Receivers Guidance for Implementing Branded Indicators for Message Identification (BIMI)
draft-brotman-ietf-bimi-guidance-00
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draft-brotman-ietf-bimi-guidance-00
Network Working Group A. Brotman
Internet-Draft Comcast, Inc
Intended status: Best Current Practice T. Zink
Expires: August 10, 2019 Zink Magical Contraptions
February 6, 2019
Receivers Guidance for Implementing Branded Indicators for Message
Identification (BIMI)
draft-brotman-ietf-bimi-guidance-00
Abstract
This document is meant to assist receivers or other mailbox providers
by providing guidance to implementing Brand Indicators for Message
Identification (BIMI). This document is a companion to the main BIMI
drafts which should first be consulted and reviewed.
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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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 August 10, 2019.
Copyright Notice
Copyright (c) 2019 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
(https://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. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Goals for BIMI . . . . . . . . . . . . . . . . . . . . . . . 3
3. Should your site implement BIMI? . . . . . . . . . . . . . . 3
3.1. If your site satisfies the requirements, this is likely a
"yes". . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Site implementations . . . . . . . . . . . . . . . . . . . . 4
5. Validation of a BIMI message . . . . . . . . . . . . . . . . 4
5.1. BIMI Site Requirements . . . . . . . . . . . . . . . . . 5
5.2. BIMI Certificate Validation . . . . . . . . . . . . . . . 6
6. Communicating BIMI results between the MTA and the MUA . . . 6
6.1. Image Retrieval . . . . . . . . . . . . . . . . . . . . . 6
6.2. TTL of cached images . . . . . . . . . . . . . . . . . . 7
6.3. Privacy Concerns . . . . . . . . . . . . . . . . . . . . 7
6.4. Basic flow example . . . . . . . . . . . . . . . . . . . 7
7. Domain Reputation . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Rolling up based upon domain vs organizational domain . . 9
8. Working with MVAs . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Resolving disputes . . . . . . . . . . . . . . . . . . . 11
9. Troubleshooting BIMI . . . . . . . . . . . . . . . . . . . . 11
10. Public documentation . . . . . . . . . . . . . . . . . . . . 12
10.1. For Brands: . . . . . . . . . . . . . . . . . . . . . . 12
10.2. For users: . . . . . . . . . . . . . . . . . . . . . . . 12
11. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 13
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 14
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
14. Normative References . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The Brand Indicators for Message Identification (BIMI) specification
introduces a method by which Mail User Agent (MUA, e.g, an email
client) providers combine DMARC-based message authentication in
addition to cryptographic methods to ensure the identity of a sender,
and then to retrieve iconography that the sender has selected. The
iconography can then be displayed within the MUA. The displayed
iconography grants the sender brand impressions via the BIMI-capable
MUA, and should be a driving factor for the adoption of authenticated
email.
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1.1. Terminology
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
[BCP 14] [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Goals for BIMI
As stated in other BIMI drafts, BIMI intends to advance email
authentication by granting a sending party brand impressions as long
as the message passes authentication mechanisms and and meets other
receiver qualifications (reputation, encryption, whitelisting, et
cetera). DMARC currently has wide adoption by some of the
InternetaEUR[TM]s larger brands, but there is still a long tail of
small-to-medium size brands (and many large ones) that do not have
it. Because BIMI provides a visual presence in the inbox, and
because visual impressions are desirable for brands, BIMI provides an
incentive for marketers to spur DMARC adoption, whereas a concern
purely from security may not.
3. Should your site implement BIMI?
3.1. If your site satisfies the requirements, this is likely a "yes".
As email has evolved over the past three decades, it is no longer a
medium of merely exchanging text, but of enabling people to build
rich experiences on top of it. BIMI provides an incentive for brands
to send email more securely because the desired behavior - a visual
imprint in the inbox - first requires DMARC adoption.
#Terminology
The following terms are used throughout this document.
o MTA
o MUA
o DKIM
o SPF
o DMARC
o Alignment
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o BIMI Certificates
o IMAP
o Recipient Domain
o Sending Domain
o MVA
o FBL
For definitions of these terms, see the Appendix.
4. Site implementations
In order for a site to correctly implement BIMI, the receiver must be
able to perform the following:
o Validate SPF
o Validate DKIM signatures
o Validate DMARC
o Validate a BIMI Certificate (a new kind of Extended Validation
(EV) certificate)
o Fetch an image located at an https location
o For some receivers, an additional requirement is a BIMI-capable
IMAP daemon, or another method of a mail server signaling to an
MUA that it is safe to load a BIMI image , as well as securely
pointing to the BIMI location to pull it from.
A site may wish to implement URI alteration and image caching for
hosted recipients. By implementing BIMI, a site agrees that through
some combination of trust mechanisms, it will instruct a BIMI-capable
MUA to display the image fetched from a URI within the message
headers. This URI is created after the MTA authenticates a message,
and is also able to authenticate the BIMI certificate associated with
the sending domain.
5. Validation of a BIMI message
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5.1. BIMI Site Requirements
In the BIMI specification, a message MUST be authenticated via DMARC.
As stated in the DMARC draft, this requires that only one of DKIM or
SPF must successfully pass validation. However, for additional local
security measures, a receiving site may create additional
requirements for senders in order to verify BIMI (that is, indicate
to a downstream MUA that it is safe to load a BIMI logo in the email
client)
This may include, but is not limited to:
o Requiring both DKIM and SPF to validate and align with the
organizational domain in the From: address (whereas DMARC only
requires one of SPF or DKIM to align with the From: domain)
o A DMARC policy of quarantine or reject
o SPF "strength" requirements (e.g., requiring "-all", disallowing
usage of "?all" or "+all", or not allowing inclusion of overly
large address spaces)
o SMTP delivery via TLS
o Feedback Loop registration or other method of registration with
the receiving site.
These localized requirements are at the discretion of the receiving
site. In general, the stricter the criteria, the less chance there
is of an MUA erroneously showing a logo and giving the wrong signal
to a user.
Upon receipt of an email, a receiver that implements BIMI should
remove or rename any previously existing BIMI-* headers other than
BIMI-Selector, as they may have come from an attacker (as long as the
BIMI-Selector is covered by the DKIM signature; if not, it should be
removed, renamed, or ignored).
Additionally:
o It may be useful to have messages exiting a site to have those
BIMI-* headers removed as well.
o It is useful for a site that has not implemented BIMI to remove
those headers so that an MUA that does make use of those headers
would not accidentally display a BIMI image when the message has
not been properly authenticated by the email receiver (even though
an MUA should not make use of BIMI headers and instead rely upon
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settings from the mailstore, it is possible that some MUAs will
nevertheless use headers without taking appropriate precautions).
5.2. BIMI Certificate Validation
(Currently, see document in Reference below)
6. Communicating BIMI results between the MTA and the MUA
In order for a receiver that has implemented BIMI to notify an MUA
that it should display the images:
o An MTA must verify BIMI and if successful, write to the mail store
(where the messages are saved) that the message passed BIMI, and
it is safe to load the logo. For example, in an IMAP mailstore, a
flag on the message could be set that indicates that the message
passed BIMI, and a second flag that tells the MUA where to get the
BIMI logo from.
o When displaying a message, the MUA does not look for any BIMI
headers stamped by the MTA, but instead relies upon the mailstore
flags or message properties that a message passed BIMI, and use
that to decide show the logo. The MUA then pulls the required
image and displays it as appropriate.
Alternatively, the MUA may also look for the flag in the mailstore
and then attempt to extract the key/value pairs from the BIMI-
Location headers. In either case, the MUA must first check to see if
a message passed BIMI before loading the BIMI image.
While the MTA MAY stamp BIMI-related information in the message
headers, they should not be relied upon by an MUA.
6.1. Image Retrieval
A core part of the BIMI specification is that the MUA will retrieve
an image file to display for each BIMI-validated message. There are
multiple ways to accomplish this, for example:
o In its most basic setup, a BIMI-capable MUA could retrieve that
image file directly from the site specified in the BIMI record.
o Other providers may choose to cache the associated images in a
local store which could be used as the BIMI resource address in
the headers of a BIMI-approved message in a sort of proxy
configuration.
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6.2. TTL of cached images
In some circumstances it is necessary to cache the images that an MUA
would want to load. For example, if a domain owner has a short TTL
time, it would force the MUA to look it up in an unreasonably short
period of time. In this case, a receiver may want to set its own
TTL.
One option is to set it to several hours, or a day; another option is
to set the TTL to the same as the expiration period in the BIMI
certificate that points to the BIMI image. The downside is that the
caching mechanism might need to check for certificate revocation, and
then re-fetch images.
6.3. Privacy Concerns
There is some concern that the retrieval of the iconography could
result in a privacy leak.
As the images are retrieved, it's possible that the image provider
could track the retrieving system in some way. This has implications
whether it be the sender or provider that is hosting the image. For
example, a sender could include a singular selector for a single
recipient, or a provider could append a tracking string to the image
URI in the header.
An in-depth discussion of all the potential privacy leaks with
respect to loading or embedding images is outside the scope of this
document.
6.4. Basic flow example
One sample implementation of BIMI by a receiver, who does everything
on-the-fly, is as following:
o An email receiver has established a relationship with several
MVAs, trusts them, and has received their public keys for
verifying BIMI certificates. The email receiver makes these keys
available to its mail servers (e.g., by distributing local copies
to each server). [NOTE: Use of MVA above per Thede]
o Upon receipt of a message, the receiver checks to see if the
message passes aligned-SPF or DKIM, and DMARC, and ensures that
the sending domain has a DMARC policy of "quarantine" or "reject"
per local receiver policy, while properly applying the appropriate
DMARC policy to the message.
o If the message passes prior checks, the receiver will then check
to see if the domain in the From: address has a BIMI record (or,
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if the message has a BIMI-Selector header that is covered by the
DKIM-Signature, uses that to do the BIMI query in DNS).
o If a BIMI record is found, the receiver then retrieves the BIMI
certificate from the location that the BIMI record points to, and
attempts to verify the BIMI cert with each public key it has from
the MVAs that it works with.
o Upon successful verification of the cert, the receiver checks to
see if the signed image hash in the BIMI cert matches any of the
hashes of the images that the BIMI record points to (the receiver,
in this instance, is not storing any of the images locally, but
instead is downloading them on-the-fly). If a hash of a
downloaded image from the BIMI record matches the hash in the BIMI
cert, this is a successful BIMI verification.
o If the BIMI verification does not verify, then the MTA must not
indicate to the MUA to show a BIMI image. The MUA MAY show a
default image such as a set of initials, or unidentified sender.
o The email receiver then does the rest of its anti-spam, anti-
malware, and anti-phishing checks (these checks may be performed
before BIMI is verified). If a message fails a phishing or
malware checks, the email receiver must not say the message passed
BIMI. If a message is neither malware nor phishing but is
detected as spam (meaning that the message comes from a known
brand, but contains spammy content), then the email receiver may
optionally say that the message passed BIMI (and therefore a
receiver should show the image) but it is up to the receiver.
o The email receiver then sets either the appropriate IMAP flags, or
other mailstore flag, or other message property that signals to a
downstream email client that the message passed BIMI and is safe
to load the logo, along with a pointer to the logo (e.g., to the
https location specified in the BIMI record).
o What eventually happens is the email client then looks at the
flags set by the email receiver (MTA). If the flags are set to
show a BIMI logo, then the email client downloads the image and
displays it in the sender photo (or however else it chooses to
render the BIMI logo in conjunction with the message).
7. Domain Reputation
Receivers are advised to consider incorporating local sources of
domain trust intelligence into the processes which ultimately
determine whether or not BIMI logos are displayed. Simply because a
sending domain passes BIMI requirements does not mean the images
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should automatically be displayed in the MUA; a site may impose
further restrictions based on domain reputation.
One source of additional reputation intelligence could be a platform
that the email provider has created to calculate domain trust based
on historical traffic; another is an explicit list of trusted domains
that has been curated by an individual provider; a third is a list
that is purchased from a vendor that might be a pass/fail or a scored
list; another option is some mix of any of the previous three.
7.1. Rolling up based upon domain vs organizational domain
BIMI is designed to be able to work on selectors, and so in theory a
brand/domain could specify multiple BIMI logos and differentiate them
on a per-domain (per-selector) basis. The advantage for the brand is
that they can choose the image they want the user to see depending
upon various conditions (e.g., seasonal images, regional images,
etc.).
However, for an email receiver, it may be easier to roll up BIMI
logos on an organizational domain basis. One reason may be for the
purposes of reputation, another may be for simplifying management of
images. In this case, it would need to be made clear to brands that
this is how the loading of BIMI images works. This documentation
could live on a postmaster site, under technical documentation, or
other official page maintained by the receiver. It could then be
referred to when sending organizations ask about how to on-board to
BIMI at the receiver, and provide official guidance about the way it
works at the site.
If rolling up by organizational domain, then it may make sense to use
a "lowest common denominator" approach. That is, an organizational
domain must meet all the requirements for BIMI, rather than only a
subdomain. The reason for this is that if sub.brand.com gets an
image due to having strong authentication policies, but brand.com
does not, then this may cause confusion because a user may learn to
associate sub.brand.com and its image with brand.com; and if
brand.com can be spoofed even though sub.brand.com cannot, that can
lead to users becoming more susceptible to phishing from brand.com.
To alleviate this, receivers may wish to show logos only for domains
that have organizational domains with strong DMARC policies. Or, if
an organizational domain does not have a strong DMARC policy but a
subdomain does, then it may treat the organizational domain as if it
does have a strong DMARC policy so as to prevent a phisher or spammer
from impersonating the brand or any of its subdomains.
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8. Working with MVAs
Email receivers need to know whether or not itaEUR[TM]s safe to
download and display an image. That is, an attacker could go through
the trouble of creating a BIMI logo and uploading it, but the logo
may look visually similar to a real brand. For example, a spammer or
phisher could create a lookalike domain for a well-known brand such
as Paypal, then copy/paste (or slightly modify) the logo.
To prevent this, an email receiver could choose to verify logos of
known brands by themselves (do it all in-house) and establish its own
internal processes, or it could use a Mark Verifying Authority (MVA).
The receiver could then outsource the maintenance of the list of
trusted brands to the MVA, and simply download the list of brands and
images from the MVA and display the logos in its email clients.
However, even here a receiver would need to exercise caution. It
needs to ensure that MVAs follow best practices, respond to
complaints, and do a good job of vetting brands. If users ultimately
end up getting phished because they trust signals in the email
client, then it is the email receiver that will suffer the brunt of
the complaints and loss of reputation, rather than the MVA.
Therefore, an email receiver still needs to track complaints from its
users, especially with respect to phishing and impersonation, and
then send the feedback back to the MVA. If an MVA still generates
too many complaints, this could be indicative of a rogue MVA (one
that intentionally signs up malicious accounts), or a
aEURoesloppyaEUR MVA (one with internal processes that not
rigorous enough, or are designed to maximize revenue at the cost of
lax security).
An email receiver should use multiple MVAs to reduce the risk of
becoming too reliant upon a single MVA in case they have to stop
using it, and therefore lose many dozens, hundreds, or thousands of
images with no replacement and thereby contributing to user
dissatisfaction confusion. Furthermore, because MVAs may be revoked,
brands may wish to diversify their own risk by getting certified by
at least two MVAs. The reason for doing this is that if the MVA they
use ever gets revoked by an email receiver because of its bad
practices, then their own brand will suffer penalties (not having a
logo displayed) despite never having done anything wrong. By
researching multiple MVAs, a brand can reduce the chances that losing
one by a receiver affects their brand.
For this reason, brands are encouraged to get certified at multiple
MVAs, and receivers are encouraged to use multiple MVAs.
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8.1. Resolving disputes
From time to time, disputes may arise between brands where one brand
says that another is infringing on its logo.
A brand owner would want to have all email receivers stop showing
logos for the infringing brand because it could damage its own
brandaEUR[TM]s reputation. However, an email receiver is not
necessarily in a good position to determine what constitutes
legitimate usage of a logo, nor determine ownership of a logo, nor
may want the legal risk associated with making this determination.
Therefore, email receivers are strongly encouraged to partner with
Dispute Resolution Agencies. These agencies specialize in copyright
infringement resolution. An affected party would then contact the
Dispute Resolution Agency, rather than the email receiver, who would
then make the decision about if use of the logo were legitimate.
Then, they would publish the result of the dispute publicly where it
could be viewed by anyone.
MVAs should respect the decision of the courts and any brand found to
be infringing ought to be removed from their list of domains for
which they load BIMI logos for. The issuing MVA of the infringing
brandaEUR[TM]s BIMI Certificate should formally revoke it. However,
this is not guaranteed in the case of a rogue MVA or a sloppy MVA.
Therefore, email receivers should also pay attention to the Dispute
Resolution Agencies, and any results that they say are infringing
should be prevented from loading in their email clients. The email
receiver should also keep track of how often disputes occur and are
found against various MVAs, as an MVA with too many disputes ruled
against it could be evidence of a sloppy MVA or a rogue MVA.
9. Troubleshooting BIMI
There are several factors to consider for email receivers on things
that can go wrong, below are a handful of considerations:
o Failing to verify BIMI certs when they otherwise should be. This
can be caused by:
o Not having the key to a corresponding MVA
o Not having access to the key when required
o The wrong key is associated with the wrong MVA
o Failing to load a logo in the email client
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o Failing to access the logo (e.g., permissions errors)
o Connectivity problems to the logo
o Failing to display a correct logo in the email client
o Having the wrong logo stored for a brand (i.e., uploading it to a
local store but associating it with the wrong brand)
o Caching a logo for too long after it has updated
There are many reasons why a logo may fail to load; having tools to
investigate (logs, headers in messages, internal documentation that
is clearly written, having the knowledge pushed out to multiple
escalation channels) are important for investigation.
10. Public documentation
10.1. For Brands:
It is ideal to publish the criteria that is used by your site to
determine when BIMI will be displayed. It is fine to say that you
use some internal domain reputation metrics as additional criteria to
determine whether or not a logo should be displayed, and it
isnaEUR[TM]t necessary to give away the exact nature of the algorithm
other than to say "You must maintain good sending practices."
If you use an explicit whitelist, a site may want to list the minimum
requirements, and the method of applying to be whitelisted.
Similarly, a provider may wish to state what type of activity will
revoke the decision to display logos previously approved.
10.2. For users:
BIMI is not meant to instill additional trust in messages, and it is
important to make this known to your users. All messages, even those
with logos, should still be treated with (mild) skepticism, and any
action regarding the message should still be individually evaluated.
ItaEUR[TM]s possible for a site that has a high trust value to become
compromised and send fraudulent messages that could compromise a
useraEUR[TM]s system. Ensure your customers have a place that
documents BIMI and demonstrates how to check messages for fraud.
11. Appendix
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11.1. Glossary
o MUA - Mail User Agent - The application used to read messages by
the end user. This could be a thick client or a web-based
application.
o MTA - Mail Transfer Agent - Software used to transfer messages
between two systems, typically between two sites, using SMTP as
the protocol.
o SPF - SPF is a framework that designates which systems should be
sending for a given domain. This can be a list of IPs, CIDRs, or
references to DNS records. As the sender should be controlling
their DNS, they should understand which IPs should be sending as
their domain.
o DKIM - DKIM is a system by which a chosen set of headers, combined
with the message contents, are cryptographically signed, and then
validated by the receiving system. Using DNS, the receiving
system can retrieve a public key, and then validate the signature
within the headers of a message. When implemented properly, the
systems responsible for sending the messages for a given domain
name should be the only ones capable of creating messages that
correctly validates. Provided that certain restrictions are met,
DKIM is one possible technology a receiver could utilize to
authenticate messages in the context of BIMI.
o DMARC - DMARC is a message authentication mechanism that works
with SPF and DKIM. The BIMI specification requires that a message
passes DMARC. In order for a message to pass DMARC, one of SPF or
DKIM must successfully validate, and the domain in the From:
address must align with the domain that passed SPF or DKIM.
o MVA - Mark Verifying Authority - An entity that a receiver uses to
certify that the iconography that they intend to use with BIMI is
properly/legally licensed for their use.
o DRA - Dispute Resolution Authority - This organization will
moderate between two entities that believe they are both entitled
to use a logo. Receivers should then abide by the decision of the
DRA as it pertains to logo usage in the MUA.
o Alignment - Alignment refers to the organizational domain, as
defined by DMARC, of the domain in the From: address being the
same as the organizational domain that passed SPF or DKIM. For
example, foo.example.com has an organizational domain of
example.com; bar.foo.example.com also has an organizational domain
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of example.com. It aligns with org.example.com, because both have
the same organizational domain.
o BIMI Certificates - An Extended Validation Certificate is used in
conjunction with BIMI to create a place where information
pertaining to iconography for a sending domain can be securely
verified. In the case of BIMI, hashes for an MVA-approved set of
iconography will be stored in a field within the certificate.
This should allow a receiver site to validate the retrieved
imagery before putting the BIMI image URI into the message
headers.
o Terry Zink - Alex BrotmanaEUR[TM]s best friend.
12. Contributors
TBD
13. References
The full BIMI verification spec can be found at:
<https://github.com/authindicators/rfc-brand-indicators-for-message-
identification>
Verified Mark Certificates Usage: <https://docs.google.com/document/
d/1OzL9FqexZpZJQuoqAK2E3sXjOwEcLNCvXW7e88Olt2I/edit>
14. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Authors' Addresses
Alex Brotman
Comcast, Inc
Email: alex_brotman@comcast.com
Brotman & Zink Expires August 10, 2019 [Page 14]
Internet-Draft BIMI-RG February 2019
Terry Zink
Zink Magical Contraptions
Email: tzink@terryzink.com
Brotman & Zink Expires August 10, 2019 [Page 15]