Service accounts

This page describes how service accounts work with Compute Engine.

To learn how to create and use service accounts, read the Creating and enabling service accounts for instances documentation.

To learn about best practices for creating and managing service accounts, read the Best practices for working with service accounts documentation.

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What is a service account?

A service account is a special kind of account used by an application or compute workload, rather than a person. Service accounts are managed by Identity and Access Management (IAM).

How Compute Engine uses service accounts

Compute Engine uses two types of service accounts:

A user-managed service account can be attached to a Compute Engine instance to provide credentials to applications running on the instance. These credentials are used by the application for authentication to Google Cloud APIs, and authorization to access Google Cloud resources. Only user-managed service accounts can be attached to an instance, and an instance can have only one attached service account. You can change the service account that is attached to an instance at creation time or later on.

Google-managed service accounts are used by the instance to access internal processes on your behalf.

In addition, you can create firewall rules that allow or deny traffic to and from instances based on the service account that you associate with each instance.

How authorization is determined

The authorization provided to applications hosted on a Compute Engine instance is limited by two separate configurations: the roles granted to the attached service account, and the access scopes that you set on the instance. Both of these configurations must allow access before the application running on the instance can access a resource.

Suppose you have an app that reads and writes files on Cloud Storage, it must first authenticate to the Cloud Storage API. You can create an instance with the cloud-platform scope and attach a service account to the instance. You can then grant IAM roles to the service account to give your app access to the appropriate resources. Your app uses the service account credentials to authenticate to the Cloud Storage API without embedding any secret keys or user credentials in your instance, image, or app code. Your app also uses the authorization provided by the IAM roles on the service account to access resources. For more information about authorization, see Authorization on this page.

User-managed service accounts

User-managed service accounts include new service accounts that you explicitly create and the Compute Engine default service account.

New service accounts

You can create and manage your own service accounts using IAM. After you create an account, you grant the account IAM roles and set up instances to run as the service account. Apps running on instances with the service account attached can use the account's credentials to make requests to other Google APIs.

To create and set up a new service account, see Creating and enabling service accounts for instances.

Compute Engine default service account

New projects that have enabled the Compute Engine API have a Compute Engine default service account, which has the following email:

PROJECT_NUMBER-compute@developer.gserviceaccount.com

Google creates the Compute Engine default service account and adds it to your project automatically but you have full control over the account.

The Compute Engine default service account is created with the IAM basic Editor role, but you can modify your service account's roles to control the service account's access to Google APIs.

You can disable or delete this service account from your project, but doing so might cause any applications that depend on the service account's credentials to fail. If you accidentally delete the Compute Engine default service account, you can try to recover the account within 30 days. For more information, see Creating and managing service accounts.

In summary, the Compute Engine default service account has the following attributes:

  • Automatically created, with an autogenerated name and email address, and added to your project when you enable the Compute Engine API.
  • Automatically granted the IAM basic Editor role, if you have not disabled this behavior.
  • Attached by default to all instances created by the Google Cloud CLI and the console. You can override this behavior by specifying a different service account when you create the instance, or by explicitly specifying that no service account be attached to the instance.

Google-managed service accounts

These service accounts (sometimes known as service agents) are created and managed by Google and assigned to your project automatically. These accounts represent different Google services and each account has some level of access to your Google Cloud project.

You cannot attach Google-managed service accounts to a Compute Engine instance.

Google APIs Service Agent

Apart from the default service account, all projects enabled with Compute Engine come with a Google APIs Service Agent, identifiable using the email:

PROJECT_NUMBER@cloudservices.gserviceaccount.com

This service account is designed specifically to run internal Google processes on your behalf. The account is owned by Google and is not listed in the Service Accounts section of console. By default, the account is automatically granted the project editor role on the project and is listed in the IAM section of console. This service account is only deleted when the project is deleted. However, you can change the roles granted to this account, including revoking all access to your project.

Certain resources rely on this service account and the default editor permissions granted to the service account. For example, managed instance groups and autoscaling uses the credentials of this account to create, delete, and manage instances. If you revoke permissions to the service account, or modify the permissions in such a way that it does not grant permissions to create instances, this will cause managed instance groups and autoscaling to stop working.

For these reasons, you should not modify this service account's roles unless a role recommendation explicitly suggests that you modify them.

Compute Engine Service Agent

All projects that have enabled the Compute Engine API have a Compute Engine Service Agent, which has the following email:

service-PROJECT_NUMBER@compute-system.iam.gserviceaccount.com

This service account is designed specifically for Compute Engine to perform its service duties on your project. It relies on the Service Agent IAM Policy granted on your Google Cloud Project. It is also the service account Compute Engine uses to access the customer-owned service account on VM instances. Google owns this account, but it is specific to your project. This account is hidden from the IAM page in the console unless you select Include Google-provided role grants. By default, the account is automatically granted the compute.serviceAgent role on your project.

This service account is deleted only when you delete your project. You can change the roles granted to this account and revoke all access to your project from the account. Revoking or changing the permissions for this service account prevents Compute Engine from accessing the identities of your service accounts on your VMs, and can cause outages of software running inside your VMs.

For these reasons, you should avoid modifying the roles for this service account as much as possible.

Attaching a service account to an instance

To avoid providing an application with excess permissions, we recommend that you create a user-managed service account, grant it only the roles your application needs to function properly, and attach it to your Compute Engine instance. Your code can then use Application Default Credentials to authenticate with the credentials provided by the service account.

You can attach a service account to a Compute Engine instance when you create the instance, or later. Only one service account can be attached to an instance at a time; if you attach a service account to an instance that already has a service account attached, the previous service account is no longer used by that instance.

When you attach a service account to a Compute Engine instance, you must also ensure that the scopes set on the instance are correct. Otherwise, your app might not be able to access all of the APIs it needs. For more information, see Access scopes on this page.

For step-by-step information about attaching a service account to a Compute Engine instance, see Creating and enabling service accounts for instances.

Authorization

When you set up an instance to run as a service account, you determine the level of access the service account has by the IAM roles that you grant to the service account. If the service account has no IAM roles, then no resources can be accessed using the service account on that instance.

Furthermore, an instance's access scopes determine the default OAuth scopes for requests made through the gcloud CLI and client libraries on the instance. As a result, access scopes potentially further limit access to API methods when authenticating through OAuth. However, they do not extend to other authentication protocols like gRPC.

The best practice is to set the full cloud-platform access scope on the instance, then control the service account's access using IAM roles.

Essentially:

  • IAM restricts access to APIs based on the IAM roles that are granted to the service account.
  • Access scopes potentially further limit access to API methods.

Both access scopes and IAM roles are described in detail in the sections below.

IAM roles

You must grant the appropriate IAM roles to a service account to allow that service account access to relevant API methods.

For example, you can grant a service account the IAM roles for managing Cloud Storage objects, or for managing Cloud Storage buckets, or both, which limits the account to the permissions granted by those roles.

When you grant an IAM role to a service account, any application running on an instance that has that service account attached will have the authorization conferred by that role.

Some things to keep in mind:

  • Some IAM roles are in Beta.

    If there isn't a predefined role for the access level you want, you can create and grant custom roles.

  • You must set access scopes on the instance to authorize access.

    While a service account's access level is determined by the roles granted to the service account, an instance's access scopes determine the default OAuth scopes for requests made through the gcloud CLI and client libraries on the instance. As a result, access scopes potentially further limit access to API methods when authenticating through OAuth.

Access scopes

Access scopes are the legacy method of specifying authorization for your instance. They define the default OAuth scopes used in requests from the gcloud CLI or the client libraries. (Access scopes do not apply for calls made using gRPC.)

Access scopes apply on a per-instance basis. You set access scopes when creating an instance and the access scopes persists only for the life of the instance.

Generally, the documentation for each API method lists the scopes required for that method. For example, the instances.insert method provides a list of valid scopes in its authorization section.

Access scopes have no effect if you have not enabled the related API on the project that the service account belongs to. For example, granting an access scope for Cloud Storage on a virtual machine instance allows the instance to call the Cloud Storage API only if you have enabled the Cloud Storage API on the project.

Default scopes

When you create a new Compute Engine instance, it is automatically configured with the following access scopes:

  • Read-only access to Cloud Storage:
    https://www.googleapis.com/auth/devstorage.read_only
  • Write access to write Compute Engine logs:
    https://www.googleapis.com/auth/logging.write
  • Write access to publish metric data to your Google Cloud projects:
    https://www.googleapis.com/auth/monitoring.write
  • Read-only access to Service Management features required for Google Cloud Endpoints(Alpha):
    https://www.googleapis.com/auth/service.management.readonly
  • Read/write access to Service Control features required for Google Cloud Endpoints(Alpha):
    https://www.googleapis.com/auth/servicecontrol
  • Write access to Cloud Trace allows an application running on a VM to write trace data to a project.
    https://www.googleapis.com/auth/trace.append

Scopes best practice

There are many access scopes available to choose from, but a best practice is to set the cloud-platform access scope, which is an OAuth scope for most Google Cloud services, and then control the service account's access by granting it IAM roles.

https://www.googleapis.com/auth/cloud-platform

Scopes examples

Following the scopes best practice, if you enabled the cloud-platform access scope on an instance and then granted the following predefined IAM roles:

  • roles/compute.instanceAdmin.v1
  • roles/storage.objectViewer
  • roles/compute.networkAdmin

Then the service account has only the permissions included in those three roles. Applications impersonating that service account cannot perform actions outside of these roles despite the Google Cloud access scope.

On the other hand, if you grant a more restrictive scope on the instance, like the Cloud Storage read-only scope (https://www.googleapis.com/auth/devstorage.read_only), and set the roles/storage.objectAdmin administrator role on the service account, then by default, requests from the gcloud CLI and the client libraries would not be able to manage Cloud Storage objects from that instance, even though you granted the service account the roles/storage.ObjectAdmin role. This is because the Cloud Storage read-only scope does not authorize the instance to manipulate Cloud Storage data.

Example access scopes include the following:

  • https://www.googleapis.com/auth/cloud-platform. View and manage your data across most of the Google Cloud services in the specified Google Cloud project.
  • https://www.googleapis.com/auth/compute. Full control access to Compute Engine methods.
  • https://www.googleapis.com/auth/compute.readonly. Read-only access to Compute Engine methods.
  • https://www.googleapis.com/auth/devstorage.read_only. Read-only access to Cloud Storage.
  • https://www.googleapis.com/auth/logging.write. Write access to the Compute Engine logs.

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