I work with many customers where Security is no longer a second-thought, but the key driver for any conversation/engagement. One of the many Governance standards and policies that typically comes up during these conversations, is to ensure their Azure environment resources can only have approved extensions deployed. The policy also falls under the highly recommended CIS benchmarks. Ensuring your environment is following CIS benchmarks and controls is a very good starting point.
First, you need to understand which extensions are available. To get the list of all extensions available, within your Azure region, and dump it to a file, you can use the following PowerShell code:
Get-AzVmImagePublisher -Location "canadacentral" | Get-AzVMExtensionImageType | Get-AzVMExtensionImage | Select Type, PublisherName, Version | Format-Table -AutoSize | Out-File -FilePath .\VMextensionsCanadaCentral.txt
Next, now that we know which extensions are available, we need to understand which extensions we want to allow. Once we have that list, it is simply adding the list of extensions (separated with a semicolon; ensure no whitespace exists!!).
Once you have the list, deploy the Policy “Only approved VM extensions should be installed” and set it to ‘Deny‘. Or on the flipside, you can set the Policy to ‘Disabled’, which would allow the Policy to allow extensions, but the following within the list. Depending on your security stance, you can either allow explicit extensions and deny all by default. Or, allow any extension by default, however disable only the following extensions based on your list. I would think the former provides more control.
Once you have the policy deployed, configured and enabled, your users will get an error something like this, if the extension the VM is requesting is not a part of the approved list…
To get this policy in your environment, you can either go to the Azure Policy service and look up the definition, or visit the following link HERE, and access the JSON/Policy from Azure’s GitHub repo.