bugtraq - Re: Is predictable spam filtering a vulnerability? (silently dropping messages)

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Date: Wed, 23 Jun 2004 18:46:55 -0400 (EDT)
From: der Mouse <mouse@...ents.Montreal.QC.CA>
To: bugtraq@...urityfocus.com
Subject: Re: Is predictable spam filtering a vulnerability? (silently dropping
 messages)

OK, I'll brave the storm of broken autoresponders one more time....

>> IMHO 3: If user Joe gets 10 delivery failures of messages that he
>>         has not sent and one delivery failure of message that he has
>>         actually sent, it is worse than if he gets nothing.

> This is indeed a problem, and it's a loophole that needs to be
> closed.

Unfortunately it's difficult for most people to close.

> There needs to be a way for an SMTP server to correlate a bounce
> message with a sent message, and reject the bounce message if it
> wasn't caused by a validly-sent message.  Proposals like SPF can help
> a little.

A little.  But there also is a need to _identify_ bounce messages.

A few years back, I got joed - some lamer forged my address into the
from-line of what appears to have been an entire spamrun.  I got some
small number of thousands of bounces before I taught my mailer to pick
apart multipart/report bounces and reject them if the bounced message
doesn't show certain signs that all messages I send show.  This helped
immensely, and when the modern crop of from-line forging malware showed
up, my defenses were already in place and functioning.

Today, I occasionally get bounces for malware with my address forged
into the fromline.  I respond to them with a more or less stock
response that goes something like

	If you _must_ do accept-and-bounce (something which is
	increasingly "part of the problem" in today's net), please at
	least make sure your bounces are proper multipart/report
	bounces, so they can be mechanically identified and treated
	appropriately.  (See RFC 3462 for more on multipart/report.)

I've been doing this only a little while.  If there comes to be a site
which is a persistent source of bounces and also persistently ignores
that request, I'm prepared to block it entirely.

/~\ The ASCII				der Mouse
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