# 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