Azure Runbooks are usually run in the cloud (on an automatically assigned ‘Microsoft’ host) or on a Hybrid Worker Group.

Hybrid Worker Groups consist of 1 or more machines, but there are also ‘System hybrid workers’, which are machines monitored by OMS. If you want to execute a Powershell script directly on a specific System hybrid worker, or on a specific group member of a worker group, you can use Powershell and specify the host instead of the group:

Start-AzureRmAutomationRunbook -Name “RunbookName” -RunOn hybridWorkerName -AutomationAccountName “automationaccount” -ResourceGroupName “resourcegroup”

If you try this on a System Hybrid Worker, you’ll get an error on the device itself and in the runbook results:

“Invalid Runbook xxx Authenticode signature status – NotSigned”.

This can be ‘fixed’ by setting the following registry key to ‘False’:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\HybridRunbookWorker\GuidOfYourWorker\EnableSignatureValidation

Et voila, the runbook runs nicely. I do not recommend disabling this key in production, this article is purely to share knowledge, and if someone knows how to do this without disabling this key, I’d love to hear it!