* To: debian-kr-user@lists.debian.or.kr * Subject: 유용한 EXIM filter * From: Seongtae Yoo * Date: Thu, 13 Sep 2001 20:16:47 +0900 * List-Id: Debian 한국 사용자 메일링 리스트 * References: <001c01c13c02$9280cbc0$1654f2d3@lopa.co.kr> * Sender: debian-kr-user-admin@lists.debian.or.kr 첨부 파일로 심심찮게 들어오는 바이러스 메일을 막을 길이 없나 고민하다고 exim 문서의 filter를 읽고 나름대로 filtering을 하였으나 실력의 역부족을 느끼고 인터넷 상에 훨씬 정교한 filter가 있을 것이라 생각하고 exim 메일링 리스트를 뒤졌더니 아니나 다를까 멋진 filter가 있네요. 첨부 파일로 보냅니다. 파일의 끝에 간단한 설치 방법도 적혀 있습니다. 설치랄 것도 없긴 하지만서도... 아쉬운 점은 데비안을 대충 깔아서 그런지 한글 필터링이 안된다는 것, 메세지 내용이 매우 많은 경우 첨부 파일을 제대로 검색할 수 없다는 점입니다. 하루에 주고 받는 메일이 백통에 지나지 않는 경우, message_body_visible 변수를 100000 정도로 해 놓아면 웬만큼 메세지 내용이 많아도 첨부 파일로 붙어오는 바이러스는 검색할 수 있을 것이라 여겨집니다. # Exim filter ## Version: 0.15 # $Id: system_filter.exim,v 1.9 2001/08/17 12:47:26 nigel Exp $ ## Exim system filter to refuse potentially harmful payloads in ## mail messages ## (c) 2000-2001 Nigel Metheringham ## ## 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 "Your 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 "Your 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 "(?:file)?name=(\"[^\"]+\\\\.(?:vb[se]|ws[fh]|jse?|exe|com|cmd|shs|hta|bat|scr|lnk|pif)\")" then fail text "Your 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 "(?:file)?name=(\\\\S+\\\\.(?:vb[se]|ws[fh]|jse?|exe|com|cmd|shs|hta|bat|scr|lnk|pif))" then fail text "Your 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 "(?:Content-(?:Type:(?>\\\\s*)[\\\\w-]+/[\\\\w-]+|Disposition:(?>\\\\s*)attachment);(?>\\\\s*)(?:file)?name=|begin(?>\\\\s+)[0-7]{3,4}(?>\\\\s+))(\"[^\"]+\\\\.(?:vb[se]|ws[fh]|jse?|exe|com|cmd|shs|hta|bat|scr|lnk|pif)\")[\\\\s;]" then fail text "Your 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 [body_unquoted_fn_match] if $message_body matches "(?:Content-(?:Type:(?>\\\\s*)[\\\\w-]+/[\\\\w-]+|Disposition:(?>\\\\s*)attachment);(?>\\\\s*)(?:file)?name=|begin(?>\\\\s+)[0-7]{3,4}(?>\\\\s+))(\\\\S+\\\\.(?:vb[se]|ws[fh]|jse?|exe|com|cmd|shs|hta|bat|scr|lnk|pif))[\\\\s;]" then fail text "Your 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 ## ----------------------------------------------------------------------- #### 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 # 0.15 17 August, 2001 # Chnaged the . in filename detect to \S (stops it going mad) # #### 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. # # (?:Content- # start of content header # (?:Type: (?>\s*) # rest of c/t header # [\w-]+/[\w-]+ # content-type (any) # |Disposition: (?>\s*) # content-disposition hdr # attachment) # content-disposition # ;(?>\s*) # ; space or newline # (?:file)?name= # filename=/name= # |begin (?>\s+) [0-7]{3,4} (?>\s+)) # begin octal-mode # (\"[^\"]+\. # quoted filename. # (?:vb[se] # list of extns # |ws[fh] # |jse? # |exe # |com # |cmd # |shs # |hta # |bat # |scr # |lnk # |pif) # \" # end quote # ) # end of filename capture # [\s;] # trailing ;/space/newline # # ### [End]