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
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provisions of BCP 78 and BCP 79.
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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.
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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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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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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.
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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