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DKIM, as read by rfc4871, states to make address forgery more difficult besides protecting sender identity and integrity of the mail.

I have been searching for the details, which precise validation step prevents From:-header spoofing, counting in the fact that Dave Rand and Doug Otis of Trend Micro argue that there's a weakness in DKIM when prepending a second From: header. The opposite means that it's not possible with only one From:-Header.

Please uncover my error in my reasoning:

  1. Assuming I am a spammer who has registered the domain spammailer.com
  2. Because I'm the owner, I'm authorized to setup DKIM-records in DNS for domain spammailer.com.
  3. Now I'm sending an email from my mailserver which has the private key to [email protected]:

Header:

Return-Path: <[email protected]>
Received:...
Received:...
Date: Wed, 17 Oct 2018 15:11:08 +0000
From: [email protected]
To: <[email protected]>
Message-ID: <[email protected]>
Subject: pay rise of 5%
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=spammailer.com;
    s=bostonceltics; t=1519400838;
    h=from:subject:date:message-id:to:mime-version:content-type:content-transfer-encoding;
    bh=kpYel1IlDvqXEUc0SyIpXbMte3XpQOCXHl+zTyHQvGc=;
    b=NEUyWUoeKEoKAYTY8g04o73j+wrYUcEGSq7uwpbsAGo0OzuuIBluEfG1MbGF/Tf6yxxJB4
    gTDD3sqb19EsQxv39QsAwgddAz01Osw5LKU0MjLZpxw6NA8zLllJUsrNdNQAYSII9ip4xX
    ImU7+KFOEF+gmxR5aseUt5H6JT/aOmhPE9xsSyg9wLf0Bikyy5Cgh+Ay7AHQLMZogbTi9W
    dAPpZZcZs0pTwhcard6SaesypJ+xZNna+BA+C1vXrGDc+9stYZVi+Zufh6zlZo1E/sQSRL
    jowB1mjv1vjINRY30aq0rh4dT8RHe38/PKFf8vQHOSOKvjIKv984UeOTIFIUHw==

As far as I understand, the verification process now does the following:

  1. getting the public key from the sender domain d=spammailer.com+separator
  2. decrypting the DKIM signature with the spammer's public key which passes (I suppose!) because the mail is not altered in any way.

Now the recipient at IBM accounting recognizes that there's an email from the CEO which is displayed as the From:-address in most MUAs.

1 Answer 1

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DKIM by itself does not protect against spoofing of the From: field in the mail header (i.e. RFC822.From). It only adds a signature which shows that significant parts of the mail header were not changed and that the mail passed through the mail server responsible for a specific domain. In your specific case the DKIM signature shows that the mail passed through the mail server for spammailer.com but not through the mail server for ibm.com as proper mails from [email protected] would probably do.

The protection against spoofing of the From: field is only added by DMARC. Providing that the mail domain has a DMARC policy (as ibm.com currently does which dig txt _dmarc.ibm.com shows) the receiving system can check if there is an aligned DKIM signature or SPF result. Aligned regarding DKIM means that the domain given in the from From: field matches the domain of the DKIM signature (either exactly or by allowing sub-domains, depending on the DMARC policy).

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  • Thanks a lot for your explanation. That was also my understanding, but could not get a reason why Otis and Rand mentioned the missing verification of headers such as a seconf From: Header (besides other weaknesses), see on: tools.ietf.org/id/draft-otis-dkim-harmful-04.html The effort is not even neccessary to put a second From-Header into the email when you want to forge an address. So my understanding of what the term forgery means because the from:header is visible to the enduser and not the return path, sender id or MAIL FROM-Header.
    – Toni
    Commented Oct 22, 2018 at 8:14

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