Resolving Host Guardian Service (HGS) TPM Attestation Breakdown Post-Hardware Maintenance on Guarded Hosts

Question

Resolving Host Guardian Service (HGS) TPM Attestation Breakdown Post-Hardware Maintenance on Guarded Hosts

George Wilson 60

Dear Colleagues,

We are currently investigating an issue within our guarded fabric where Shielded Virtual Machines are failing to boot on a specific Guarded Host. The root cause has been isolated to a Host Guardian Service (HGS) TPM attestation failure triggered by a recent motherboard replacement on the hypervisor.

Because the underlying Trusted Platform Module (TPM 2.0) chip was swapped alongside the system board, the host's Endorsement Key (EK) and measured boot baseline no longer match the trusted hardware identities stored in the HGS attestation database.

I would appreciate guidance from anyone with hands-on experience on the following:

TPM EK Certificate Extraction: The recommended methodology (via Get-PlatformIdentifier or equivalent cmdlet) to capture the new EK public key certificate chain off the serviced host.
HGS Baseline Re-registration: The proper sequence using Add-HgsAttestationTpmHost and Set-HgsAttestationTpmPolicy to invalidate the old TPM baseline and commit the updated host security posture, ensuring seamless operational recovery for the affected Shielded VMs.

Many thanks in advance for your insights.

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Answer 1

Hello George Wilso,

When a motherboard replacement introduces a new TPM 2.0 chip, the host’s Endorsement Key and measured boot values no longer match what the Host Guardian Service has stored, so attestation fails and Shielded VMs cannot boot. The first step is to extract the new TPM EK certificate from the serviced host. You can run Get-PlatformIdentifier -Name <HostName> or Get-HgsAttestationTpmHost locally, which will return the EK public key and certificate chain. This output is what you need to register with HGS.

Next, you must re‑register the host against HGS with the updated TPM baseline. Use Add-HgsAttestationTpmHost -Name <HostName> -Path <EKCertFile> to add the new EK identity, then update the attestation policy with Set-HgsAttestationTpmPolicy so that the old baseline is invalidated and the new one is trusted. Once the policy is committed, the Guarded Host will pass attestation again and Shielded VMs will resume normal boot. Be sure to confirm that the host is listed as compliant with Get-HgsAttestationReport before returning it to production.

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Thank you,

HL.