a Diameter EAP-Master-Session-Key AVP.
o If the Diameter-EAP-Answer will have a successful result code, the
saved state (see above) can be used to construct an
Accounting-EAP-Auth-Method AVP.
6.3. Accounting Requests
In Accounting-Requests, the vendor-specific RADIUS MS-Acct-EAP-Type
attribute [RFC2548] can be translated to a Diameter
Accounting-EAP-Auth-Method AVP, and vice versa.
When translating from Diameter to RADIUS, note that the
MS-Acct-EAP-Type attribute does not support expanded EAP types. Type
values greater than 255 should be translated to type 254.
7. IANA Considerations
This document does not create any new namespaces to be maintained by
IANA, but it requires new values in namespaces that have been defined
in the Diameter Base protocol and RADIUS specifications.
o This document defines one new Diameter command (in Section 3)
whose Command Code is allocated from the Command Code namespace
defined in [BASE]. The Command Code for DER / DEA is 268.
o This document defines four new AVPs whose AVP Codes are allocated
from the AVP Code namespace defined in [BASE] as follows:
462 for EAP-Payload (defined in Section 4.1.1),
463 for EAP-Reissued-Payload (defined in Section 4.1.2),
464 for EAP-Master-Session-Key (defined in Section 4.1.3), and
465 for Accounting-EAP-Auth-Method (defined in Section 4.1.5).
o This document defines one new AVP (attribute) whose AVP Code
(Attribute Type) is to be allocated from the Attribute Type
namespace defined in [RFC2865] and [RFC3575]. The Radius
Attribute Type for EAP-Key-Name (defined in Section 4.1.4) is 102.
o This document defines one new Diameter application (in
Section 2.1) whose Application ID is to be allocated from the
Application Identifier namespace defined in [BASE]. The
Application ID for Diameter EAP is 5.
8. Security Considerations
8.1. Overview
Diameter peer-to-peer connections can be protected with IPsec or TLS.
These mechanisms are believed to provide sufficient protection under
the normal Internet threat model, that is, assuming the authorized
nodes engaging in the protocol have not been compromised, but the
attacker has complete control over the communication channels between
them. This includes eavesdropping, message modification, insertion,
man-in-the-middle and replay attacks. The details and related
security considerations are discussed in [BASE].
In addition to authentication provided by IPsec or TLS, authorization
is also required. Here, authorization means determining if a
Diameter message received from an authenticated Diameter peer should
be accepted (and not authorization of users requesting network access
from a NAS). In other words, when a Diameter server receives a
Diameter-EAP-Request, it has to decide if the client is authorized to
act as a NAS for the specific user, service type, and so on.
Correspondingly, when a NAS contacts a server to send a
Diameter-EAP-Request, it has to determine whether the server is
authorized to act as home server for the realm in question.
Authorization can involve local Access Control Lists (ACLs),
information contained in certificates, or some other means. See
[BASE] for more discussion and related security considerations. Note
that authorization issues are particularly relevant when Diameter
redirects are used. While redirection reduces the number of nodes
which have access to the contents of Diameter messages, a compromised
Diameter agent may not supply the right home server’s address. If
the Diameter client is unable to tell whether this particular server
is authorized to act as the home server for this particular user, the
security of the communications rests on the redirect agent.
The hop-by-hop security mechanisms (IPsec and TLS) combined with
proper authorization provide good protection against "outside"
attackers, except for denial-of-service attacks. The remaining part
of this section deals with attacks by nodes that have been properly
authorized (to function as a NAS, Diameter agent, or Diameter
server), but abuse their authorization or have been compromised. In
general, it is not possible to completely protect against attacks by
compromised nodes, but this section offers advice on limiting the
extent of the damage.
Attacks involving eavesdropping or modification of EAP messages are
beyond the scope of these document. See [EAP] for discussion of
these security considerations (including method negotiation,
dictionary attacks, and privacy issues). While these attacks can be
carried out by an attacker between the client and the NAS,
compromised NASes and Diameter agents are naturally also in a good
position to modify and eavesdrop on the EAP messages.
Similarly, attacks involving the link layer protocol used between the
client and the NAS, such as PPP or IEEE 802.11, are beyond the scope
of this document.
8.2. AVP Editing
Diameter agents can modify, insert, and delete AVPs. Diameter agents
are usually meant to modify AVPs, and the protocol cannot distinguish
well-intentioned and malicious modifications (see [RFC2607] for more
discussion). Similarly, a compromised NAS or server can naturally
include a different set of AVPs than expected.
Therefore, the question is what an attacker who compromises an
authorized NAS, agent, or server can do using Diameter EAP messages.
Some of the consequences are rather obvious. For instance, a
Diameter agent can give access to unauthorized users by changing the
Result-Code to DIAMETER_SUCCESS. Other consequences are less obvious
and are discussed below and authentication method negotiation attacks
are discussed in the next section.
By including suitable AVPs in an AA-Answer/Diameter-EAP-Answer
messages, an attacker may be able (depending on implementation and
configuration details) to:
o Give unauthorized users access, or deny access to authorized users
(Result-Code).
o Give an attacker a login session to a host otherwise protected by
firewalls, or redirect an authorized user’s login session to a
host controlled by the attacker (Login-Host).
o Route an authorized user’s traffic through a host controlled by
the attacker (various tunneling AVPs).
o Redirect an authorized user’s DNS requests to a malicious DNS
server (various vendor-specific AVPs).
o Modify routing tables at the NAS and thus redirect packets
destined for someone else (Framed-Route, Framed-Routing).
o Remove packet filters and other restrictions for user (Filter,
Callback, various vendor-specific AVPs).
o Cause the NAS to call some number, possibly an expensive toll
number controlled by the attacker (callback AVPs).
o Execute Command Line Interface (CLI) commands on the NAS (various
vendor-specific attributes).
By modifying an AA-Request/Diameter-EAP-Request, an attacker may be
able to:
o Change NAS-Identifier/NAS-Port/Origin-Host (or another attribute)
so that a valid user appears to be accessing the network from a
different NAS than in reality.
o Modify Calling-Station-ID (either to hide the true value, gain
access, or frame someone else).
o Modify password change messages (some vendor-specific attributes).
o Modify usage information in accounting messages.
o Modify contents of Class and State AVPs.
Some of these attacks can be prevented if the NAS or server is
configured to not accept some particular AVPs, or accepts them only
from some nodes.
8.3. Negotiation Attacks
This section deals with attacks where the NAS, any Diameter agents,
or Diameter server attempt to cause the authenticating user to choose
some authentication method other than EAP, such as PAP or CHAP
(negotiation attacks within EAP are discussed in [EAP], Section 7.8).
The vulnerability can be mitigated via implementation of a per-
connection policy by the authenticating peer, and a per-user policy
by the Diameter server. For the authenticating peer, the
authentication policy should be set on a per-connection basis.
With a per-connection policy, an authenticating peer will only
attempt to negotiate EAP for a session in which EAP support is
expected. As a result, it is presumed that an authenticating peer
selecting EAP requires that level of security. If it cannot be
provided, there is likely a misconfiguration, or the authenticating
peer may be contacting the wrong server. In this case, the
authenticating peer simply disconnects.
Similarly, with a per-user policy, the home server will not accept
authentication methods other than EAP for users for which EAP support
is expected.
For a NAS, it may not be possible to determine whether a peer is
required to authenticate with EAP until the peer’s identity is known.
For example, for shared-uses NASes one reseller may implement EAP
while another does not. Alternatively, some peer might be
authenticated locally by the NAS while other peers are authenticated
via Diameter. In such cases, if any peers of the NAS MUST do EAP,
then the NAS MUST attempt to negotiate EAP for every session. This
avoids forcing a peer to support more than one authentication type,
which could weaken security.
8.4. Session Key Distribution
Since there are currently no end-to-end (NAS-to-home server) security
mechanisms specified for Diameter, any agents that process
Diameter-EAP-Answer messages can see the contents of the
EAP-Master-Session-Key AVP. For this reason, this specification
strongly recommends avoiding Diameter agents when they cannot be
trusted to keep the keys secret.
In environments where agents are present, several factors should be
considered when deciding whether the agents that are authorized (and
considered "trustworthy enough") to grant access to users and specify
various authorization and tunneling AVPs are also "trustworthy
enough" to handle the session keys. These factors include (but are
not limited to) the type of access provided (e.g., public Internet or
corporate internet), security level of the agents, and the
possibilities for attacking user’s traffic after it has been
decrypted by the NAS.
Note that the keys communicated in Diameter messages are usually
short-term session keys (or short-term master keys that are used to
derive session keys). To actually cause any damage, those session
keys must end up with some malicious party that must be able to
eavesdrop, modify, or insert traffic between the user and the NAS
during the lifetime of those keys (for example, in 802.11i the
attacker must also eavesdrop the "four-way handshake").
8.5. Privacy Issues
Diameter messages can contain AVPs that can be used to identify the
user (e.g., User-Name) and approximate location of the user (e.g.,
Origin-Host for WLAN access points, Calling-Station-Id for fixed
phone lines). Thus, any Diameter nodes that process the messages may
be able to determine the geographic location of users.
Note that in many cases, the user identity is also sent in clear
inside EAP-Payload AVPs, and it may be possible to eavesdrop this
between the user and the NAS.
This can be mitigated somewhat by using EAP methods that provide
identity protection (see [EAP], Section 7.3), and using Session-Id or
pseudonyms for accounting.
8.6. Note about EAP and Impersonation
If the EAP method used does not provide mutual authentication,
obviously anyone can impersonate the network to the user. Even when
EAP mutual authentication is used, it occurs between the user and the
Diameter home server. See [EAPKey] for an extensive discussion about
the details and their implications.
One issue is worth pointing out here. As described in [EAPKey], the
current EAP architecture does not allow the home server to restrict
what service parameters or identities (such as SSID or BSSID in
802.11 wireless LANs) are advertised by the NAS to the client. That
is, a compromised NAS can change its BSSID or SSID, and thus appear
to offer a different service than intended. Even if these parameters
are included in Diameter-EAP-Answer messages, the NAS can tell
different values to the client.
Therefore, the NAS’s possession of the session keys proves that the
user is talking to an authorized NAS, but a compromised NAS can lie
about its exact identity. See [EAPKey] for discussion on how
individual EAP methods can provide authentication of NAS service
parameters and identities.
Note that the usefulness of this authentication may be rather limited
in many environments. For instance, in wireless LANs the user does
not usually securely know the identity (such as BSSID) of the "right"
access point; it is simply picked from a beacon message that has the
correct SSID and good signal strength (something that is easy to
spoof). Thus, simply authenticating the identity may not allow the
user to distinguish the "right" access point from all others.
9. Acknowledgements
This Diameter application relies heavily on earlier work on Diameter
NASREQ application [NASREQ] and RADIUS EAP support [RFC3579]. Much
of the material in this specification has been copied from these
documents.
The authors would also like to acknowledge the following people for
their contributions to this document: Bernard Aboba, Jari Arkko,
Julien Bournelle, Pat Calhoun, Henry Haverinen, John Loughney,
Yoshihiro Ohba, and Joseph Salowey.
10. References
10.1. Normative References
[BASE] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and
J. Arkko, "Diameter Base Protocol", RFC 3588,
September 2003.
[EAP] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[NASREQ] Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
"Diameter Network Access Server Application", RFC
4005, August 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[EAPKey] Aboba, B., Simon, D., Arkko, J., Eronen, P., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)
Key Management Framework", Work in Progress, July
2004.
[IEEE-802.1X] Institute of Electrical and Electronics Engineers,
"Local and Metropolitan Area Networks: Port-Based
Network Access Control", IEEE Standard 802.1X,
September 2001.
[IEEE-802.11i] Institute of Electrical and Electronics Engineers,
"IEEE Standard for Information technology -
Telecommunications and information exchange between
systems - Local and metropolitan area networks -
Specific requirements - Part 11: Wireless Medium
Access Control (MAC) and Physical Layer (PHY)
Specifications: Amendment 6: Medium Access Control
(MAC) Security Enhancements", IEEE Standard
802.11i-2004, July 2004.
[IKEv2] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", Work in Progress, June 2004.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS
Attributes", RFC 2548, March 1999.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and
Policy Implementation in Roaming", RFC 2607,
June 1999.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote
Authentication Dial In User Service)", RFC 3575,
July 2003.
[RFC3576] Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
Aboba, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)",
RFC 3576, July 2003.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote
Authentication Dial In User Service) Support For
Extensible Authentication Protocol (EAP)", RFC 3579,
September 2003.
[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G., and J.
Roese, "IEEE 802.1X Remote Authentication Dial In User
Service (RADIUS) Usage Guidelines", RFC 3580,
September 2003.
Authors’ Addresses
Pasi Eronen (editor)
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland
EMail: pasi.eronen@nokia.com
Tom Hiller
Lucent Technologies
1960 Lucent Lane
Naperville, IL 60566
USA
Phone: +1 630 979 7673
EMail: tomhiller@lucent.com
Glen Zorn
Cisco Systems
500 108th Avenue N.E., Suite 500
Bellevue, WA 98004
USA
Phone: +1 425 344 8113
EMail: gwz@cisco.com
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