Subject: Re: [secdir] Secdir last call review of draft-ietf-dmarc-rfc7601bis-03

From: "Murray S. Kucherawy" <[email-masked]>
Date: Sat, 3 Nov 2018 13:00:27 +0900

On Thu, Nov 1, 2018 at 2:56 AM Rifaat Shekh-Yusef <[email-masked]>
wrote:

> Section 7.1.  Forged Header Fields
>
> In addition to a recommended solution, this section has list a potential
> alternative solutions which the document then states that it is not
> appropriate
> for this document to specify which mechanism should be used.
>
> Since an implementer is not expected to do anything with this information,
> it
> might be more appropriate for this to be moved to an appendix at the end
> of
> document.
>

Implementers need to be aware of these things in order to make informed
handling decisions, but we also acknowledge that we can't propose a single
solution that will work for all operational environments.  That's what this
text means to convey.

By my read of RFC3552, that's what this section is for, rather than an
appendix.

Section 7.2.  Misleading Results, First paragraph, last sentence
>
>    "In particular, this issue is not resolved by forged header field
> removal
>    discussed above."
>
> which seems to be in conflict with the following statement from section 5:
>
>    "For simplicity and maximum security, a border MTA could remove all
>    instances of this header field on mail crossing into its trust
>    boundary."
>

They're not in conflict.  Even if I remove all instances of this header
field at ingress and then evaluate DKIM (for example), I have no idea if a
valid signature from "example.com" should be interpreted such that I trust
that message more (or less).

The two paragraphs you're talking about solve different problems.

> Section 7.2.  Misleading Results, Second paragraph
>
>    "Hence, MUAs and downstream filters must take some care with use of
>    this header even after possibly malicious headers are scrubbed."
>
> How do you expect an MUA or downstream filter to act on "take some care"?
> Can you elaborate on that?
>

If you are a spammer or malware distributor, you can elicit a DKIM "pass"
by simply creating your own domain and signing your bad email with that
domain.  The fact that you got a "pass" doesn't tell you anything about
which domain's signature succeeded, nor does it confirm the message content
is safe or trustworthy in any way.

"take some care" means "keep this in mind while deciding how to render a
message to end users, who might not know to even look at this".

7.3.  Header Field Position
>
> This section explains that headers fields are *not* guaranteed to be in a
> specific order. The section then states that "there will be *some*
> indication..."
>
> Since the order is not guaranteed, what do you expect an implementer to
> take
> away from this?
>

"in the general case" and "but most do not".

So: Most of the time, you can rely on header field ordering to determine
the sequence of handling.  You are at least certain about whether you can
trust the tail end of that, because you know your own environment from the
ingress point.

7.8.  Intentionally Malformed Header Fields
>
> This is a general issue with any header. Is there anything specific to
> this
> header that an implementer should pay attention to?
>

No, not really, but at the time this text was originally written (see
RFC5451; this is about the fourth version of this material), it was felt
this was worth adding.

-MSK

---

From: Rifaat Shekh-Yusef <[email-masked]>
Date: Sun, 4 Nov 2018 16:59:01 -0500

On Sat, Nov 3, 2018 at 12:00 AM Murray S. Kucherawy <[email-masked]>
wrote:

>> Since an implementer is not expected to do anything with this
>> information, it
> valid signature from "example.com" should be interpreted such that I
> trust that message more (or less).
>

I thought the DKIM signature is part of this header, but I now understand
that it is a separate header.

> I guess what I am looking for is a statement about the best case scenario
for an MUA to be able to display a message with some confidence given the
current state of affair.
For example, if there is a valid DKIM, an Authentication-Results header
with dkim pass,and the header was provided by a trusted entity?

> Fair enough; I think it is worth adding this sentence to make it clear.

> RFC5451; this is about the fourth version of this material), it was felt
I guess it does hurt to remind implementer to pay attention to this issue,
but I think it might be useful to state that there is nothing special about
this header to avoid possible question like this in the future.

Regards,
 Rifaat

---

From: "Murray S. Kucherawy" <[email-masked]>
Date: Mon, 5 Nov 2018 13:04:04 +0900

> with dkim pass,and the header was provided by a trusted entity?

I would argue that the text that's there does give the best case scenario:
Absent a reputation system (which doesn't exist publicly, so the lowest
common denominator doesn't have this), it's dangerous for any component of
a mail system to trust a message just because some authentication system
passed.  The "value" of the attesting domain is what matters, and for the
most part you don't know what that value is.  Really large operators (e.g.,
Gmail) do have reputation systems, but your typical home open source
installation does not.

> Fair enough; I think it is worth adding this sentence to make it clear.

Doesn't the last sentence of that paragraph already say exactly that?

> this header to avoid possible question like this in the future.

Adjusted the text accordingly.

-MSK