In September 2023, the SMIME Baseline Requirements (BRs) officially became a requirement for Certificate Authorities (CAs) issuing S/MIME certificates (for more details, visit CA/Browser Forum S/MIME BRs).

The definition of these BRs served two main purposes. Firstly, they established a standard profile for CAs to follow when issuing S/MIME certificates. Secondly, they detailed the necessary steps for validating each certificate, ensuring a consistent level of validation was performed by each CA.

One of the new validation methods introduced permits mail server operators to verify a user’s control over their mailbox. Considering that these services have ownership and control over the email addresses, it seems only logical for them to be able to do domain control verification on behalf of their users since they could bypass any individual domain control challenge anyway. This approach resembles the HTTP-01 validation used in ACME (RFC 8555), where the server effectively ‘stands in’ for the user, just as a website does for its domain.

Another new validation method involves delegating the verification of email addresses through domain control, using any approved TLS domain control methods. Though all domain control methods are allowed for in TLS certificates as supported its easiest to think of the DNS-01 method in ACME here. Again the idea here is straightforward: if someone can modify a domain’s TXT record, they can also change MX records or other DNS settings. So, giving them this authority suggests they should reasonably be able to handle certificate issuance.

Note: If you have concerns about these two realities, it’s a good idea to take a few steps. First, ensure that you trust everyone who administers your DNS and make sure it is securely locked down. 

To control the issuance of S/MIME certificates and prevent unauthorized issuance, the Certification Authority Authorization (CAA) record can be used. Originally developed for TLS, its recently been enhanced to include S/MIME (Read more about CAA and S/MIME).

Here’s how you can create a CAA record for S/MIME: Suppose an organization, let’s call it ‘ExampleCo’, decides to permit only a specific CA, ‘ExampleCA’, to issue S/MIME certificates for its domain ‘example.com’. The CAA record in their DNS would look like this:

example.com. IN CAA 0 smimeemail "ExampleCA.com"

This configuration ensures that only ‘ExampleCA.com’ can issue S/MIME certificates for ‘example.com’, significantly bolstering the organization’s digital security.

If you wanted to stop any CA from issuing a S/MIME certificate you would create a record that looks like this: 

example.com. IN CAA 0 issuemail ";"

Another new concept introduced in this round of changes is a new concept called an account identifier in the latest CAA specification. This feature allows a CA to link the authorization to issue certificates to a specific account within their system. For instance:

example.com. IN CAA 0 issue "ca-example.com; account=12345"

This means that ‘ca-example.com’ can issue certificates for ‘example.com’, but only under the account number 12345.

This opens up interesting possibilities, such as delegating certificate management for S/MIME or CDNs to third parties. Imagine a scenario where a browser plugin, is produced and managed by a SaaS on behalf of the organization deploying S/MIME. This plug-in takes care of the initial enrollment, certificate lifecycle management, and S/MIME implementation acting as a sort of S/MIME CDN.

This new capability, merging third-party delegation with specific account control, was not feasible until now. It represents a new way for organizations to outsource the acquisition and management of S/MIME certificates, simplifying processes for both end-users and the organizations themselves.

To the best of my knowledge, no one is using this approach yet, and although there is no requirement yet to enforce CAA for SMIME it is in the works. Regardless the RFC has been standardized for a few months now but despite that, I bet that CAs that were issuing S/MIME certificates before this new CAA RFC was released are not respecting the CAA record yet even though they should be. If you are a security researcher and have spare time that’s probably a worthwhile area to poke around 😉