Exim: establecer filtros anti-spam para entrada y salida de correo

eximExim nos brinda la posibilidad de establecer filtros para todos los correos que circulan por el servidor, tanto entrada como salida. Estos filtros son personalizables y permiten crear reglas que revisen las cabeceras, asunto y cuerpo de los correos en busca de ciertos patrones maliciosos conocidos, expresiones regulares, etc.

Por defecto, en servidores con cPanel disponemos de una base de reglas en el fichero /etc/antivirus.exim en el cual podemos ver bastantes reglas ya establecidas y usarlas como base para las nuestras. Si queremos activar estos filtros antes debemos acceder a WHM -> Exim Configuration Editor y modificar la opción:

** System Filter File

The system filter file is usually stored as /etc/cpanel_exim_system_filter. [INFO] Custom values must be existing files.

En el cuadro de texto ponemos la ruta contra el fichero /etc/antivirus.exim.

Una vez activado, simplemente queda modificar, borrar o añadir nuestras reglas personalizadas, la sintaxis y estructura a utilizar es sencilla:

if $message_body matches "XXXXXXXXX"
then
  fail text "Texto que aparece en el log\n\
             cuando se filtra un correo\n\
             con esta regla, se pueden \n\
             utilizar variables $1 $2 $3.\n\
  seen finish
endif
if $header_content-type: matches "XXXXXXXX"
then
  fail text "Texto que aparece en el log\n\
             cuando se filtra un correo\n\
             con esta regla, se pueden \n\
             utilizar variables $1 $2 $3.\n\
  seen finish
endif

A partir de aquí todo depende de la pericia de cada uno con las expresiones regulares. Se puede activar un log independiente para el registro de todos los correos electrónicos filtrados, establecemos la siguiente directiva al comienzo del fichero:

logfile /var/log/filtros-exim.log 0744

Y creamos el fichero de log:

# touch /var/log/filtros-exim.log
# chown cpaneleximfilter. /var/log/filtros-exim.log
# chmod 0744 /var/log/filtros-exim.log

En caso de establecer un log, debemos especificar en cada una de las reglas si queremos que se registre dicho filtrado y como, se hace mediante la directiva logwrite, también se pueden establecer las variables y datos a registrar en el log:

if $header_content-type: matches "XXXXXXXX"
then
  logwrite "$tod_log $message_id from $sender_address TEXTO_PERSONALIZADO"
  fail text "Texto que aparece en el log\n\
             cuando se filtra un correo\n\
             con esta regla, se pueden \n\
             utilizar variables $1 $2 $3.\n\
  seen finish
endif

Este filtro central para Exim puede ser muy útil para evitar el spam, tanto entrante como saliente y establecer ciertas reglas puntuales para controlar el tráfico de correo en el servidor.

Este es el fichero base para aquellos que no lo tengáis:

# Exim filter
## Version: 0.17
#	$Id: system_filter.exim,v 1.11 2001/09/19 11:27:56 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 "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 "(?:file)?name=(\"[^\"]+\\\\.(?:ad[ep]|ba[st]|chm|cmd|com|cpl|crt|eml|exe|hlp|hta|in[fs]|isp|jse?|lnk|md[be]|ms[cipt]|pcd|pif|reg|scr|sct|shs|url|vb[se]|ws[fhc])\")"
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 "(?:file)?name=(\\\\S+\\\\.(?:ad[ep]|ba[st]|chm|cmd|com|cpl|crt|eml|exe|hlp|hta|in[fs]|isp|jse?|lnk|md[be]|ms[cipt]|pcd|pif|reg|scr|sct|shs|url|vb[se]|ws[fhc]))"
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 "(?:Content-(?:Type:(?>\\\\s*)[\\\\w-]+/[\\\\w-]+|Disposition:(?>\\\\s*)attachment);(?>\\\\s*)(?:file)?name=|begin(?>\\\\s+)[0-7]{3,4}(?>\\\\s+))(\"[^\"]+\\\\.(?:ad[ep]|ba[st]|chm|cmd|com|cpl|crt|eml|exe|hlp|hta|in[fs]|isp|jse?|lnk|md[be]|ms[cipt]|pcd|pif|reg|scr|sct|shs|url|vb[se]|ws[fhc])\")[\\\\s;]"
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 "(?:Content-(?:Type:(?>\\\\s*)[\\\\w-]+/[\\\\w-]+|Disposition:(?>\\\\s*)attachment);(?>\\\\s*)(?:file)?name=|begin(?>\\\\s+)[0-7]{3,4}(?>\\\\s+))(\\\\S+\\\\.(?:ad[ep]|ba[st]|chm|cmd|com|cpl|crt|eml|exe|hlp|hta|in[fs]|isp|jse?|lnk|md[be]|ms[cipt]|pcd|pif|reg|scr|sct|shs|url|vb[se]|ws[fhc]))[\\\\s;]"
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
# 0.15 17 August, 2001
#	Changed the . in filename detect to \S (stops it going mad)
# 0.16 19 September, 2001
#	Pile of new extensions including the eml in current use
# 0.17 19 September, 2001
#	Syntax fix
#
#### 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.
#		(?:ad[ep]				# list of extns
#		|ba[st]
#		|chm
#		|cmd
#		|com
#		|cpl
#		|crt
#		|eml
#		|exe
#		|hlp
#		|hta
#		|in[fs]
#		|isp
#		|jse?
#		|lnk
#		|md[be]
#		|ms[cipt]
#		|pcd
#		|pif
#		|reg
#		|scr
#		|sct
#		|shs
#		|url
#		|vb[se]
#		|ws[fhc])
#	  \"						# end quote
#	  )						# end of filename capture
#	  [\s;]						# trailing ;/space/newline

#
#
### [End]

Un comentario en “Exim: establecer filtros anti-spam para entrada y salida de correo

  1. HOLA ME PARECIO MUY INTERESANTE ESTE POST,
    PERO TENGO UNA DUDA,SI YO QUISIERA HACER UN FILTRO PARA QUE DETERMINADAS CUENTAS SOLO PUEDAN ENVIAR CORREOS A UNA CUENTA QUE YO ESTABLESCO Y AL MISMO TIEMPO SOLO PUEDAN RECIBIR CORREOS DE ESTA CUENTA, SE PUEDE HACER DE ESTE MODO???

    SALUDOS…

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