined in the next section.
This MUST be included in i1 unless an attribute specific to a
particular lower layer is included in i1.
7.2. EAP Lower-Layer Attribute
A new RADIUS attribute is defined to carry information on which EAP
lower layer is used for this EAP authentication. This attribute
provides information relating to the lower layer over which EAP is
transported. This attribute MAY be sent by the NAS to the RADIUS
server in an Access-Request or an Accounting-Request packet. A
summary of the EAP-Lower-Layer attribute format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Value (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The code is 163, the length is 6, and the value is a 32-bit unsigned
integer in network byte order. The value specifies the EAP lower
layer in use. Values are taken from the IANA registry established in
Section 11.1.
8. AAA-Layer Bindings
This section discusses which AAA attributes in a AAA Access-Request
message can and should be validated by a EAP server (i.e., data from
i2 in 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 to act 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.
NAS-IP-Address: This value is typically the IP address of the
authenticator; however, 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.
NAS-IPv6-Address: This value is typically the IPv6 address of the
authenticator; however, in a proxied connection, it likely will
not match the source IPv6 address of an Access-Request. A
consistency check MAY verify the subnet of the IPv6 address was
correct based on the last-hop proxy.
NAS-Identifier: This is an identifier populated by the NAS to
identify the NAS to the AAA server; it SHOULD be validated against
the local database.
NAS-Port-Type: This specifies the underlying link technology. It
SHOULD be validated against the value received from the peer in
the information exchange and against a database of authorized
link-layer technologies.
9. Security Considerations
This section discusses security considerations surrounding the use of
EAP channel bindings.
9.1. Trust Model
In the considered trust model, EAP peer and authentication server are
honest, while the authenticator is maliciously sending false
information to peer and/or server. In the model, the peer and server
trust each other, which is not an unreasonable assumption,
considering they already have a trust relationship. The following
are the trust relationships:
o The server trusts that the channel-binding information received
from the peer is the information that the peer received from the
authenticator.
o The peer 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 peer and server
o derivation of keying material including a key for integrity
protection of channel-binding messages known to the peer and EAP
server but not the authenticator
o transmission of the channel-binding request from peer to server
over an integrity-protected channel
o transmission of the channel-binding result from server to peer
over an integrity-protected channel
This trust model is a significant departure from the standard EAP
model. In many EAP deployments today, attacks where one
authenticator can impersonate another are not a significant concern
because all authenticators provide the same service. A authenticator
does not gain significant advantage by impersonating another
authenticator. The use of EAP in situations where different
authenticators provide different services may give an attacker who
can impersonate a authenticator greater advantage. The system as a
whole needs to be analyzed to evaluate cases where one authenticator
may impersonate another and to evaluate the impact of this
impersonation.
One attractive implementation strategy for channel binding is to add
channel-binding support to a tunnel method that can tunnel an inner
EAP authentication. This way, channel binding can be achieved with
any method that can act as an inner method even if that inner method
does not have native channel-binding support. The requirement for
mutual authentication and key derivation is at the layer of EAP that
actually performs the channel binding. Tunnel methods sometimes use
cryptographic binding, a process where a peer proves that the peer
for the outer method is the same as the peer for an inner method to
tie authentication at one layer together with an inner layer.
Cryptographic binding does not always provide mutual authentication;
its definition does not require the server to prove that the inner
server and outer server are the same. Even when cryptographic
binding does attempt to confirm that the inner and outer server are
the same, the Master Session Key (MSK) from the inner method is
typically used to protect the binding. An attacker such as an
authenticator that wishes to subvert channel binding could establish
an outer tunnel terminating at the authenticator. If the outer
method tunnel terminates on the authenticator, the MSK is disclosed
to the authenticator, which can typically attack cryptographic
binding. If the authenticator controls cryptographic binding, then
it typically controls the channel-binding parameters and results. If
the channel-binding process is used to differentiate one
authenticator from another, then the authenticator can claim to
support services that it was not authorized to. This attack was not
in scope for existing threat models for cryptographic binding because
differentiated authenticators was not a consideration. Thus,
existing cryptographic binding does not typically provide mutual
authentication of the inner-method server required for channel
binding. Other methods besides cryptographic binding are available
to provide mutual authentication required by channel binding. As an
example, if server certificates are validated and names checked,
mutual authentication can be provided directly by the tunnel.
9.2. Consequences of Trust Violation
If any of the trust relationships listed in Section 9.1 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.
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. A peer could potentially gain an
advantage in auditing or charging if both are valid and information
from i1' is used for auditing or charging. Such peers can be
detected by including the information in i2 and checking i1 against
i2.
If information from i1 does not validate, an EAP server cannot
generally determine whether the authenticator advertised incorrect
information or whether the peer is dishonest. This should be
considered before using channel-binding validation failures to
determine the reputation either of the peer or authenticator.
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 or not
channel binding is offered. On the other hand, dishonest servers can
claim a successful validation even if i1 contains invalid
information. 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 that make 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 cannot manipulate a
single channel-binding message or 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.
9.3. Bid-Down Attacks
EAP methods that add channel binding will typically negotiate its
use. Even for entirely new EAP methods designed with channel binding
from the first version, some deployments may not use it. It is
desirable to protect against attacks on the negotiation of channel
bindings. An attacker including the NAS SHOULD NOT be able to
prevent a peer and server who support channel bindings from using
them.
Unfortunately, existing EAP methods may make it difficult or
impossible to protect against attacks on negotiation. For example,
many EAP state machines will accept a success message at any point
after key derivation to terminate authentication. EAP success
messages are not integrity protected; an attacker who could insert a
message can generate one. The NAS is always in a position to
generate a success message. Common EAP servers take advantage of
state machines accepting success messages even in cases where an EAP
method might support a protected indication of success. It may be
challenging to define channel-binding support for existing EAP
methods in a manner that permits peers to distinguish an old EAP
server that sends a success indication and does not support channel
binding from an attacker injecting a success indication.
9.4. Privacy Violations
While the channel-binding information exchanged between EAP peer and
EAP server (i.e., i1 and the 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. If encryption is not
available, private information is not sent as part of the channel-
binding information, as described in Section 6.1.
Privacy implications of attributes selected for channel binding need
to be considered. Consider channel binding the username attribute.
A peer sends a privacy protecting anonymous identifier in its EAP
identity message, but sends the full username in the protected i1
message. However, the authenticator would like to learn the full
username. It makes a guess and sends that in i2 rather than the
anonymous identifier. If the EAP server validates this attribute and
fails when the username from the peer mismatches i2, then the EAP
server confirms the authenticator's guess. Similar privacy exposures
may result whenever one party is in a position to guess channel-
binding information provided by another party.
10. Operations and Management Considerations
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. This section
discusses 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 database may already
exist on the AAA server, in which case the EAP server passes the
parameters to the AAA server for validation. The EAP methods need to
be able to export received channel-binding information to the EAP
server so it can be validated.
11. IANA Considerations
A new top-level registry has been created for "Extensible
Authentication Protocol (EAP) Channel Binding Parameters". This
registry consists of several sub-registries.
The "EAP Channel-Binding Codes" sub-registry defines values for the
code field in the channel-binding data and channel-binding response
packet. See the table in Section 5.3.1 for initial registrations.
This registry requires Standards Action [RFC5226] for new
registrations. Early allocation [RFC4020] is allowed. An additional
reference column has been added to the table for the registry,
pointing all codes in the initial registration to this specification.
Valid values in this sub-registry range from 0-255; 0 is reserved.
The "EAP Channel-Binding Namespaces" sub-registry contains
registrations for the NSID field in the channel-binding data and
channel-binding response. Initial registrations are found in the
table in Section 5.3.2. Registrations in this registry require IETF
Review. Valid values range from 0-255; 0 is reserved. As with the
"EAP Channel-Binding Codes" sub-registry, a reference column has been
included to point to this document for initial registrations.
11.1. EAP Lower Layers Registry
A new sub-registry in the EAP Numbers registry at
http://www.iana.org/assignments/eap-numbers has been created for EAP
Lower Layers. Registration requires Expert Review [RFC5226]; the
primary role of the expert is to prevent multiple registrations for
the same lower layer.
The following table gives the initial registrations for this
registry.
+-------+----------------------------------------+
| Value | Lower Layer |
+-------+----------------------------------------+
| 1 | Wired IEEE 802.1X |
| 2 | IEEE 802.11 (no-pre-auth) |
| 3 | IEEE 802.11 (pre-authentication) |
| 4 | IEEE 802.16e |
| 5 | IKEv2 |
| 6 | PPP |
| 7 | PANA (no pre-authentication) [RFC5191] |
| 8 | GSS-API [GSS-API-EAP] |
| 9 | PANA (pre-authentication) [RFC5873] |
+-------+----------------------------------------+
11.2. RADIUS Registration
A new RADIUS attribute is registered with the name EAP-Lower-Layer;
163. The RADIUS attributes are in the registry at
http://www.iana.org/assignments/radius-types.
12. Acknowledgments
The authors and editor would like to thank Bernard Aboba, Glen Zorn,
Joe Salowey, Stephen Hanna, and Klaas Wierenga for their valuable
inputs that helped to improve and shape this document over the time.
Sam Hartman's work on this specification is funded by JANET(UK).
The EAP-Lower-Layer attribute was taken from "RADIUS Attributes for
IEEE 802 Networks" [RADIUS-WLAN].
13. References
13.1. Normative References
[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.
[RFC4020] Kompella, K. and A. Zinin, "Early IANA Allocation of
Standards Track Code Points", BCP 100, RFC 4020,
February 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008.
13.2. Informative References
[AAA-PAY] Clancy, T., Lior, A., Ed., Zorn, G., and K. Hoeper,
"EAP Method Support for Transporting AAA Payloads",
Work in Progress, May 2010.
[GSS-API-EAP] Hartman, S., Ed. and J. Howlett, "A GSS-API Mechanism
for the Extensible Authentication Protocol", Work in
Progress, June 2012.
[HC07] Hoeper, K. and L. Chen, "Where EAP Security Claims
Fail", Institute for Computer Sciences, Social
Informatics and Telecommunications Engineering
(ICST), The Fourth International Conference on
Heterogeneous Networking for Quality, Reliability,
Security and Robustness (QShine 2007), August 2007.
[RADIUS-WLAN] Aboba, B., Malinen, J., Congdon, P., and J. Salowey,
"RADIUS Attributes for IEEE 802 Networks", Work in
Progress, October 2011.
[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.
[RFC5191] Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and
A. Yegin, "Protocol for Carrying Authentication for
Network Access (PANA)", RFC 5191, May 2008.
[RFC5296] Narayanan, V. and L. Dondeti, "EAP Extensions for EAP
Re-authentication Protocol (ERP)", RFC 5296,
August 2008.
[RFC5873] Ohba, Y. and A. Yegin, "Pre-Authentication Support for
the Protocol for Carrying Authentication for Network
Access (PANA)", RFC 5873, May 2010.
Appendix A. Attacks Prevented by Channel Bindings
In the following appendix, 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). This document
only provides part of the solution necessary to realize a defense
against these attacks. In addition, lower-layer protocols need to
describe what attributes should be included in channel-binding
requests. EAP methods need to be updated in order to describe how
the channel-binding request and response are carried. In addition,
deployments may need to decide what information is populated in the
local database. The following sections describe types of attacks
that can be prevented by this framework with appropriate lower-layer
attributes carried in channel bindings, EAP methods with channel-
binding support, and appropriate local database information at the
EAP server.
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 peers to believe that they are accessing
the network over secure connections and, e.g., transmit confidential
information that they normally would not send over a weakly protected
connection. This attack works under the conditions that peers 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
peers to connect to the guest network, because the peers used
credentials and/or an EAP method that is associated with the
corporate network.
A.2. Forced Roaming
Mobile phone providers boosting their cell towers' 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 that they are roaming.
Channel bindings would detect the bogus network identifier because
the network identifier sent to the authentication server in i1 will
match neither 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 the EAP MD5 Challenge Handshake
Authentication Protocol (EAP-MD5-CHAP), the Flexible Authentication
via Secure Tunneling EAP (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. This presumes that all
acceptable EAP methods support channel binding and that an attacker
cannot break the EAP method in real-time.
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 document, 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
peer believe it is connected to a network different from the one the
peer 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 peer got access to.
Both attacks can be prevented by implementing channel bindings,
because the server can compare the information sent to the peer, the
information it received from the authenticator during the AAA
communication, and the information stored in the policy database.
Authors' Addresses
Sam Hartman (editor)
Painless Security
356 Abbott St.
North Andover, MA 01845
USA
EMail: hartmans-ietf@mit.edu
T. Charles Clancy
Virginia Polytechnic Institute and State University
Electrical and Computer Engineering
900 North Glebe Road
Arlington, VA 22203
USA
EMail: tcc@vt.edu
Katrin Hoeper
Motorola Solutions, Inc.
1301 E. Algonquin Road
Schaumburg, IL 60196
USA
EMail: khoeper@motorolasolutions.com
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