Microsoft Windows AppXsvc Deployment Extension - Privilege Escalation Vulnerability

# Exploit Title: Microsoft Windows AppXsvc Deployment Extension - Privilege Escalation
# Exploit Author: Abdelhamid Naceri
# Vendor Homepage: www.microsoft.com
# Tested on: Windows 10 1903
# CVE : CVE-2019-1385


Windows: "AppX Deployment Service" (AppXSVC) elevation of privilege vulnerability

Class: Local Elevation of Privileges

Description:
This Poc is exploiting a vulnerability in (AppXSvc) , abusing this vulnerability 
could allow an attacker to overwrite\create file as SYSTEM which can result in EOP .
The're is 2 way to abuse the issue .
Step To Reproduce :
[1] For An Arbitrary File Creation
1-Turn %userprofile%\AppData\Local\Microsoft\WindowsApps\Backup Into a Junction To
your target directory example "c:\"
2-Open Powershell and execute the command Add-AppxPackage -RegisterByFamilyName -ForceApplicationShutdown -MainPackage Microsoft.MicrosoftEdge_8wekyb3d8bbwe
3-Check the directory the file should be created now
4-Enjoy:)
[2] To Overwrite File 
1-Create a temp dir in %temp%\
2-Create a hardlink to your target file in the temp created dir
3-Turn %userprofile%\AppData\Local\Microsoft\WindowsApps\Backup Into a junction to
your temp created dir
4-Open Powershell and execute the command Add-AppxPackage -RegisterByFamilyName -ForceApplicationShutdown -MainPackage Microsoft.MicrosoftEdge_8wekyb3d8bbwe
5-Check the file again
Limitation :
when 'MicrosoftEdge.exe' is created it would inherit the directory permission which
mean the file wouldnt be writtable in majority of cases but a simple example of 
abusement in the directory "c:\" <- the default acl is preventing Athenticated Users
from creating file but not modifying them so if we abused the vulnerability in "c:\"
we will have an arbitrary file created and also writeable from a normal user .
also you cant overwrite file that are not writable by SYSTEM , i didnt make a check
in the poc because in if the file is non readable by the current user the check will
return false even if the file is writtable by SYSTEM . NOTE : you can also overwrite
file which you cant even read them .
In the file creation make sure the path is writtable by SYSTEM otherwise the poc will
fail . I think 99% of folders are writtable by SYSTEM
Platform:
This has been tested on a fully patched system (latest patch -> November 2019) :
OS Edition:              Microsoft Windows 10 Home
Os Version:              1903
OS Version Info:         18362.418

Additional Info
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\BuldLabEx  = 18362.1.amd64fre.19h1_release.190318-1202


Expected result:
The Deployment Process should fail with "ERROR_ACCESS_IS_DENIED"
Observed result :
The Deployment Process is overwritting or creating an arbitrary file as 
"LOCAL SYSTEM"

NOTE : It was patched on 7/11/19