Use of Transport Layer Security (TLS) in the Network News Transfer Protocol (NNTP)
draft-elie-nntp-tls-recommendations-05

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 August 11, 2017.

Copyright Notice

Copyright (c) 2017 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. Updates/Changes to RFC 4642
• 3. Recommendations
• 3.1. Compression
• 3.2. Protocol Versions and Security Preferences
• 3.3. Server Name Indication
• 3.4. Prevention of SSL Stripping
• 3.5. Authenticated Connections
• 3.6. Human Factors
• 4. Security Considerations
• 5. IANA Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Appendix A. Detailed Changes to RFC 4642
• A.1. Related to TLS-level Compression
• A.2. Related to Implicit TLS
• A.3. Related to RC4 Cipher Suites
• A.4. Related to Server Name Indication
• A.5. Related to Certificate Verification
• A.6. Related to Other Obsolete Wording
• Appendix B. Acknowledgments
• Appendix C. Document History (to be removed by RFC Editor before publication)
• C.1. Changes since -04
• C.2. Changes since -03
• C.3. Changes since -02
• C.4. Changes since -01
• C.5. Changes since -00
• 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, TLS negotiation, TLS-level compression, TLS session resumption, cipher suites, public key lengths, forward secrecy, hostname validation, certificate verification, and other aspects of using TLS with NNTP.

[Q1]For RFC Editor: Throughout the document, should [RFC7525] be referenced as [BCP195] or [RFC7525]? Same question for other BCP documents.

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].

When this document uses the terms "implicit TLS", it refers to TLS negotiation immediately upon connection on a separate port.

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:

• 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 implicit TLS and therefore use strict TLS configuration (Section 3.2 of [RFC7525]), that is to say they SHOULD use a port 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). Implicit TLS is the preferred way of using TLS with NNTP for the same reasons, transposed to NNTP, as those given in Appendix A of [MUA-STS]. (Note that [MUA-STS] and [RFC4642] have one author in common.)
• 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 given in Sections 4.2 and 4.2.1 of [RFC7525] instead. The mandatory-to-implement cipher(s) suite(s) depend on the TLS protocol version. For instance, when TLS 1.2 is used, the TLS_RSA_WITH_AES_128_CBC_SHA cipher suite MUST be implemented (Section 9 of [RFC5246]).
• All NNTP clients and any NNTP server that is known by multiple names MUST support the Server Name Indication (SNI) extension defined in Section 3 of [RFC6066], in conformance with Section 3.6 of [RFC7525]. It was only a "SHOULD" in Section 2.2.2 of [RFC4642].
• NNTP implementations and deployments MUST follow the rules and guidelines defined in [RFC6125] and [RFC5280] for hostname validation and certificate verification. Part of Section 5 of [RFC4642] is therefore rationalized in favour of following those two documents.

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 [RFC8054], 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 Compression Ratio Info-leak Made Easy (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. Prevention of SSL Stripping

In order to help prevent SSL Stripping attacks (Section 2.1 of [RFC7457]), NNTP implementations and deployments MUST follow the recommendations provided in Section 3.2 of [RFC7525]. Notably, in case implicit TLS is not used, news clients SHOULD attempt to negotiate TLS even if the server does not advertise the STARTTLS capability label in response to the CAPABILITIES command (Section 2.1 of [RFC4642]).

3.5. Authenticated Connections

[RFC4642] already provides recommendations and requirements for certificate validation in the context of checking the client or the server's identity. Those requirements are strengthened by Appendix A.5 of this document.

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], and in accordance with opportunistic security principles [RFC7435]). 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].

3.6. Human Factors

NNTP clients SHOULD provide ways for end users (and NNTP servers SHOULD provide ways for administrators) to complete at least 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).
• Be warned if the version of TLS used for encryption of a given stream is not secure enough.
• If authenticated encryption is used, determine how the connection was authenticated or verified.
• Be warned if the certificate offered by an NNTP server cannot be verified.
• Be warned if the cipher suite used to encrypt a connection is not secure enough.
• Be warned if the certificate changes for a given server.
• When a security layer is not already in place, be warned if a given server stops advertising the STARTTLS capability label in response to the CAPABILITIES command (Section 2.1 of [RFC4642]) whereas it advertised the STARTTLS capability label during any previous connection within a (possibly configurable) time frame. (Otherwise, a human might not see the warning the first time, and the warning would disappear immediately after that.)
• 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 implicit TLS is used, and that the penultimate task helps prevent an attack known as SSL Stripping (Section 2.1 of [RFC7457]).

4. Security Considerations

Beyond the security considerations already described in [RFC4642], [RFC6125] and [RFC7525], the following caveat is worth mentioning when not using implicit TLS: NNTP servers need to 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 does not change the formal definition of the STARTTLS extension (Section 6 of [RFC4642]). Nonetheless, as implementations of the STARTTLS extension should follow this document, IANA will add its reference to the existing STARTTLS label in the NNTP capability labels registry contained in the Network News Transfer Protocol (NNTP) Parameters registry:

6. References

6.1. Normative References

6.2. Informative References

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 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 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.

A.2. Related to Implicit TLS

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 immediate TLS negotiation on a separate port (referred to in this document as "implicit TLS") is the preferred way of using TLS with NNTP.
• If a host wishes to offer separate servers for transit and reading clients (Section 3.4.1 of [NNTP]), TCP port 563 SHOULD be used for implicit TLS with the reading server, and an unused port of its choice different than TCP port 433 SHOULD be used for implicit TLS with the transit server. The ports used for implicit TLS should be clearly communicated to the clients, and specifically that no plain-text communication occurs before the TLS session is negotiated.
• As some existing implementations negotiate TLS via a dynamic upgrade from unencrypted to TLS-protected traffic during an NNTP session on well-known TCP ports 119 or 433, this specification formalizes the STARTTLS command in use for that purpose. However, as already mentioned above, implementations SHOULD use implicit TLS on a separate port.
• Note: a common alternative to protect NNTP exchanges with transit servers that do not implement TLS is the use of IPsec with encryption [RFC4301].

An additional informative reference to [RFC4301] is therefore added to Section 7.2 of [RFC4642].

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 Certificate Verification

The text between "During the TLS negotiation" and "identity bindings)." in Section 5 of [RFC4642] is replaced with the following text:

• During TLS negotiation, the client MUST verify the server's identity in order to prevent man-in-the-middle attacks. The client MUST follow the rules and guidelines defined in [RFC6125], where the reference identifier MUST be the server hostname that the client used to open the connection, and that is also specified in the TLS "server_name" extension [RFC6066]. The following NNTP-specific consideration applies: DNS domain names in server certificates MAY contain the wildcard character "*" as the complete left-most label within the identifier.
• If the match fails, the client MUST follow the recommendations in Section 6.6 of [RFC6125] regarding certificate pinning and fallback.
• Beyond server identity checking, clients also MUST apply the procedures specified in [RFC5280] for general certificate validation (e.g., certificate integrity, signing, and path validation).

Additional normative references to [RFC5280] (replacing [PKI-CERT] it obsoletes), [RFC6066], and [RFC6125] are therefore added to Section 7.1 of [RFC4642].

A.6. 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 7.4.1.2 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: Stéphane Bortzmeyer, Ben Campbell, Viktor Dukhovni, Stephen Farrell, Sabahattin Gucukoglu, Richard Kettlewell, Jouni Korhonen, Mirja Kühlewind, David Eric Mandelberg, Matija Nalis, Chris Newman, and Peter Saint‑Andre.

Special thanks to Michael Bäuerle, for shepherding this document, and to the Responsible Area Director, Alexey Melnikov, for sponsoring it. They both significantly helped to increase its quality.

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

C.1. Changes since -04

• Take into account the remarks received during IESG telechat.
• Mention the Document Shepherd, Michael Bäuerle.
• Update the reference [NNTP-COMPRESS] to [RFC8054], now it has been released.
• Add a reference to [RFC7435].
• Move [RFC4422], [RFC4643], [RFC5536], and [RFC5537] from informative to normative references.
• A few wording improvements.

C.2. Changes since -03

• Improve wording to make clear that the server hostname that the client used to open the connection is the same as the one specified in the TLS "server_name" extension.
• Move [RFC5280], [RFC6125] and [RFC7525] to normative references.
• In detailed changes of [RFC4642], use [NNTP] instead of [RFC3977] as this RFC is referenced as [NNTP] in [RFC4642]. Also mention obsolete [PKI-CERT].

C.3. Changes since -02

• Use (and define) the "implicit TLS" terminology instead of "strict TLS". The language in [RFC7525] is unfortunate since "strict TLS" is not clearly defined in that document, and the name suggests that it is an alternative to "opportunistic TLS", rather than an alternative to STARTTLS. While STARTTLS is often used opportunistically, that is not always the case.
• Mention SSL Stripping in Section 3.6 with a reference to Section 2.1 of [RFC7457] because the intent of the related task may not have been clear enough. Reported by Matija Nalis.
• Add Section 3.4 about how to prevent SSL stripping, notably by an attempt to negotiate TLS even if STARTTLS is not advertised, when implicit TLS is not used.
• Strengthen the requirements on hostname validation and certificate verification, by referencing [RFC6125] and [RFC5280].
• Ask IANA to add this document to the NNTP capabilily labels registry.
• Reference the security considerations of [RFC6125].
• Mention informative and normative references to add to [RFC4642].

C.4. Changes since -01

• Take into account all the remarks sent during IETF Last Call.
• Move the part about [RFC4642] from Introduction to a new dedicated Section named "Updates/Changes to RFC 4642" so as to make the document a bit more structured.
• The warning about lack of STARTTLS is expanded in scope to say "during any previous connection within a (possibly configurable) time frame" instead of "during the previous connection".
• Remove Appendix about export restrictions on crypto. It is useless since RFC 2804.
• Add wording about the use of strict TLS for transit. Mention the use of a port other than 433 for strict TLS between two peers, and add a note about a possible use of IPsec [RFC4301] for transit. Do not only speak about port 563.
• Explicitly mention the mandatory-to-implement cipher suite for TLS 1.2.
• Do not keep the paragraph about TLS Client Hello messages and Server Name Indication (SNI) in [RFC4642]. Support for SNI [RFC6066] is now a MUST, and not a SHOULD.
• Reference [RFC7457] for the STARTTLS command injection vulnerability.
• Add notes to RFC Editor to ask that [MUA-STS] and [NNTP-COMPRESS] references be changed to their [RFCxxxx] form, once published, and whether [BCP195] should be used instead of [RFC7525].
• Move [RFC5246] (TLS) to a normative reference.
• Minor other wording improvements.

C.5. Changes since -00

• Clarify in the introduction of Section 3 that NNTP implementations compliant with this document are REQUIRED to also comply with [RFC7525].
• Improve the wording of Section 3.2 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.

Author's Address