Dear all,
The vendor of our CA system has started planning for the implementation of DNSSEC validation of Domain Validation and CAA (SC-085v2).
On point came up: We do currently support domain validation method 3.2.2.4.4 Constructed Email to Domain Contact. This is implemented in a way that the CA system has the mail-server configured where it hands off the emails to be sent out.
It's now not really clear how to handle this with respect to DNSSEC validation. Is the expectation of the community that the sending mail-server will have to do DNSSEC validation as described in the TLS BR?
If so, that would have the side-effect that when such a DNSSEC validation fails, the mail-server currently has no way of signaling this failure back to the CA system. This in turn would mean that the customer would simply not receive the constructed email with the token and the domain validation would remain in a "pending" state.
1. What is the community's expectation regarding DNSSEC checks for email-based domain validation methods?
2. How are other CA's implementing this case?
Thanks for any feedback and experience sharing!
Kind regards
Roman
Roman Fischer
Information Security Manager
+41 76 310 12 66
SwissSign AG
Sägereistrasse 25
Postfach
CH-8152 Glattbrugg
swisssign.com
Nichts mehr verpassen: Folgen Sie uns auf LinkedIn!
Abonnieren Sie unseren Newsletter oder besuchen Sie unseren Blog.
--
You received this message because you are subscribed to the Google Groups "Server Certificate WG (CA/B Forum)" group.
To unsubscribe from this group and stop receiving emails from it, send an email to servercert-w...@groups.cabforum.org.
To view this discussion visit https://groups.google.com/a/groups.cabforum.org/d/msgid/servercert-wg/ZR0P278MB01708F040851FCAA255AFC2EFA30A%40ZR0P278MB0170.CHEP278.PROD.OUTLOOK.COM.
-- Dimitris Zacharopoulos CA/B Forum SCWG Chair
Dear all,
Following up on a discussion during the last face-2-face meeting, we are asking the community for feedback on (realistic) threat vectors that could abuse the situation where DNSSEC would not be checked for E-Mail based Domain Control Validation while DNSSEC would be checked for CAA checks.
The reason we're looking for such threat vectors is to decide if a temporary exclusion of DNSSEC check for e-mail based DCV would present a risk that is not sufficiently mitigated by doing CAA checks with DNSSEC validation during certificate issuance.
Kind regards
Roman
DZ.
Oct 23, 2025 12:12:43 'Martijn Katerbarg' via Server Certificate WG (CA/B Forum) <server...@groups.cabforum.org>:
DZ.
Oct 23, 2025 13:22:31 'Martijn Katerbarg' via Server Certificate WG (CA/B Forum) <server...@groups.cabforum.org>:
Indeed it is!
>I guess from this point on, the big question is if the WG considers this risk acceptable in light of the deprecation of email/phone methods, and allow these methods to continue without DNSSEC validation, or not.
On 10/23/2025 9:56 PM, Michael Richardson wrote: > 'Dimitris Zacharopoulos' via Server Certificate WG (CA/B Forum) <server...@groups.cabforum.org> wrote: > > Thanks Martijn, let me try to describe the threat in my own words to > > make sure I understood. During the DCV phase, the attacker sends an > > unsigned fake DNS response about the MX record associated with a Domain > > Name (example.com), the CA does not check DNSSEC errors and sends the > > challenge to the inbox of -say- admini...@example.com which ends up > > to the mail server of the attacker and completes the DCV. > > Yes. So it seems like if there is DNSSEC, then it needs to be validated. > Seems like an own goal by the CA, as example.com did the right thing with DNSSEC. > > > Then the attacker stops the DNS attack and sends a request to issue the > > certificate, in which case the CA will query the real DNSSEC-signed > > zone, verify there is no CAA record (or a CAA that allows issuance) and > > issues the certificate. Is that a fair description of the threat > > scenario? > > > If this is a correct description, I would like to ask Henry who's > > tested equally-specific prefix BGP attacks before (or anyone else with > > similar experience), how likely and easy it is for such a threat to be > > executed considering the fact that the fake mail server must be up for > > some time in order to receive the challenge email. > > I don't know why you need a BGP attack. > The mail server can be at fubar.attacker.example, and it can just remain live > for as long as they like. I'm not sure if mailenator.com will accept all > email, or just mail to @mailenator.com, but if it accepts anything, then one > could just use that. I originally thought one would need to perform a BGP attack to hijack the IP address of the official mail server but you're right, if the attacker can spoof the DNS it can change the MX record value to anything under the attacker's control. So, assuming the CA performs the DCV process far apart from the certificate issuance (when it checks CAA), this method does not protect domain owners as well as the other methods. I guess from this point on, the big question is if the WG considers this risk acceptable in light of the deprecation of email/phone methods, and allow these methods to continue without DNSSEC validation, or not. Dimitris. -- You received this message because you are subscribed to the Google Groups "Server Certificate WG (CA/B Forum)" group. To unsubscribe from this group and stop receiving emails from it, send an email to servercert-w...@groups.cabforum.org. To view this discussion visit https://urldefense.com/v3/__https://groups.google.com/a/groups.cabforum.org/d/msgid/servercert-wg/0c434c43-22cd-4e0d-8848-a854d237392f*40harica.gr__;JQ!!J5K_pWsD!3_YHRDKMRVzPFnfjQEFPTjSsZYh1olCrg2qzYMSOBtMqMTWAHEKDJkCoV6eY9y-D8le7ZNhpS8389mkEkop-T_HdnFvHcHk4QhosTHE$.
Hi Martijn,
Just regarding the multiple DNS resolvers thing: At least in our environment we already use two resolvers. One is built-in to the CA system and the other is used for everything else, including outbound e-mail. My guess is that some CAs may even have (or want to) offload the e-mail sending to a cloud service which would also use a separate DNS resolver.
Rgds
Roman
Yes, I'm aware of the Delegated Third Party issue. 👍 Just brought it up because of the recent discussions about possible usage of cloud services. 😉
-Roman
Hi Dimitris,
>Is that a fair description of the threat scenario?Indeed it is!>I guess from this point on, the big question is if the WG considers this risk acceptable in light of the deprecation of email/phone methods, and allow these methods to continue without DNSSEC validation, or not.Agreed. I think you know my opinion 😉.
I would add that a CA not wanting to do DNSSEC for email methods, might need to introduce more complexity into their systems. I would assume most CAs use 1 pair of DNS resolvers, managed by themselves, for any DCV related activities. If DNSSEC checking is enforced on that resolver, then for any CA to not do DNSSEC for email, they would need to setup an additional pair or resolvers. This increases the risk for misconfigurations and a CA might suddenly find themselves in a spot where they’ve used the wrong one for the wrong method.
The recent analysis provided by the Chrome team in the SC-90 discussion was very useful. The conclusion I made from this analysis is that the email validation methods are not considered "broken" but not "upgrade-able", and not able to improve their security posture compared to the other DNS/HTTP-based methods which have gained so much from MPIC and DNSSEC. Chris, is that a fair statement?
If that's the case and the deprecation path and timeline for SC-90 is something the WG can agree to, is the DNSSEC enforcement for email DCV methods really necessary at this point, considering the fact that CAs which have not implemented DNSSEC to their email servers will have to make changes, only to deprecate these methods two years later?
Does it make sense to add some language in SC-90 to allow an exception for DNSSEC enforcement in the email DCV methods until these methods are deprecated?
To view this discussion visit https://groups.google.com/a/groups.cabforum.org/d/msgid/servercert-wg/d84d3489-8be1-4272-a48e-6abb95693eb2%40harica.gr.
The recent analysis provided by the Chrome team in the SC-90 discussion was very useful. The conclusion I made from this analysis is that the email validation methods are not considered "broken" but not "upgrade-able", and not able to improve their security posture compared to the other DNS/HTTP-based methods which have gained so much from MPIC and DNSSEC. Chris, is that a fair statement?The “not upgrade-able” characterization seems accurate, but it’s also a symptom of the larger issue, which is that these validation methods are already at a deficit from a security-by-design perspective.
They rely on a less direct proof of control, inherit systemic vulnerabilities from the entire email ecosystem, and present a broader attack surface than the alternatives.
If that's the case and the deprecation path and timeline for SC-90 is something the WG can agree to, is the DNSSEC enforcement for email DCV methods really necessary at this point, considering the fact that CAs which have not implemented DNSSEC to their email servers will have to make changes, only to deprecate these methods two years later?
Does it make sense to add some language in SC-90 to allow an exception for DNSSEC enforcement in the email DCV methods until these methods are deprecated?SC-090's objective is the full deprecation of those methods due to their inherent risk. While it may have gone unnoticed to some, SC-090’s preamble acknowledges the DNSSEC challenges for email-based methods. The ballot attempts to mitigate this risk by discouraging, but not prohibiting, their use upon SC-085 becoming effective (example). This intends to balance short-term tradeoffs while realizing the long-term benefit.
Generally speaking, we think ecosystem resources should focus on continued migration to more secure, modern methods, rather than temporarily hardening phone- and email-based ones.
Regardless, if the community feels explicit exemptions for SC-085 are necessary, we believe they should occur in a separate and targeted ballot rather than being commingled with SC-090's existing text. This approach is consistent with past working group discussions on managing ballot scope and complexity, just as the community agreed that SC-091's contents, though closely related to SC-090, were best served by a separate ballot.
Hi Dimitris,
To clarify our perspective, when we discussed SC-090 at the 2025-09-04 Validation Subcommittee, we also considered including the contents of what is now SC-091 within SC-090’s scope. The group concluded that despite these work items being related and motivated by a shared problem, narrowly focused ballots are best and allow targeted discussion.
To maintain consistency with that principle of creating narrowly focused ballots, while also avoiding unnecessary or risky dependencies, we still think it would be most effective to address potential DNSSEC exception(s) as a separate work item, especially since:
The idea of an exception was just recently raised in this thread about 2 weeks ago, and consensus on the path forward is not yet clear.
The relevant DNSSEC requirements do not take effect until March 2026, allowing ample time for a thorough, dedicated discussion.
Thank you
-Chris
--
You received this message because you are subscribed to the Google Groups "Server Certificate WG (CA/B Forum)" group.
To unsubscribe from this group and stop receiving emails from it, send an email to servercert-w...@groups.cabforum.org.
To view this discussion visit https://groups.google.com/a/groups.cabforum.org/d/msgid/servercert-wg/591e7b90-f09e-4f21-b504-f30c0e8a3db0%40harica.gr.