On Tue, Jun 20, 2023 at 11:18 AM Scott Kitterman <skl...@kitterman.com> wrote:
> I am starting a separate thread, because this isn't just about SPF. > > I think the criticism of the reliability of SPF data is valid, but I think > DKIM is similarly problematic. Once you get rid of SPF, you'll find you > haven't really solved much. The next logical step will be to get rid of > DKIM. > > DKIM has man wonderful features, but the bottom line for DMARC is a valid > DKIM > signature indicates that the mail was sent (at some point) from an MTA > that > was authorized by the signing domain. This is what a SPF pass result > means > too. DKIM has broader utility in DMARC because it can persist through > some > indirect mail flows, but either way, an SPF pass and a valid DKIM > signature > both mean the message was authorized by the domain in the relevant > identity. > > The particular SPF problem that's being the focus of some much attention > is > addressed in the RFC 7208 security considerations (See section 11.4). > > DKIM has a similar, but different problem. The DKIM replay problem is bad > enough that the IETF has chartered a working group to try to address it: > > https://datatracker.ietf.org/doc/charter-ietf-dkim/ > > I'll lay out examples to demonstrate: > > Case 1 - First Party Only > > An organization only authorizes email to be sent from MTAs it directly > controls (I know this mostly doesn't exist, but it's useful to show where > the > problems are). The organization does not send email for any third parties. > > In this case, as long as the organization can limit sending to authorized > users, the quality of both SPF and DKIM pass results should be well > aligned > with the nature of the mail the organization sends. > > Case 2 - Sender For Others, Own Domain > > An domain uses it's own domain to send mail for others (like all webmail > providers). The domain's email is sent using it's own infrastructure, so > it > doesn't need to worry directly about third parties. > > In this case, there's still no real risk of external forgery, so an SPF or > DKIM pass means it was sent by the domain. The risk is to the domain's > reputation if users sign up and use the service to send "bad" mail. > > If the domain fails to prevent 'bad' messages from being sent, then these > 'bad > messages get a DMARC pass. The mail is sent through the authorized email > server, so it gets SPF pass and a valid DKIM signature. > > This is a particular threat for DKIM, because once this succeeds for a > single > message, the user can replay the message on third party infrastructure to > send > it to large numbers of recipients. This is the core issue the DKIM > working > group was resurrected to address. It is not, however, clear that there's > a > protocol solution to this problem and the group has been pretty quiet > recently. > > Case 3 - Sender For Others, Their Domain > > When organizations authorized third parties to send on their behalf (by > publishing an SPF record or a DKIM public key record in DNS), they are > trusting their domain's reputation to those third parties. > > In particular, they are at risk of those third parties doing poor inbound > validation and other customers of the authorized third parties injecting > 'bad' > mail authorized by their domain. > > This is where I think the focus of recent discussion has been. > > In these cases, email is emitted via third parties and passes SPF (may > also be > DKIM signed, but I suspect it's less common because replay is easier), so > it's > authorized, but unwanted. > > Case 2 and Case 3 are both real problems, but only because people are > trying > to make DMARC pass a positive assertion. In order to do that reliably, > organizations need to be more like Case 1 and be very careful vetting > third > parties that they authorize for. I don't think that's a protocol problem. > I think this is a key factor for the calculus of value in DMARC " ...but only because people are trying to make DMARC pass a positive assertion." I agree that it's not a protocol implementation. It really can't/shouldn't be used to try and make positive assertions (even though people attempt to do so). Apart from the cases Scott points out, there is the very real situation of when good domains go bad, whether because the good domain has been subverted, sold or for other reasons. If we move away from attempting to use DMARC to make a positive assertion then the issues around configurations/implementations become less problematic. Perfection is the enemy of good. Michael Hammer
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