Executive Summary

Active Directory Forest trusts provide a secure way for resources in a forest to trust identities from another forest. This trust is directional; a trusted forest can authenticate its users to the trusting forest without allowing the reverse. 

A feature, Enforcement for forest boundary for Kerberos full delegation, was introduced in Windows Server 2012 that allows an administrator of the trusted forest to configure whether Ticket-Granting Tickets (TGTs) may be delegated to a service in a trusting forest. 

An unsafe default configuration for this feature exists when setting up inbound trusts that lets an attacker in the trusting forest request delegation of a TGT for an identity from the trusted forest. 

This advisory addresses the issue by recommending a new safe default configuration for unconstrained Kerberos delegation across Active Directory forest trusts that supersedes the original unsafe configuration.

Recommended Actions

Customers should review Knowledge Base Article 4490425 and take appropriate action.

The enforcement for forest boundary for Kerberos full delegation will be available as an update for all supported versions of Windows Server starting in the March 2019 Security Update and is currently available for Server 2012 and newer. We recommend that you set the feature on incoming forest trusts.

FAQ

1. What is unconstrained delegation?
Unconstrained delegation is when a service can acquire a copy of your TGT to act on your behalf when authenticating to other services. Unconstrained delegation lets the service authenticate to any other service which can lead to security issues such as elevation of privilege. Unconstrained delegation has been replaced by constrained delegation which limits which services can receive tickets on behalf of a user.

2. What is TGT delegation?
TGT delegation allows a service to acquire a TGT from a domain with an inbound trust. This allows any service within an untrusted forest to acquire a TGT to the trusted forest. A feature was introduced in Windows Server 2012 to disable this capability.

3. Why is TGT delegation enabled by default?
Applications may rely on unconstrained delegation across inbound trusts and disabling delegation may lead to outages.

4. How do I determine if TGT delegation is enabled?
You can check that the flag is set on the trust using PowerShell.

Get-AdTrust -filter {Direction -eq "Inbound" -and TGTDelegation -eq $true}

5. How do I disable TGT delegation?
You can set the EnableTGTDelegation to NO using Netdom. See the KB article for more details.

netdom.exe trust fabrikam.com /domain:contoso.com /EnableTGTDelegation:No

6. What is the security risk of leaving TGT delegation enabled?
If an attacker can enable unconstrained delegation of any principal in an untrusted forest and request a service ticket to the trusted forest, they can also request a TGT from the trusted forest. An attacker can then impersonate the user in the trusted forest from within the untrusted forest leading to elevation of privilege.

Security Updates

To determine the support life cycle for your software version or edition, see the Microsoft Support Lifecycle.

Product Platform Article Download Impact Severity Supersedence
Windows 10 for 32-bit Systems None None
Windows 10 for x64-based Systems None None
Windows 10 Version 1607 for 32-bit Systems None None
Windows 10 Version 1607 for x64-based Systems None None
Windows 10 Version 1703 for 32-bit Systems None None
Windows 10 Version 1703 for x64-based Systems None None
Windows 10 Version 1709 for 32-bit Systems None None
Windows 10 Version 1709 for 64-based Systems None None
Windows 10 Version 1709 for ARM64-based Systems None None
Windows 10 Version 1803 for 32-bit Systems None None
Windows 10 Version 1803 for ARM64-based Systems None None
Windows 10 Version 1803 for x64-based Systems None None
Windows 10 Version 1809 for 32-bit Systems None None
Windows 10 Version 1809 for ARM64-based Systems None None
Windows 10 Version 1809 for x64-based Systems None None
Windows 7 for 32-bit Systems Service Pack 1 None None
Windows 7 for x64-based Systems Service Pack 1 None None
Windows 8.1 for 32-bit systems None None
Windows 8.1 for x64-based systems None None
Windows RT 8.1 None None
Windows Server 2008 for 32-bit Systems Service Pack 2 None None
Windows Server 2008 for 32-bit Systems Service Pack 2 (Server Core installation) None None
Windows Server 2008 for Itanium-Based Systems Service Pack 2 None None
Windows Server 2008 for x64-based Systems Service Pack 2 None None
Windows Server 2008 for x64-based Systems Service Pack 2 (Server Core installation) None None
Windows Server 2008 R2 for Itanium-Based Systems Service Pack 1 None None
Windows Server 2008 R2 for x64-based Systems Service Pack 1 None None
Windows Server 2008 R2 for x64-based Systems Service Pack 1 (Server Core installation) None None
Windows Server 2012 None None
Windows Server 2012 (Server Core installation) None None
Windows Server 2012 R2 None None
Windows Server 2012 R2 (Server Core installation) None None
Windows Server 2016 None None
Windows Server 2016 (Server Core installation) None None
Windows Server 2019 None None
Windows Server 2019 (Server Core installation) None None
Windows Server, version 1709 (Server Core Installation) None None
Windows Server, version 1803 (Server Core Installation) None None

Mitigations

Microsoft has not identified any mitigating factors for this vulnerability.

Workarounds

Microsoft has not identified any workarounds for this vulnerability.

Acknowledgements



See acknowledgements for more information.

Disclaimer

The information provided in the Microsoft Knowledge Base is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.

Revisions

Version Date Description
1.0 02/12/2019 Information published.

Source: https://portal.msrc.microsoft.com/en...sory/ADV190006