OK, I admit to being dumb. I got another scam email yesterday of the same formulation as the earlier ones (mail From: me@mydomain, To: me@mydomain) attempting to extort bitcoin from me.
How? What had I missed this time?
Well, this was slightly different. Checking the mail headers (and my logs) showed that the email had a valid “Sender” address (some bozo calling themselves “email@example.com”) so my earlier “check_sender_access” test would obviously have allowed the email to pass. But what I hadn’t considered was that the sender might then spoof the From: address in the data portion of the email (which is trivially easy to do).
Dumb, so dumb. So what to do to stop this?
Postfix allows for quite a lot of further directives to manage senders through the smtpd_sender_restrictions and mine were still not tight enough to stop this form of abuse. One additional check is offered by the reject_sender_login_mismatch directive which will:
“Reject the request when $smtpd_sender_login_maps specifies an owner for the MAIL FROM address, but the client is not (SASL) logged in as that MAIL FROM address owner; or when the client is (SASL) logged in, but the client login name doesn’t own the MAIL FROM address according to $smtpd_sender_login_maps.”
Now since I store all my user details in a mysql database called “virtual_mailbox_maps” it is simple enough to tell postfix to use that database as the “smtpd_sender_login_map” and check the “From” address against that, That way only locally authenticated valid users can specify a local “From:” address. Why I missed that check is just beyond me.
My postfix configuration now includes the following:
smtpd_sender_login_maps = $virtual_mailbox_maps
smtpd_sender_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_non_fqdn_sender, reject_unauthenticated_sender_login_mismatch, check_sender_access hash:/etc/postfix/localdomains
(Note that I chose to use the “reject_unauthenticated_sender_login_mismatch” rather than the wider “reject_sender_login_mismatch” because I only care about outside unauthenticated senders abusing my system. I can deal with authenticated users differently…)
Now let’s see what happens.