krb5-enum-users NSE Script

local asn1 = require "asn1"
local coroutine = require "coroutine"
local nmap = require "nmap"
local os = require "os"
local shortport = require "shortport"
local stdnse = require "stdnse"
local string = require "string"
local table = require "table"
local unpwdb = require "unpwdb"

description = [[
Discovers valid usernames by brute force querying likely usernames against a Kerberos service.
When an invalid username is requested the server will respond using the
Kerberos error code KRB5KDC_ERR_C_PRINCIPAL_UNKNOWN, allowing us to determine
that the user name was invalid. Valid user names will illicit either the
TGT in a AS-REP response or the error KRB5KDC_ERR_PREAUTH_REQUIRED, signaling
that the user is required to perform pre authentication.

The script should work against Active Directory and ?
It needs a valid Kerberos REALM in order to operate.
]]

---
-- @usage
-- nmap -p 88 --script krb5-enum-users --script-args krb5-enum-users.realm='test'
--
-- @output
-- PORT   STATE SERVICE      REASON
-- 88/tcp open  kerberos-sec syn-ack
-- | krb5-enum-users:
-- | Discovered Kerberos principals
-- |     administrator@test
-- |     mysql@test
-- |_    tomcat@test
--
-- @args krb5-enum-users.realm this argument is required as it supplies the
--       script with the Kerberos REALM against which to guess the user names.
--

--
--
-- Version 0.1
-- Created 10/16/2011 - v0.1 - created by Patrik Karlsson <[email protected]>
--

author = "Patrik Karlsson"
license = "Same as Nmap--See https://nmap.org/book/man-legal.html"
categories = {"auth", "intrusive"}


portrule = shortport.port_or_service( 88, {"kerberos-sec"}, {"udp","tcp"}, {"open", "open|filtered"} )

-- This an embryo of a Kerberos 5 packet creation and parsing class. It's very
-- tiny class and holds only the necessary functions to support this script.
-- This class be factored out into its own library, once more scripts make use
-- of it.
KRB5 = {

  -- Valid Kerberos message types
  MessageType = {
    ['AS-REQ'] = 10,
    ['AS-REP'] = 11,
    ['KRB-ERROR'] = 30,
  },

  -- Some of the used error messages
  ErrorMessages = {
    ['KRB5KDC_ERR_C_PRINCIPAL_UNKNOWN'] = 6,
    ['KRB5KDC_ERR_PREAUTH_REQUIRED'] = 25,
    ['KDC_ERR_WRONG_REALM'] = 68,
  },

  -- A list of some ot the encryption types
  EncryptionTypes = {
    { ['aes256-cts-hmac-sha1-96'] = 18 },
    { ['aes128-cts-hmac-sha1-96'] = 17 },
    { ['des3-cbc-sha1'] = 16 },
    { ['rc4-hmac'] = 23 },
    -- { ['des-cbc-crc'] = 1 },
    -- { ['des-cbc-md5'] = 3 },
    -- { ['des-cbc-md4'] = 2 }
  },

  -- A list of principal name types
  NameTypes = {
    ['NT-PRINCIPAL'] = 1,
    ['NT-SRV-INST'] = 2,
  },

  -- Creates a new Krb5 instance
  -- @return o as the new instance
  new = function(self)
    local o = {}
    setmetatable(o, self)
    self.__index = self
    return o
  end,

  -- A number of custom ASN1 decoders needed to decode the response
  tagDecoder = {

    ["\x18"] = function( self, encStr, elen, pos )
      return string.unpack("c" .. elen, encStr, pos)
    end,

    ["\x1B"] = function( ... ) return KRB5.tagDecoder["\x18"](...) end,

    ["\x6B"] = function( self, encStr, elen, pos )
      return self:decodeSeq(encStr, elen, pos)
    end,

    -- Not really sure what these are, but they all decode sequences
    ["\x7E"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA0"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA1"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA2"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA3"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA4"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA5"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA6"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA7"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA8"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xA9"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xAA"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,
    ["\xAC"] = function( ... ) return KRB5.tagDecoder["\x6B"](...) end,

  },

  -- A few Kerberos ASN1 encoders
  tagEncoder = {

    ['table'] = function(self, val)

      local types = {
        ['GeneralizedTime'] = 0x18,
        ['GeneralString'] = 0x1B,
      }

      local len = asn1.ASN1Encoder.encodeLength(#val[1])

      if ( val._type and types[val._type] ) then
        return string.pack("B", types[val._type]) .. len .. val[1]
      elseif ( val._type and 'number' == type(val._type) ) then
        return string.pack("B", val._type) .. len .. val[1]
      end

    end,
  },

  -- Encodes a sequence using a custom type
  -- @param encoder class containing an instance of a ASN1Encoder
  -- @param seqtype number the sequence type to encode
  -- @param seq string containing the sequence to encode
  encodeSequence = function(self, encoder, seqtype, seq)
    return encoder:encode( { _type = seqtype, seq } )
  end,

  -- Encodes a Kerberos Principal
  -- @param encoder class containing an instance of ASN1Encoder
  -- @param name_type number containing a valid Kerberos name type
  -- @param names table containing a list of names to encode
  -- @return princ string containing an encoded principal
  encodePrincipal = function(self, encoder, name_type, names )
    local princ = {}

    for i, n in ipairs(names) do
      princ[i] = encoder:encode( { _type = 'GeneralString', n } )
    end

    princ = self:encodeSequence(encoder, 0x30, table.concat(princ))
    princ = self:encodeSequence(encoder, 0xa1, princ)
    princ = encoder:encode( name_type ) .. princ

    -- not sure about how this works, but apparently it does
    princ = stdnse.fromhex( "A003") .. princ
    princ = self:encodeSequence(encoder,0x30, princ)

    return princ
  end,

  -- Encodes the Kerberos AS-REQ request
  -- @param realm string containing the Kerberos REALM
  -- @param user string containing the Kerberos principal name
  -- @param protocol string containing either of "tcp" or "udp"
  -- @return data string containing the encoded request
  encodeASREQ = function(self, realm, user, protocol)

    assert(protocol == "tcp" or protocol == "udp",
      "Protocol has to be either \"tcp\" or \"udp\"")

    local encoder = asn1.ASN1Encoder:new()
    encoder:registerTagEncoders(KRB5.tagEncoder)

    local data = {}

    -- encode encryption types
    for _,enctype in ipairs(KRB5.EncryptionTypes) do
      for k, v in pairs( enctype ) do
        data[#data+1] = encoder:encode(v)
      end
    end

    data = self:encodeSequence(encoder, 0x30, table.concat(data) )
    data = self:encodeSequence(encoder, 0xA8, data )

    -- encode nonce
    local nonce = 155874945
    data = self:encodeSequence(encoder, 0xA7, encoder:encode(nonce) ) .. data

    -- encode from/to
    local fromdate = os.time() + 10 * 60 * 60
    local from = os.date("%Y%m%d%H%M%SZ", fromdate)
    data = self:encodeSequence(encoder, 0xA5, encoder:encode( { from, _type='GeneralizedTime' })) .. data

    local names = { "krbtgt", realm }
    local sname = self:encodePrincipal( encoder, KRB5.NameTypes['NT-SRV-INST'], names )
    sname = self:encodeSequence(encoder, 0xA3, sname)
    data = sname .. data

    -- realm
    data = self:encodeSequence(encoder, 0xA2, encoder:encode( { _type = 'GeneralString', realm })) .. data

    local cname = self:encodePrincipal(encoder, KRB5.NameTypes['NT-PRINCIPAL'], { user })
    cname = self:encodeSequence(encoder, 0xA1, cname)
    data = cname .. data

    -- forwardable
    local kdc_options = 0x40000000
    data = string.pack(">I4", kdc_options) .. data

    -- add padding
    data = '\0' .. data

    -- hmm, wonder what this is
    data = stdnse.fromhex( "A0070305") .. data
    data = self:encodeSequence(encoder, 0x30, data)
    data = self:encodeSequence(encoder, 0xA4, data)
    data = self:encodeSequence(encoder, 0xA2, encoder:encode(KRB5.MessageType['AS-REQ'])) .. data

    local pvno = 5
    data = self:encodeSequence(encoder, 0xA1, encoder:encode(pvno) ) .. data

    data = self:encodeSequence(encoder, 0x30, data)
    data = self:encodeSequence(encoder, 0x6a, data)

    if ( protocol == "tcp" ) then
      data = string.pack(">s4", data)
    end

    return data
  end,

  -- Parses the result from the AS-REQ
  -- @param data string containing the raw unparsed data
  -- @return status boolean true on success, false on failure
  -- @return msg table containing the fields <code>type</code> and
  --         <code>error_code</code> if the type is an error.
  parseResult = function(self, data)

    local decoder = asn1.ASN1Decoder:new()
    decoder:registerTagDecoders(KRB5.tagDecoder)
    decoder:setStopOnError(true)
    local result = decoder:decode(data)
    local msg = {}


    if ( #result == 0 or #result[1] < 2 or #result[1][2] < 1 ) then
      return false, nil
    end

    msg.type = result[1][2][1]

    if ( msg.type == KRB5.MessageType['KRB-ERROR'] ) then
      if ( #result[1] < 5 and #result[1][5] < 1 ) then
        return false, nil
      end

      msg.error_code = result[1][5][1]
      return true, msg
    elseif ( msg.type == KRB5.MessageType['AS-REP'] ) then
      return true, msg
    end

    return false, nil
  end,

}

-- Checks whether the user exists or not
-- @param host table as received by the action method
-- @param port table as received by the action method
-- @param realm string containing the Kerberos REALM
-- @param user string containing the Kerberos principal
-- @return status boolean, true on success, false on failure
-- @return state VALID or INVALID or error message if status was false
local function checkUser( host, port, realm, user )

  local krb5 = KRB5:new()
  local data = krb5:encodeASREQ(realm, user, port.protocol)
  local socket = nmap.new_socket()
  local status = socket:connect(host, port)

  if ( not(status) ) then
    return false, "ERROR: Failed to connect to Kerberos service"
  end

  socket:send(data)
  status, data = socket:receive()

  if ( port.protocol == 'tcp' ) then data = data:sub(5) end

  if ( not(status) ) then
    return false, "ERROR: Failed to receive result from Kerberos service"
  end
  socket:close()

  local msg
  status, msg = krb5:parseResult(data)

  if ( not(status) ) then
    return false, "ERROR: Failed to parse the result returned from the Kerberos service"
  end

  if ( msg and msg.error_code ) then
    if ( msg.error_code == KRB5.ErrorMessages['KRB5KDC_ERR_PREAUTH_REQUIRED'] ) then
      return true, "VALID"
    elseif ( msg.error_code == KRB5.ErrorMessages['KDC_ERR_WRONG_REALM'] ) then
      return false, "Invalid Kerberos REALM"
    end
  elseif ( msg.type == KRB5.MessageType['AS-REP'] ) then
    return true, "VALID"
  end
  return true, "INVALID"
end

-- Checks whether the Kerberos REALM exists or not
-- @param host table as received by the action method
-- @param port table as received by the action method
-- @param realm string containing the Kerberos REALM
-- @return status boolean, true on success, false on failure
local function isValidRealm( host, port, realm )
  return checkUser( host, port, realm, "nmap")
end

-- Wraps the checkUser function so that it is suitable to be called from
-- a thread. Adds a user to the result table in case it's valid.
-- @param host table as received by the action method
-- @param port table as received by the action method
-- @param realm string containing the Kerberos REALM
-- @param user string containing the Kerberos principal
-- @param result table to which all discovered users are added
local function checkUserThread( host, port, realm, user, result )
  local condvar = nmap.condvar(result)
  local status, state = checkUser(host, port, realm, user)
  if ( status and state == "VALID" ) then
    table.insert(result, ("%s@%s"):format(user,realm))
  end
  condvar "signal"
end

local function fail (err) return stdnse.format_output(false, err) end

action = function( host, port )

  local realm = stdnse.get_script_args("krb5-enum-users.realm")
  local result = {}
  local condvar = nmap.condvar(result)

  -- did the user supply a realm
  if ( not(realm) ) then
    return fail("No Kerberos REALM was supplied, aborting ...")
  end

  -- does the realm appear to exist
  if ( not(isValidRealm(host, port, realm)) ) then
    return fail("Invalid Kerberos REALM, aborting ...")
  end

  -- load our user database from unpwdb
  local status, usernames = unpwdb.usernames()
  if( not(status) ) then return fail("Failed to load unpwdb usernames") end

  -- start as many threads as there are names in the list
  local threads = {}
  for user in usernames do
    local co = stdnse.new_thread( checkUserThread, host, port, realm, user, result )
    threads[co] = true
  end

  -- wait for all threads to finish up
  repeat
    for t in pairs(threads) do
      if ( coroutine.status(t) == "dead" ) then threads[t] = nil end
    end
    if ( next(threads) ) then
      condvar "wait"
    end
  until( next(threads) == nil )

  if ( #result > 0 ) then
    result = { name = "Discovered Kerberos principals", result }
  end
  return stdnse.format_output(true, result)
end