Azure Arc for servers security overview

This article describes the security configuration and considerations you should evaluate before deploying Azure Arc-enabled servers in your enterprise.

Identity and access control

Each Azure Arc-enabled server has a managed identity as part of a resource group inside an Azure subscription. That identity represents the server running on-premises or other cloud environment. Access to this resource is controlled by standard Azure role-based access control. From the Access Control (IAM) page in the Azure portal, you can verify who has access to your Azure Arc-enabled server.

Azure Arc-enabled server access control

Users and applications granted contributor or administrator role access to the resource can make changes to the resource, including deploying or deleting extensions on the machine. Extensions can include arbitrary scripts that run in a privileged context, so consider any contributor on the Azure resource to be an indirect administrator of the server.

The Azure Connected Machine Onboarding role is available for at-scale onboarding, and is only able to read or create new Azure Arc-enabled servers in Azure. It cannot be used to delete servers already registered or manage extensions. As a best practice, we recommend only assigning this role to the Azure Active Directory (Azure AD) service principal used to onboard machines at scale.

Users as a member of the Azure Connected Machine Resource Administrator role can read, modify, reonboard, and delete a machine. This role is designed to support management of Azure Arc-enabled servers, but not other resources in the resource group or subscription.

Agent security and permissions

To manage the Azure Connected Machine agent (azcmagent) on Windows, your user account needs to be a member of the local Administrators group. On Linux, you must have root access permissions.

The Azure Connected Machine agent is composed of three services, which run on your machine.

  • The Hybrid Instance Metadata Service (himds) service is responsible for all core functionality of Arc. This includes sending heartbeats to Azure, exposing a local instance metadata service for other apps to learn about the machine‚Äôs Azure resource ID, and retrieve Azure AD tokens to authenticate to other Azure services. This service runs as an unprivileged virtual service account on Windows, and as the himds user on Linux.

  • The Guest Configuration service (GCService) is responsible for evaluating Azure Policy on the machine.

  • The Guest Configuration Extension service (ExtensionService) is responsible for installing, updating, and deleting extensions (agents, scripts, or other software) on the machine.

The guest configuration and extension services run as Local System on Windows, and as root on Linux.

Using a managed identity with Azure Arc-enabled servers

By default, the Azure Active Directory system assigned identity used by Arc can only be used to update the status of the Azure Arc-enabled server in Azure. For example, the last seen heartbeat status. You can optionally assign other roles to the identity if an application on your server uses the system assigned identity to access other Azure services. To learn more about configuring a system-assigned managed identity to access Azure resources, see Authenticate against Azure resources with Azure Arc-enabled servers.

While the Hybrid Instance Metadata Service can be accessed by any application running on the machine, only authorized applications can request an Azure AD token for the system assigned identity. On the first attempt to access the token URI, the service will generate a randomly generated cryptographic blob in a location on the file system that only trusted callers can read. The caller must then read the file (proving it has appropriate permission) and retry the request with the file contents in the authorization header to successfully retrieve an Azure AD token.

  • On Windows, the caller must be a member of the local Administrators group or the Hybrid Agent Extension Applications group to read the blob.

  • On Linux, the caller must be a member of the himds group to read the blob.

To learn more about using a managed identity with Arc-enabled servers to authenticate and access Azure resources, see the following video.

Using disk encryption

The Azure Connected Machine agent uses public key authentication to communicate with the Azure service. After you onboard a server to Azure Arc, a private key is saved to the disk and used whenever the agent communicates with Azure. If stolen, the private key can be used on another server to communicate with the service and act as if it were the original server. This includes getting access to the system assigned identity and any resources that identity has access to. The private key file is protected to only allow the himds account access to read it. To prevent offline attacks, we strongly recommend the use of full disk encryption (for example, BitLocker, dm-crypt, etc.) on the operating system volume of your server.

Next steps

  • Before evaluating or enabling Azure Arc-enabled servers across multiple hybrid machines, review Connected Machine agent overview to understand requirements, technical details about the agent, and deployment methods.

  • Review the Planning and deployment guide to plan for deploying Azure Arc-enabled servers at any scale and implement centralized management and monitoring.