Beware, KB3135173 for Windows 10 May Reset Default Apps (Again)

Apparently, many users have reported problems with one of the recent patches for Windows 10.

Reports are coming in that the recent patch for Windows 10 with the identifier KB3135173 may reset default applications on systems it is installed on. We have talked about Windows 10 resetting default applications before, the first time happening shortly after release of the operating system.

The changes did not affect all users of Windows 10 back then who installed the responsible update at that time but those who were affected reported that default apps for web browsing, pdf viewing and photo viewing were reset, and that programs were removed from the system as well.


KB 3135173 is a cumulative update for Windows 10 that Microsoft released on February 9, 2016. It fixes vulnerabilities reported in the security bulletins MS16-022, MS16-021, MS16-016, MS16-014, MS16-013, MS16-011 and MS16-009.

You find information about these security patches in our patch day overview for February 2016.

Users noticed that something was not right after the patches were installed and the machine rebooted for the first time.

The Action Center started to display one or multiple notifications on the screen that app defaults were reset listing the file type that was reset, and the default system application it was mapped to.

An app default was reset. An app caused a problem with the default setting for .avi files so it was reset to Films & TV.

This happened for custom file associates for file types such as pdf, avi, mp4, jpg or png, and each time the file type was reset to one of the default applications Windows 10 ships with.

Even worse, while it is seemingly possible to associate different apps or programs with the reset file types, Windows 10 would reset those associations a moment later making it impossible to use custom file associations on affected PCs.

Temporary solutions

an app default was reset

So what is the solution for users who are affected by this issue? One course of action is to uninstall the update and block it from running, but it is not suggested since it patches security issues. Removing it will make the system vulnerable to attacks targeting those vulnerabilities.

The Winhelponline Blog created a Registry file that supposedly fixes the issue. I was not able to test it as my test systems were not affected by the issue.

After running the REG fix, open Control Panel > Default Programs and set file associations or default programs as you prefer. None of the built-in Universal Apps should reset the associations from now on. This method has been tested (and works fine) in Windows 10 Build 10586.

Changing file associations on Windows 10

windows 10 default apps

Windows 10 offers several options to choose default applications for file types. The starting point for all of them is the operating system’s Settings application.

  1. Tap on Windows-I to open the Settings application.
  2. Navigate to System > Default Apps.

There you find programs or apps associated with popular application types such as email, web browsing or music playing.

You can associate programs with those types right away, but may want to use the three options listed below those for finer controls.

  • Choose default apps by file type displays all known file types and the applications they have been associated with.
  • Choose default apps by protocol displays protocols, such as http or ftp and the applications or programs they are associated with.
  • Set defaults by app finally opens the classic “set default programs” dialog of the Control Panel. Select a program from the list, and associate all file types and protocols with it that it supports (or customize the selection).

set default programs

Closing Words

The resetting of custom file and protocol associations should never happen on its own, and it seems that Microsoft needs to adjust the parameters that are responsible for the resetting to make sure it won’t happen again in the future.

For now, it is wise to check default applications, file associations and even installed programs after updates regularly to make sure none were affected by it.

Windows 10 users may also want to create system backups, or at least System Restore points, before running any updates that Microsoft put out for the operating system.

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