Internet DRAFT - draft-bkl-bimi-overview
draft-bkl-bimi-overview
Network Working Group S. Blank
Internet-Draft Valimail
Intended status: Informational N. Kumaran
Expires: September 12, 2019 Google
J. Levine, Ed.
Standcore LLC
March 11, 2019
An Overview of the Design of BIMI
draft-bkl-bimi-overview-00
Abstract
Brand Indicators for Message Identification (BIMI) provides a
mechanism for mail senders to publish a validated logotype that mail
receivers can display with the senders' messages. This document
provides a brief overview of BIMI and examines some of the trade offs
and decisions in its design.
Discussion venue
Comments on this draft may be directed to the BIMI list at
bimi@ietf.org.
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 September 12, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. High level architecture and data flow . . . . . . . . . . . . 3
3. Risks and problems of BIMI . . . . . . . . . . . . . . . . . 4
3.1. Private club . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Inconsistent validation . . . . . . . . . . . . . . . . . 5
3.3. Pay to Play . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. User Security . . . . . . . . . . . . . . . . . . . . . . 6
4. Indicator Publishing Options . . . . . . . . . . . . . . . . 7
4.1. Message header . . . . . . . . . . . . . . . . . . . . . 7
4.2. S/MIME signatures . . . . . . . . . . . . . . . . . . . . 7
4.3. DNS assertion records . . . . . . . . . . . . . . . . . . 8
5. Validation Requirements . . . . . . . . . . . . . . . . . . . 8
5.1. Indicator Usage and Rights Validation Scenarios . . . . . 9
5.2. Registered Mark . . . . . . . . . . . . . . . . . . . . . 9
5.3. Registered Mark (untrusted jurisdiction) . . . . . . . . 9
5.4. Common Use Mark . . . . . . . . . . . . . . . . . . . . . 10
5.5. Common Use Mark (untrusted jurisdiction) . . . . . . . . 10
5.6. New/Rebranded Mark . . . . . . . . . . . . . . . . . . . 10
5.7. Mildly Altered Mark . . . . . . . . . . . . . . . . . . . 10
5.8. Multiple Marks . . . . . . . . . . . . . . . . . . . . . 11
5.9. Derivative Mark . . . . . . . . . . . . . . . . . . . . . 11
5.10. Co-marketing . . . . . . . . . . . . . . . . . . . . . . 11
5.11. Franchisee . . . . . . . . . . . . . . . . . . . . . . . 12
6. Validator options . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Pros and cons of validation approaches . . . . . . . . . 13
6.2. Receiver Managed Reputation . . . . . . . . . . . . . . . 13
6.3. Remote Reputation . . . . . . . . . . . . . . . . . . . . 14
6.4. Centralized system . . . . . . . . . . . . . . . . . . . 14
6.5. Self validation . . . . . . . . . . . . . . . . . . . . . 14
6.6. Third Party Validation Publishing Options . . . . . . . . 15
6.6.1. Publishing validation: Certificates . . . . . . . . . 15
6.6.2. Publishing validation: API . . . . . . . . . . . . . 15
6.7. General Issues of Validation . . . . . . . . . . . . . . 16
7. Consuming the indicator . . . . . . . . . . . . . . . . . . . 16
7.1. Issues . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Reporting . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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9. Threats and Remediation . . . . . . . . . . . . . . . . . . . 17
9.1. Bad indicators . . . . . . . . . . . . . . . . . . . . . 17
9.2. Revoking Certificates . . . . . . . . . . . . . . . . . . 18
9.3. Geographic scope . . . . . . . . . . . . . . . . . . . . 18
9.4. IMAP flags . . . . . . . . . . . . . . . . . . . . . . . 18
10. Informative References . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
Brand Indicators for Message Identification (BIMI) provides a
mechanism for mail senders to publish a validated indicator, which is
typically a logotype, that mail receivers can display with the
senders' messages.
For senders, an indicator published with BIMI is available to any
recipient without having to make individual arrangements with each
recipient system. BIMI indicators are only displayed with messages
that are validated with DMARC [RFC7489]. For senders that want to
have their indicator displayed ("brand impression") this may provide
an incentive to make their mail streams DMARC compliant.
For mail systems that display indicators next to messages, BIMI
provides an automatic way to obtain indicators that have been
validated by third parties, without having to individually research
or contact each sender.
2. High level architecture and data flow
A sender publishes its indicator by putting a file containing the
indicator's image on the web, or possibly several images if they want
to show it in various forms. The sender may arrange for a third
party to verify that the sender is authorized to use the indicator.
The sender then publishes one or more DNS records with names like
selector._bimi.example.com, containing tags with the URIs of the
indicator(s), and URIs of validation information if any. If it wants
to associate different indicators with different messages, it can
publish several DNS records with the various indicator URIs, and the
message can pick a selector (see below.)
Then the sender sends mail the usual way, arranging for it to be
DMARC validated. If it wants to pick a particular indicator, a
message can include a BIMI-selector header to pick a particular
selector. Otherwise it implicitly uses the "default" selector.
(Note that this allows mail to use BIMI without having to include any
new BIMI material at all.)
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The recipient system receives the mail and does DMARC validation as
usual. If DMARC passes, it does a DNS lookup of the BIMI record
using the DMARC validated domain and the appropriate selector. It
fetches the indicator(s) using the URIs in the DNS record, may check
the third party validation, and may make other checks to decide
whether the indicator is usable.
Assuming that all worked, the recipient system adds the usual
Authentication-Results header with the DMARC and BIMI validation
results, and a new BIMI-Location header that contains the URIs of the
indicator files to the message. An MUA, either one integrated into a
webmail system or a separate one, can use the BIMI-Location header
URIs to fetch the indicators to display to the user, per its own
policy.
3. Risks and problems of BIMI
BIMI inherently assumes that showing indicators with mail messages is
desirable, and standardizing how this is done is of value for senders
and perhaps for recipients. Some mail systems have been showing
logos and other per-sender images such as avatars or pictures of the
sender for a long time.
Since indicator validation requires human effort, there will always
be avenues to show indicators that are confusing or malicious. BIMI
attempts to contain those abuse vectors in its validation process and
ensure they are mitigatable at scale. Even assuming abuse is
successfully contained within the validation mechanisms, there are
other inherent risks and potential problems that could arise out of
the usage of BIMI if it is improperly architected or implemented.
Some of the most concerning are detailed below. The trade offs
between potential problems and benefits remains unclear.
3.1. Private club
Currently large mail systems show indicators of large senders using
fragmented methods to identify the senders' mail and the indicators
to use. One of the goals of BIMI is to open up the process to
organizations that don't have access to representatives of receiving
systems, or the ability to go through a separate process for each
receiver, but if implemented poorly, it just changes the secret
handshake. While many domains may publish validated logos via BIMI,
others will be unable to or intentionally decline to participate.
Similarly BIMI can identify some set of indicators to tie to senders,
but will never handle a complete set of validated indicators either.
Depending on DMARC means that only senders with their own domain can
use BIMI. It can't identify many small senders that share their
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providers' mail domains, e.g., a girl scout troop using a single
address from their provider, such as troop42@bigisp.com, even though
the troop has a license to use the Scouts' trefoil logo. A modified
design could address this, described later.
Conversely, if the indicator has to be a registered trademark, that
limits BIMI to organizations big enough to pay for and maintain a
trademark registration (in the US typically several thousand dollars
to get a trademark, and several hundred dollars every few years for
maintenance.) Some countries have no trademark registries, or do no
meaningful examination, which means that people in those countries
are out of luck. On the other hand, if the indicator doesn't have to
be registered, see the next section.
3.2. Inconsistent validation
To the extent that BIMI has multiple validation methods, it will
produce different results on different mail systems. If systems use
their own reputation data to decide whose indicators to show,
indicators that appear on one system may not appear on another. If
there are third party validators (certificate authorities or the
like), there's no reason to assume that everyone will accept the same
set of validators, any more than every HTTPS client trusts the same
set of CA's.
Determining whether an indicator in an SVG file is the same as one in
a trademark registry is non-deterministic. The picture in a
trademark registry is typically a low resolution monochrome image.
(For example, see the copy of the IETF logo at
https://www.tmdn.org/tmview/trademark/thumbnail/US500000078755496).
There is no mechanical way to decide whether two images in different
resolutions and different sizes and colors drawn different ways are
"the same". At some point, people will have to look at them and
decide, and they will not always decide the same way. There is
limited existing practice to work from, primarily trademark
examination to determine whether a proposed trademark is
"substantially similar" to one registered elsewhere. Trademark legal
disputes are invariably about whether marks are likely to be
confused, not whether they are the same, and in any event the
resolution of the dispute generally involves lawyers and expert
witnesses. Dispute resolution about indicators is unlikely to work
at Internet scale.
For the vast numbers of organizations that don't have registered
trademarks, the problem is even worse since there's no agreed way to
find the reference mark to compare the indicator to.
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And, of course, there's no guarantee that the third party validators
will be consistently honest, competent, and diligent. Experience
with certificate authorities suggests that is improbable.
3.3. Pay to Play
DMARC started with limited use to protect heavily phished domains,
with paypal.com being the usual example. As DMARC rolled out in
other fashions (such as AOL and Yahoo! using it to deal with address
book theft), it soon became widely used and increasingly required.
This is because DMARC is a fairly effective tool to deter exact
domain phishing attacks and email-borne fraud. However, DMARC has
proved difficult to deploy for many organizations. While there are
some free resources to help people get their DMARC set up, in
practice it means that businesses pay someone to make their DMARC
work. Similarly, although the current plan is for BIMI to be totally
optional, it's not hard to see it going down the same path,
especially if logo validation is perceived by senders as a signal to
receiving systems to help disambiguate fraudulent mail streams. Even
if there are paths to getting a validated BIMI indicator without a
full trademark registration, it's likely to be complex enough that
most organizations will need a consultant. Hence it would end up as
another expensive hoop to jump through just so you can send some mail
to your clients.
Even if you do your BIMI implementation yourself, third party
validators will not be free. Validation is labor intensive, both the
process of determining that your domain matches the organization that
owns the indicator, and the process of comparing the reference
indicator to the proposed indicator. If it's a derivative or
otherwise modified mark (the girl scout troop combines the trefoil
with a picture of a local landmark) the process is even more labor
intensive. The work is similar to what trademark examiners do, and
they are attorneys with experience in trademark law.
3.4. User Security
In the current draft, every time BIMI causes an MTA or MUA to fetch a
remote resource, whoever runs that resource can tell something about
the recipient. As noted below, fetches in the MTA right after the
message has been received leak relatively little, particularly on
large systems where the resources are likely to be cached locally.
Fetches from the URL in the BIMI-Location header are a definite web
bug. This can be fixed relatively easily as described later. The
risk here is with smaller mail systems or MUAs that do not have means
to cache the logos.
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4. Indicator Publishing Options
The indicator(s) are published on the web as SVG files or potentially
other formats.
For a domain owner that wishes to assert a BIMI indicator, the
process must be as lightweight and transparent as possible. There
are multiple options, each with its own inherent value and threats,
that are possible to make this assertion.
4.1. Message header
BIMI indicators could be published by adding them in a new header
field in participating messages, along the lines of a validated
X-Face header. This mechanism is simple, but has several
disadvantages and threats:
o It requires all participating messages to have the header field;
which is neither lightweight nor transparent.
o It requires per-message validation of the header field, instead of
being cached at the domain level.
o It does not allow for the indicators to be pre-fetched and
validated in advance, which opens attack vectors from CGI scripts
and the like.
4.2. S/MIME signatures
In principle Indicators could be asserted through S/MIME
certificates. They don't share the same attack vectors as asserting
through a naked header field, and they're self-validating to the
extent the MUA trusts the signer. S/MIME is unlikely to work at
scale for a variety of reasons:
o Many senders don't have the skill to do the signing,
o major webmail systems don't interpret S/MIME signatures,
o and S/MIME certificates have all of the process issues of any
other certificates.
Requiring S/MIME for BIMI to function is neither lightweight nor
transparent for domain owners.
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4.3. DNS assertion records
Publishing an assertion record in DNS has several advantages:
o There is exactly one mechanism for both simple and complex
policies to be published.
o Operational complexity is reduced, and MTAs only need to check a
single record per sending domain in a consistent manner to enforce
policy.
o Indicators can be verified and/or cached in advance, so that
malicious headers cannot be used as an attack vector.
o By publishing a single DNS record, the indicator can be consumed
by all receiving mail systems without needing to make any
modifications to the email in transit, allowing for transparency
regardless of the source of email.
o This looks and feels similar to a DMARC record, meaning a domain
owner that is already using DMARC is familiar with the mechanism
already.
The downsides and threats of publishing an indicator in the DNS are
that this requires BIMI be a domain-based standard, and can be
compromised if a domain's DNS is compromised or hijacked.
5. Validation Requirements
For an indicator to be considered validated, there must be a
confirmed mapping among organization, domains, and indicators. To
prevent fraudulent representation of any of this mapping, all of the
following criteria must be met:
o The organization is legitimate
o The domain names are controlled by the organization
o The party requesting validation is currently authorized to do so
by the organization
o The party requesting the validation is who they say they are
o The organization has current rights to display the indicator
The final requirement, that the organization has rights to display
the indicator, is fraught with nuance, and BIMI could become a vector
for phishing unless this is handled properly. What follows are the
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different types of indicators an organization might wish to prove
they have the rights for, and how they can attest to those rights.
None of the below attestation of rights are full proof; meeting these
requirements does not mean a mail system will consider that
sufficient evidence to display the indicator. It is expected that
both validators and receiving systems will choose which of these use
cases they will support and which they will not.
5.1. Indicator Usage and Rights Validation Scenarios
There is a wide variety of ways that an indicator might be associated
with an organization and its domain(s), and how the association can
be verified.
The intent is not for a domain owner to know which of these use case
their indicator falls into and make a request accordingly, but for a
validator to make sure they use appropriate validation criteria,
given the type of indicator they are provided with.
Further, in the short term, validators may choose to only validate
indicators that can be verified using the highest standards, waiting
to verify more complicated use cases until they're certain of their
verification processes and confident they can prevent fraudulent
requests against new types of indicators.
5.2. Registered Mark
Usable when an indicator has been registered in a well known and
trustworthy jurisdiction.
To prove a logo is associated with the organization, the
registration, jurisdiction, issuance date, and expiration must be
provided, and the registrant of that mark must match the organization
requesting indication validation.
The problem with registered marks, beyond what is discussed in
Section 3.3, is that trademarks are not anti-phishing mechanisms.
Rather, they are anti-confusion mechanisms within tightly scoped
silos and jurisdictions, and they see frequent abuse and have slow
and costly remediation processes.
5.3. Registered Mark (untrusted jurisdiction)
When a registered mark in a jurisdiction that is untrusted is
provided, all the evidence should be reviewed, but validation should
proceed as if a Common Use Mark had been provided, not a Registered
one.
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5.4. Common Use Mark
A widely used but unregistered indicator. Without providing for
Common Use Marks, BIMI will certainly become a private club. This
is, however, the most difficult use case to provide rights for. You
must prove active usage of the indicator in the real world, and
provide documentation showing that the rights to the logo have been
properly assigned to your organization.
The threat here is cousin or lookalike marks. While BIMI has no
direct protections for lookalike marks, allowing for Common Use Marks
makes it far easier to validate such a mark.
5.5. Common Use Mark (untrusted jurisdiction)
Validation similar to above.
The threat in untrusted jurisdictions for lookalike or cousin domains
is much higher.
5.6. New/Rebranded Mark
A Registered or Common Use Mark that is not in wide enough use to
pass the Common Use requirements.
Many organizations come out with new products or wholly rebrand from
time to time. These common changes should be able to be reflected in
BIMI, or systems that displayed BIMI indicators could be out of date
for significant period of time until Common Usage could be proven or
even longer until a registration was completed.
Validation is the same as for Common Use, but instead of proving
usage in the real world, you must prove intent to use.
The threat here is that there is that bad actors could attempt to use
this use case to validate arbitrary logos.
5.7. Mildly Altered Mark
For an indicator to look good within a BIMI context sometimes mild
changes are required (i.e. to take a long logo and stack it so that
it will fit within a square, to remove a background color, etc.).
Specifically, a Mildly Altered Mark is a logo of one of the preceding
types of marks that has been altered minimally (recolored, cropped,
rotated, stretched, or reformatted or sliced and repositioned to fit
in a different aspect ratio). These mild modifications aren't new
indicators worthy of registration or wouldn't pass common usage tests
of their own.
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To validate a mildly altered mark, you must validate the unaltered
mark, and the validator must further attest that the modifications
are minimal per the above list and only for the purpose of working
better within a BIMI context.
The threat with mildly altered marks is that any defenses depending
on automated detection will be confused by the alterations, which bad
actors would presumably take advantage of. Further, this relies on a
human saying that an alteration is mild, which different people will
do differently.
5.8. Multiple Marks
A logo comprised of more than one of the preceding types of marks,
all from the same organization, with one or more marks potentially
obscured by others.
Validation requires validating each primary mark individually, and
the validator attesting that any obscured mark is still
distinguishable at its original self. Again, automated detection
could be confused by this, which bad actors can take advantage of.
It is also possible that obscured marks could be a vector to create
confusable indicators.
5.9. Derivative Mark
A logo comprised of more than one of the preceding types of marks, to
which new imagery has been added, potentially obscuring the initial
mark(s). Derivative marks are common, as organizations frequently
slap other images on top of their logos, such as in seasonal or
geographic campaigns. Providing this functionality dramatically
expands the approachability of BIMI for many organizations' needs.
As with Multiple Marks, anything that obscures the original mark
could create a new confusable indicator.
5.10. Co-marketing
Multiple marks or derivative marks, but from separate organizations,
where there is joint permission for use. This occurs frequently with
partnerships.
Validation is the same as for multiple marks, but each organization
must prove all validation requirements for its mark, and all
organizations must provide proof that their indicator can be used in
conjunction with the other. The expiration of the co-marketing must
also be provided for.
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Validating a co-marketing indicator would likely depend on validating
the underlying marks first.
Threats here are again related to one indicator obscuring another, or
two marks being put together in such a way as to become confusable
with a more well known one. .
5.11. Franchisee
It is common for an organization to have the rights to display an
indicator without being the owner of the indicator.
To validate this, the initial indicator must be validated, and the
franchisee must prove they have the rights to display and prove they
are in good standing with the licensor and those rights have not
expired.
The indicator would likely have to be validated by its owner before
franchisees can be granted their own validated indicator.
Threats are franchisees who were legitimate when they got the
indicator validated, but then lost standing and continued to use the
indicator.
6. Validator options
There are multiple ways that a recipient system might validate a
logo.
o Third party validation: a mutually trusted third party attests
that the domain is authorized to use the logo. There are various
ways the third party could publish the attestation.
o Local reputation: large systems track sender reputation, and could
accept logos for senders with good reputations.
o Remote reputation: There might be a shared reputation service, or
receivers could use sender characteristics like a domain in a TLD
such as .BANK or .NGO that limits registrants to known
Organizations.
o Self validation: some systems know their own customers and/or are
whitelist based, and may not require validation for published BIMI
indicators so long as DMARC passes.
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6.1. Pros and cons of validation approaches
Third party validator
Pros:
o Scalable solution that takes much of the burden off of senders and
receivers
o Senders bear a predictable cost for vetting, rather than receivers
bearing the bulk of the burden, which allows smaller receivers to
participate
o More flexibility and lower technical bar for domain owners
Cons:
o Some cost component (e.g. senders still have to pay.)
o Does not exist today; (almost) entirely new.
o Smaller receivers may not be able to tell which third parties are
reliable.
6.2. Receiver Managed Reputation
Pros:
o If a receiver already trusts a brand, validation may be more
straightforward
o Easy to set up; no additional steps on the sender side
Cons:
o Impractical for smaller receivers due to lack of adequate local or
shared reputation data.
o Reputation can be gamed.
o Lack of transparency and consistency
o Smaller senders may not send enough mail to generate trust data.
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6.3. Remote Reputation
Some registries limit registrants to verified members of a group such
as NGOs, co-ops, or banks.
Pros:
o Leverage existing validation at TLDs or similar groups.
o Easy to use, just see if the domain's in a validated name tree.
Cons:
o Limited coverage
o Unknown and highly variable quality of remote reputation
validator. (Some TLDs only validate at application time, never
check again later.)
6.4. Centralized system
Similar to government, e.g. USPTO, JIPDEC
Cons:
o Risk of too much concentration of power, too slow in terms of
responsiveness to customers and ability to execute, lack of
incentives.
6.5. Self validation
Pros:
o No overhead to publishing an indicator
Cons:
o No validation; no assurances about indicator
o Too easy to publish fraudulent indicators
o No meaningful way to evaluate as a receiving system unless the
domain is on a whitelist, in which case it's like remote
reputation.
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6.6. Third Party Validation Publishing Options
Once a third party has validated an indicator, how can it tell
consuming systems about it?
6.6.1. Publishing validation: Certificates
A third party validator could act as a CA and sign certificates that
contain the sender's domain and logo, or for large logos perhaps a
hash of the logo. See [BIMICERT].
Pros:
o Open standard, well defined and understood
o Costs borne by sender, certificates published by sender. No
centralized trusted authority that may fail catastrophically
o Technical security of certificates is well understood
o Certificate Transparency could be utilized, including
prepublication for challenge.
Cons:
o CAs have ongoing history of mismanagement, lack of trust and
responsiveness.
o Revocation doesn't really work. (See below Section 9.2.)
6.6.2. Publishing validation: API
Third party validator could provide an API where receivers could send
queries with the domain and indicator and get back yes or no, or
perhaps send the domain and get back a list of indicator hashes.
Pros:
o Doesn't depend on CAs.
o Revocation is easy.
o More flexible than certificates (easier to add or remove domains
that an indicator is validated for)
Cons:
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o Doesn't exist yet, needs to be defined, implemented, and
discoverable.
o Easily subverted into closed club or expensive (to receivers)
service.
o Could end up with many of the same issues that CAs have
6.7. General Issues of Validation
Is there a reproducible way an authority can decide whether a
proposed SVG image is close enough to an image in a registry that is
likely lower resolution and not in color? Does the expertise exist
and if so is it widespread enough to support an adequate number of
validation organizations?
How if at all to audit validation. CT logs or the like could
prepublish certificates, and log all issued certificates to aid in
forensics in case of mistakes.
How is conflict of interest handled? If third parties are paid to
validate, what is their motivation to turn away business when that
business appears to be fraudulent?
7. Consuming the indicator
The MTA fetches the certificate(s) and indicator(s) using the URIs in
the DNS record, and checks the validation.
Assuming that all worked, the recipient adds a new BIMI-Location
header to the message that contains the URIs of the indicator files.
7.1. Issues
If MUAs fetch the indicator using a URI in the BIMI-Location header,
that's a web bug with privacy issues. If the MTA put a copy of the
actual indicator as a data: URI in the header rather than an https
pointer, that would solve the web bug problem and would also make the
MUA faster. If data: is too bulky, the MTA could cache the indicator
and put a local URL in the header. The MTA still has to fetch the
indicator when the message is received, but since the message has
just arrived, the sender knows the message has been received. Since
the indicator fetch is by the MTA before the message is delivered,
the sender can't use the indicator fetch to tell when the user looks
at it in an MUA. The sender may be able to use the fetch to tell
that the message got far enough to be worth BIMI validation.
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8. Reporting
BIMI currently has no provision for reporting its use, but indicator
publishers say they want it.
o It must not be a mechanism for tracking whether messages were
delivered or opened. Caching certificates and indicators helps
here.
o It must not be a mechanism for exposing the internals of mail
systems, e.g, by leaking data about what of messages were
delivered vs not.
One possibility would be a BIMI tag similar to DMARC reporting tags
that reports how many BIMI eligible messages for a domain were
received, but not what the MTA did with them.
9. Threats and Remediation
BIMI should have some mechanism for remediating problems that one
receiver sees to make that available to all consumers in a timely
manner.
9.1. Bad indicators
An obvious threat is that a corrupt or incompetent validator approves
a indicator that a domain owner shouldn't display. There are two and
a half somewhat different scenarios here:
1. The actual owner of the domain gets a validation with his domain
and someone else's indicator.
2. A malicious third party tricks a validator into approving a
totally fraudulent validation for someone other than the owner of
the domain involved.
3. A malicious third party tricks a validator into validating an
indicator with a lookalike domain (ub3r.com vs. uber.com).
The second scenario doesn't seem important, because you can't use the
certificate unless you can send DMARC aligned mail, which you can't
do unless you control the actual domain's DNS. (If the bad guy can
take over your DNS all bets are off.)
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9.2. Revoking Certificates
If BIMI validations are published with certificates, sometimes the
validations will be wrong and a certificate will need to be revoked.
As has been widely observed, certificate revocation has never worked
very well. But since this would be a fairly unusual application of
certificates, maybe some other approaches would work if the
validators issue certificates:
1. Taking a tip from Let's Encrypt, reissue the certificates every
week. Another possibility would be some sort of stapling, but if
the CA has to do something anyway, it might as well reissue
2. Serve the certificates from the CA rather than from the sender.
(Put something in the certificate that has to match the domain in
the URI used to fetch it.) Then if the certificate turns out to
be bad, the CA can stop serving it. This is essentially the API
approach, but serving certificates rather than hashes. Or under
the assumption that most unexpired certificates are still good,
perhaps just send a hash of the certificate and get back an
answer whether it's still good, which is a lot less data.
3. Since the certificates will be validated in tens of thousands of
MTAs rather than in a billion PCs and phones, some sort of
revocation list that doesn't scale to a billion might still scale
to tens of thousands.
9.3. Geographic scope
Trademarks and indicators have geographic scope. A BIMI certificate
could presumably have a list of country or other geographic codes but
what is the recipient supposed to do with them? Does the MDA attempt
to know where its users live? Does the code go into the BIMI-
Location header (or in a certificate the header points to) so the MUA
on my phone shows me different indicators in New York or Paris? This
is a user interface issue beyond the protocol, but there should be a
plausible story that could calm the lawyers who'd be signing off on
indicator publication.
9.4. IMAP flags
BIMI invents a flag for IMAP that the MDA can set on a message to
mark that the BIMI-Location header is real. It presumably stays with
the message if it's moved to other folders. This opens a narrow
abuse channel if a malicious IMAP client can put messages with fake
BIMI-Location headers and the IMAP flag, but if your client can do
that, you will have far worse problems than bogus MUA indicators.
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An alternative approach would be for the MDA to add a DKIM signature
with recipient system's d= that includes the new BIMI-Location header
and the MUA can check that signature to decide whether to believe it.
This would require some careful sanitizing to prevent spoofing but I
think it's doable.
10. Informative References
[BIMICERT]
Chuang, W., Ed. and T. Loder, Ed., "Brand Indicator for
Message Identification in X.509 certificates", internet-
draft draft-chuang-bimi-certificate-00.txt, May 2018.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
Authors' Addresses
Seth Blank
Valimail
Email: seth@valimail.com
Neil Kumaran
Google
Email: nmk@google.com
John Levine (editor)
Standcore LLC
PO Box 727
Trumansburg, NY 14886
Phone: +1 6465701224
Email: standards@standcore.com
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