[Smcwg-public] Proposed method for "validating applicant as operator of associated mail servers"

Fri Jan 21 14:53:59 UTC 2022

Hello Stephen,

It would probably be helpful to understand the underlying principles of the
validation method in 3.2.2.1 and the proposed validation method to fully
understand the risks and decide on the validation reuse.

Method 3.2.2.1 (Enterprise RA) validates that the Applicant has control
over the domain portion of the email address. The underlying principle is
that since the Applicant has control over the domain, they can change the
MX pointers to point to another mail server that they control, and
therefore control the mailbox. In fact, there is a concern here on whether
a validation method that verifies operation of the domain by verifying that
the Applicant controls the content served over HTTPS, such as 3.2.2.4.6
(Agreed‑Upon Change to Website), can be used to validate control of the
mailboxes.

The proposed method directly validates control of the mail server by
performing resolution of the SMTP FQDN. Therefore, the underlying principle
is the same.

Since the security properties of both methods are the same, I believe that
the reuse of the validation data should be the same, unless there is
something I am missing.

Best regards,

Fotis

On Thu, Jan 20, 2022 at 11:54 PM Stephen Davidson via Smcwg-public <
smcwg-public at cabforum.org> wrote:

> Thanks Dimitris.
>
>
>
> You can see the PR at https://github.com/cabforum/smime/pull/34/files,
> which has some related comments in place.
>
>
>
> Your comment is also related to the upcoming topic on reuse of validation
> data.
>
>
>
> When (email) domain validation is used as described in Section 3.2.2.1 we
> will adopt the TLS BR standard allowing reuse for all corresponding
> Certificate Requests within a period up to a maximum of 398 days.
>
>
>
> I suggest that when mailbox validation is used as described in Section
> 3.2.2.2 and in this MX proposal:
>
> 1) the check must be checked for each Certificate Request, and
>
> 2) A successful validation for each Certificate Request is valid for up to
> 30 days.
> An alternative would allow reuse of the successful verification for all
> corresponding Certificate Requests up to 30 days (ie, not be required every
> time).
>
>
>
> Happy to hear alternate view points on this.
>
>
>
> Best regards, Stephen
>
>
>
>
>
> *From:* Smcwg-public <smcwg-public-bounces at cabforum.org> *On Behalf Of *Dimitris
> Zacharopoulos (HARICA) via Smcwg-public
> *Sent:* Wednesday, January 19, 2022 1:37 PM
> *To:* smcwg-public at cabforum.org
> *Subject:* Re: [Smcwg-public] Proposed method for "validating applicant
> as operator of associated mail servers"
>
>
>
>
> This method has several challenges some of which have already been
> discussed in the validation subcommittee and the server certificate working
> group. It's basically the fact that the control of the domain name is
> performed by a different entity, which is different than the actual
> "Applicant". It is like the Applicant is delegating control over to a
> different entity.
>
> Assuming we overcome these issues, if we accept the fact that the operator
> of a server in the MX records of a Domain Name is by virtue *authorized *to
> issue any S/MIME certificate that contains any email address that contains
> the domain part of the validated domain name, this validation evidence *should
> not be allowed to be reused*. That is because the Domain Name owner could
> decide to switch to another mail provider, change the MX records and
> wouldn't want the previous mail provider to be authorized to issue
> certificates under their Domain Name.
>
> This means that, similarly to the CAA mandatory checking requirement in
> the TLS BRs, the CA would need to always check the MX records for every
> Domain Name being validated using this method.
>
>
> Dimitris.
>
> On 19/1/2022 7:17 μ.μ., Stephen Davidson via Smcwg-public wrote:
>
> Hello all:
>
>
>
> Per our discussion today of the draft text of section 3.2.2 “Validation
> of mailbox authorization or control
> <https://github.com/cabforum/smime/blob/preSBR/SBR.md#322-validation-of-mailbox-authorization-or-control>”
> Fotis Loukos has proposed a new method for "validating applicant as
> operator of associated mail servers."  This would apply in cases where I
> have my own domain but redirect/outsource the operation of my entire email
> domain to a service.
>
>
>
> I believe that we should accommodate this common use case in the S/MIME
> BR, but know that it’s different from our previous discussions on mailbox
> verification which centered mainly on familiar methods from the TLS BR.  As
> such, I attach the proposed text below and hope that WG members can review
> the associated RFC 5321 Section 5.1 and provide feedback on list.  For
> example, this may tie in with another agenda item on the reuse periods for
> different types of verification. If needed we can schedule specific time
> (perhaps at the F2F) to work on this as well.
>
>
>
> It was a useful call today; thank you.
>
> Regards, Stephen
>
>
>
>
>
>
>
> *#### 3.2.2.3 Validating applicant as operator of associated mail
> server(s)*
>
>
>
> Confirming the Applicant's control over the rfc822Name email address by
> confirming control of the SMTP FQDN to which a message delivered to the
> email address should be directed. The SMTP FQDN MUST be identified using
> the address resolution algorithm defined in RFC 5321 Section 5.1 which
> determines which SMTP FQDNs are authoritative for a given email address. If
> more than one SMTP FQDNs have been discovered, the CA MUST verify control
> of an SMTP FQDN following the selection process at RFC 5321 Section 5.1.
>
>
>
> When confirming the Applicant's control of the SMTP FQDN, the CA MUST use
> the methods described in Section 3.2.2.4 of the TLS Baseline Requirements.
>
>
>
> This method is suitable for validating control of all email addresses
> under a single domain.
>
>
>
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-- 

Fotis Loukos |  Security Engineer |  fotisl at google.com |
Brandschenkestrasse 110, 8002 Zurich, Switzerland
Company Identifikationsnummer: CH-020.4.028.116-1

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