bugtraq - RE: Is predictable spam filtering a vulnerability?

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Message-ID: <66C2E02AE076D711B07A00508B938E3105E98E@hqmail.bakbone.com>
Date: Fri, 18 Jun 2004 16:29:40 -0700
From: Lance James <lance.james@...bone.com>
To: 'Aaron Cake' <aaron@...pm.com>, bugtraq@...urityfocus.com
Subject: RE: Is predictable spam filtering a vulnerability?

Your point on the Rejected messages is that it does happen. Reverse-NDR's
are a real problem - and are a loophole since NDR is a smtp spec.

On a second note, your comment on filters, (not involving the me2 spam
filter companies) filters do not stop spam, and sometimes they are truly
more detrimental.

There was an article posted on security focus that brought up all of these
issues with security and spam.

www.securityfocus.com/infocus/1763 (it's two parts, that's the first one, it
leads to the other one.).

The social engineering attack would have to be well surveilled to pull off,
but it is definitely possible by a long term and determined attacker.

Lance

-----Original Message-----
From: Aaron Cake [mailto:aaron@...pm.com] 
Sent: Thursday, June 17, 2004 7:19 AM
To: bugtraq@...urityfocus.com
Subject: RE: Is predictable spam filtering a vulnerability?

> During a recent email conversation with several participants, we
> discovered that the email service of one participant silently
> dropped legitimate emails that happened to contain certain
> combinations of words common in spam. I believe this sort of
> filter is common practice, and in fact even in place for some of
> my own email addresses.
>
> However, this experience made me think: isn't predictable spam
> filtering in general a vulnerability that could be used as a hoax
> device?

Certainly. I have brought this issue up with several other ISPs who insist
on blocking my personal domain because I'm a "little guy". They can't prove
that I don't spam, so they default to blocking everything that comes from me
instead. AOL is the biggest and perhaps most annoying offender.

I personally see this as a denial of service attack against MYSELF.
Obviously not meant to be malicious in nature, but quite effective
regardless.

Imagine if I decided to use a spam fitler against someone else...I make an
email that contains known rejected words. I send that email, setting the
"FROM" address and header to be that of my victim. If I send out hundreds of
these messages, I can use someone else's spam filter to mail-bomb my victim
with "rejected" messages.

The REAL issue is that any email filter that silently drops messages can
easily mistake legitimate mail for spam. The user never knows, sometimes the
sender doesn't know, and the braindead admins who set up the filter think
they've done their job. What is even more useless is when the message is
bounced with instructions on how to get off their block list. You send an
email to their admin, yet it is bounced!

Spam filters are often worse then the spam problem itself.

---
Aaron Cake
Technical Services
Advanced Computer Ideas
Phone: 1-519-433-0279
Fax:   1-519-433-5413