Independent Submission J. Élie
Internet-Draft December 21, 2016
Updates: 4642 (if approved)
Intended status: Standards Track
Expires: June 24, 2017

Use of Transport Layer Security (TLS) in the Network News Transfer Protocol (NNTP)


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

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This Internet-Draft will expire on June 24, 2017.

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Table of Contents

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.

[Q1]For RFC Editor: Throughout the document, should [RFC7525] be referenced as [BCP195] or [RFC7525]?

[Q2]For RFC Editor: Throughout the document, the references to [MUA-STS] (draft-ietf-uta-email-deep) and [NNTP-COMPRESS] (draft-murchison-nntp-compress) should be referenced as their equivalent [RFCxxxx], once published.

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" with an acute accent wherever possible -- it is U+00C9 ("É" in XML). The third letter of "Stephane" and the penultimate letter of "allee" similarly have an acute accent (U+00E9, "é" in XML). Also, the letters "ae" in "Baeuerle" should be written as an a-umlaut (U+00E4, "ä" in XML).

2. Updates/Changes to RFC 4642

This document updates [RFC4642] in the following aspects:

Appendix A of this document gives detailed changes with regards to the wording of [RFC4642].

3. 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 are 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.

3.1. 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 the use of the NNTP COMPRESS extension.

3.2. Protocol Versions and Security Preferences

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 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.

3.3. Server Name Indication

The TLS extension for Server Name Indication (SNI) defined in Section 3 of [RFC6066] MUST be implemented by all news clients. It also MUST be implemented by any news server that is known by multiple names. (Otherwise, it is not possible for a server with several hostnames to present the correct certificate to the client.)

3.4. 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:

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.

Note: 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].

3.5. 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:

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

4. 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 (Section 2.2 of [RFC7457]). 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.

5. IANA Considerations

This document has no actions for IANA.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3977] Feather, C., "Network News Transfer Protocol (NNTP)", RFC 3977, DOI 10.17487/RFC3977, October 2006.
[RFC4642] Murchison, K., Vinocur, J. and C. Newman, "Using Transport Layer Security (TLS) with Network News Transfer Protocol (NNTP)", RFC 4642, DOI 10.17487/RFC4642, October 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011.

6.2. Informative References

[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", Ekoparty Security Conference, 2012.
[MUA-STS] Moore, K. and C. Newman, "Mail User Agent Strict Transport Security (MUA-STS)", July 2016.
[NNTP-COMPRESS] Murchison, K. and J. Élie, "Network News Transfer Protocol (NNTP) Extension for Compression", October 2016.
[RFC3749] Hollenbeck, S., "Transport Layer Security Protocol Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, December 2005.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, DOI 10.17487/RFC4422, June 2006.
[RFC4643] Vinocur, J. and K. Murchison, "Network News Transfer Protocol (NNTP) Extension for Authentication", RFC 4643, DOI 10.17487/RFC4643, October 2006.
[RFC5386] Williams, N. and M. Richardson, "Better-Than-Nothing Security: An Unauthenticated Mode of IPsec", RFC 5386, DOI 10.17487/RFC5386, November 2008.
[RFC5536] Murchison, K., Lindsey, C. and D. Kohn, "Netnews Article Format", RFC 5536, DOI 10.17487/RFC5536, November 2009.
[RFC5537] Allbery, R. and C. Lindsey, "Netnews Architecture and Protocols", RFC 5537, DOI 10.17487/RFC5537, November 2009.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014.
[RFC7457] Sheffer, Y., Holz, R. and P. Saint-Andre, "Summarizing Known Attacks on Transport Layer Security (TLS) and Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457, February 2015.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, DOI 10.17487/RFC7465, February 2015.
[RFC7525] Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015.
[RFC7590] Saint-Andre, P. and T. Alkemade, "Use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June 2015.

Appendix A. Detailed Changes to RFC 4642

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

A.1. Related to TLS-level Compression

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

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

A.2. Related to Strict TLS

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

A.3. Related to RC4 Cipher Suites

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 (Section 9 of [RFC5246]) and Sections 4.2 and 4.2.1 of [RFC7525].

A.4. Related to Server Name Indication

The last two sentences of the seventh paragraph in Section 2.2.2 of [RFC4642] are removed. Section 3.6 of [RFC7525] apply.

A.5. Related to Other Obsolete Wording

The first two sentences of the seventh paragraph in Section 2.2.2 of [RFC4642] are removed. There is no special requirement for NNTP with regards to TLS Client Hello messages. Section and Appendix E of [RFC5246] apply.

Appendix B. Acknowledgments

This document draws heavily on ideas in [RFC7590] by Peter Saint‑Andre and Thijs Alkemade; 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, Stéphane Bortzmeyer, Sabahattin Gucukoglu, Richard Kettlewell, Jouni Korhonen, David Eric Mandelberg, and Chris Newman.

Many thanks to the Responsible Area Director, Alexey Melnikov, for reviewing and sponsoring this document.

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

C.1. Changes since -01

C.2. Changes since -00

Appendix D. Issue to Address

Author's Address

Julien Élie 10 allée Clovis Noisy-le-Grand, 93160 France EMail: URI: