[cabf_validation] [EXTERNAL]-Re: Question about MPDV and CAA
Dimitris Zacharopoulos (HARICA)
dzacharo at harica.gr
Mon Jun 26 09:29:57 UTC 2023
After revisiting this issue, I somewhat agree with Pedro's assessment
that for the "Agreed-upon change to website" and "DNS" methods, checking
CAA DNS records from multiple vantage points is redundant and will only
create additional cost and delays without any significant security
improvements.
To give an example, assume a CA receives a request to include 50 Domain
Names and the Applicant chooses to use the "DNS Change" Domain
Validation method. Some of those Domain Names may contain the same Base
Domain Name. In that case, it is likely that the CA will only need to
validate the Base Domain Names, but the CAA query must take place for
all Domain Names requested.
The DNS checks for the Random Value or Request Token would be optimized
and would result to the least amount of traffic and delay. MPDV would
protect against BGP hijacking so the Domain Validation is covered, even
if the CAA records are assumed to be "hijacked".
If the CA is using 3 locations for MPDV (in addition to the main audited
location), the CAA checks will result in 4 times more traffic (200
queries instead of 50), just for the CAA. And that's just for 1
certificate.
Therefore, I would recommend removing the mandatory requirement to check
CAA records via multiple vantage points in the first implementation of
the MPDV concept, in order to first evaluate the effectiveness and
possible issues that will come up. If the group determines that MPDV
works as designed/expected and that the additional traffic caused is not
an issue, we could explore adding multiple vantage point checks for CAA
records as well, but focusing more on the non-DNS/agreed-upon change to
website methods.
It would also be helpful to hear from CAs that already implement MPDV
(Let's Encrypt ?) if they also query CAA records from multiple vantage
points for each issuance and how this has affected their traffic volume
and resources.
Best regards,
Dimitris.
On 21/6/2023 11:38 μ.μ., Pedro FUENTES via Validation wrote:
> Thanks, Martin, for the additional insights.
>
> Yes, tricking CAA to use mail validation is the risk I can see, as
> other methods would rely on MPDV and issuance seems to be prevented as
> far I can see. I was trying to figure out the other situations
> identified below.
>
> Now about “Since Attacker can perform a BGP hijack on CA B, they’re
> able to reroute the email to their own servers”… I think this is not
> so straightforward as it sounds, because nowadays is more and more
> rare that companies run their own mail servers, but there’s a trend to
> use cloud services for email… so the BGP hijack would need to attack
> the service provider (Amazon, Microsoft and the like), and that’s not
> so easy as just rerouting IP traffic in the CA endpoint.
>
> Anyway, if the conclusion here is that the reason to apply MPDV to CAA
> is that we thunk that e-mail based DCV is unsafe, then I guess we’re
> opening up a Pandora Box… :)
>
> Cheers,
> Pedro
>
>> On 21 Jun 2023, at 22:08, Martijn Katerbarg
>> <martijn.katerbarg at sectigo.com> wrote:
>>
>> Pedro,
>> Just adding my thoughts here. In my opinion items 1, 2 and 3 all tie
>> together.
>> Say that Company X wants their domains completely secured. They lock
>> in on 1 CA, thus setting a CAA record allowing issuance only by CA A.
>> They’ve chosen CA A, because CA A has disabled DCV through email
>> validation (or, have found a way to make this MPDV compliant) and has
>> implemented MPDV for both CAA record checking, as well as their DCV
>> options.
>>
>> So far so good.
>>
>> But now, Attacker wants to try to obtain a certificate for one of
>> Company X their domains. Upon investigation, they notice that Company
>> X only allows issuance by CA A, and due to MPDV, they cannot use BGP
>> hijacking as an attack vector.
>>
>> So, Attacker goes instead to CA B. CA B has implemented MPDV for
>> several options, but not for email validation. Since Attacker can
>> perform a BGP hijack on CA B, they’re able to reroute the email to
>> their own servers. Now as Attacker has done BGP Hijacking, they’re
>> also able to spoof the CAA DNS record, thus, showing CA B a valid CAA
>> record, for CA B to issue the certificate.
>>
>> As such, while Company X has taken all precautions, a weak link in
>> this chain, can circumvent the entry of the checks.
>> (And as such, in time I expect we will see Email DCV go away, but
>> probably not as the first step of enabling MPDV)
>> Regards,
>>
>> Martijn
>> *From:*Validation <validation-bounces at cabforum.org>*On Behalf
>> Of*Pedro FUENTES via Validation
>> *Sent:*Wednesday, 21 June 2023 19:19
>> *To:*Ryan Dickson <ryandickson at google.com>
>> *Cc:*CA/Browser Forum Validation SC List <validation at cabforum.org>
>> *Subject:*Re: [cabf_validation] [EXTERNAL]-Re: Question about MPDV
>> and CAA
>> CAUTION: This email originated from outside of the organization. Do
>> not click links or open attachments unless you recognize the sender
>> and know the content is safe.
>> Hi Ryan,
>> Thanks for the reply. This make things much clear to me.
>> My interpretation of your risk scenarios are:
>>
>> 1. Enabling a CA that allows email validation… this implies that the
>> attacker not only hijacks the network, but also controls the
>> validation email. For me this risk effectively exists, but I’d
>> assume low likelihood (else the email validations methods should
>> be considered insecure and disallowed in the BR).
>> 2. Disabling CAA checks such as account binding… considering that
>> MPDV must still be used to prove domain control, I still fail to
>> see the risk here…For example, the attacker could skip some ACME
>> security features… but I’d say that the issuance would still be
>> prevented thanks to MPDV.
>> 3. “no-issue” CAA record… again, given than MPDV must still be used,
>> I fail to see the risk.
>>
>> Can you please elaborate your comments con scenarios #2 and #3… How
>> the issuance could succeed given the fact that MPDV must be still
>> used for domain control validation?
>> Any security countermeasure must necessarily exist to mitigate a risk
>> that, not only exists, but it has a level (combination of impact and
>> likelihood) that justifies its mitigation… it’s evident that the
>> impact is very high (misissuance), but I’d like to fully understand
>> the risk level.
>> Just in case… we are fine deploying distributed CAA checks if
>> required, but I’d like to fully understand the rational here… mostly
>> because CA systems that are more complex and with critical components
>> distributed in the cloud will also increase the attack surface and
>> risks for the CA itself.
>> Best,
>> Pedro
>>
>>
>> On 21 Jun 2023, at 15:18, Ryan Dickson <ryandickson at google.com>
>> wrote:
>> Hi Pedro,
>>
>> Thanks for taking the time to review the proposal - and for your
>> question. Also, sorry for the delayed response - I’ve been out of
>> the office for a few days.
>>
>> The researchers at Princeton initially advised us to evaluate
>> both domain validation challenges and CAA records to add
>> resilience to the issuance process. Specifically, the goal was to
>> make it more difficult for an adversary to launch what they
>> described as a “downgrade attack.”
>>
>> For example, let’s consider a domain owner who used CAA to
>> restrict issuance to a set of CAs that do not support email-based
>> domain control validation because they do not want to allow
>> validation of their domain to occur via email. Also, suppose we
>> aren’t checking CAA from multiple Network Perspectives. In this
>> case, it’d be easier for an adversary to downgrade the issuance
>> process because they would only need to launch one successful
>> attack (the perspective used by the CA to establish the primary
>> determination) to subvert CAA and allow a CA not included in the
>> set of CAs permitted to issue a certificate to the domain to do
>> so. In contrast, if CAA was checked across multiple Network
>> Perspectives, the adversary would need to launch a global BGP
>> attack to obtain a certificate for the target domain (harder to
>> accomplish and not always viable by the adversary, for example,
>> they might only have the means to accomplish a local or regional
>> attack).
>>
>> We also see layers of security built on top of CAA, for example,
>> Account Binding and Validation Method Binding as specified by RFC
>> 8657. These extensions allow organizations to restrict issuance
>> to specific 1) account IDs or 2) ACME domain validation methods.
>> Not checking CAA records from multiple perspectives allows an
>> adversary to more easily downgrade these additional security
>> measures (based on the same approach described above) and then
>> target the added attack surface these records were intended to
>> eliminate.
>>
>> The “no-issue” CAA record (i.e., CAA 0 issue “;”) is another
>> example where the CAA record has a significant impact on issuance
>> behavior. If CAA is not being checked from multiple perspectives,
>> this is another security control that can be more easily
>> downgraded than if CAA is being checked from multiple perspectives.
>>
>> As always, other considerations and perspectives are welcome!
>>
>> Thanks,
>> Ryan
>> On Fri, Jun 16, 2023 at 6:43 AM Pedro FUENTES via Validation
>> <validation at cabforum.org> wrote:
>>
>> Hello,
>> Sorry as most likely this has been already discussed, but as
>> I came “late to the party”, there are things that I surely
>> missed.
>> About the need to consider CAA also in the MPDV… I’m thinking
>> about this and I fail to see the risk we’re managing by doing
>> it. My rational is that MPDV, once verifies the domain
>> ownership/control, would also imply that records in the DNS
>> (i.e. CAA) are legit.
>> The only situation I see where this could apply, is when
>> someone could trick a CAA record during the reuse period of a
>> previously validated domain, so MPDV could verify proper
>> domain control, but the CAA check that must be done for each
>> issuance is faked, but I’d say that faking the CAA could have
>> as only logic purpose to enable another CA to issue the
>> certificate, and that CA would also need to check the domain
>> control using MPDV.
>> When you decided to include CAA in the game… what was the
>> logic behind?
>> Most likely there’s a good reason that clever people has
>> discussed already, so I’d appreciate if you can help me
>> understand better.
>> Thanks!
>> Pedro
>> *
>> WISeKey SA*
>> *Pedro Fuentes
>> *CSO - Trust Services Manager
>> Office:+ 41 (0) 22 594 30 00 <tel:+41%2022%20594%2030%2000>
>> Mobile: + 41 (0) 791 274 790
>> Address: Avenue Louis-Casaï 58 | 1216 Cointrin | Switzerland
>> *Stay connected with WISeKey
>> <https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.wisekey.com%2F&data=05%7C01%7Cmartijn.katerbarg%40sectigo.com%7C453206dbe65948fc195908db727b9eea%7C0e9c48946caa465d96604b6968b49fb7%7C0%7C0%7C638229647491007545%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=RCkHjL9dW1XoPxlnQis2WJJq6Lo%2BOnG%2FU4BGa7yTwTE%3D&reserved=0>
>> *
>>
>> *THIS IS A TRUSTED MAIL*: This message is digitally signed
>> with a WISeKey identity. If you get a mail from WISeKey
>> please check the signature to avoid security risks
>> *CONFIDENTIALITY: *This email and any files
>> transmitted with it can be confidential and it’s intended
>> solely for the use of the individual or entity to which they
>> are addressed. If you are not the named addressee you should
>> not disseminate, distribute or copy this e-mail. If you have
>> received this email in error please notify the sender
>> *DISCLAIMER: *WISeKey does not warrant the accuracy
>> or completeness of this message and does not accept
>> any liability for any errors or omissions herein as this
>> message has been transmitted over a public network. Internet
>> communications cannot be guaranteed to be secure or
>> error-free as information may be intercepted, corrupted,
>> or contain viruses. Attachments to this e-mail are checked
>> for viruses; however, we do not accept any liability for any
>> damage sustained by viruses and therefore you are kindly
>> requested to check for viruses upon receipt.
>> _______________________________________________
>> Validation mailing list
>> Validation at cabforum.org
>> https://lists.cabforum.org/mailman/listinfo/validation
>> <https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Furldefense.proofpoint.com%2Fv2%2Furl%3Fu%3Dhttps-3A__lists.cabforum.org_mailman_listinfo_validation%26d%3DDwMFaQ%26c%3DeuGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM%26r%3DAFTYu1HAQdkStwzgxyDbKOLyDwTHEezL5yeqoxeZ0fc%26m%3DDsWs3jyPLw-2N-fE3sPErWvVPxoF-LYnrcdR9WhZMYA%26s%3DGNSe8xPelzQlUicnJyZ8iRap81lwJpyA2HYpTw9jJsI%26e%3D&data=05%7C01%7Cmartijn.katerbarg%40sectigo.com%7C453206dbe65948fc195908db727b9eea%7C0e9c48946caa465d96604b6968b49fb7%7C0%7C0%7C638229647491007545%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=KdHwP%2F6wqizcIHozfWKf022XyDxDxTYfjYhPTLDdXtg%3D&reserved=0>
>>
>> *
>> WISeKey SA*
>> *Pedro Fuentes
>> *CSO - Trust Services Manager
>> Office: + 41 (0) 22 594 30 00
>> Mobile: + 41 (0) 791 274 790
>> Address: Avenue Louis-Casaï 58 | 1216 Cointrin | Switzerland
>> *Stay connected with WISeKey
>> <https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.wisekey.com%2F&data=05%7C01%7Cmartijn.katerbarg%40sectigo.com%7C453206dbe65948fc195908db727b9eea%7C0e9c48946caa465d96604b6968b49fb7%7C0%7C0%7C638229647491007545%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=RCkHjL9dW1XoPxlnQis2WJJq6Lo%2BOnG%2FU4BGa7yTwTE%3D&reserved=0>
>> *
>>
>> *THIS IS A TRUSTED MAIL*: This message is digitally signed with a
>> WISeKey identity. If you get a mail from WISeKey please check
>> the signature to avoid security risks
>>
>>
>> *CONFIDENTIALITY: *This email and any files transmitted with it can
>> be confidential and it’s intended solely for the use of
>> the individual or entity to which they are addressed. If you are not
>> the named addressee you should not disseminate, distribute or copy
>> this e-mail. If you have received this email in error please notify
>> the sender
>> *DISCLAIMER: *WISeKey does not warrant the accuracy or completeness
>> of this message and does not accept any liability for any errors or
>> omissions herein as this message has been transmitted over a public
>> network. Internet communications cannot be guaranteed to be secure or
>> error-free as information may be intercepted, corrupted, or contain
>> viruses. Attachments to this e-mail are checked for viruses;
>> however, we do not accept any liability for any damage sustained by
>> viruses and therefore you are kindly requested to check for viruses
>> upon receipt.
>
> *
> WISeKey SA
> *
> *Pedro Fuentes
> *CSO - Trust Services Manager
> Office: + 41 (0) 22 594 30 00
> Mobile: + 41 (0) 791 274 790
> Address: Avenue Louis-Casaï 58 | 1216 Cointrin | Switzerland
> *Stay connected with WISeKey <http://www.wisekey.com>
> *
> *THIS IS A TRUSTED MAIL*: This message is digitally signed with a
> WISeKey identity. If you get a mail from WISeKey please check
> the signature to avoid security risks
>
> *CONFIDENTIALITY: *This email and any files transmitted with it can be
> confidential and it’s intended solely for the use of the individual or
> entity to which they are addressed. If you are not the named addressee
> you should not disseminate, distribute or copy this e-mail. If
> you have received this email in error please notify the sender
>
> *DISCLAIMER: *WISeKey does not warrant the accuracy or completeness of
> this message and does not accept any liability for any errors or
> omissions herein as this message has been transmitted over a public
> network. Internet communications cannot be guaranteed to be secure or
> error-free as information may be intercepted, corrupted, or contain
> viruses. Attachments to this e-mail are checked for viruses;
> however, we do not accept any liability for any damage sustained by
> viruses and therefore you are kindly requested to check for viruses
> upon receipt.
>
>
> _______________________________________________
> Validation mailing list
> Validation at cabforum.org
> https://lists.cabforum.org/mailman/listinfo/validation
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.cabforum.org/pipermail/validation/attachments/20230626/2fe5c9bb/attachment-0001.html>
More information about the Validation
mailing list