Internet DRAFT - draft-otis-dmarc-author-align
draft-otis-dmarc-author-align
dmarc D. Otis
Internet-Draft Trend Micro
Intended status: Experimental March 4, 2015
Expires: September 5, 2015
DMARC Author Align
draft-otis-dmarc-author-align-01
Abstract
Deals with DMARC acceptance failures disrupting legitimate and valid
message distribution affecting millions while attempting to exclude
From header field domains not aligned with email acceptance methods
in a manner incompatible with normal email conventions. DMARC does
not accommodate recent message structure underscoring the erroneous
premise of a store-and-forward transport enforcing non-negotiable
message structure. Active exploitation of DMARC dependency on flawed
acceptance practices are further aggravated when its feedback is sent
to malefactors. Risks prevented by careful accommodation of
legitimate and valid messages also better ensures economic, social,
and civic benefits derived from an open exchange of email.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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/.
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 September 5, 2015.
Copyright Notice
Otis Expires September 5, 2015 [Page 1]
Internet-Draft DMARC-Author-Align March 2015
Copyright (c) 2015 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Domain Authorization Issues . . . . . . . . . . . . . . . . . . 3
3. Methods For Preventing Disruption . . . . . . . . . . . . . . . 4
3.1. Past solution used with Sender-ID . . . . . . . . . . . . . 5
4. Why DMARC . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Privacy Considerations . . . . . . . . . . . . . . . . . . 7
4.2. Security Considerations . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
Otis Expires September 5, 2015 [Page 2]
Internet-Draft DMARC-Author-Align March 2015
1. Introduction
SMTP security via opportunistic DANE TLS
[I-D.ietf-dane-smtp-with-dane] should soon offer a secure global host
identity scheme for email. DNSSEC/DANE overcomes security weaknesses
found in both routing and message exchange. While some claim DNSSEC/
DANE is not practical they also misrepresent weaker methods based on
IP address authorization or signed message fragments as representing
domain authentication. IP address based authorization or potentially
malformed message fragments can not safely verify a domain is bound
with a message. DMARC even offers malefactors feedback which can
enhance the exploit effectiveness or leak relationship information
used to facilitate deceptions. Misleading use of the term
"authentication" which conflicts with [RFC3552] and [RFC4949] occurs
with [RFC7001] and [I-D.kucherawy-dmarc-base].
2. Domain Authorization Issues
The domain referenced by SPF [RFC7208] or the domain of a DKIM
[RFC6376] signature has not been a problem since seldom was
acceptance based on From header field Domain Alignment with those
used by these two methods. However, when acceptance is based on From
header field alignment in the case of [I-D.kucherawy-dmarc-base]
using either SPF or DKIM related domains, this now disrupts many
Third-Party Services and deprecates the use of the From header field
of being able to retain the role of Author. The disruption becomes
egregious when messages from the domain's own users are rejected
based on the erroneous level of this domain's asserted alignment
practices. At the strictest alignment level, erroneous assertions
not only disrupt messages from their users, it also affects
subscriptions or services for other users of the Third-Party Service.
SPF normally provides a form of authorization by listing IP addresses
of authorized outbound servers. In many cases, these servers
represent a shared resource used by perhaps thousands of domains.
SPF is unable to verify an IP address represents the actions of a
claimed domain which does not meet the definition of "$
authentication" in [RFC4949].
DKIM intended to establish increased levels of trust based upon
verified DKIM signatures controlling acceptance and what a user sees
within the FROM header field. But DKIM failed to guard against pre-
pended header fields to ensure acceptance is not based on verified
DKIM signatures that don't prevent header field spoofing, especially
that of the FROM. This weakness allows malefactors to exploit DKIM
signature acceptance established by high-volume DKIM domains to spoof
ANY other domain, even when prohibited within the Signer's network.
Otis Expires September 5, 2015 [Page 3]
Internet-Draft DMARC-Author-Align March 2015
Some argued a requirement that DKIM validation include assurances the
signed header fields are not invalidly repeated represents a protocol
layer violation. Reporting a signature verified by a process that
MUST examine the entire header field stack as needing such a
recommended validation be made by a prior unreported and unknown
process suggests a greater violation, if not in protocol, in trust.
It took several years for one of the largest service providers to
notice this oversight long after arguments were made about this risk.
Ignoring essential header field stack validation that MUST occur
represents an oversight in the DKIM deployment specifications that at
one time had been partially addressed by the earlier DMARC
specifications. It seems even this validation has been removed by
what might be described as a misguided insistence such processing
remains the responsibility of the transport.
[RFC5321] Section 3.3 clearly indicates messages SHOULD NOT be
rejected based on perceived defects in [RFC5322] message structure.
Section 7.1 also warns against preventing spoofing within the SMTP
transport and suggests much safer PGP or S/MIME, both of which
benefit by deployment of DANE. DMARC was developed to curtail
phishing attempts leading to user attrition with high volume
transactional services. Unfortunately, DMARC is being (ab)used to
lessen phishing attempts related to general user accounts where there
seems little interest at finding a solution for the problems this
creates.
3. Methods For Preventing Disruption
Conditionally permit Sender header field alignment: For domains
handling normal user email, a special DMARC assertion could allow
policy be established by the Sender header field when present with
an assumption their users employ Mail User Agents displaying the
Sender header field.
Define a new Author header: A new "Author" header field can be used
to re-establish the Author role for RFC5322.From domains affected
by those handling normal user email employing Third-Party services
sending their messages on their behalf. For this provision to be
effective, a practice of re-locating the Author role to this new
header needs to be established by these third-party services.
Establishing a new header prevents confusion caused by the use of
unknown alternatives, such as Reply-To, or Original-From, or
indirectly with use of Original Authentication Results Header.
Third-Party Authorization: A different domain is specifically
Otis Expires September 5, 2015 [Page 4]
Internet-Draft DMARC-Author-Align March 2015
excluded from actions caused by non-alignment when authorized by
the DMARC domain using [I-D.otis-tpa-label].
A few large domains have had a high percentage of user accounts
compromised. These events gave malefactors access to prior private
exchanges and contact lists. Even after accounts were reclaimed,
malefactors continue sending convincing spoofed messages from other
sources. To mitigate harm, some domains have asserted DMARC
Alignment policies similar to those used by domains that only emit
transactional messaging where a DMARC recommendation of restricted
use is normally heeded. In addition, some domains also recommended
"reject" rather than "quarantine" as a misalignment response. In
conjunction with misleading DMARC alignment assertions, rejection
becomes a highly disruptive choice.
Currently, the least disruptive adjustment made by receivers faced
with Third-Party services used by a RFC5322.From domain is to
override their policy of "reject" with "quarantine" to allow delivery
of the message causing users to search through their "quarantine"
folder for otherwise lost messages. Alternatively, the From header
field may replace the Author role with that of the Sender. Some have
suggested the From header field contents could be retained in the
Reply-To header. Others suggested creating an X-Original-From header
field which could be given the name Author.
3.1. Past solution used with Sender-ID
During the development of Sender-ID, some Mail User Agents combined
the Sender header field with that of the From header field when
alignment matched the Sender rather than the From. The synthesized
header became "sender address" on behalf of "from address". A more
understandable approach would be to always display the Sender header
field when present and not matching the domain of the From header
field. This change would include policy following that of the
Sender, which is the domain really being trusted and most likely in
alignment with the acceptance mechanisms. This change would
eliminate most of the disruptions now resulting from DMARC while also
improving security.
DMARC could include an alignment assertion permitting policy to be
based on the Sender header. Those domains that failed to ensure
against the use of Third party services, could have their policy
altered by the receivers to permit Sender alignment. In most cases,
this would remove the need of users to risk recovering messages from
their "quarantine" folder.
Otis Expires September 5, 2015 [Page 5]
Internet-Draft DMARC-Author-Align March 2015
It is unfair to place a large burden on receivers and expect them to
remain cooperative. Prior to making alignment assertions likely to
disrupt services handling legitimate messages, it is possible for
RFC5322.From domains to make assertions which allow compliance with
normal email handling. When RFC5322.From domains proactively guard
against disrupting legitimate messages, receivers are more likely to
cooperate with their recommendations. When the asserted policies
prove disruptive over time, DMARC should offer receivers reasonable
overrides.
4. Why DMARC
Deterrents based upon reputation and/or path based scoring strategies
that utilize a variety of originating header fields has proved
ineffective. These header fields often remain invisible to
recipients, and contain domains exploited for periods measured in
hours, to avoid any Whack-A-Mole like response. Even long term
reputations have issues due to an intermix of messages from
compromised accounts. Content filtering is unable to keep up with
the polymorphic abuse. Few recipients will inspect the stack of
message header fields, or be able to draw useful conclusions from a
profusion of unfriendly information. As a result, many recipients
deal with abuse by sorting messages into groups based on assumed
sources found in a few originating header fields.
DMARC represents an open registry that offers domain specific
guidance for DKIM/SPF alignment sending practices to determine
whether messages should be delivered, quarantined, or refused.
However, appropriate actions become unclear whenever Third-Party
Services are involved. Although DMARC warns of a potential for
disruption, the specific handling requested by DMARC is very limited.
DMARC expects receivers to devise their own special handling to
mitigate disruptions that DMARC assertions might cause for legitimate
messaging. This is unfortunate, since the necessary feedback is
given to the DMARC asserting domain and not to the cooperating
receivers.
When a Third Party domain does not employ DKIM or SPF or does not
include Authentication-Results header fields [RFC7001] or perhaps
[I-D.kucherawy-original-authres] (OAR) or its "X-" version could
allow authorizations to be exploited. For Third Party domains not
applying DMARC but capture the OAR, past compliance with DMARC based
on the OAR can be made a requirement for authorization.
While conceivably Domain Alignment might just rely on the content of
the Original-Authentication-Results header, whether to trust this, or
any other message content can not be based on the mere acceptance of
Otis Expires September 5, 2015 [Page 6]
Internet-Draft DMARC-Author-Align March 2015
the message alone. Whether false content even effects message
acceptance would be difficult to determine. Only the DMARC asserting
domain is able to make this type of determination based on their
knowledge of outbound messages and corrections needed based on DMARC
feedback.
4.1. Privacy Considerations
Unless all valid Third-Party Domains have been authorized or allowed
a suitable From header field alternative, personally identifiable
information will be exchanged within the DMARC feedback. This
feedback can unintentionally expose private exchanges made on behalf
of the RFC5322.From domain's users. To the greatest extent possible,
this feedback information should not be shared with other domains not
offering the information. This feedback can even identify mailing-
list subscribers that never sent any message to the list, or invoices
made on behalf of an accountant's client.
4.2. Security Considerations
This draft extends Domain Alignment validation practices that depend
on DKIM [RFC6376] or SPF [RFC7208]. Most related security matters
are discussed in those specifications. Additional considerations are
also included in [RFC6377]. Some receivers mistakenly bypass
validation of the [RFC5322] header fields because a signature from a
Trusted Domain had been confirmed as perhaps suggested in [RFC5863].
Validation of the header stack MUST NOT be omitted unless the message
is not accepted for other reasons.
Services that depend only upon path authorizations might permit the
RFC5322.From domain to be spoofed and obtain acceptance. During such
events, the RFC5322.From domain might need to retract its
authorization from the service. For this reason, path related
validation based on IP addresses should only be used as a carefully
monitored interim solution.
5. Acknowledgements
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Otis Expires September 5, 2015 [Page 7]
Internet-Draft DMARC-Author-Align March 2015
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", RFC 3552, July 2003.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
September 2011.
[RFC7001] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7001, September 2013.
6.2. Informative References
[I-D.ietf-dane-smtp-with-dane]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-14
(work in progress), February 2015.
[I-D.kucherawy-dmarc-base]
Kucherawy, M. and E. Zwicky, "Domain-based Message
Authentication, Reporting and Conformance (DMARC)",
draft-kucherawy-dmarc-base-13 (work in progress),
February 2015.
[I-D.kucherawy-original-authres]
Otis Expires September 5, 2015 [Page 8]
Internet-Draft DMARC-Author-Align March 2015
Chew, M. and M. Kucherawy, "Original-Authentication-
Results Header Field", draft-kucherawy-original-authres-00
(work in progress), February 2012.
[I-D.otis-tpa-label]
Otis, D. and D. Black, "Third-Party Authorization Label",
draft-otis-tpa-label-05 (work in progress), March 2015.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System (AS)",
BCP 6, RFC 1930, March 1996.
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006.
[RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker,
"DomainKeys Identified Mail (DKIM) Development,
Deployment, and Operations", RFC 5863, May 2010.
[RFC6377] Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
Mailing Lists", BCP 167, RFC 6377, September 2011.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
April 2014.
Author's Address
Douglas Otis
Trend Micro
Otis Expires September 5, 2015 [Page 9]
Internet-Draft DMARC-Author-Align March 2015
10101 N. De Anza Blvd
Cupertino, CA 95014
USA
Phone: +1.408.257-1500
Email: doug_otis@trendmicro.com
Otis Expires September 5, 2015 [Page 10]
