From: Victor Duchovni (Victor.DuchovniMorganStanley.com)
Date: Tue Oct 05 2004 - 11:45:09 CDT

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On Tue, Oct 05, 2004 at 10:26:04AM -0400, Rob Foehl wrote:

> On Tue, 5 Oct 2004, Pierre Fortin wrote:
>
> >Oct 4 18:08:25 bones postfix/local[27027]: BDC00C553:
> >to=<pfortinpfortin.com>, relay=local, delay=5, status=bounced (mail
> >forwarding loop for pfortinpfortin.com)
> [...]
> >Is this an indication of a possible exploit..?
>
> No, it's an indication of a forwarding loop. Provide postconf -n and
> details on where you expect the above address to be delivered (aliases,
> etc).
>

Not necessarily. The "Delivered-To:" header can be forged. Whether
protecting against this "attack" warrants new code is not quite clear yet.

The attacker could for example push the mailbox over quota, and then send
a flood of mail that will bounce. Postfix would then need an over quota
cache that rate limits over-quota bounces to 1 per TTL (with rejects
between TTL expiration). This would (hypothetically) work with local(8)
but cannot solve over-quota problems with 3rd party MDAs, because there
is no standard (sysexits.h) error code for mailbox over quota.

So if someone is hell-bent on getting your system to generate a bounce,
they can probably find a way to do it... If this becomes a major problem,
defending against it requires more than just "fixing" Delivered-To.

--
        Viktor.

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