pigeonhole question: filtering on delivered-to in case of fetchmail

Trever L. Adams trever at middleearth.sapphiresunday.org
Wed Jul 17 19:03:04 EEST 2019

> On 15 Jul 2019, at 18:11, Trever L. Adams via dovecot <dovecot at dovecot.org <https://dovecot.org/mailman/listinfo/dovecot>> wrote:
> >/So, one of the problems I am seeing is that people are trying to fake />/users into revealing information by sending from an outside domain but />/with an internal reply to address and claiming to be administration, IT />/or what not. /
> You should not accept external mail claiming to be from your domain unless that mail comes via authenticated submission. But if the reply to is going to an internal address… 
> I’m puzzled by exactly what you mean here. Are you saying that users on your system are trying to phish other users on your system?
> >/I can set up something that will reject if from is outside the domain by />/reply to is internal. The problem is in some setups, there are fetchmail />/setups. I do not want to reject these with a message. Which is what I am />/currently doing for the others. Maybe I should discard them all without />/rejecting. /
> I haven’t used fetch mail in many many years, so I can’t answer anything specifically about it, but if you use it to allow external senders to send mail via your system in a way that is not authenticated then you should not do that.

I do NOT allow email claiming to be from my domains. The problem is
"forgery" of Reply-To headers. It isn't really forgery as far as I know
there is now method to check this anywhere. People are allowed to put
what they want there. The setups in question do NOT allow
unauthenticated submission with a FROM from the internal domain.

I have erased the email in question, so I cannot give an exact example
but it is something like this:

From: something at devcubesomething.org (I remember cube and dev in the domain)

To: trever at thedomain (yes it was sent to me, thankfully not one of the
other users)

Reply-To: info at thedomain (yes, stupid account to use, but that was it)

Subject: Your account will be deleted/deactivated

Some nonsense about having failed to follow directions and if I don't
click the link below, the account would be deleted. It was NOT talking
about an account on another system, but the email account itself.

So, as you see, it is coming from an outside domain. As the sieve code
showed, I am testing for where reply-to claims to be for internal
domain, but the from is NOT from it. This email was a good example of that.

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