On 29 Oct 2004, Peter Hicks wrote:
> I recently had a situation where a user had a catch-all setup that was
> being forwarded to our domain from another server. Needless to say, this
> account attracted a lot of spam and virii. Part of our exim.conf file
> had this in the data acl:
>
> deny message = $found_extension files are not accepted here
> demime = com:vbs:bat:pif:scr
>
> This lead to a situation where an email with a pif attachment was denied
> with a 5xx code, and the server that was doing the forwarding sent a
> bounce to the local account on their machine, which was then forwarded
> back to our system. The bounce included the original mail as an
> attachment, so this created a nested attachement. This happened over and
> over again in an infinite loop, creating an email with hundreds and
> hundreds of nested attachments.
Surely this means that the remote system was badly misconfigured, because
when the first bounce that was generated by them was forwarded to you, it
should have had a null return path. When you rejected it for the second
time, there should have therefore been nowhere to deliver the "bounced
bounce" to, and no loop. For there to have been a loop, the remote system
must somehow be ending up sending the bounce with a non-null sender.
> I have therefore changed the deny to discard.
I think you should get the remote end to fix their broken system, instead
of breaking yours.
> Are there any unseen ramifications for doing this? Do people have
> ligitmate reasons to send the above attachments? If they do, then it
> would be nice if they received the error message.
No doubt they do get sent around occasionally, but I should imagine it's
very rare, especially considering that blocking this sort of stuff has
been standard on many systems for a long time.
Tim
