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16 Jul 2014

The mail forwarding threat model Email

The recent DMARC kerfluffle has brought new attention to mail forwarders that send mail on behalf of other people. We've been giving a lot of thought to ways to tell nice forwarders from nasty ones, so that mail systems can deliver mail from the nice ones and filter the nasty ones. It occurs to me that there are several scenarios for the way that forwarders work, so I've collected them in a little chart.

We assume that forwarders can sign the mail they send, so there's no problem telling that mail from the forwarder really came from them. We also crudely divide agents into Good ones that send mail that the recipients generally want, and Bad ones that send mail that the recipients don't want.

Each row of the table starts with three letters. They mean:

  • G or B, the forwarder is Good or Bad
  • A or U, the original message was Authenticated or Unauthenticated before it was forwarded. Note that Unauthenticated doesn't mean "forged", since there are many ways a user can send mail that is legitimate yet isn't authenticated.
  • G or B, the original sender was Good or Bad

GAGSubscriber sending mail through a mailing list
GUGNewspaper forward-an-article, or ESP mailing for a customer who can't provide a signing key.
GABCompromised subscriber sending mail through a mailing list, or spammer sends to list that doesn't limit mail to subscribers
GUBSpammer who's stolen a user's address book sending mail to a list to which the victim subscribes
BAGFormerly legit list goes rogue (never seen it)
BUGSpammer sending modified copies of mail scraped from an archive
BABCompromised user sending through malicious list (unlikely)
BUBRegular old spam with fake return address.

posted at: 19:43 :: permanent link to this entry :: 0 comments
posted at: 19:43 :: permanent link to this entry :: 0 comments

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