+# Exim filter
+## Version: 0.14
+# $Id: system_filter.exim,v 1.4 2001/05/22 08:18:31 nigel Exp $
+
+## Exim system filter to refuse potentially harmful payloads in
+## mail messages
+## (c) 2000-2001 Nigel Metheringham <nigel@exim.org>
+##
+## This program is free software; you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation; either version 2 of the License, or
+## (at your option) any later version.
+##
+## This program is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program; if not, write to the Free Software
+## Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+## -A copy of the GNU General Public License is distributed with exim itself
+
+## -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+## If you haven't worked with exim filters before, read
+## the install notes at the end of this file.
+## The install notes are not a replacement for the exim documentation
+## -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+
+## -----------------------------------------------------------------------
+# Only run any of this stuff on the first pass through the
+# filter - this is an optomisation for messages that get
+# queued and have several delivery attempts
+#
+# we express this in reverse so we can just bail out
+# on inappropriate messages
+#
+if not first_delivery
+then
+ finish
+endif
+
+## -----------------------------------------------------------------------
+# Check for MS buffer overruns as per BUGTRAQ.
+# http://www.securityfocus.com/frames/?content=/templates/article.html%3Fid%3D61
+# This could happen in error messages, hence its placing
+# here...
+# We substract the first n characters of the date header
+# and test if its the same as the date header... which
+# is a lousy way of checking if the date is longer than
+# n chars long
+if ${length_80:$header_date:} is not $header_date:
+then
+ fail text "This message has been rejected because it has\n\
+ an overlength date field which can be used\n\
+ to subvert Microsoft mail programs\n\
+ The following URL has further information\n\
+ http://www.securityfocus.com/frames/?content=/templates/article.html%3Fid%3D61"
+ seen finish
+endif
+
+## -----------------------------------------------------------------------
+# These messages are now being sent with a <> envelope sender, but
+# blocking all error messages that pattern match prevents
+# bounces getting back.... so we fudge it somewhat and check for known
+# header signatures. Other bounces are allowed through.
+if $header_from: contains "@sexyfun.net"
+then
+ fail text "This message has been rejected since it has\n\
+ the signature of a known virus in the header."
+ seen finish
+endif
+if error_message and $header_from: contains "Mailer-Daemon@"
+then
+ # looks like a real error message - just ignore it
+ finish
+endif
+
+## -----------------------------------------------------------------------
+# Look for single part MIME messages with suspicious name extensions
+# Check Content-Type header using quoted filename [content_type_quoted_fn_match]
+if $header_content-type: matches "[[content_type_quoted_fn_match]]"
+then
+ fail text "This message has been rejected because it has\n\
+ potentially executable content $1\n\
+ This form of attachment has been used by\n\
+ recent viruses or other malware.\n\
+ If you meant to send this file then please\n\
+ package it up as a zip file and resend it."
+ seen finish
+endif
+# same again using unquoted filename [content_type_unquoted_fn_match]
+if $header_content-type: matches "[[content_type_unquoted_fn_match]]"
+then
+ fail text "This message has been rejected because it has\n\
+ potentially executable content $1\n\
+ This form of attachment has been used by\n\
+ recent viruses or other malware.\n\
+ If you meant to send this file then please\n\
+ package it up as a zip file and resend it."
+ seen finish
+endif
+
+
+## -----------------------------------------------------------------------
+# Attempt to catch embedded VBS attachments
+# in emails. These were used as the basis for
+# the ILOVEYOU virus and its variants - many many varients
+# Quoted filename - [body_quoted_fn_match]
+if $message_body matches "[[body_quoted_fn_match]]"
+then
+ fail text "This message has been rejected because it has\n\
+ a potentially executable attachment $1\n\
+ This form of attachment has been used by\n\
+ recent viruses or other malware.\n\
+ If you meant to send this file then please\n\
+ package it up as a zip file and resend it."
+ seen finish
+endif
+# same again using unquoted filename [body_unquoted_fn_match]
+if $message_body matches "[[body_unquoted_fn_match]]"
+then
+ fail text "This message has been rejected because it has\n\
+ a potentially executable attachment $1\n\
+ This form of attachment has been used by\n\
+ recent viruses or other malware.\n\
+ If you meant to send this file then please\n\
+ package it up as a zip file and resend it."
+ seen finish
+endif
+## -----------------------------------------------------------------------
+
+
+#### Version history
+#
+# 0.01 5 May 2000
+# Initial release
+# 0.02 8 May 2000
+# Widened list of content-types accepted, added WSF extension
+# 0.03 8 May 2000
+# Embedded the install notes in for those that don't do manuals
+# 0.04 9 May 2000
+# Check global content-type header. Efficiency mods to REs
+# 0.05 9 May 2000
+# More minor efficiency mods, doc changes
+# 0.06 20 June 2000
+# Added extension handling - thx to Douglas Gray Stephens & Jeff Carnahan
+# 0.07 19 July 2000
+# Latest MS Outhouse bug catching
+# 0.08 19 July 2000
+# Changed trigger length to 80 chars, fixed some spelling
+# 0.09 29 September 2000
+# More extensions... its getting so we should just allow 2 or 3 through
+# 0.10 18 January 2001
+# Removed exclusion for error messages - this is a little nasty
+# since it has other side effects, hence we do still exclude
+# on unix like error messages
+# 0.11 20 March, 2001
+# Added CMD extension, tidied docs slightly, added RCS tag
+# ** Missed changing version number at top of file :-(
+# 0.12 10 May, 2001
+# Added HTA extension
+# 0.13 22 May, 2001
+# Reformatted regexps and code to build them so that they are
+# shorter than the limits on pre exim 3.20 filters. This will
+# make them significantly less efficient, but I am getting so
+# many queries about this that requiring 3.2x appears unsupportable.
+# 0.14 15 August,2001
+# Added .lnk extension - most requested item :-)
+# Reformatted everything so its now built from a set of short
+# library files, cutting down on manual duplication.
+# Changed \w in filename detection to . - dodges locale problems
+# Explicit application of GPL after queries on license status
+#
+#### Install Notes
+#
+# Exim filters run the exim filter language - a very primitive
+# scripting language - in place of a user .forward file, or on
+# a per system basis (on all messages passing through).
+# The filtering capability is documented in the main set of manuals
+# a copy of which can be found on the exim web site
+# http://www.exim.org/
+#
+# To install, copy the filter file (with appropriate permissions)
+# to /etc/exim/system_filter.exim and add to your exim config file
+# [location is installation depedant - typicaly /etc/exim/config ]
+# in the first section the line:-
+# message_filter = /etc/exim/system_filter.exim
+# message_body_visible = 5000
+#
+# You may also want to set the message_filter_user & message_filter_group
+# options, but they default to the standard exim user and so can
+# be left untouched. The other message_filter_* options are only
+# needed if you modify this to do other functions such as deliveries.
+# The main exim documentation is quite thorough and so I see no need
+# to expand it here...
+#
+# Any message that matches the filter will then be bounced.
+# If you wish you can change the error message by editing it
+# in the section above - however be careful you don't break it.
+#
+# After install exim should be restarted - a kill -HUP to the
+# daemon will do this.
+#
+#### LIMITATIONS
+#
+# This filter tries to parse MIME with a regexp... that doesn't
+# work too well. It will also only see the amount of the body
+# specified in message_body_visible
+#
+#### BASIS
+#
+# The regexp that is used to pickup MIME/uuencoded body parts with
+# quoted filenames is replicated below (in perl format).
+# You need to remember that exim converts newlines to spaces in
+# the message_body variable.
+#
+[<body_quoted_fn_match>]
+#
+#
+### [End]