General Guidance for Implementing Branded Indicators for Message Identification (BIMI)

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.¶

3.1. Brands

Organizations take great care to create and promote the image associated with their brand. By implementing BIMI, and creating additional impressions, an organization can foster a stronger tie with customers. In exchange for positive authentication, and strong DMARC policies, the MBP and MUA may show the associated logos with those messages. It should be noted that the domain holder must implement those strong policy on not just a sub-domain, but also the Organizational Domain.¶

As a Brand holder, you may need to satisfy these requirements:¶

• Ability to alter DNS to host a new TXT record¶
• A web server to host one or two files, depending on your implementation¶
• If you choose to obtain an evidence document, you will need a person to act as a representative for your company¶
• The desire to have DMARC enforcement (quarantine/reject) policies on both the organizational and sub-domains. (ex., example.com and sub.example.com)¶
• In the DMARC record, pct must be absent or 100%¶

However, also note that BIMI may not be for every domain. For example, it seems unlikely that a domain would want to implement BIMI for person-to-person correspondence. Or if a domain is not meant to send email, the domain holder may want to explicitly ensure the domain is exempted from BIMI via the BIMI DNS record.¶

3.2. Receiver

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.¶

3.3. MUA Authors

The Mail User Agent (MUA) is ultimately responsible for displaying BIMI logos. This could be an in-house/proprietary MUA, or something more generally available. While the MUA may enable the display of the logos, the responsibility for validating inbound messages lies with the Receiver/MBP. MUA Authors should also allow users the option to disable BIMI logo display.¶

3.4. MTA Authors

The receiving MTA at the destination is the system that is best suited to evaluate message authentication, as well as the DMARC and BIMI policies. The MTA would also be responsible for creating the additional headers that the MUA is meant to utilize. In an ideal world, all MTAs would support BIMI and allow the individual MBPs on deploying BIMI. The MTA would also ideally allow the MBP to alternately utilize a proxy instead of the direct URL retrieved from the BIMI record or evidence document.¶

5.1. Site implementations

In order for a site to correctly implement BIMI, the receiver must be able to perform the following:¶

• Validate SPF¶
• Validate DKIM signatures¶
• Validate DMARC¶
• Discover and fetch a BIMI assertion record using DNS¶
• Fetch a SVG using HTTPS¶
• Validate a SVG using a profile¶
• Add Authentication-Results and BIMI-* Headers to a message¶

Optionally, for a site to correctly implement BIMI evidence document (VMC is one example) verification, the receiver must be able to perform the following:¶

• Fetch the document using HTTPS¶
• Validate the evidence document¶
• Extract a SVG from the evidence document¶

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 (optionally) able to authenticate the evidence document associated with the sending domain. Discussion of these trust mechanisms is beyond the scope of this document.¶

5.2. Validation of a BIMI message

5.3. 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:¶

• An MTA must verify BIMI, and if it passes, add additional headers containing the logo to be displayed.¶

The MUA must 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 without additional checks to make sure they were added by a trusted source, for example, making sure the MTA strips existing headers on ingress, or by checking for a bimi pass in a trusted Authentication-Results header.¶

5.4. Leveraging ARC for MTA MUA communication

If both the MTA and MUA support ARC then this MAY be used by the MUA to check that the BIMI evaluation was undertaken by a trusted MTA. In this case the MTA MUST add bimi entries to the Authentication-Results and ARC-Authentication-Results headers. The MUA MUST evaluate ARC, and only use ARC sets which have passed and were added by known good servers. This evaluation MUST stop at the first ARC fail.¶

If the MUA is configured to require an evidence document then it MAY check for a policy.authority=pass in the bimi Authentication-Results and decline to show a logo if that is not present.¶

If the MTA has added a BIMI-Indicator header containing the encoded SVG then the MTA SHOULD also add a short hashed checksum of this SVG into the Authentication-Results set. The MUA MUST disregard any BIMI-Indicator headers which do not have a matching hash in the Authentication-Results headers. The hashed checksum for the Indicator MAY be added in the policy.indicator-hash entry. If no BIMI-Indicator is present, or if the hash does not match then the MUA MAY retrieve the indicator from the evidence document at URL specified in the policy.authority-uri if present, from the SVG at the URL specified in the policy.indicator-uri if present and if the MUA does not require a verified evidence document, or by evaluating BIMI directly using the domain and selector from the bimi Authentication-Results entry. The BIMI-Location header is not protected from forgery in the ARC set, and MUST NOT be used.¶

NOTE: This needs to be added to draft-brand-indicators-for-message-identification and relevant entries registered with IANA A hashed Indicator will need to be added to the AR set policy.indicator-uri to be added to draft¶

5.5. 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:¶

• In its most basic setup, a BIMI-capable MUA could retrieve the image file directly from the site specified in the BIMI-Location header.¶

• A BIMI capable MTA will add a header containing the Base64 encoded SVG of the image file. The MUA can use this header to retrieve the already validated image file for display. This is the recommended method of image retrieval as the work of retrieval and validation has already been done by the MTA. A consideration for this method may be the additional storage requirements for adding a base64-encoded version of the SVG, where the original file could be between 1 and 30 kilobytes, and encoding may add 35% to that size.¶

• 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.¶

5.6. Limited use of HTTP Redirects

• Receivers may choose not to follow HTTP redirects when retrieving images or evidence documents, or may choose to follow only a limited number of redirects.¶

• When setting up BIMI, senders should eliminate, or limit the use of HTTP redirects to avoid images being unretrievable by receivers who either do not support the use of HTTP redirection, or have limited its use.¶

5.7. 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 evidence document that contains the BIMI image. The downside is that the caching mechanism might need to check for certificate revocation, and then re-fetch images.¶

6.1. Image Display

Although BIMI does not define an aspect ratio for Brand Indicators it is expected that the majority of receivers will display them in a square or circular space. Is it recommended to brands that their Indicators should be constructed to display in a 1:1 aspect ratio, receivers should design the user interface display for BIMI Indicators with this in mind.¶

6.2. Security Concerns

Receivers should consider the impact of XML bomb or "billion laughs" Denial of Service attacks when handling XML documents such as when validating SVG documents. CVE-2003-1564 is an example of this attack.¶

When validating XML documents, receivers should consider the security and privacy implications of retrieving external entries referenced in those documents.¶

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.¶

A receiver may choose to track the number of selectors an organizational domain is permitted to use and deny processing if this exceeds a defined limit. Similarly, a receiver may choose to track and limit distinct Indicator URLs.¶

MTAs are encouraged to cache BIMI Records, evidence documents, and Indicators to limit tracking.¶

MUAs are encouraged to extract Indicators from the BIMI-Indicator header rather than retrieving them directly from the source, as doing so will limit any data exposure to the MTA processing the message. The BIMI approved SVG profile prohibits an SVG from loading external elements, this removes the risk of tracking when an Indicator is shown in the client.¶

An in-depth discussion of all the potential privacy leaks with respect to loading or embedding images is outside the scope of this document.¶

7.1. Logo Hosting Considerations

The logo you wish to associate with your brand can be hosted anywhere, not necessarily within the domain that will be used to send the messages. Doing so may make it easier to associate during inspection, though it is understood that not all entities have a web server at the domain associated with their email messages.¶

7.2. CDN Considerations

If the logo is behind a CDN (Content Delivery Network) this may prevent automated systems from reaching the resource. The automated systems may not appear to be a proper browser experience, and would not be able to correctly respond to a challenge that the CDN may use to protect a site, and therefore unable to retrieve the logo file. If possible, those BIMI logos/resources should be marked as unprotected, allowing any who request the resource to do so without possibility of a challenge.¶

7.3. Domains listed in your evidence document

While obtaining an evidence document, an entity is expected to provide at least one domain name. There exists the opportunity to list additional domains in the "SAN" field of the certificate. These domains may or may not match the 5322.From domain, but must match the domain being used in the BIMI assertion record. When using the organizational domain, other third-level domains can take advantage of the evidence document as well.
Within the core specification, it is discussed how the evaluator should look at the original domain being used, as well as the Organizational Domain.¶

8.1. Known Issues

8.2. Adherence to SVG P/S

There may be a few issues that designers may experience when trying to adhere to SVG P/S.¶

• SVG P/S is based on SVG Tiny 1.2, which does not allow for certain types of gradients. When trying to convert/save as SVG Tiny 1.2, it will typically result in an embedded raster file. This is not compliant with SVG P/S, and could result in display issues.¶
• When exporting to SVG Tiny 1.2 with Adobe Illustrator, the application will insert x and y attributes within the svg element. These need to be removed to comply with SVG P/S.¶

8.3. Tools

8.4. Caveats

9.1. Message Classification

The successful validation of BIMI does NOT indicate that a message is not spam, malware, or phishing.¶

It is expected that receivers undertake their usual message filtering and classification steps, and take the results of these checks into consideration when deciding if a BIMI Indicator should be shown to the user.¶

If classification is performed before BIMI is evaluated then a receiver MAY CHOOSE to skip BIMI processing for that message, in this case they SHOULD add a bimi=skipped entry to the Authentication-Results header for that message, and SHOULD add a comment stating the reasons for skipping BIMI processing.¶

If a message is classified as phishing or malware then the MUA SHOULD NOT display the logo.¶

If a message is classified as spam (meaning that the message comes from a known brand, but contains spammy content), then the email receiver MAY choose not to display the logo.¶

10.1. 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 sub-domain. 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 sub-domain 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 sub-domains.¶

A strong DMARC policy may be defined as one which has some level of enforcement. For example, a p=quarantine policy with an effective pct=100, or a p=reject policy.¶

10.2. VMC Root of Trust

VMCs are verified back to their issuing Mark Verifying Authority (MVA). Receivers may wish to maintain their own list of trusted CAs for BIMI rather than relying on a generally available bundle of trusted Root Certificates such as those distributed with browsers or operating systems. The AuthIndicators Working Group will maintain a list of known VMC Root CA Certificates to help bootstrap such a list.¶

11.1. SPF Concerns

SPF [RFC7208] is used to denote from where a message should be arriving, typically based on IP. SPF will only provide authentication for the first hop when sending from the originator to another internet mail site. There are some mechanisms within the SPF that could be misused in a number of ways:¶

• SPF allows for a +all mechanism. This effectively allows all hosts on the internet to be authenticated as this domain¶
• Some domains publish domains with includes that result in a large number of IPs that can be used by any number of other senders¶
• A sender could typo a CIDR from an ip4/ip6 statement¶
• A platform may not properly validate users are attached to a specified domain when sending¶
• A platform may allow for some loose rules relating to forwards, which could permit an attacker to misuse a domain¶

It's suggested that a receiver should be extremely careful when allowing a message to be authenticated solely on SPF. Similarly, a sender should do as much as they can to utilize both SPF and DKIM, properly aligned.¶

11.2. DKIM Concerns

DKIM [RFC6376] is a cryptographic signature meant to protect against tampering with a message. This method is the most likely to survive forwarding to another internet site.¶

• DKIM Replay is a method by which an attacker attempts to subvert a previously sent message, and use the signature to send something different. See [draft-chuang-dkim-replay-problem] for additional information.¶
• Poor selection for signature algorithm, the key length, or the length which a key has been in use¶
• Oversigning of headers (RFC6376, Section 8.15) is considered a partial protection against DKIM Replay, and should be considered by senders implementing BIMI¶
• Expiration of signatures utilizing the -x option while signing¶

13.1. Documentation 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 isn'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 allow list, a site may want to list the minimum requirements, and the method of applying to be listed. Similarly, a provider may wish to state what type of activity will revoke the decision to display logos previously approved.¶

13.2. Documentation 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. It's possible for a site that has a high trust value to become compromised and send fraudulent messages that could compromise a user's system. Ensure your customers have a place that documents BIMI and demonstrates how to check messages for fraud.¶

14.1. Glossary

• 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.¶

• MTA - Mail Transfer Agent - Software used to transfer messages between two systems, typically between two sites, using SMTP as the protocol.¶

• SPF - Sender Policy Framework - 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.¶

• DKIM - DomainKeys Identified Mail - 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.¶

• DMARC - Domain-based Message Authentication, Reporting, and Conformance - 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.¶

• 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, baz.example.com has an organizational domain of example.com; bar.foo.example.com also has an organizational domain of example.com. It aligns with org.example.com, because both have the same organizational domain. A definition of organizational domain and methods of discovery may be found in the DMARC RFC.¶

• 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.¶

• 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.¶

• VMC - Verified Mark Certificate - 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.¶