Updated – MS16-072 may break your User Group Policies “by-design”

This is a PSA for all Group Policy administrator about MS16-072 that was release yesterday. This patch fixed a man in the middle attack using Group Policy Update however it appears that it has also changed the behavior that Group Policy is applied. If you have a security filtered group policy that are applied to users AND you have also removed “Authenticated Users” group from the GPO then this GPO will no longer apply to the user.

To workaround this problem you can either remove the patch or add “read” permissions to the “Authenticated Users” group back to the GPO. This allows the computer object to read the policy setting and the policy will then work again. As a reminder I stressed back in 2010 that you should never just remove “Authenticated Users” from your GPO’s and that you should instead simply remove the “Apply” permission for the group. See http://www.grouppolicy.biz/2010/05/how-to-apply-a-group-policy-object-to-individual-users-or-computer/

No word yet if this is deliberate change in behavior to fix the man in the middle attack or if this is something that will be fixed.

Update: Thanks to Darren Mar-Elia he had discovered that this was actually a documented change in behavior

MS16-072 changes the security context with which user group policies are retrieved. This by-design behavior change protects customers’ computers from a security vulnerability. Before MS16-072 is installed, user group policies were retrieved by using the user’s security context. After MS16-072 is installed, user group policies are retrieved by using the machines security context.

Forum post https://social.technet.microsoft.com/Forums/en-US/e2ebead9-b30d-4789-a151-5c7783dbbe34/patch-tuesday-kb3159398?forum=winserverGP

76 Comments

  1. This update doesn’t change the default permissions of a gpo so that it adds “authenticated users” with a separate “read” permission. If this update did what it was designed to do, it is obviously a horrible design. Before becoming aware that this update caused an issue, I tried restoring all permissions to default and then removed “authenticated users” from the scope and re-added the user groups I wanted in the scope. This did not resolve the issue and it should have. This update should update the “default permissions” button so it adds a read permission for “authenticated users”, and a separate “apply” permission for “authenticated users” that is removed when you remove “authenticated users” from the scope. Or… changing the scope to only selected user groups should update the “delegated permissions” appropriately so the scope works correctly. I really hope Microsoft is seeing the failure in this release.

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