Use of Transport Layer Security (TLS) in the Network News Transfer Protocol (NNTP)
draft-elie-nntp-tls-recommendations-01

Abstract

This document provides recommendations for improving the security of the Network News Transfer Protocol (NNTP) when using Transport Layer Security (TLS). It modernizes the NNTP usage of TLS to be consistent with TLS best current practices. If approved, this document updates RFC 4642.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on February 6, 2017.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 1.1. Conventions Used in This Document
• 1.2. Author's Note
• 2. Recommendations
• 2.1. Compression
• 2.2. Protocol Versions and Cipher Suites
• 2.3. Authenticated Connections
• 2.4. Human Factors
• 3. Security Considerations
• 4. IANA Considerations
• 5. References
• 5.1. Normative References
• 5.2. Informative References
• Appendix A. Changes to RFC 4642
• Appendix B. Implementation Notes
• Appendix C. Acknowledgements
• Appendix D. Document History (to be removed by RFC Editor before publication)
• D.1. Changes since -00
• Appendix E. Issues to Address
• Author's Address

1. Introduction

The Network News Transfer Protocol (NNTP) [RFC3977] has been using Transport Layer Security (TLS) [RFC5246] (along with its precursor, Secure Sockets Layer or SSL) since at least year 2000. The use of TLS in NNTP was formalized in [RFC4642], providing at the same time implementation recommendations. In order to address the evolving threat model on the Internet today, this document provides stronger recommendations regarding that use.

In particular, this document updates [RFC4642] by specifying that NNTP implementations and deployments MUST follow the best current practices documented in the "Recommendations for Secure Use of TLS and DTLS" [RFC7525]. This includes stronger recommendations regarding SSL/TLS protocol versions, fallback to lower versions, strict TLS, TLS-level compression, TLS session resumption, cipher suites, public key lengths, forward secrecy, and other aspects of using TLS with NNTP.

Notably, this document updates [RFC4642] in the following aspects:

• NNTP implementations and deployments SHOULD disable TLS-level compression (Section 3.3 of [RFC7525]), thus no longer using TLS as a means to provide data compression (contrary to Abstract and Section 2.2.2 of [RFC4642]).
• NNTP implementations and deployments SHOULD prefer strict TLS configuration (Section 3.2 of [RFC7525]), that is to say they SHOULD use TCP port 563 dedicated to NNTP over TLS, and begin the TLS negotiation immediately upon connection (contrary to a dynamic upgrade from unencrypted to TLS-protected traffic via the use of the STARTTLS command, as Section 1 of [RFC4642] was encouraging). For the same reasons as those given in Appendix A of [MUA-STS] transposed to NNTP, strict TLS is the preferred way of using TLS with NNTP.
• NNTP implementations and deployments MUST NOT negotiate RC4 cipher suites ([RFC7465]) contrary to Section 5 of [RFC4642] that REQUIRED them to implement the TLS_RSA_WITH_RC4_128_MD5 cipher suite so as to ensure that any two NNTP compliant implementations can be configured to interoperate. This document removes that requirement, so that NNTP client and server implementations follow the recommendations of Section 4.2.1 of [RFC7525] instead.

1.1. Conventions Used in This Document

Any term not defined in this document has the same meaning as it does in [RFC4642] or the NNTP core specification [RFC3977].

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 [RFC2119].

1.2. Author's Note

Please write the first letter of "Elie" and the penultimate letter of "allee" with an acute accent wherever possible -- they are respectively U+00C9 ("É" in XML) and U+00E9 ("é" in XML). Also, the letters "ae" in "Baeuerle" should be written as an a-umlaut (U+00E4, "ä" in XML).

2. Recommendations

The best current practices documented in the "Recommendations for Secure Use of TLS and DTLS" [RFC7525] are included here by reference. Therefore, NNTP implementations and deployments compliant with this document is REQUIRED to also comply with [RFC7525].

Instead of repeating those recommendations here, this document mostly provides supplementary information regarding secure implementation and deployment of NNTP technologies.

2.1. Compression

NNTP supports the use of the COMPRESS command, defined in Section 2.2 of [NNTP-COMPRESS], to compress data between an NNTP client and server. Although this NNTP extension might have slightly stronger security properties than TLS-level compression [RFC3749] (since NNTP compression can be activated after authentication has completed, thus reducing the chances that authentication credentials can be leaked via for instance a CRIME attack, as described in Section 2.6 of [CRIME]), this document neither encourages nor discourages use of NNTP COMPRESS extension.

2.2. Protocol Versions and Cipher Suites

NNTP implementations of news servers are encouraged to support options to configure the minimal TLS protocol version to accept, and which cipher suites, signature algorithms or named groups (like elliptic curves) to use for incoming connections. Additional options can naturally also be supported. The goal is to enable administrators of news servers to easily and quickly strengthen security, if need be (for instance by rejecting cipher suites considered unsafe with regards to local policy). News clients may also support similar options, either configurable by the user or enforced by the news reader.

2.3. Authenticated Connections

[RFC4642] already provides recommendations and requirements for certificate validation in the context of checking the client or the server's identity.

Wherever possible, it is best to prefer certificate-based authentication (along with SASL [RFC4422]), and ensure that:

• Clients authenticate servers.
• Servers authenticate clients.
• Servers authenticate other peer servers.

This document does not mandate certificate-based authentication, although such authentication is strongly preferred. As mentioned in Section 2.2.2 of [RFC4642], the AUTHINFO SASL command (Section 2.4 of [RFC4643]) with the EXTERNAL mechanism (Appendix A of [RFC4422]) MAY be used to authenticate a client once its TLS credentials have been successfully exchanged.

Given the pervasiveness of eavesdropping [RFC7258], even an encrypted but unauthenticated connection might be better than an unencrypted connection (this is similar to the "better-than-nothing security" approach for IPsec [RFC5386]). Encrypted but unauthenticated connections include connections negotiated using anonymous Diffie‑Hellman mechanisms or using self-signed certificates, among others.

When an NNTP server receives a Netnews article, it MAY add a <diag‑match> (Section 3.1.5 of [RFC5536]), which appears as "!!" in the Path header field of that article, to indicate that it verified the identity of the client or peer server. This document encourages the construction of such Path header fields, as described in Section 3.2.1 of [RFC5537].

2.4. Human Factors

It is strongly encouraged that NNTP clients provide ways for end users (and that NNTP servers provide ways for administrators) to complete the following tasks:

• Determine if a given incoming or outgoing connection is encrypted using a security layer (either using TLS or an SASL mechanism that negotiates a security layer).
• Determine the version of TLS used for encryption of a given stream.
• If authenticated encryption is used, determine how the connection was authenticated or verified.
• Inspect the certificate offered by an NNTP server.
• Determine the cipher suite used to encrypt a connection.
• Be warned if the certificate changes for a given server.
• Be warned if a given server stops advertising the STARTTLS capability label in response to the CAPABILITIES command (of course when a security layer is not already in place) whereas it advertised the STARTTLS capability label during the previous connection.
• Be warned if a failure response to the STARTTLS command is received from the server whereas the STARTTLS capability label was advertised.

Note that the last two tasks cannot occur when strict TLS is used.

3. Security Considerations

Beyond the security considerations already described in [RFC4642] and [RFC7525], the author wishes to add the following caveat when not using strict TLS.

NNTP servers need ensure that they are not vulnerable to the STARTTLS command injection vulnerability (CERT vulnerability ID #555316). Though this command MUST NOT be pipelined, an attacker could pipeline it. Therefore, NNTP servers MUST discard any NNTP command received between the use of STARTTLS and the end of TLS negotiation.

4. IANA Considerations

This document has no actions for IANA.

5. References

5.1. Normative References

5.2. Informative References

Appendix A. Changes to RFC 4642

This section lists detailed changes this document applies to [RFC4642].

The second sentence in the Abstract of [RFC4642] is replaced with the following text:

• The primary goal is to provide encryption for single-link confidentiality purposes, but data integrity, and (optional) certificate-based peer entity authentication are also possible.

The third and fourth paragraphs in Section 1 of [RFC4642] are replaced with the following text:

• TCP port 563 is dedicated to NNTP over TLS, and registered in the IANA Service Name and Transport Protocol Port Number Registry for that usage. NNTP implementations using TCP port 563 begin the TLS negotiation immediately upon connection and then continue with the initial steps of an NNTP session. This use of strict TLS on a separate port is the preferred way of using TLS with NNTP.
• As some existing implementations negotiate TLS via a dynamic upgrade from unencrypted to TLS-protected traffic during an NNTP session, this specification formalizes the STARTTLS command in use for that purpose. However, as already mentioned above, implementations SHOULD use strict TLS on a separate port.

The second sentence of the first paragraph in Section 2.2.2 of [RFC4642] is replaced with the following text:

• The STARTTLS command is usually used to initiate session security, although it can also be used for client and/or server certificate authentication.

The third paragraph in Section 5 of [RFC4642] is removed. Consequently, NNTP no longer requires to implement any cipher suites, other than those prescribed by TLS [RFC5246] and Section 4.2.1 of [RFC7525].

Appendix B. Implementation Notes

Some governments enforce legislation prohibiting the export of strong cryptographic technologies. Nothing in this document ought to be taken as advice to violate such prohibitions.

Appendix C. Acknowledgements

This document draws heavily on ideas in [RFC7590] by Peter Saint‑Andre and Thijs Alkemade, and a large portion of this text was borrowed from that specification.

The author would like to thank the following individuals for contributing their ideas and support for writing this specification: Michael Bäuerle, Richard Kettlewell, and Chris Newman.

Appendix D. Document History (to be removed by RFC Editor before publication)

D.1. Changes since -00

• Clarify in the introduction of Section 2 that NNTP implementations compliant with this document are REQUIRED to also comply with [RFC7525].
• Improve the wording of Section 2.2 to mention that configuration is primarily intended for news servers. Also, be more consistent in the options to accept, and include signature algorithms and named groups.

Appendix E. Issues to Address

• Should the paragraph starting with "Servers MUST be able to understand backwards-compatible TLS Client Hello messages" in Section 2.2.2 of [RFC4642] remain as-is or should it be modernized with another wording? (And which one? or is it already done by the reference to [RFC7525]?)
• Should the paragraphs in Section 5 of [RFC4642] dealing with how the client checks the server hostname and the binding between the identity of servers and the public keys presented be modernized? (Obsolete them in favour of RFC 6125 for instance? or maybe [RFC7525] is enough as it also points to RFC 6125)
• Regarding peering between mode-switching news servers, should something specific be added? NNTP has port 119, and NNTP over TLS has port 563. NNSP has port 433 but no dedicated port for TLS. Shouldn't a port for NNSP over TLS be registered? Otherwise, both reading and peering are supposed to use port 563, which may be inconvenient. We could then recommend the use of stunnel with TCP wrappers, or an equivalent mechanism, listening to that new separate port for mode-switching news servers that do not natively support TLS for peering.

Author's Address