[Acme] Re: Potential issues with dns-persist-01

[zandoodle <[e-mail]>]

Thanks for the review

On 06/04/2026 19:40, Aaron Gable wrote:
> On Sat, Apr 4, 2026 at 6:47 PM zandoodle <[e-mail]> wrote:
>
>>     1. An attacker sets up an ACME server and convinces the client to use
>>     it, the client is then provided the attacker's ACME account on the CA's
>>     domain and instructs the client to setup dns-persist-01 under the
>>     attacker's account.
>>     2. An attacker sets up an ACME server and convinces the client to use
>>     it, the client is then provided the attacker's ACME account for the
>>     attacker's domain and instructs the client to setup dns-persist-01 under
>>     the attacker's account. This requires that the certificate authority allows
>>     accounts to be created under the attacker's domain e.g. creating the
>>     account under the domain in the Host header field as done by Let's Encrypt.
>>
>> A few notes here:
> First, these two scenarios are essentially identical. In both of them, the
> attacker is merely providing an attacker-controlled account URI to the
> victim client, to get them to populate it in a DNS record. The only
> difference with the second scenario is that the victim client operator
> would see that their directory URL and account URL have the same domain
> name, and might be less likely to notice that something weird is going on.
>
> Second, the second of these scenarios doesn't actually work, at least not
> against Let's Encrypt. Although we reflect the Host: header in API
> responses, we only respect the *canonical* AccountURI for the purpose of
> CAA and dns-persist-01 accounturi=. A client that populates a TXT record
> with `accounturi=malicious.ca/acme/acct/12335` would not be able to
> complete validation.
I've since found where that's implemented in Boulder and it appears to 
sufficiently protect against the second attack, the canoncalisation is 
also performed for the dns-account-01 challenge, has this already been 
discussed?
>
> Finally, the fundamental threat model here was already considered and
> mitigated in RFC 8555. One can imagine a similarly-positioned malicious
> ACME proxy-esque server which intercepts an HTTP-01 challenge and replaces
> the `token` with one of its own. However, the attacker still can't get the
> victim client to successfully complete validation because the necessary
> value presented by the client to complete the challenge consists of
> *two* pieces
> of information: the token (provided by the server) and the Key
> Authorization (computed directly by the client). The victim client can't be
> fooled into computing a Key Authorization for the malicious proxy's key.
>
> So the solution here is to do the same. The dns-persist record should
> contain a piece of information computed directly by the client, rather than
> merely provided by the (potentially malicious) server. So I agree
> with Sebastian's suggestion that the record format should support an
> `accountkey=` parameter.
>
> Aaron
>
Using the account's public key in the DNS record would prevent both 
attacks however I'm not sure what the security implications would be 
from discouraging key rollovers.

What I had proposed in another reply 
(https://mailarchive.ietf.org/arch/msg/acme/eMmmT1yr6cAOAWYZ5dyZN8Y9ufs/) 
was an CA controlled endpoint (such as the account URI) alongside a 
token parameter fetched from the CA controlled endpoint. This would make 
the client verify that its key is valid for its account with the CA over 
HTTPS, the token parameter wouldn't be strictly necessary however it 
would demonstrate to the CA that the client has verified their key.