Hi,
I am not certain on the official process for submitting suggestions to
improve the system filter file system_filter.exim.
There is a potential problem with Explorer masking the try identity of
a file, so that the file can be opened in Explorer by the wrong
application, and really be an executable virus.
The article
http://www.guninski.com/clsidext.html
gives more details, i.e.
Details:
If the file extension is certain CLSID e.g.:
testhta.txt.{3050F4D8-98B5-11CF-BB82-00AA00BDCE0B}
then Windows explorer and IE do not show the CLSID extension,
while the above file is in fact .hta file. Some exploit scenarios
include leaving such shared resources or sending them in archive by email.
To overcome this Windows issue, I suggest that two filters should be
updated to treat files that have {...} as their final extension should
be treated like executables (e.g. vbs files).
I've appended a new system_filter.exim file for review and
consideration. The file also includes code to stop the postmaster
getting bounces from spam mail with invalid return addresses, which
was discussed earlier on this list, and I've been using for a while,
but is not in the offical code.
Thanks,
Douglas.
--
================================
Dr. Douglas GRAY STEPHENS
Global Infrastructure (Directories)
Schlumberger Cambridge Research
High Cross,
Madingley Road,
Cambridge.
CB3 0EL
ENGLAND
Phone +44 1223 325295
Mobile +44 773 0051628
Fax +44 1223 311830
Email DGrayStephens@???
================================
# Exim filter
## Version: 0.14
# $Id: system_filter.exim,v 1.4 2001/06/27 22:11:41 root Exp $
## If you haven't worked with exim filters before, read
## the install notes at the end of this file.
#
# 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 latest 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
if $return_path is ""
then
seen finish
endif
fail text "This message has been rejected because it has\n\
\tan overlength date field which can be used\n\
\tto subvert Microsoft mail programs\n\
\tThe following URL has further information\n\
\thttp://www.securityfocus.com/frames/?content=/templates/article.html%3Fid%3D61"
seen finish
endif
# This is a nasty compromise.
# This crud is now being sent with a <> envelope sender, but
# blocking all error messages that pattern match prevents
# bounces getting back.... so we fudge it somewhat
if $header_from: contains "@sexyfun.net"
then
if $return_path is ""
then
seen finish
endif
fail text "This message has been rejected since it has\n\
\tthe 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|pif)\")"
then
if $return_path is ""
then
seen finish
endif
fail text "This message has been rejected because it has\n\
\tpotentially executable content $1\n\
\tThis form of attachment has been used by\n\
\trecent viruses or other malware.\n\
\tIf you meant to send this file then please\n\
\tpackage 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=([\\\\w.-]+\\\\.(?:vb[se]|ws[fh]|jse?|exe|com|cmd|shs|hta|bat|scr|pif))"
then
if $return_path is ""
then
seen finish
endif
fail text "This message has been rejected because it has\n\
\tpotentially executable content $1\n\
\tThis form of attachment has been used by\n\
\trecent viruses or other malware.\n\
\tIf you meant to send this file then please\n\
\tpackage 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
# 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|pif|{[^\"}]+})\")[\\\\s;]"
then
if $return_path is ""
then
seen finish
endif
fail text "This message has been rejected because it has\n\
\ta potentially executable attachment $1\n\
\tThis form of attachment has been used by\n\
\trecent viruses or other malware.\n\
\tIf you meant to send this file then please\n\
\tpackage 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+))([\\\\w.-]+\\\\.(?:vb[se]|ws[fh]|jse?|exe|com|cmd|shs|hta|bat|scr|pif|{[\\\\w.-]+}))[\\\\s;]"
then
if $return_path is ""
then
seen finish
endif
fail text "This message has been rejected because it has\n\
\ta potentially executable attachment $1\n\
\tThis form of attachment has been used by\n\
\trecent viruses or other malware.\n\
\tIf you meant to send this file then please\n\
\tpackage 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.
# 0.13a 28 May, 2001
# Douglas Gray Stephens add block to stop viruses with
# mail-formed return address being forwarded to the postmaster.
# 0.14 30 June, 2001
# Douglas Gray Stephens add block to attachments that mask true
# file type (see http://www.guninski.com/clsidext.html)
#
#### 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 ]
# at the top 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 parts 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
# |pif)
# \" # end quote
# |[\w.-]+\. # unquoted filename.ext
# (?:vb[se] # list of extns
# |ws[fh]
# |jse?
# |exe
# |com
# |cmd
# |shs
# |hta
# |bat
# |scr
# |pif)
# ) # end of filename capture
# [\s;] # trailing ;/space/newline
#
#
### [End]
