[cabf_validation] [EXTERNAL]-Re: Question about MPDV and CAA

Wed Jun 21 20:08:01 UTC 2023

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 <mailto: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 <mailto: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  <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> WISeKey

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 <mailto:Validation at cabforum.org> 
https://lists.cabforum.org/mailman/listinfo/validation

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  <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> WISeKey

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.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.cabforum.org/pipermail/validation/attachments/20230621/6bb793d6/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 6807 bytes
Desc: not available
URL: <http://lists.cabforum.org/pipermail/validation/attachments/20230621/6bb793d6/attachment-0001.p7s>