A DKIM signature on email by itself means very little

December 23, 2023

In yesterday's entry on what I think the SMTP Smuggling attack enables, I casually said that you were safe if you ignored SPF results and only paid attention to DKIM. As sometimes happens, this was my thoughts eliding some important qualifications that I just take as given when talking about DKIM, but that I should spell out. The most important qualification is that a (valid) DKIM signature by itself means almost nothing, which is a bit unlike how SPF works.

First off, anyone can DKIM sign a message, provided that they control a bit of DNS (you could probably even do it in a mail client). Quite a lot of people, including spammers, can even DKIM sign email that is 'aligned' with the 'From:' header, which means that the DKIM signature is from the From: domain, not just from some random domain. A valid DKIM signature does provide definite attribution, and if it's for the From: domain, it more or less identifies who authorized the mail. Also, in practice lack of a DKIM signature is itself a signal, because an increasing number of places more or less require a DKIM signature, sometimes one that is from the From: domain.

(However, some people only have SPF records and this can be deliberately used to create email that can't be easily forwarded.)

A valid DKIM signature for the From: domain is at least as strong a sign as an SPF pass result. However, this doesn't mean that the email is any good, any more than an SPF pass does; spammers can and do pass both checks. Similarly, lack of a valid DKIM signature for the From: domain doesn't mean that it's not from that domain. To have some idea of that you need to check the domain's DMARC policy. In effect, the equivalent of SPF is the combination of DKIM and DMARC (or something like it).

So when I casually wrote about (only) paying attention to DKIM, I was implicitly thinking of using DKIM along with something else to tell you when DKIM results matter. This might be specific knowledge of which important domains you deal with DKIM sign their email (including your own domain), or it might mean checking DMARC, or both. And of course you can ignore both SPF and DKIM signatures, apart perhaps from logging DKIM results.

(We don't explicitly use DKIM signatures and DMARC in our Exim configuration, but these days we use rspamd for spam scoring and I think it makes some use of DKIM and perhaps DMARC.)

Written on 23 December 2023.
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Last modified: Sat Dec 23 22:49:08 2023
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