local coroutine = require "coroutine"
local math = require "math"
local nmap = require "nmap"
local shortport = require "shortport"
local stdnse = require "stdnse"
local comm = require "comm"
local vulns = require "vulns"
local http = require "http"


description = [[
Tests a web server for vulnerability to the Slowloris DoS attack without actually launching a DoS attack.

Slowloris was described at Defcon 17 by RSnake
(see http://ha.ckers.org/slowloris/).

This script opens two connections to the server, each without
the final CRLF. After 10 seconds, second connection sends
additional header. Both connections then wait for server timeout.
If second connection gets a timeout 10 or more seconds after the
first one, we can conclude that sending additional header prolonged
its timeout and that the server is vulnerable to slowloris DoS attack.

You can specify custom http User-agent field with <code>http.useragent</code>
script argument.

Idea from Qualys blogpost:
 * https://community.qualys.com/blogs/securitylabs/2011/07/07/identifying-slow-http-attack-vulnerabilities-on-web-applications

]]

---
-- @usage
-- nmap --script http-slowloris-check  <target>
--
-- @args http.useragent Specifies custom user agent string.
--
-- @output
-- PORT   STATE SERVICE REASON
-- 80/tcp open  http    syn-ack
-- | http-slowloris-check:
-- |   VULNERABLE:
-- |   Slowloris DOS attack
-- |     State: VULNERABLE
-- |     Description:
-- |       Slowloris tries to keep many connections to the target web server open and hold them open as long as possible.
-- |       It accomplishes this by opening connections to the target web server and sending a partial request. By doing
-- |       so, it starves the http server's resources causing Denial Of Service.
-- |
-- |     Disclosure date: 2009-09-17
-- |     References:
-- |_      http://ha.ckers.org/slowloris/

author = "Aleksandar Nikolic"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
categories = {"vuln", "safe"}


portrule = shortport.http

local HalfHTTP
local Bestopt
local TimeWithout -- time without additional headers
local TimeWith     -- time with additional headers

-- does a half http request and waits until timeout
local function slowThread1(host,port)
  -- if no response was received when determining SSL
  if ( Bestopt == "none" ) then
    return
  end
  local socket,status
  local catch = function()
    TimeWithout = nmap.clock()
  end
  local try = nmap.new_try(catch)
  socket = nmap.new_socket()
  socket:set_timeout(500 * 1000)
  socket:connect(host.ip, port, Bestopt)
  socket:send(HalfHTTP)
  try(socket:receive())
  TimeWithout = nmap.clock()
end

-- does a half http request but sends another
-- header value after 10 seconds
local function slowThread2(host,port)
  -- if no response was received when determining SSL
  if ( Bestopt == "none" ) then
    return
  end
  local socket,status
  local catch = function()
    -- note the time the socket timedout
    TimeWith = nmap.clock()
    stdnse.print_debug("2 try")
  end
  local try = nmap.new_try(catch)
  socket = nmap.new_socket()
  socket:set_timeout(500 * 1000)
  socket:connect(host.ip, port, Bestopt)
  socket:send(HalfHTTP)
  stdnse.sleep(10)
  socket:send("X-a: b\r\n")
  try(socket:receive())
  TimeWith = nmap.clock()
end

action = function(host,port)

  local slowloris  = {
    title = "Slowloris DOS attack",
    description = [[
Slowloris tries to keep many connections to the target web server open and hold them open as long as possible.
It accomplishes this by opening connections to the target web server and sending a partial request. By doing
so, it starves the http server's resources causing Denial Of Service.
]],
    references = {
      'http://ha.ckers.org/slowloris/',
    },
    dates = {
      disclosure = {year = '2009', month = '09', day = '17'},
    },
    exploit_results = {},
  }

  local report = vulns.Report:new(SCRIPT_NAME, host, port)
  slowloris.state = vulns.STATE.NOT_VULN

  local _
  _, _, Bestopt = comm.tryssl(host, port, "GET / \r\n\r\n", {}) -- first determine if we need ssl
  HalfHTTP = "POST /" .. tostring(math.random(100000, 900000)) .. " HTTP/1.1\r\n" ..
    "Host: " .. host.ip .. "\r\n" ..
    "User-Agent: " .. http.USER_AGENT .. "\r\n; " ..
    "Content-Length: 42\r\n"
  -- both threads run at the same time
  local thread1 = stdnse.new_thread(slowThread1, host, port)
  local thread2 = stdnse.new_thread(slowThread2, host, port)
  while true do -- wait for both threads to die
    if coroutine.status(thread1) == "dead" and  coroutine.status(thread2) == "dead" then
      break
    end
    stdnse.sleep(1)
  end
  -- compare times
  if ( not(TimeWith) or not(TimeWithout) ) then
    return
  end
  local diff = TimeWith - TimeWithout
  stdnse.print_debug("Time difference is: %d",diff)
  -- if second connection died 10 or more seconds after the first
  -- it means that sending additional data prolonged the connection's time
  -- and the server is vulnerable to slowloris attack
  if diff >= 10 then
    stdnse.print_debug("Difference is greater or equal to 10 seconds.")
    slowloris.state = vulns.STATE.VULN
  end
  return report:make_output(slowloris)
end