Use of Transport Layer Security (TLS) in the Network News Transfer Protocol (NNTP)
draft-elie-nntp-tls-recommendations-00

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Author Julien ÉLIE 
Last updated 2016-07-23
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Independent Submission                                           J. Elie
Internet-Draft                                             July 23, 2016
Updates: 4642 (if approved)
Intended status: Standards Track
Expires: January 24, 2017

                 Use of Transport Layer Security (TLS)
              in the Network News Transfer Protocol (NNTP)
                 draft-elie-nntp-tls-recommendations-00

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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 24, 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
     1.2.  Author's Note . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Compression . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Protocol Versions and Cipher Suites . . . . . . . . . . .   4
     2.3.  Authenticated Connections . . . . . . . . . . . . . . . .   4
     2.4.  Human Factors . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Changes to RFC 4642  . . . . . . . . . . . . . . . .   9
   Appendix B.  Implementation Notes . . . . . . . . . . . . . . . .   9
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Appendix D.  Issues to Address  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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:

   o  NNTP implementations and deployments SHOULD disable TLS-level
      compression (Section 3.3 of [RFC7525]), thus no longer using TLS

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      as a means to provide data compression (contrary to Abstract and
      Section 2.2.2 of [RFC4642]).

   o  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.

   o  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).

2.  Recommendations

   The best current practices documented in the "Recommendations for
   Secure Use of TLS and DTLS" [RFC7525] are included here by reference.
   Instead of repeating those recommendations here, this document mostly
   provides supplementary information regarding secure implementation
   and deployment of NNTP technologies.

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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 are encouraged to support options to configure
   the minimal TLS protocol version to accept, and the cipher suites not
   to accept.  Additional options can naturally also be supported.  The
   goal is to enable news administrators to easily and quickly
   strengthen security, if need be.

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:

   o  Clients authenticate servers.

   o  Servers authenticate clients.

   o  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

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   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:

   o  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).

   o  Determine the version of TLS used for encryption of a given
      stream.

   o  If authenticated encryption is used, determine how the connection
      was authenticated or verified.

   o  Inspect the certificate offered by an NNTP server.

   o  Determine the cipher suite used to encrypt a connection.

   o  Be warned if the certificate changes for a given server.

   o  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.

   o  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.

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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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3977]  Feather, C., "Network News Transfer Protocol (NNTP)",
              RFC 3977, DOI 10.17487/RFC3977, October 2006,
              <http://www.rfc-editor.org/info/rfc3977>.

   [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, <http://www.rfc-editor.org/info/rfc4642>.

5.2.  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. Elie, "Network News Transfer Protocol
              (NNTP) Extension for Compression", June 2016.

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   [RFC3749]  Hollenbeck, S., "Transport Layer Security Protocol
              Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May
              2004, <http://www.rfc-editor.org/info/rfc3749>.

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,
              <http://www.rfc-editor.org/info/rfc4422>.

   [RFC4643]  Vinocur, J. and K. Murchison, "Network News Transfer
              Protocol (NNTP) Extension for Authentication", RFC 4643,
              DOI 10.17487/RFC4643, October 2006,
              <http://www.rfc-editor.org/info/rfc4643>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5386]  Williams, N. and M. Richardson, "Better-Than-Nothing
              Security: An Unauthenticated Mode of IPsec", RFC 5386,
              DOI 10.17487/RFC5386, November 2008,
              <http://www.rfc-editor.org/info/rfc5386>.

   [RFC5536]  Murchison, K., Ed., Lindsey, C., and D. Kohn, "Netnews
              Article Format", RFC 5536, DOI 10.17487/RFC5536, November
              2009, <http://www.rfc-editor.org/info/rfc5536>.

   [RFC5537]  Allbery, R., Ed. and C. Lindsey, "Netnews Architecture and
              Protocols", RFC 5537, DOI 10.17487/RFC5537, November 2009,
              <http://www.rfc-editor.org/info/rfc5537>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,
              <http://www.rfc-editor.org/info/rfc7465>.

   [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, <http://www.rfc-editor.org/info/rfc7525>.

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   [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, <http://www.rfc-editor.org/info/rfc7590>.

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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.

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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.

Appendix D.  Issues to Address

   o  Should the paragraph starting with "Servers MUST be able to
      understand backwards-compatible TLS Client Hello messages" in
      Section 2.2.2 of [RFC4642] be modernized with current TLS
      practices?  (And which ones?)

   o  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?)

   o  Section 3.2 of [RFC7525] applied to NNTP adds the following
      requirement: a client SHOULD attempt to negotiate TLS even if the
      STARTTLS capability label is not advertised by the news server.
      The goal is to help prevent SSL Stripping.  Yet, an attacker who
      can strip STARTTLS from the capability list could easily ensure
      that 502 is answered to that command.  So, should we all the same
      keep that requirement for NNTP?  (I would suggest not to keep it.)

   o  Regarding peering between mode-switching news servers, should
      something specific be added?  (e.g., as strict TLS is the
      preferred way to negotiate TLS, innfeed would connect to port 563
      of a news server, and innd would also listen on port 563.  Or
      should we ask the registration of a new port for that purpose,
      NNSP over TLS, like port 433 already dedicated to NNSP?  Or should
      we recommend the use of stunnel with TCP wrappers, or an
      equivalent mechanism, in case using a separate port is not
      possible?)

Author's Address

   Julien Elie
   10 allee Clovis
   Noisy-le-Grand  93160
   France

   EMail: julien@trigofacile.com
   URI:   http://www.trigofacile.com/

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