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*** Spammers Use New Technique To Evade Filters ***

PC World -- 05/03/2007
Spammers have stepped up efforts to use encrypted attachments
to evade filtering systems, service provider Email Systems has reported.

The technique relies on the fact that many spam systems can't scan inside
emails containing encrypted or password-protected attachment, and work
out that they are not legitimate. Without a rule to block such attachments,
most systems will pass on the email to recipients, handing spammers an
important victory in the battle to get spam through.

In recent weeks, Email Systems detected a small but steady stream of
such spam emanating from bot-compromised hosts, containing a zipped
-up version of the pervasive 'Storm' bot-loading Trojan that plagued
Internet users in January.

Recipients would have been able to inadvertently unzip the Trojan
using an embedded password, after being attracted by a number of
eye-catching subject lines, including 'Worm Detected!', 'Virus Detected!',
'Spyware Alert!' and 'Warning!'

Although the technique has been around for some months, spammers
appear to be stepping up their attempts to use it, said Greg Miller of
Email Systems. The company had quarantined hundreds of thousands
of copies of attachment spam, up from levels a tenth this volume some
months ago.

"We have moved on from spam being just a guy sending out huge
amounts of spam," said Miller. The vast bulk of spam was now automated
via bots, and this made finding new infection methods even more critical
to the spam economy. "Every six months or so we see a new attack that
is very successful," he said.

As anti-spam systems adapted to popular techniques such as image
spam, criminals were having to look further to engineer spam stealthiness.

The easiest means of detecting the current encrypted file attacks would
be the attachment's filesize, 77KB, but this could be varied in future
attacks quite easily. The best approach was simply to disallow encrypted
emails to pass through the system at all.
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