Re: [Uta] New proposal: SMTP Strict Transport Security

[Chris Newman <[e-mail]>]

This document is providing STS functionality for SMTP relay, while DEEP is providing STS functionality for SMTP submission (plus IMAP & POP). I believe it’s important to align these two proposals and am open to changing DEEP to do so (https://tools.ietf.org/html/draft-ietf-uta-email-deep-04).

Note that early versions of DEEP also provided STS for SMTP relay but solving the STS problem for MUAs is sufficiently difficult and different that I think it’s good to have a separate document for SMTP relay STS. If you are open to editing this as a WG document with rough consensus changes, I would welcome the collaboration.

I agree with most of Viktor’s comments so I won’t repeat them. Here are some new comments:

1. I personally dislike using DNS records for any of this proposal. I believe SMTP security policy is best communicated within SMTP as this minimizes attack surface, eliminates the need for TOFU in some scenarios, and puts the policy configuration closer to the server operator. We can use the PKIX trust model with SMTP/STARTTLS just as we use it with HTTPS (with Viktor’s notable caveats) so HTTPS is not needed and shouldn’t be used to validate SMTP security policy.

2. I think using HTTPS to validate MX records in the non-DNSSEC scenario is a very interesting idea. But MX records are routing rather than policy and thus don’t belong in a policy record. How about just defining a well-known HTTPS URL that contains a copy of the actual binary MX record for the domain (or a semantically-identical syntax transformation of that binary record)? Then we just need the SMTP server to advertise the binary policy flag Viktor mentioned to get a non-DNSSEC model to reasonably trustworthy MX records.

3. DEEP goes through a lot of trouble to create an extensible framework for policy by creating a registry (while still keeping the model simple). Your proposal notes the need for future policy work but lacks a model to add new policies. I think the SMTP STS document is incomplete without an extensibility model and you can get it by referencing DEEP (and if we need to change the syntax or model in DEEP to make that more palatable, I’m open to that discussion).

4. UTA already discussed the idea of timeouts for security policy in DEEP and reached rough consensus not to include timeouts. I doubt the SMTP relay use case is sufficiently different to alter that rough consensus. So I suspect we can drop timeouts from SMTP STS after a discussion, but if there is rough consensus to keep timeouts in SMTP STS then I’d like to reconsider that decision for DEEP on a proposal-alignment basis.

5. DEEP’s reporting mechanism for SMTP submission (the CLIENT command) is an as-it-happens rather than after-the-fact mechanism. I’d like to investigate the idea of an after-the-fact mechanism for MUAs although I’m not sure it’s a good fit. Regardless, we need to use the same reporting syntax for submission SMTP and relay SMTP, so I’m open to changing DEEP’s current submission reporting syntax as needed to align them.

6. I think JSON would be a better format than XML for reporting. The reports are coming from an untrusted source and will be parsed. JSON has a much smaller attack surface than XML and as this is part of a security mechanism, I think that’s an important consideration. The one potential advantage I see to XML is the wide availability of SAX-style parsers that can handle large volumes of data, but I think the attack surface argument should win in this case. I know a number of XML parser libraries are insecure-by-default due to schema URI resolution. Regardless, this is an interesting design trade-off for a rough consensus decision.

7. I think your document should reference DEEP section 6 so SMTP relay cipher usage is recorded for trace purposes; that’s another aspect of your transparency goal.

7. I note in passing that DEEP allows use of the PKIX trust model and the DANE trust model at the same time. I think that’s good.

I’m wondering if I should rename DEEP to MUA STS?

		- Chris

On March 21, 2016 at 20:11:06 , Daniel Margolis ([e-mail]) wrote:

Thanks for the feedback to both of you. I don't disagree; I think Viktor makes a very solid point in favor of simplicity. In addition, a report-only protocol could be extended to support arbitrary error reporting; an out-of-band (e.g. HTTP) channel to share delivery failures between domains strikes me as useful in the general case. 

Separately, because we're already assuming providers (both sending and receiving) make a choice on implementing DANE and/or webPKI, I don't think actually splitting the two makes it any more or less complex to implement, or should discourage adoption of one or the other mechanism. 

So I would say I'm feeling a bit in favor of Viktor's suggestion, but I'd like to chat a bit more with my co-authors and think about it first. ;) 

Viktor, as an aside regarding the hosted mail scenario, we already had the suggestion to move the HTTPS endpoint to something like "_smtp_sts.example.com/current". If we do that, the customer (example.com) can make this a CNAME for "_smtp_sts.hostingdomain.com", who can use SNI to serve the policy with the customer's cert (assuming the customer trusts the hosting provider with this; for domains that don't operate their own HTTPS endpoint this seems to me to be likely). For the more complex case, the cron setup you describe doesn't seem too onerous, I agree. 

Thanks again for the feedback. 

On Tue, Mar 22, 2016 at 10:21 AM, Neil Cook <[e-mail]> wrote:

> On 22 Mar 2016, at 08:49, Viktor Dukhovni <[e-mail]> wrote:
>
> On Tue, Mar 22, 2016 at 08:58:25AM +0100, Daniel Margolis wrote:
>
> My (strong) suggestion: use DNS for just cache invalidation, and
> perhaps also publication (via a separate record) of the "rua"
> reporting URI.  Do not duplicate data which one must in any case
> obtain and cache via HTTPS in DNS.
>
> Do not attempt to hedge your bets and support DANE/DNSSEC via STS,
> I don't think that makes much sense either.
>

I agree with the “don’t hedge your bets” part. I was quite surprised to see all the justification for STS in the first part of the document, including “the mechanism described here presents a variant for systems not yet supporting DNSSEC”, and yet then goes on to include DNSSEC as one of the policy authentication mechanisms.

>    * Allow (DANE or other) domains to publish just the RUA,
>      the feature is not STS-specific.
>
+1

Neil


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