Microsoft Guidance for cleaning up orphaned keys generated on vulnerable TPMs and used for Windows Hello for Business

Microsoft is aware of an issue in Windows Hello for Business (WHfB) with public keys that persist after a device is removed from Active Directory, if the AD exists. After a user sets up Windows Hello for Business (WHfB), the WHfB public key is written to the on-premises Active Directory. The WHfB keys are tied to a user and a device that has been added to Azure AD, and if the device is removed, the corresponding WHfB key is considered orphaned. However, these orphaned keys are not deleted even when the device it was created on is no longer present. Any authentication to Azure AD using such an orphaned WHfB key will be rejected. However, some of these orphaned keys could lead to the following security issue in Active Directory 2016 or 2019, in either hybrid or on-premises environments.

An authenticated attacker could obtain orphaned keys created on TPMs that were affected by CVE-2017-15361 (ROCA), discussed in Microsoft Security Advisory ADV170012 to compute their WHfB private key from the orphaned public keys. The attacker could then impersonate the user by using the stolen private key to authenticate as the user within the domain using Public Key Cryptography for Initial Authentication (PKINIT).

This attack is possible even if firmware and software updates have been applied to TPMs that were affected by CVE-2017-15361 because the corresponding public keys might still exist in Active Directory.

This advisory provides guidance for cleaning up any orphaned public keys that were generated with an unpatched TPM (before firmware updates discussed in ADV170012 were applied). Follow this guidance to identify and remove orphaned WHfB keys.

This particular issue with orphaned public keys can be present when WHfB is set up in the following configurations:

• WHfB is deployed on Active Directory 2016 or 2019, either in hybrid mode or on-premises only.
• Currently have or have had in the past, WHfB keys generated on TPMs that were affected by CVE-2017-15361.

Important: Azure Active Directory (Azure AD) and Active Directory Federation Services (AD FS) are not affected by this issue. However, we strongly recommend that you follow theRecommended Actionssection of ADV170012, and apply any firmware updates supplied by your TPM OEM, to avoid any exposure to Azure AD and AD FS. See Step #4 of theRecommended Actions for a list of OEMs and links to information for their updates.