Re: DNS records and delegation
There's a specific provision in the CAA checking algorithm that allows CAs to
not even bother checking CAA records if CA operates the nameservers for the
domain.
On Monday, 14 October 2019 04:28:19 UTC+2, Clint Wilson wrote:
> On Thu, Oct 10, 2019 at 11:32 PM Ryan Sleevi via dev-security-policy <
> dev-security-policy@lists.mozilla.org> wrote:
>
> > On Thu, Oct 10, 2019 at 11:42 PM Jeremy Rowley via dev-security-policy <
> > dev-security-policy@lists.mozilla.org> wrote:
> >
> > > Question, is there any prohibition against demonstration of domain
> > control
> > > being delegated to a third party or even the CA itself? I don't think so,
> > > but figured we've discussed differences in interpretation a lot lately so
> > > wanted to see if people agreed.
> > >
> >
> > Huge thanks for bringing this up early :) Definitely the example for all
> > CAs :)
> >
>
> Thank you for the response, insight, and feedback here! Really, really
> helpful in clarifying some of the open questions/discussions we've been
> having.
>
>
> >
> > That said, you probably could have worded better, "demonstration of domain
> > control being delegated to a third party" is... probably not the best way
> > of framing ;)
> >
> >
> > > I mean, the obvious issue is the customer.com domain would need to want
> > > to delegate this domain.com. But if you had a pretty non-technical
> > person
> > > operating the DNS, they could set it to the domain.com name and leave
> > > their DNS settings forever.
> > >
> > > This looks allowed under the BRs, but should it be? Or is it like key
> > > escrow - okay if a reseller does it (but frowned upon). Totally not cool
> > if
> > > the CA does it.
> > >
> >
> > This is definitely a good way of looking at it!
> >
> > I'm familiar with at least one CA using an approach like this - Amazon
> > Trust Services, with
> > https://docs.aws.amazon.com/acm/latest/userguide/gs-acm-validate-dns.html
> > .
> > There's subtlety here, though, which is why I like how you drew the
> > comparison to key escrow.
> >
> > On first glance, it seems Amazon Trust Services is issuing certificates to
> > Amazon Web Services, both (through a complex structure) Affiliates, on
> > behalf of the user. And so it might seem that the user is pointing their
> > DNS to the CA, and that's how it's working.
> >
> > However, when you look through the CCADB disclosures, along with the
> > certificates actually issued, it's clearer that the user is pointing those
> > records to Amazon Web Services, but the actual issuing CA is DigiCert (as
> > disclosed at https://www.amazontrust.com/repository/ ). It's also the case
> > that, if you look at how things are setup, the domain holder isn't the one
> > applying for the certificate - it's Amazon Web Services applying for the
> > certificate (i.e. they are the Applicant, not the domain owner), and AWS
> > configuring their own DNS records to contain a random value, to demonstrate
> > to the CA (DigiCert) that AWS, the Applicant, is authorized for the domain.
> >
> > It's a really elegant solution, but it turns out it's not a "CA
> > self-dealing" kind of thing, which would have been useful as an example if
> > it was already allowed.
> >
> > Now let's discuss how this idea could all go wrong, because it teases out a
> > little why the question of who is Applicant is relevant and important to
> > understanding.
> >
> > Let's imagine that I setup the following (and I'm going to butcher this
> > pseudo-code, so bear with me)
> > _validation.sleevi.example 3600 IN CNAME .ca.example
> > .ca.example 1 IN CNAME .random.ca.example
> > .random.ca.example 1 IN TXT "{the CA challenge goes here}"
> >
> > Now, whenever the CA receives a request to validate "sleevi.example", they
> > map that to a domain-id. They then update the CNAME, and the TXT record, to
> > match the CA-defined challenge value. Then, they issue a TXT record lookup
> > for _validation.sleevi.example, get the challenge value back, and huzzah,
> > the cert is authorized!
> >
> > But there's a small problem with this. Who was the Applicant requesting
> > sleevi.example? From the CA's perspective, how does it distinguish me, the
> > legit domain holder, from 'evil hacker'? In the Amazon case, AWS is the
> > Applicant, and AWS is the one doing this fancy DNS stuff, so it's clear -
> > only AWS can do this, and they're who the user delegated to.
> >
> > However, if the "party doing this domain dance" is != "the applicant", then
> > a lot can go wrong here, because they might do the dance for two different
> > applicants. The easiest example would be if the CA, for every CSR it
> > received, regardless of the Applicant, it updated the for
> > sleevi.example - at that point, anyone in the world could get a cert for my
> > domain from that CA, and that would not be the good result.
> >
>
> Apologies, but this isn't entirely clear to me. I'm guessing (hoping) my
> misunderstanding centers around a difference between the Applicant fully
>
Re: DNS records and delegation
On Thu, Oct 10, 2019 at 11:32 PM Ryan Sleevi via dev-security-policy <
dev-security-policy@lists.mozilla.org> wrote:
> On Thu, Oct 10, 2019 at 11:42 PM Jeremy Rowley via dev-security-policy <
> dev-security-policy@lists.mozilla.org> wrote:
>
> > Question, is there any prohibition against demonstration of domain
> control
> > being delegated to a third party or even the CA itself? I don't think so,
> > but figured we've discussed differences in interpretation a lot lately so
> > wanted to see if people agreed.
> >
>
> Huge thanks for bringing this up early :) Definitely the example for all
> CAs :)
>
Thank you for the response, insight, and feedback here! Really, really
helpful in clarifying some of the open questions/discussions we've been
having.
>
> That said, you probably could have worded better, "demonstration of domain
> control being delegated to a third party" is... probably not the best way
> of framing ;)
>
>
> > I mean, the obvious issue is the customer.com domain would need to want
> > to delegate this domain.com. But if you had a pretty non-technical
> person
> > operating the DNS, they could set it to the domain.com name and leave
> > their DNS settings forever.
> >
> > This looks allowed under the BRs, but should it be? Or is it like key
> > escrow - okay if a reseller does it (but frowned upon). Totally not cool
> if
> > the CA does it.
> >
>
> This is definitely a good way of looking at it!
>
> I'm familiar with at least one CA using an approach like this - Amazon
> Trust Services, with
> https://docs.aws.amazon.com/acm/latest/userguide/gs-acm-validate-dns.html
> .
> There's subtlety here, though, which is why I like how you drew the
> comparison to key escrow.
>
> On first glance, it seems Amazon Trust Services is issuing certificates to
> Amazon Web Services, both (through a complex structure) Affiliates, on
> behalf of the user. And so it might seem that the user is pointing their
> DNS to the CA, and that's how it's working.
>
> However, when you look through the CCADB disclosures, along with the
> certificates actually issued, it's clearer that the user is pointing those
> records to Amazon Web Services, but the actual issuing CA is DigiCert (as
> disclosed at https://www.amazontrust.com/repository/ ). It's also the case
> that, if you look at how things are setup, the domain holder isn't the one
> applying for the certificate - it's Amazon Web Services applying for the
> certificate (i.e. they are the Applicant, not the domain owner), and AWS
> configuring their own DNS records to contain a random value, to demonstrate
> to the CA (DigiCert) that AWS, the Applicant, is authorized for the domain.
>
> It's a really elegant solution, but it turns out it's not a "CA
> self-dealing" kind of thing, which would have been useful as an example if
> it was already allowed.
>
> Now let's discuss how this idea could all go wrong, because it teases out a
> little why the question of who is Applicant is relevant and important to
> understanding.
>
> Let's imagine that I setup the following (and I'm going to butcher this
> pseudo-code, so bear with me)
> _validation.sleevi.example 3600 IN CNAME .ca.example
> .ca.example 1 IN CNAME .random.ca.example
> .random.ca.example 1 IN TXT "{the CA challenge goes here}"
>
> Now, whenever the CA receives a request to validate "sleevi.example", they
> map that to a domain-id. They then update the CNAME, and the TXT record, to
> match the CA-defined challenge value. Then, they issue a TXT record lookup
> for _validation.sleevi.example, get the challenge value back, and huzzah,
> the cert is authorized!
>
> But there's a small problem with this. Who was the Applicant requesting
> sleevi.example? From the CA's perspective, how does it distinguish me, the
> legit domain holder, from 'evil hacker'? In the Amazon case, AWS is the
> Applicant, and AWS is the one doing this fancy DNS stuff, so it's clear -
> only AWS can do this, and they're who the user delegated to.
>
> However, if the "party doing this domain dance" is != "the applicant", then
> a lot can go wrong here, because they might do the dance for two different
> applicants. The easiest example would be if the CA, for every CSR it
> received, regardless of the Applicant, it updated the for
> sleevi.example - at that point, anyone in the world could get a cert for my
> domain from that CA, and that would not be the good result.
>
Apologies, but this isn't entirely clear to me. I'm guessing (hoping) my
misunderstanding centers around a difference between the Applicant fully
delegating DNS to the CA vs the Applicant only configuring a single CNAME
record? If the Applicant has configured
_validation.sleevi.example 3600 IN CNAME .ca.example
then the CA wouldn't be able to use any other value to complete
the full lookup to include the TXT record, without the Applicant directly
changing the CNAME value.
However, if the CA is fully managing this DNS, and therefore able to
independently reconfigure:
Re: DNS records and delegation
On Fri, Oct 11, 2019 at 3:14 PM Doug Beattie wrote: > Ryan, > > Are you recommending that: > a) we need a new domain validation method that describes this, or > b) those CAs that want to play with fire can go ahead and do that based on > their own individual security analysis, or > c) we need a clear policy/guideline in the BRs or root program that MUST be > followed when the CAs (maybe other entities) are updating DNS with random > values? > > I'm pretty sure I know the answer (none of the above). > I'm suggesting the BRs forbid this (and the other examples) from being done by CAs today, so (b) is not an option. This is the same as the BRs forbidding a CA from, say, using the serial number in a certificate they issue to act as a 'continuous authorization' for issuance. I'm totally open for a discussion about (a)/(c) - done correctly, it would allow us to achieve the ideal property, that: 1) We constantly go back to the DNS as the source of truth. For example, we could remove the reuse of validation data entirely here / reduce it to something small (like 5 minutes). 2) The domain name owner can voluntarily decide to make a continual authorization to a CA /for their account/, just like they can make a continual authorization to their hosting provider or other authorized entity There are a variety of challenges that could cause things to go wrong. We'd need to address those. They might be insurmountable, but it's worth discussing. This seems like potentially something suited for the IETF ACME WG, especially for the nuances of the security properties and ensuring wide review, not just among CAs and a few browsers. ___ dev-security-policy mailing list dev-security-policy@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-security-policy
RE: DNS records and delegation
Ryan, Are you recommending that: a) we need a new domain validation method that describes this, or b) those CAs that want to play with fire can go ahead and do that based on their own individual security analysis, or c) we need a clear policy/guideline in the BRs or root program that MUST be followed when the CAs (maybe other entities) are updating DNS with random values? I'm pretty sure I know the answer (none of the above). -Original Message- From: dev-security-policy On Behalf Of Ryan Sleevi via dev-security-policy Sent: Friday, October 11, 2019 2:40 PM To: Clint Wilson Cc: Ryan Sleevi ; mozilla-dev-security-policy ; Jeremy Rowley Subject: Re: DNS records and delegation On Fri, Oct 11, 2019 at 2:10 PM Clint Wilson wrote: > Apologies, but this isn't entirely clear to me. I'm guessing (hoping) > my misunderstanding centers around a difference between the Applicant > fully delegating DNS to the CA vs the Applicant only configuring a > single CNAME record? If the Applicant has configured > _validation.sleevi.example 3600 IN CNAME .ca.example then > the CA wouldn't be able to use any other value to complete > the full lookup to include the TXT record, without the Applicant > directly changing the CNAME value. > > However, if the CA is fully managing this DNS, and therefore able to > independently reconfigure: > _validation.sleevi.example 3600 IN CNAME .ca.example to > _validation.sleevi.example 3600 IN CNAME > .ca.example > that's clearly a very different story. > Is it correct to think of these as two different scenarios? In my > mind, the first scenario is the one I'm most interested in. > It's my fault for not being clearer of the example. You can think of as an ID computed from the domain being requested - e.g. "sleevi.example" - rather than the account doing the requesting (the Applicant). In that scenario, when Evil Hacker, the Applicant, requests a cert for sleevi.example, the CA doesn't look at the Applicant-ID. They look to see if the domain - e.g. sleevi.example - is one that is signed up to use their service. If so, they modify the records, and now Evil Hacker has access. I tried to clarify this later on, that it's possible to design around; for example, by using .ca.example. This is closer to what AWS does. There are /still/ risks with that approach though, in terms of greater centralization of risk. Using the AWS example, if you wanted to get a malicious cert for sleevi.example, you'd either need to compromise my DNS provider, my DNS registrar, or my AWS account. Using the example, with the CA hosting, you'd either need to compromise my DNS provider, my DNS registrar, or... well, the CA's systems. Allowing the CA to do this sort of flow creates real challenges, because they're the only ones that can issue certs. For example, the CA could refuse to use that TXT method for anyone who doesn't point to them (e.g. who uses AWS instead of the CA). It might seem odd to suggest that CAs might refuse issuance, but we see it all the time. After all, the whole reason we have so many trusted CAs is because there are a number (particularly the European CAs) that want to refuse issuance on grounds of who is applying or how they're applying (e.g. ETSI EN 319 411-et-al forbidding automation entirely!). So we know CAs can try to lock folks in, and we also know that CAs' incentives, around user/account security, are not necessarily the same as might exist with a third-party provider like AWS. I realize that seems like I'm suggesting a lot of ill-intent. I'm simply trying to threat-model both the systemic weaknesses and the economic incentives. If we allow the CAs to do this, it seems like we'd need even stronger rules regarding Subscriber/CA authentication and identification, and that... would likely be controversial and complex, to say the least ;) Would there be a benefit to something like having (or > as discussed further below) published in CAA as well? i.e. > the Applicant publishes a whitelist of valid values to > be used in this type of validation-delegation-schema? I haven't > thought this through fully, but it seems like it could help with > explicitly codifying the requirements, especially if the CAA record is > published at sleevi.example? > That's functionally what AWS is doing - it's using instead of as the binding, and pushing that in the hop. > This would suggest that we don't want the CA doing it, because the CA > is >> not the Applicant, and the goal of 3.2.2.4 is to make sure the >> Applicant can demonstrate control. >> >> Another way of looking at this is imagining the following? >> - Do we think it's allowed by the BRs for the domain operator to set >> their MX to be the CA, so the CA can auto-answer their own emails >> sent under 3.2.2.4.4.? >> > - Do we think it's allowed by the BRs for th
Re: DNS records and delegation
On Fri, Oct 11, 2019 at 2:10 PM Clint Wilson wrote: > Apologies, but this isn't entirely clear to me. I'm guessing (hoping) my > misunderstanding centers around a difference between the Applicant fully > delegating DNS to the CA vs the Applicant only configuring a single CNAME > record? If the Applicant has configured > _validation.sleevi.example 3600 IN CNAME .ca.example > then the CA wouldn't be able to use any other value to > complete the full lookup to include the TXT record, without the Applicant > directly changing the CNAME value. > > However, if the CA is fully managing this DNS, and therefore able to > independently reconfigure: > _validation.sleevi.example 3600 IN CNAME .ca.example > to > _validation.sleevi.example 3600 IN CNAME > .ca.example > that's clearly a very different story. > Is it correct to think of these as two different scenarios? In my mind, > the first scenario is the one I'm most interested in. > It's my fault for not being clearer of the example. You can think of as an ID computed from the domain being requested - e.g. "sleevi.example" - rather than the account doing the requesting (the Applicant). In that scenario, when Evil Hacker, the Applicant, requests a cert for sleevi.example, the CA doesn't look at the Applicant-ID. They look to see if the domain - e.g. sleevi.example - is one that is signed up to use their service. If so, they modify the records, and now Evil Hacker has access. I tried to clarify this later on, that it's possible to design around; for example, by using .ca.example. This is closer to what AWS does. There are /still/ risks with that approach though, in terms of greater centralization of risk. Using the AWS example, if you wanted to get a malicious cert for sleevi.example, you'd either need to compromise my DNS provider, my DNS registrar, or my AWS account. Using the example, with the CA hosting, you'd either need to compromise my DNS provider, my DNS registrar, or... well, the CA's systems. Allowing the CA to do this sort of flow creates real challenges, because they're the only ones that can issue certs. For example, the CA could refuse to use that TXT method for anyone who doesn't point to them (e.g. who uses AWS instead of the CA). It might seem odd to suggest that CAs might refuse issuance, but we see it all the time. After all, the whole reason we have so many trusted CAs is because there are a number (particularly the European CAs) that want to refuse issuance on grounds of who is applying or how they're applying (e.g. ETSI EN 319 411-et-al forbidding automation entirely!). So we know CAs can try to lock folks in, and we also know that CAs' incentives, around user/account security, are not necessarily the same as might exist with a third-party provider like AWS. I realize that seems like I'm suggesting a lot of ill-intent. I'm simply trying to threat-model both the systemic weaknesses and the economic incentives. If we allow the CAs to do this, it seems like we'd need even stronger rules regarding Subscriber/CA authentication and identification, and that... would likely be controversial and complex, to say the least ;) Would there be a benefit to something like having (or > as discussed further below) published in CAA as well? i.e. > the Applicant publishes a whitelist of valid values to be > used in this type of validation-delegation-schema? I haven't thought this > through fully, but it seems like it could help with explicitly codifying > the requirements, especially if the CAA record is published at > sleevi.example? > That's functionally what AWS is doing - it's using instead of as the binding, and pushing that in the hop. > This would suggest that we don't want the CA doing it, because the CA is >> not the Applicant, and the goal of 3.2.2.4 is to make sure the Applicant >> can demonstrate control. >> >> Another way of looking at this is imagining the following? >> - Do we think it's allowed by the BRs for the domain operator to set their >> MX to be the CA, so the CA can auto-answer their own emails sent under >> 3.2.2.4.4.? >> > - Do we think it's allowed by the BRs for the domain operator to give the >> CA FTP/SSH/file upload access to /.well-known/pki-validation, so that the >> CA can place the answer file on the server, and then request it as the CA, >> under 3.2.2.4.6? >> - Do we think it's allowed by the BRs for the domain owner to set an email >> of domain-id@ca.example using 3.2.2.4.13 / 3.2.2.4.14, and allowing the >> CA >> to self-acknowledge that e-mail? >> >> I seem to recall that there is a provision in the BRs that would prohibit >> this, at least in that none of the above (in the CA-controlled example) >> are >> demonstrating the Applicant themselves has control. And we certainly know >> from the above example that it could go quite poorly if naively >> implemented, so that's probably the right thing. >> > > Agreed with the conclusion, though I can't find a clear statement to this > effect so it remains
Re: DNS records and delegation
Hello,
I just want to add that Let's Encrypt also allows for this (at least if I
understand what you correctly)
This following is from https://letsencrypt.org/docs/challenge-types/
> Since Let’s Encrypt follows the DNS standards when looking up TXT records
for DNS-01 validation, you can use CNAME records or NS records to delegate
answering the challenge to other DNS zones. This can be used to delegate
the _acme-challenge subdomain to a validation-specific server or zone. It
can also be used if your DNS provider is slow to update, and you want to
delegate to a quicker-updating server.
Now you don't delegate it to Let's Encrypt, however you can delegate it to
a hosting provider or similar, which seems to be about the same as what AWS
is doing if I understand you correctly.
- Cynthia Revström
On Fri, Oct 11, 2019 at 7:32 AM Ryan Sleevi via dev-security-policy <
dev-security-policy@lists.mozilla.org> wrote:
> On Thu, Oct 10, 2019 at 11:42 PM Jeremy Rowley via dev-security-policy <
> dev-security-policy@lists.mozilla.org> wrote:
>
> > Question, is there any prohibition against demonstration of domain
> control
> > being delegated to a third party or even the CA itself? I don't think so,
> > but figured we've discussed differences in interpretation a lot lately so
> > wanted to see if people agreed.
> >
>
> Huge thanks for bringing this up early :) Definitely the example for all
> CAs :)
>
> That said, you probably could have worded better, "demonstration of domain
> control being delegated to a third party" is... probably not the best way
> of framing ;)
>
>
> > I mean, the obvious issue is the customer.com domain would need to want
> > to delegate this domain.com. But if you had a pretty non-technical
> person
> > operating the DNS, they could set it to the domain.com name and leave
> > their DNS settings forever.
> >
> > This looks allowed under the BRs, but should it be? Or is it like key
> > escrow - okay if a reseller does it (but frowned upon). Totally not cool
> if
> > the CA does it.
> >
>
> This is definitely a good way of looking at it!
>
> I'm familiar with at least one CA using an approach like this - Amazon
> Trust Services, with
> https://docs.aws.amazon.com/acm/latest/userguide/gs-acm-validate-dns.html
> .
> There's subtlety here, though, which is why I like how you drew the
> comparison to key escrow.
>
> On first glance, it seems Amazon Trust Services is issuing certificates to
> Amazon Web Services, both (through a complex structure) Affiliates, on
> behalf of the user. And so it might seem that the user is pointing their
> DNS to the CA, and that's how it's working.
>
> However, when you look through the CCADB disclosures, along with the
> certificates actually issued, it's clearer that the user is pointing those
> records to Amazon Web Services, but the actual issuing CA is DigiCert (as
> disclosed at https://www.amazontrust.com/repository/ ). It's also the case
> that, if you look at how things are setup, the domain holder isn't the one
> applying for the certificate - it's Amazon Web Services applying for the
> certificate (i.e. they are the Applicant, not the domain owner), and AWS
> configuring their own DNS records to contain a random value, to demonstrate
> to the CA (DigiCert) that AWS, the Applicant, is authorized for the domain.
>
> It's a really elegant solution, but it turns out it's not a "CA
> self-dealing" kind of thing, which would have been useful as an example if
> it was already allowed.
>
> Now let's discuss how this idea could all go wrong, because it teases out a
> little why the question of who is Applicant is relevant and important to
> understanding.
>
> Let's imagine that I setup the following (and I'm going to butcher this
> pseudo-code, so bear with me)
> _validation.sleevi.example 3600 IN CNAME .ca.example
> .ca.example 1 IN CNAME .random.ca.example
> .random.ca.example 1 IN TXT "{the CA challenge goes here}"
>
> Now, whenever the CA receives a request to validate "sleevi.example", they
> map that to a domain-id. They then update the CNAME, and the TXT record, to
> match the CA-defined challenge value. Then, they issue a TXT record lookup
> for _validation.sleevi.example, get the challenge value back, and huzzah,
> the cert is authorized!
>
> But there's a small problem with this. Who was the Applicant requesting
> sleevi.example? From the CA's perspective, how does it distinguish me, the
> legit domain holder, from 'evil hacker'? In the Amazon case, AWS is the
> Applicant, and AWS is the one doing this fancy DNS stuff, so it's clear -
> only AWS can do this, and they're who the user delegated to.
>
> However, if the "party doing this domain dance" is != "the applicant", then
> a lot can go wrong here, because they might do the dance for two different
> applicants. The easiest example would be if the CA, for every CSR it
> received, regardless of the Applicant, it updated the for
> sleevi.example - at that point, anyone in the world could get a cert for my
Re: DNS records and delegation
On Thu, Oct 10, 2019 at 11:42 PM Jeremy Rowley via dev-security-policy <
dev-security-policy@lists.mozilla.org> wrote:
> Question, is there any prohibition against demonstration of domain control
> being delegated to a third party or even the CA itself? I don't think so,
> but figured we've discussed differences in interpretation a lot lately so
> wanted to see if people agreed.
>
Huge thanks for bringing this up early :) Definitely the example for all
CAs :)
That said, you probably could have worded better, "demonstration of domain
control being delegated to a third party" is... probably not the best way
of framing ;)
> I mean, the obvious issue is the customer.com domain would need to want
> to delegate this domain.com. But if you had a pretty non-technical person
> operating the DNS, they could set it to the domain.com name and leave
> their DNS settings forever.
>
> This looks allowed under the BRs, but should it be? Or is it like key
> escrow - okay if a reseller does it (but frowned upon). Totally not cool if
> the CA does it.
>
This is definitely a good way of looking at it!
I'm familiar with at least one CA using an approach like this - Amazon
Trust Services, with
https://docs.aws.amazon.com/acm/latest/userguide/gs-acm-validate-dns.html .
There's subtlety here, though, which is why I like how you drew the
comparison to key escrow.
On first glance, it seems Amazon Trust Services is issuing certificates to
Amazon Web Services, both (through a complex structure) Affiliates, on
behalf of the user. And so it might seem that the user is pointing their
DNS to the CA, and that's how it's working.
However, when you look through the CCADB disclosures, along with the
certificates actually issued, it's clearer that the user is pointing those
records to Amazon Web Services, but the actual issuing CA is DigiCert (as
disclosed at https://www.amazontrust.com/repository/ ). It's also the case
that, if you look at how things are setup, the domain holder isn't the one
applying for the certificate - it's Amazon Web Services applying for the
certificate (i.e. they are the Applicant, not the domain owner), and AWS
configuring their own DNS records to contain a random value, to demonstrate
to the CA (DigiCert) that AWS, the Applicant, is authorized for the domain.
It's a really elegant solution, but it turns out it's not a "CA
self-dealing" kind of thing, which would have been useful as an example if
it was already allowed.
Now let's discuss how this idea could all go wrong, because it teases out a
little why the question of who is Applicant is relevant and important to
understanding.
Let's imagine that I setup the following (and I'm going to butcher this
pseudo-code, so bear with me)
_validation.sleevi.example 3600 IN CNAME .ca.example
.ca.example 1 IN CNAME .random.ca.example
.random.ca.example 1 IN TXT "{the CA challenge goes here}"
Now, whenever the CA receives a request to validate "sleevi.example", they
map that to a domain-id. They then update the CNAME, and the TXT record, to
match the CA-defined challenge value. Then, they issue a TXT record lookup
for _validation.sleevi.example, get the challenge value back, and huzzah,
the cert is authorized!
But there's a small problem with this. Who was the Applicant requesting
sleevi.example? From the CA's perspective, how does it distinguish me, the
legit domain holder, from 'evil hacker'? In the Amazon case, AWS is the
Applicant, and AWS is the one doing this fancy DNS stuff, so it's clear -
only AWS can do this, and they're who the user delegated to.
However, if the "party doing this domain dance" is != "the applicant", then
a lot can go wrong here, because they might do the dance for two different
applicants. The easiest example would be if the CA, for every CSR it
received, regardless of the Applicant, it updated the for
sleevi.example - at that point, anyone in the world could get a cert for my
domain from that CA, and that would not be the good result.
This would suggest that we don't want the CA doing it, because the CA is
not the Applicant, and the goal of 3.2.2.4 is to make sure the Applicant
can demonstrate control.
Another way of looking at this is imagining the following?
- Do we think it's allowed by the BRs for the domain operator to set their
MX to be the CA, so the CA can auto-answer their own emails sent under
3.2.2.4.4.?
- Do we think it's allowed by the BRs for the domain operator to give the
CA FTP/SSH/file upload access to /.well-known/pki-validation, so that the
CA can place the answer file on the server, and then request it as the CA,
under 3.2.2.4.6?
- Do we think it's allowed by the BRs for the domain owner to set an email
of domain-id@ca.example using 3.2.2.4.13 / 3.2.2.4.14, and allowing the CA
to self-acknowledge that e-mail?
I seem to recall that there is a provision in the BRs that would prohibit
this, at least in that none of the above (in the CA-controlled example) are
demonstrating the Applicant themselves has control.
