UNC Hardening

A couple months ago, Microsoft published a couple of Windows patches to address some vulnerabilities found in the way that Windows machines access UNC paths over the network.



Guidance on Deployment of MS15-011 and MS15-014 by AskPFE Platforms

This is essentially another man-in-the-middle style SMB hijack, and these types of attacks have been well-known for a long time, maybe second only behind pass the hash stuff.  One of the countermeasures that we admins have had for years to help combat these sorts of SMB proxy attacks, is SMB signing:

Of course I'd recommend enabling this everywhere - on both domain controllers and domain members - but that's no longer quite enough.  Security researchers found a way of bypassing or disabling SMB signing, which is what prompted Microsoft to release those two security patches I mentioned above.  One of those hotfixes comes with a new Group Policy configuration setting, called UNC Hardening.

You can find this new setting in Computer Configuration > Policies > Administrative Templates > Network > Network Provider:

So keep in mind that just applying the patch alone doesn't award you any of the benefits of Hardened UNC Paths.  There is additional GPO configuration you must do to enable it.

In the GPO, an admin would specify the types of UNCs that he or she wanted to harden, so that when a client connects to a UNC that matches a certain pattern, that client applies additional security policies to that connection.

Wildcards are supported, but you must supply either a server name or share name, so no, you cannot do \\* or \\*\*.

To get the two most important UNC paths in an Active Directory domain, you'd configure the GPO thusly:

\\*\NETLOGON  RequireMutualAuthentication=1, RequireIntegrity=1
\\*\SYSVOL    RequireMutualAuthentication=1, RequireIntegrity=1

This additional layer of security costs very little, relative to the benefit of ensuring all your Windows clients will only connect to genuine, mutually authenticated domain controllers to get their Group Policies and logon scripts.  Especially if you have mobile clients on the go that connect from coffee shops and hotels!

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