[Mimedefang] Content-Transfer-Encoding header filter
dm at davidmeissner.com
dm at davidmeissner.com
Tue May 25 21:54:34 EDT 2004
Sorry if this gets posted twice - I had the wrong from address in my first posting.
I have noticed that some spam seems to come from mailers that have trouble
generating correct Content-Transfer-Encoding headers. There seems to be a flood
taking place right now from a mailer that uses "Content-Transfer-Encoding: plain" for
the header. In the past I have seen things like "quoted-pintable" and "8Bits".
To catch the easy ones, I am testing the following function for my filter:
sub filter_bad_encoding ($) {
my($entity) = @_;
my($bad_enc);
$bad_enc = '(plain|quoted-pintable)';
$enc = $entity->head->mime_encoding;
if ($enc =~ /$bad_enc/i) {
return 1;
}
return 0;
}
A few lines in the filter() function call this, and bounce messages that return the error.
I could add some of the additional malformed headers that I come across to the list
of bad types, but I'd like the opinion of the experts: since there is a fairly small number of
valid encoding types, would it make sense to test for only the valid ones and bounce
all other messages?
I realize that there are many malformed clients out there, so the drawback might be
that too many valid messages would get bounced. I'm just wondering how much of a
problem that might be.
-David Meissner
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