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# Exploit Title: AppXSvc 17763 - Arbitrary File Overwrite (DoS)  
# Date: 2019-10-28  
# Exploit Author: Gabor Seljan  
# Vendor Homepage: https://www.microsoft.com/  
# Version: 17763.1.amd64fre.rs5_release.180914-1434  
# Tested on: Windows 10 Version 1809 for x64-based Systems  
# CVE: CVE-2019-1476  
  
# Summary:  
# AppXSvc improperly handles file hard links resulting in a low privileged user  
# being able to overwrite an arbitrary file leading to elevation of privilege.  
  
# Description:  
  
# An elevation of privilege vulnerability exists when the AppX Deployment Server  
# (AppXSvc) improperly handles file hard links. While researching CVE-2019-0841  
# originally reported by Nabeel Ahmed, I have found that AppXSvc can be forced  
# to overwrite an arbitrary file by deleting all registry data files before  
# creating the file hard link. As Nabeel Ahmed described in his write-up of  
# CVE-2019-0841, if the settings.dat file is corrupted it will be replaced with  
# the original settings.dat template. However, additional settings.dat.LOG1 and  
# settings.dat.LOG2 files are also created during the initialization process.  
# Substituting the settings.dat.LOG1 or the settings.dat.LOG2 file with a hard  
# link allows a low privileged user to overwrite an arbitrary file with registry  
# data or just simply empty it, respectively. A low privileged user could exploit  
# this vulnerability to cause denial of service by overwriting critical system  
# files.  
  
Steps to reproduce:  
1. Terminate Paint 3D processes.  
2. Delete settings.* files in Microsoft.MSPaint_8wekyb3d8bbwe\Settings folder.  
3. Create a hard link from settings.dat.LOG1 to C:\Windows\win.ini.  
4. Execute the start ms-paint: command to run Paint 3D.  
5. Terminate Paint 3D processes.  
  
Expected result:  
It isn't possible to overwrite a file not writable by a low privileged user.  
  
Observed result:  
C:\Windows\win.ini file is overwritten with registry data.  
  
References:  
https://github.com/sgabe/CVE-2019-1476  
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2019-1476  
https://krbtgt.pw/dacl-permissions-overwrite-privilege-escalation-cve-2019-0841