Update to the EAP Applicability Statement for ABFAB
draft-ietf-abfab-eapapplicability-02

Abstract

This document updates the Extensible Authentication Protocol (EAP) applicability statement from RFC3748 to reflect recent usage of the EAP protocol in the Application Bridging for Federated Access Beyond web (ABFAB) working group.

Status of This Memo

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Uses of EAP for Application-Layer Access
• 2.1. Retransmission
• 2.2. Re-Authentication
• 3. Revised EAP applicability statement
• 4. Security Considerations
• 5. IANA Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informational References
• Authors' Addresses

1. Introduction

The EAP applicability statement in [RFC3748] defines the scope of the Extensible Authentication Protocol to be "for use in network access authentication, where IP layer connectivity may not be available.", and states that "Use of EAP for other purposes, such as bulk data transport, is NOT RECOMMENDED.".

While some of the recommendation against usage of EAP for bulk data transport is still valid, some of the other provisions in the applicability statement have turned out to be too narrow. Section 2 describes the example where EAP is used to authenticate application layer access. Section 3 provides new text to update the paragraph 1.3. "Applicability" in [RFC3748].

1.1. Requirements Language

In this document, several words are used to signify the requirements of the specification. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. [RFC2119]

2. Uses of EAP for Application-Layer Access

Ongoing work in the IETF (abfab working group) specifies the use of EAP over GSSAPI for generic application layer access. In the past, using EAP in this context has met resistance due to the lack of channel bindings [RFC6677]. Without channel bindings, a peer cannot verify if an authenticator is authorized to provide an advertised service.

However as additional services use EAP for authentication, the distinction of which service is being contacted becomes more important. Application services might have different properties. Consider an environment with multiple printers some of which provide a confidential service to output documents to a controlled location. If a peer sent a document to the wrong service then potentially sensitive information might be printed in an uncontrolled location and be disclosed. In addition, it might be more likely that a low-value service is compromised than some high value service. If the high-value service could be impersonated by a low-value service then the security of the overall system would be limited by the security of the lower value service.

This distinction is present in any environment where peers' security depends on which service they reach. However it is particularly acute in a federated environment where multiple organizations are involved. It is very likely that these organizations will have different security policies and practices. It is very likely that the goals of these organizations will not entirely be aligned. In many situations one organization could gain value by being able to impersonate another. In this environment, authenticating the EAP server is insufficient: the peer must also validate that the contacted host is authorized to provide the requested service.

For these reasons, channel binding MUST be implemented by peers, EAP servers and AAA servers in environments where EAP authentication is used to access application layer services. In addition, channel binding MUST default to being required by peers for non-network authentication. If the EAP server is aware that authentication is for something other than a network service, it too MUST default to requiring channel binding. Operators need to carefully consider the security implications before relaxing these requirements. One potentially serious attack exists when channel binding is not required and EAP authentication is introduced into an existing non-network service. A device can be created that impersonates a Network Access Service to peers, but actually proxies the authentication to the service that newly accepts EAP authentications may decrease the security of this service even for users who previously used non-EAP means of authentication to the service.

It is REQUIRED for the application layer to prove possession of the EAP MSK between the EAP Peer and EAP Authenticator. Failing to validate the possession of the EAP MSK can allow an attacker to insert himself into the conversation and impersonate the peer or authenticator. In addition, the application should define an channel binding attributes that are sufficient to validate that the application service is being correctly represented to the peer.

2.1. Retransmission

In EAP, the authenticator is responsible for retransmission. By default EAP assumes that the lower layer (the application in this context) is unreliable. The authenticator can send a packet whenever its retransmission timer triggers. In this mode, applications need to be able to receive and process EAP messages at any time during the authentication conversation.

Alternatively, EAP permits a lower layer to set the retransmission timer to infinite. When this happens, the lower layer becomes responsible for reliable delivery of EAP messages. Applications that use a lock-step or client-driven authentication protocol might benefit from this approach.

In addition to retransmission behavior applications need to deal with discarded EAP messages. For example, whenever some EAP methods receive erroneous input, these methods discard the input rather than generating an error response. If the erroneous input was generated by an attacker, legitimate input can sometimes be received after the erroneous input. Applications MUST handle discarded EAP messages, although the specific way in which discarded messages will be handled depends on the characteristics of the application. Options include failing the authentication at the application level, requesting an EAP retransmit and waiting for additional EAP input.

Applications designers that incorporate EAP into their application need to determine how retransmission and message discards are handled.

2.2. Re-Authentication

EAP lower layers MAY provide a mechanism for re-authentication to happen within an existing session [RFC3748]. Re-authentication permits security associations to be updated without establishing a new session. For network access, this can be important because interrupting network access can disrupt connections and media.

Some applications might not need re-authentication support. For example if sessions are relatively short-lived or if sessions can be replaced without significant disruption, re-authentication might not provide value. Protocols like Hypertext Transfer Protocol (HTTP) and Simple Mail Transport Protocol (SMTP) are examples of protocols where establishing a new connection to update security associations is likely to be sufficient.

Re-authentication is likely to be valuable if sessions or connections are long-lived or if there is a significant cost to disrupting them.

Another factor may make re-authentication important. Some protocols only permit one part in a protocol (for example the client) to establish a new connection. If another party in the protocol needs the security association refreshed then re-authentication can provide a mechanism to do so.

Application designers need to determine whether re-authentication support is needed and which parties can initiate it.

3. Revised EAP applicability statement

The following text is added to the EAP applicability statement in [RFC3748].

In cases where EAP is used for application authentication, support for EAP Channel Bindings is REQUIRED on the EAP Peer and EAP Server to validate that the host is authorized to provide the services requested. In addition, the application MUST define channel binding attributes that are sufficient to validate that the application service is being correctly represented to the peer. The protocol carrying EAP MUST prove possession of the EAP MSK between the EAP Peer and EAP Authenticator. In the context of EAP for application access the application is providing the EAP Lower Layer. Applications protocols vary so their specific behavior and transport characteristics needs to be considered when determining their retransmission and re-authentication behavior.

4. Security Considerations

In addition to the requirements discussed in the main sections of the document applications should take into account how server authentication is achieved. Some deployments may allow for weak server authentication that is then validated with an additional existing exchange that provides mutual authentication. In order to fully mitigate the risk of NAS impersonation when these mechanisms are used, it is RECOMMENDED that mutual channel bindings be used to bind the authentications together as described in [I-D.ietf-emu-crypto-bind]. When doing channel binding it is REQUIRED that the authenticator is not able to modify the channel binding data passed between the peer to the authenticator as part of the authentication process.

5. IANA Considerations

This document has no actions for IANA.

6. Acknowledgements

Large amounts of helpful text and insightful thoughts were contributed by Sam Hartman, Painless Security.

7. References

7.1. Normative References

7.2. Informational References

Authors' Addresses