Защита от вирусов шифровальщиков

На базе почтового шлюза Postfix. Данная статья пойдет и для обычного рядового сервера на базе Postfix.

Для этой задачи прекрасно справляется фильтр Amavisd-new. Ставим его yum install amavisd-new. Его конфигурационный файл:

use strict;

# COMMONLY ADJUSTED SETTINGS:

@bypass_virus_checks_maps = (1); # controls running of anti-virus code
@bypass_spam_checks_maps = (1); # controls running of anti-spam code
#$bypass_decode_parts = 1; # controls running of decoders&dearchivers

$max_servers = 2; # num of pre-forked children (2..30 is common), -m
$daemon_user = ‘amavis’; # (no default; customary: vscan or amavis), -u
$daemon_group = ‘amavis’; # (no default; customary: vscan or amavis), -g

$mydomain = ‘domain.ru’; # a convenient default for other settings

$MYHOME = ‘/var/spool/amavisd’; # a convenient default for other settings, -H
$TEMPBASE = «$MYHOME/tmp»; # working directory, needs to exist, -T
$ENV{TMPDIR} = $TEMPBASE; # environment variable TMPDIR, used by SA, etc.
$QUARANTINEDIR = undef; # -Q
# $quarantine_subdir_levels = 1; # add level of subdirs to disperse quarantine
# $release_format = ‘resend’; # ‘attach’, ‘plain’, ‘resend’
# $report_format = ‘arf’; # ‘attach’, ‘plain’, ‘resend’, ‘arf’

# $daemon_chroot_dir = $MYHOME; # chroot directory or undef, -R

$db_home = «$MYHOME/db»; # dir for bdb nanny/cache/snmp databases, -D
# $helpers_home = «$MYHOME/var»; # working directory for SpamAssassin, -S
$lock_file = «/var/run/amavisd/amavisd.lock»; # -L
$pid_file = «/var/run/amavisd/amavisd.pid»; # -P
#NOTE: create directories $MYHOME/tmp, $MYHOME/var, $MYHOME/db manually

$log_level = 0; # verbosity 0..5, -d
$log_recip_templ = undef; # disable by-recipient level-0 log entries
$DO_SYSLOG = 1; # log via syslogd (preferred)
$syslog_facility = ‘mail’; # Syslog facility as a string
# e.g.: mail, daemon, user, local0, … local7
$syslog_priority = ‘debug’; # Syslog base (minimal) priority as a string,
# choose from: emerg, alert, crit, err, warning, notice, info, debug

$enable_db = 1; # enable use of BerkeleyDB/libdb (SNMP and nanny)
$enable_global_cache = 1; # enable use of libdb-based cache if $enable_db=1
$nanny_details_level = 2; # nanny verbosity: 1: traditional, 2: detailed
$enable_dkim_verification = 1; # enable DKIM signatures verification
$enable_dkim_signing = 1; # load DKIM signing code, keys defined by dkim_key

@local_domains_maps = ( [«.$mydomain»] ); # list of all local domains

@mynetworks = qw( 127.0.0.0/8 [::1] [FE80::]/10 [FEC0::]/10
10.0.0.0/8 172.16.0.0/12 192.168.0.0/16 );

$unix_socketname = «$MYHOME/amavisd.sock»; # amavisd-release or amavis-milter
# option(s) -p overrides $inet_socket_port and $unix_socketname

$inet_socket_bind = ‘127.0.0.1’;
$inet_socket_port = 10024; # listen on this local TCP port(s)
# $inet_socket_port = [10024,10026]; # listen on multiple TCP ports

$policy_bank{‘MYNETS’} = { # mail originating from @mynetworks
originating => 1, # is true in MYNETS by default, but let’s make it explicit
os_fingerprint_method => undef, # don’t query p0f for internal clients
};

# it is up to MTA to re-route mail from authenticated roaming users or
# from internal hosts to a dedicated TCP port (such as 10026) for filtering
$interface_policy{‘10026’} = ‘ORIGINATING’;

$policy_bank{‘ORIGINATING’} = { # mail supposedly originating from our users
originating => 1, # declare that mail was submitted by our smtp client
allow_disclaimers => 1, # enables disclaimer insertion if available
# notify administrator of locally originating malware
virus_admin_maps => [«virusalert\@$mydomain»],
spam_admin_maps => [«virusalert\@$mydomain»],
warnbadhsender => 1,
# forward to a smtpd service providing DKIM signing service
# forward_method => ‘smtp:[127.0.0.1]:10027’,
# force MTA conversion to 7-bit (e.g. before DKIM signing)
smtpd_discard_ehlo_keywords => [‘8BITMIME’],
bypass_banned_checks_maps => [1], # allow sending any file names and types
terminate_dsn_on_notify_success => 0, # don’t remove NOTIFY=SUCCESS option
};

$interface_policy{‘SOCK’} = ‘AM.PDP-SOCK’; # only applies with $unix_socketname

# Use with amavis-release over a socket or with Petr Rehor’s amavis-milter.c
# (with amavis-milter.c from this package or old amavis.c client use ‘AM.CL’):
$policy_bank{‘AM.PDP-SOCK’} = {
protocol => ‘AM.PDP’,
auth_required_release => 0, # do not require secret_id for amavisd-release
};

$sa_tag_level_deflt = -999; # add spam info headers if at, or above that level
$sa_tag2_level_deflt = 5.0; # add ‘spam detected’ headers at that level
$sa_kill_level_deflt = -999; # triggers spam evasive actions (e.g. blocks mail)
$sa_dsn_cutoff_level = 10; # spam level beyond which a DSN is not sent
$sa_crediblefrom_dsn_cutoff_level = 18; # likewise, but for a likely valid From
# $sa_quarantine_cutoff_level = 25; # spam level beyond which quarantine is off
$penpals_bonus_score = 8; # (no effect without a @storage_sql_dsn database)
$penpals_threshold_high = $sa_kill_level_deflt; # don’t waste time on hi spam
$bounce_killer_score = 100; # spam score points to add for joe-jobbed bounces

$sa_mail_body_size_limit = 400*1024; # don’t waste time on SA if mail is larger
$sa_local_tests_only = 0; # only tests which do not require internet access?

# @lookup_sql_dsn =
# ( [‘DBI:mysql:database=mail;host=127.0.0.1;port=3306’, ‘user1’, ‘passwd1’],
# [‘DBI:mysql:database=mail;host=host2’, ‘username2’, ‘password2’],
# [«DBI:SQLite:dbname=$MYHOME/sql/mail_prefs.sqlite», », »] );
# @storage_sql_dsn = @lookup_sql_dsn; # none, same, or separate database

# $timestamp_fmt_mysql = 1; # if using MySQL *and* msgs.time_iso is TIMESTAMP;
# defaults to 0, which is good for non-MySQL or if msgs.time_iso is CHAR(16)

$virus_admin = undef; # notifications recip.

$mailfrom_notify_admin = undef; # notifications sender
$mailfrom_notify_recip = undef; # notifications sender
$mailfrom_notify_spamadmin = undef; # notifications sender
$mailfrom_to_quarantine = »; # null return path; uses original sender if undef

@addr_extension_virus_maps = (‘virus’);
@addr_extension_banned_maps = (‘banned’);
@addr_extension_spam_maps = (‘spam’);
@addr_extension_bad_header_maps = (‘badh’);
# $recipient_delimiter = ‘+’; # undef disables address extensions altogether
# when enabling addr extensions do also Postfix/main.cf: recipient_delimiter=+

$path = ‘/usr/local/sbin:/usr/local/bin:/usr/sbin:/sbin:/usr/bin:/bin’;
# $dspam = ‘dspam’;

$MAXLEVELS = 14;
$MAXFILES = 1500;
$MIN_EXPANSION_QUOTA = 100*1024; # bytes (default undef, not enforced)
$MAX_EXPANSION_QUOTA = 300*1024*1024; # bytes (default undef, not enforced)

$undecipherable_subject_tag = undef;

$sa_spam_subject_tag = ‘***SPAM***’;
$defang_virus = 1; # MIME-wrap passed infected mail
$defang_banned = 1; # MIME-wrap passed mail containing banned name
# for defanging bad headers only turn on certain minor contents categories:
$defang_by_ccat{+CC_BADH.»,3″} = 1; # NUL or CR character in header
$defang_by_ccat{+CC_BADH.»,5″} = 1; # header line longer than 998 characters
$defang_by_ccat{+CC_BADH.»,6″} = 1; # header field syntax error
# OTHER MORE COMMON SETTINGS (defaults may suffice):

$myhostname = ‘mail.energokab.ru’; # must be a fully-qualified domain name!

$notify_method = ‘smtp:[127.0.0.1]:10025’;
$forward_method = ‘smtp:[127.0.0.1]:10025′; # set to undef with milter!

$final_virus_destiny = D_BOUNCE;
$final_banned_destiny = D_BOUNCE;
$final_spam_destiny = D_PASS;
$final_bad_header_destiny = D_PASS;
# $bad_header_quarantine_method = undef;

#$hdrfrom_notify_sender = «\»You tried to send the forbidden investment! Thank you.\» «;

$mailfrom_to_quarantine = »;
# SOME OTHER VARIABLES WORTH CONSIDERING (see amavisd.conf-default for all)

# $warnbadhsender,
# $warnvirusrecip, $warnbannedrecip, $warnbadhrecip, (or @warn*recip_maps)
#
# @bypass_virus_checks_maps, @bypass_spam_checks_maps,
# @bypass_banned_checks_maps, @bypass_header_checks_maps,
#
# @virus_lovers_maps, @spam_lovers_maps,
# @banned_files_lovers_maps, @bad_header_lovers_maps,
#
# @blacklist_sender_maps, @score_sender_maps,
#
# $clean_quarantine_method, $virus_quarantine_to, $banned_quarantine_to,
# $bad_header_quarantine_to, $spam_quarantine_to,
#
# $defang_bad_header, $defang_undecipherable, $defang_spam

@bypass_virus_checks_maps = (
\%bypass_virus_checks, \@bypass_virus_checks_acl, \$bypass_virus_checks_re);

#$warnvirussender = 1;
#$warnspamsender = 1;
$warnbannedsender = 1;
$warnbadhsender = 1;

$warnvirusrecip = 1;
$warnbannedrecip = 1;

# REMAINING IMPORTANT VARIABLES ARE LISTED HERE BECAUSE OF LONGER ASSIGNMENTS

@keep_decoded_original_maps = (new_RE(
qr’^MAIL$’, # retain full original message for virus checking
qr’^MAIL-UNDECIPHERABLE$’, # recheck full mail if it contains undecipherables
qr’^(ASCII(?! cpio)|text|uuencoded|xxencoded|binhex)’i,
# qr’^Zip archive data’, # don’t trust Archive::Zip
));
# for $banned_namepath_re (a new-style of banned table) see amavisd.conf-sample

$banned_filename_re = new_RE(

### BLOCKED ANYWHERE
# qr’^UNDECIPHERABLE$’, # is or contains any undecipherable components
qr’^\.(exe-ms|dll)$’, # banned file(1) types, rudimentary
# qr’^\.(exe|lha|cab|dll)$’, # banned file(1) types

### BLOCK THE FOLLOWING, EXCEPT WITHIN UNIX ARCHIVES:
# [ qr’^\.(gz|bz2)$’ => 0 ], # allow any in gzip or bzip2
[ qr’^\.(rpm|cpio|tar)$’ => 0 ], # allow any in Unix-type archives

qr’.\.(pif|scr)$’i, # banned extensions — rudimentary
# qr’^\.zip$’, # block zip type

### BLOCK THE FOLLOWING, EXCEPT WITHIN ARCHIVES:
# [ qr’^\.(zip|rar|arc|arj|zoo)$’=> 0 ], # allow any within these archives

qr’^application/x-msdownload$’i, # block these MIME types
qr’^application/x-msdos-program$’i,
qr’^application/hta$’i,

# qr’^message/partial$’i, # rfc2046 MIME type
# qr’^message/external-body$’i, # rfc2046 MIME type

# qr’^(application/x-msmetafile|image/x-wmf)$’i, # Windows Metafile MIME type
# qr’^\.wmf$’, # Windows Metafile file(1) type

# block certain double extensions in filenames
qr’^(?!cid:).*\.[^./]*[A-Za-z][^./]*\.\s*(exe|vbs|pif|scr|bat|cmd|com|cpl|dll)[.\s]*$’i,

qr’.\.(exe|vbs|pif|scr|cpl)$’i, # banned extension — basic
qr’.\.(exe|vbs|pif|scr|cpl|bat|cmd|com)$’i, # banned extension — basic+cmd
qr’.\.(ade|adp|app|bas|bat|chm|cmd|com|cpl|crt|exe|fxp|grp|hlp|hta|inf|ins|isp|js|jse|

lnk|mda|mdb|mde|mdw|mdt|mdz|msc|msi

|msp|mst|ops|pcd|pif|prg|reg|scr|sct|shb|shs|vb|vbe|vbs|wsc|wsf|wsh)$’ix, # banned ext — long разрешил wmf, emf
qr’.\.(ani|cur|ico)$’i, # banned cursors and icons filename
qr’^\.ani$’, # banned animated cursor file(1) type

qr’.\.(mim|b64|bhx|hqx|xxe|uu|uue)$’i, # banned extension — WinZip vulnerab.
);
# See http://support.microsoft.com/default.aspx?scid=kb;EN-US;q262631
# and http://www.cknow.com/vtutor/vtextensions.htm
# ENVELOPE SENDER SOFT-WHITELISTING / SOFT-BLACKLISTING

@score_sender_maps = ({ # a by-recipient hash lookup table,
# results from all matching recipient tables are summed

# ## per-recipient personal tables (NOTE: positive: black, negative: white)
# ‘user1@example.com’ => [{‘bla-mobile.press@example.com’ => 10.0}],
# ‘user3@example.com’ => [{‘.ebay.com’ => -3.0}],
# ‘user4@example.com’ => [{‘cleargreen@cleargreen.com’ => -7.0,
# ‘.cleargreen.com’ => -5.0}],

## site-wide opinions about senders (the ‘.’ matches any recipient)
‘.’ => [ # the _first_ matching sender determines the score boost

new_RE( # regexp-type lookup table, just happens to be all soft-blacklist
[qr’^(bulkmail|offers|cheapbenefits|earnmoney|foryou)@’i => 5.0],
[qr’^(greatcasino|investments|lose_weight_today|market\.alert)@’i=> 5.0],
[qr’^(money2you|MyGreenCard|new\.tld\.registry|opt-out|opt-in)@’i=> 5.0],
[qr’^(optin|saveonlsmoking2002k|specialoffer|specialoffers)@’i => 5.0],
[qr’^(stockalert|stopsnoring|wantsome|workathome|yesitsfree)@’i => 5.0],
[qr’^(your_friend|greatoffers)@’i => 5.0],
[qr’^(inkjetplanet|marketopt|MakeMoney)\d*@’i => 5.0],
),

# read_hash(«/var/amavis/sender_scores_sitewide»),

{ # a hash-type lookup table (associative array)
‘nobody@cert.org’ => -3.0,
‘cert-advisory@us-cert.gov’ => -3.0,
‘owner-alert@iss.net’ => -3.0,
‘slashdot@slashdot.org’ => -3.0,
‘securityfocus.com’ => -3.0,
‘ntbugtraq@listserv.ntbugtraq.com’ => -3.0,
‘security-alerts@linuxsecurity.com’ => -3.0,
‘mailman-announce-admin@python.org’ => -3.0,
‘amavis-user-admin@lists.sourceforge.net’=> -3.0,
‘amavis-user-bounces@lists.sourceforge.net’ => -3.0,
‘spamassassin.apache.org’ => -3.0,
‘notification-return@lists.sophos.com’ => -3.0,
‘owner-postfix-users@postfix.org’ => -3.0,
‘owner-postfix-announce@postfix.org’ => -3.0,
‘owner-sendmail-announce@lists.sendmail.org’ => -3.0,
‘sendmail-announce-request@lists.sendmail.org’ => -3.0,
‘donotreply@sendmail.org’ => -3.0,
‘ca+envelope@sendmail.org’ => -3.0,
‘noreply@freshmeat.net’ => -3.0,
‘owner-technews@postel.acm.org’ => -3.0,
‘ietf-123-owner@loki.ietf.org’ => -3.0,
‘cvs-commits-list-admin@gnome.org’ => -3.0,
‘rt-users-admin@lists.fsck.com’ => -3.0,
‘clp-request@comp.nus.edu.sg’ => -3.0,
‘surveys-errors@lists.nua.ie’ => -3.0,
’emailnews@genomeweb.com’ => -5.0,
‘yahoo-dev-null@yahoo-inc.com’ => -3.0,
‘returns.groups.yahoo.com’ => -3.0,
‘clusternews@linuxnetworx.com’ => -3.0,
lc(‘lvs-users-admin@LinuxVirtualServer.org’) => -3.0,
lc(‘owner-textbreakingnews@CNNIMAIL12.CNN.COM’) => -5.0,

# soft-blacklisting (positive score)
‘sender@example.net’ => 3.0,
‘.example.net’ => 1.0,

},
], # end of site-wide tables
});
@decoders = (
[‘mail’, \&do_mime_decode],
[‘asc’, \&do_ascii],
[‘uue’, \&do_ascii],
[‘hqx’, \&do_ascii],
[‘ync’, \&do_ascii],
[‘F’, \&do_uncompress, [‘unfreeze’,’freeze -d’,’melt’,’fcat’] ],
[‘Z’, \&do_uncompress, [‘uncompress’,’gzip -d’,’zcat’] ],
[‘gz’, \&do_uncompress, ‘gzip -d’],
[‘gz’, \&do_gunzip],
[‘bz2’, \&do_uncompress, ‘bzip2 -d’],
[‘lzo’, \&do_uncompress, ‘lzop -d’],
[‘rpm’, \&do_uncompress, [‘rpm2cpio.pl’,’rpm2cpio’] ],
[‘cpio’, \&do_pax_cpio, [‘pax’,’gcpio’,’cpio’] ],
[‘tar’, \&do_pax_cpio, [‘pax’,’gcpio’,’cpio’] ],
[‘deb’, \&do_ar, ‘ar’],
# [‘a’, \&do_ar, ‘ar’], # unpacking .a seems an overkill
[‘zip’, \&do_unzip],
[‘7z’, \&do_7zip, [‘7zr’,’7za’,’7z’] ],
[‘rar’, \&do_unrar, [‘rar’,’unrar’] ],
[‘arj’, \&do_unarj, [‘arj’,’unarj’] ],
[‘arc’, \&do_arc, [‘nomarch’,’arc’] ],
[‘zoo’, \&do_zoo, [‘zoo’,’unzoo’] ],
[‘lha’, \&do_lha, ‘lha’],
# [‘doc’, \&do_ole, ‘ripole’],
[‘cab’, \&do_cabextract, ‘cabextract’],
[‘tnef’, \&do_tnef_ext, ‘tnef’],
[‘tnef’, \&do_tnef],
# [‘sit’, \&do_unstuff, ‘unstuff’], # broken/unsafe decoder
[‘exe’, \&do_executable, [‘rar’,’unrar’], ‘lha’, [‘arj’,’unarj’] ],
);
@av_scanners = (

);
@av_scanners_backup = (

);

#@av_scanners = (

# [‘AVG Anti-Virus’,
# \&ask_daemon, [«SCAN {}\n», ‘127.0.0.1:54322’],
# qr/^200 [oO][kK]/m, qr/^403/m, qr/^403 .*?: ([^\r\n]+)/m ],

# );
1; # insure a defined return value

Весь фокус в том, что в конфигурации стоит запрет на определенные разрешения файлов вложений, а также что самое главное  на двойное расширение.

И дешево и сердито.

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