Monday, July 25, 2016

June 2016 Security Update Breaks Group Policy

So, this was a thing back in the middle of June and I missed it at the time. Looking back there are a few articles about it, but I just ran into this with a client (happily, before the update was applied) and I think it's worth raising awareness of.

In some security updates for Vista and later released on June 14, 2016 the process for Group Policy object (GPO) downloading has been changed. To make the download process more secure, the computer account is now responsible for downloading all GPOs. In the past, the user account could download the GPOs also.

The result of this change is that any computer that is downloading GPOs needs to have Read access to the GPO. The computer accounts do not need Apply permission, only Read is required.

By default the Authenticated Users group has Read and Apply permission for all GPOs. The Authenticated Users group includes all users and all computer accounts. So, if you haven't changed this, then you'll have no issues when these security updates are applied.

Even though you don't think you modified the permissions on your GPOs, you might have. If you use security filtering to control GPO application, that modifies the GPO permissions. When you remove Authenticated Users for security filtering, you also remove Read permission for Authenticated Users.

The quick fix is to add Authenticated Users or Domain Computers with Read permission to all of your GPOs. This doesn't modify which users or computers apply the GPOs, but it does give the computer accounts the necessary permissions to download the GPOs. Some of the links below show how to automate this process.

If you'd like to verify whether you have any GPOs without any permissions assigned to Authenticated Users, I've created a short script that looks for that and dumps the list of GPOs and their current permissions to a file.

 #Script Requires the GroupPolicy cmdlets  
 #A manual import of the GroupPolicy module is required for pre-Win2012  
 Import-Module GroupPolicy  
 #Define badGPO as an array or you get one big string  
 #that's hard to work with  
 $badGPO = @()  
 #Get a list of all Group Policy objects  
 $gpo = Get-GPO -All  
 #Find Group Policy objects that don't have permissions  
 #assigned to Authenticated Users  
 Foreach ($g in $gpo) {  
      Try { Get-GPPermissions -GUID $ -TargetName "Authenticated Users" -TargetType Group -ErrorAction Stop }  
      Catch { $BadGPO += $g.DisplayName }  
 Write-Host "List of GPOs without Authenticated Users permissions"  
 #Create File for Report  
 "Permissions Report of GPOs without Authenticated Users" | Out-File GPO-NoAuthUser.txt  
 Foreach ($b in $badGPO) {  
      "GPO: $b" | Out-File GPO-NoAuthUser.txt -NoClobber -Append  
      Get-GPPermissions -Name $b -All | Out-File GPO-NoAuthUser.txt -NoClobber -Append  
 Write-Host "Created GPO-NoAuthUser.txt with permissions report"  
 Write-Host "Use this report to verify that computer accounts"  
 Write-Host "have read access to the GPO for application after"  
 Write-Host "applying June 2016 Security Updates. For more info"  
 Write-Host "see:"  

Some links with more information:

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