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10 allée Clovis 93160 Noisy-le-Grand France julien@trigofacile.com http://www.trigofacile.com/

Applications Independent Submission NNTP Usenet NetNews TLS STARTTLS 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.

The Network News Transfer Protocol (NNTP) has been using Transport Layer Security (TLS) (along with its precursor, Secure Sockets Layer or SSL) since at least year 2000. The use of TLS in NNTP was formalized in , 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 by specifying that NNTP implementations and deployments MUST follow the best current practices documented in the "Recommendations for Secure Use of TLS and DTLS" . 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. For RFC Editor: Throughout the document, should [RFC7525] be referenced as [BCP195] or [RFC7525]? Same question for other BCP documents. 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.

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

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).

This document updates in the following aspects: NNTP implementations and deployments SHOULD disable TLS-level compression (Section 3.3 of ), thus no longer using TLS as a means to provide data compression (contrary to Abstract and Section 2.2.2 of ). NNTP implementations and deployments SHOULD prefer implicit TLS and therefore use strict TLS configuration (Section 3.2 of ), 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 was encouraging). For the same reasons, transposed to NNTP, as those given in Appendix A of (whose one of the authors was also one of the authors of ), implicit TLS is the preferred way of using TLS with NNTP. NNTP implementations and deployments MUST NOT negotiate RC4 cipher suites () contrary to Section 5 of 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 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 ). NNTP implementations and deployments MUST support the Server Name Indication (SNI) extension defined in Section 3 of , contrary to Section 2.2.2 of for which it was only a SHOULD. All clients and servers known by multiple names MUST support the SNI extension, in conformance with Section 3.6 of . NNTP implementations and deployments MUST follow the rules and guidelines defined in and for hostname validation and certificate verification. Part of Section 5 of is therefore rationalized in favour of following those two documents. of this document gives detailed changes with regards to the wording of .

The best current practices documented in the "Recommendations for Secure Use of TLS and DTLS" are included here by reference. Therefore, NNTP implementations and deployments compliant with this document are REQUIRED to also comply with . Instead of repeating those recommendations here, this document mostly provides supplementary information regarding secure implementation and deployment of NNTP technologies.

NNTP supports the use of the COMPRESS command, defined in Section 2.2 of , to compress data between an NNTP client and server. Although this NNTP extension might have slightly stronger security properties than TLS-level compression (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 ), this document neither encourages nor discourages the use of the NNTP COMPRESS extension.

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.

The TLS extension for Server Name Indication (SNI) defined in Section 3 of 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.)

In order to help prevent SSL Stripping attacks (Section 2.1 of ), NNTP implementations and deployments are encouraged to follow the recommendations provided in Section 3.2 of . 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 ).

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 of this document. Wherever possible, it is best to prefer certificate-based authentication (along with SASL ), 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 , the AUTHINFO SASL command (Section 2.4 of ) with the EXTERNAL mechanism (Appendix A of ) MAY be used to authenticate a client once its TLS credentials have been successfully exchanged. Given the pervasiveness of eavesdropping , 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 ). 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 ), 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 .

It is strongly encouraged that NNTP clients provide ways for end users (and that NNTP servers 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). 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. 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 ) 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 ).

Beyond the security considerations already described in , and , the author wishes to add the following caveat when not using implicit TLS. NNTP servers need ensure that they are not vulnerable to the STARTTLS command injection vulnerability (Section 2.2 of ). 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.

This document does not change the formal definition of the STARTTLS extension (Section 6 of ). 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: Label Meaning Reference STARTTLS Transport layer security [RFC4642][RFC-to-be]

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Network News Transfer Protocol (NNTP) has been in use in the Internet for a decade, and remains one of the most popular protocols (by volume) in use today. This document is a replacement for RFC 977, and officially updates the protocol specification. It clarifies some vagueness in RFC 977, includes some new base functionality, and provides a specific mechanism to add standardized extensions to NNTP. [STANDARDS-TRACK] This memo defines an extension to the Network News Transfer Protocol (NNTP) that allows an NNTP client and server to use Transport Layer Security (TLS). The primary goal is to provide encryption for single-link confidentiality purposes, but data integrity, (optional) certificate-based peer entity authentication, and (optional) data compression are also possible. [STANDARDS-TRACK] This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] This document provides specifications for existing TLS extensions. It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2". The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK] The Transport Layer Security (TLS) protocol (RFC 2246) includes features to negotiate selection of a lossless data compression method as part of the TLS Handshake Protocol and to then apply the algorithm associated with the selected method as part of the TLS Record Protocol. TLS defines one standard compression method which specifies that data exchanged via the record protocol will not be compressed. This document describes an additional compression method associated with a lossless data compression algorithm for use with TLS, and it describes a method for the specification of additional TLS compression methods. [STANDARDS-TRACK] This document describes an updated version of the "Security Architecture for IP", which is designed to provide security services for traffic at the IP layer. This document obsoletes RFC 2401 (November 1998). [STANDARDS-TRACK] The Simple Authentication and Security Layer (SASL) is a framework for providing authentication and data security services in connection-oriented protocols via replaceable mechanisms. It provides a structured interface between protocols and mechanisms. The resulting framework allows new protocols to reuse existing mechanisms and allows old protocols to make use of new mechanisms. The framework also provides a protocol for securing subsequent protocol exchanges within a data security layer.This document describes how a SASL mechanism is structured, describes how protocols include support for SASL, and defines the protocol for carrying a data security layer over a connection. In addition, this document defines one SASL mechanism, the EXTERNAL mechanism.This document obsoletes RFC 2222. [STANDARDS-TRACK] This document defines an extension to the Network News Transfer Protocol (NNTP) that allows a client to indicate an authentication mechanism to the server, to perform an authentication protocol exchange, and optionally to negotiate a security layer for subsequent protocol interactions during the remainder of an NNTP session.This document updates and formalizes the AUTHINFO USER/PASS authentication method specified in RFC 2980 and deprecates the AUTHINFO SIMPLE and AUTHINFO GENERIC authentication methods. Additionally, this document defines a profile of the Simple Authentication and Security Layer (SASL) for NNTP. [STANDARDS-TRACK] This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document specifies how to use the Internet Key Exchange (IKE) protocols, such as IKEv1 and IKEv2, to setup "unauthenticated" security associations (SAs) for use with the IPsec Encapsulating Security Payload (ESP) and the IPsec Authentication Header (AH). No changes to IKEv2 bits-on-the-wire are required, but Peer Authorization Database (PAD) and Security Policy Database (SPD) extensions are specified. Unauthenticated IPsec is herein referred to by its popular acronym, "BTNS" (Better-Than-Nothing Security). [STANDARDS-TRACK] This document specifies the syntax of Netnews articles in the context of the Internet Message Format (RFC 5322) and Multipurpose Internet Mail Extensions (MIME) (RFC 2045). This document obsoletes RFC 1036, providing an updated specification to reflect current practice and incorporating incremental changes specified in other documents. [STANDARDS-TRACK] This document defines the architecture of Netnews systems and specifies the correct manipulation and interpretation of Netnews articles by software that originates, distributes, stores, and displays them. It also specifies the requirements that must be met by any protocol used to transport and serve Netnews articles. [STANDARDS-TRACK] Many application technologies enable secure communication between two entities by means of Internet Public Key Infrastructure Using X.509 (PKIX) certificates in the context of Transport Layer Security (TLS). This document specifies procedures for representing and verifying the identity of application services in such interactions. [STANDARDS-TRACK] Pervasive monitoring is a technical attack that should be mitigated in the design of IETF protocols, where possible. Over the last few years, there have been several serious attacks on Transport Layer Security (TLS), including attacks on its most commonly used ciphers and modes of operation. This document summarizes these attacks, with the goal of motivating generic and protocol-specific recommendations on the usage of TLS and Datagram TLS (DTLS). This document requires that Transport Layer Security (TLS) clients and servers never negotiate the use of RC4 cipher suites when they establish connections. This applies to all TLS versions. This document updates RFCs 5246, 4346, and 2246. Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the last few years, several serious attacks on TLS have emerged, including attacks on its most commonly used cipher suites and their modes of operation. This document provides recommendations for improving the security of deployed services that use TLS and DTLS. The recommendations are applicable to the majority of use cases. This document provides recommendations for the use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP). This document updates RFC 6120.

This section lists detailed changes this document applies to .

The second sentence in the Abstract of 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 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 and fourth paragraphs in Section 1 of 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 ), 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 . An additional informative reference to is therefore added to Section 7.2 of .

The third paragraph in Section 5 of is removed. Consequently, NNTP no longer requires to implement any cipher suites, other than those prescribed by TLS (Section 9 of ) and Sections 4.2 and 4.2.1 of .

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

The text between "During the TLS negotiation" and "identity bindings)." in Section 5 of 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 , where the reference identifier MUST be the server hostname that the client used to open the connection (or the hostname specified in the TLS "server_name" extension ). 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 regarding certificate pinning and fallback. Beyond server identity checking, clients also MUST apply the procedures specified in for general certificate validation (e.g., certificate integrity, signing, and path validation). Additional normative references to (replacing the one it obsoletes), , and are therefore added to Section 7.1 of .

The first two sentences of the seventh paragraph in Section 2.2.2 of 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 apply.

This document draws heavily on ideas in 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, Viktor Dukhovni, Sabahattin Gucukoglu, Richard Kettlewell, Jouni Korhonen, David Eric Mandelberg, Matija Nalis, Chris Newman, and Peter Saint‑Andre. Many thanks to the Responsible Area Director, Alexey Melnikov, for reviewing and sponsoring this document.

Use (and define) the "implicit TLS" terminology instead of "strict TLS". The language in 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 with a reference to Section 2.1 of because the intent of the related task may not have been clear enough. Reported by Matija Nalis. Add 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 and . Ask IANA to add this document to the NNTP capabilily labels registry. Reference the security considerations of . Mention informative and normative references to add to .

Take into account all the remarks sent during IETF Last Call. Move the part about 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 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 . Support for SNI is now a MUST, and not a SHOULD. Reference 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 (TLS) to a normative reference. Minor other wording improvements.

Clarify in the introduction of that NNTP implementations compliant with this document are REQUIRED to also comply with . Improve the wording of 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.