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Create and use minimally privileged Service accounts to run GKE cluster nodes instead of using the Compute Engine default Service account. Unnecessary permissions could be abused in the case of a node compromise.
A GCP service account (as distinct from a Kubernetes ServiceAccount) is an identity that an instance or an application can be used to run GCP API requests. This identity is used to identify virtual machine instances to other Google Cloud Platform services. By default, Kubernetes Engine nodes use the Compute Engine default service account.
This account has broad access by default, as defined by access scopes, making it useful to a wide variety of applications on the VM, but it has more permissions than are required to run your Kubernetes Engine cluster.
A minimally privileged service account should be created and used to run the Kubernetes Engine cluster instead of using the Compute Engine default service account, and create separate service accounts for each Kubernetes Workload (See recommendation 5.2.2).
Kubernetes Engine requires, at a minimum, the node service account to have the monitoring.viewer, monitoring.metricWriter, and logging.logWriter roles. Additional roles may need to be added for the nodes to pull images from GCR.
Note
Note
By default, nodes use the Compute Engine default service account when you create a new cluster.

Impact

Instances are automatically granted the cloud-platform scope platform scope to allow full access to all Google Cloud APIs. This is so that the IAM permissions of the instance are completely determined by the IAM roles of the Service account.
Thus if Kubernetes workloads were using cluster access scopes to perform actions using Google APIs, they may no longer be able to, if not permitted by the permissions of the Service account. To remediate, follow recommendation 5.2.2. The Service account roles listed here are the minimum required to run the cluster.
Additional roles may be required to pull from a private instance of Google Container Registry (GCR).

Audit

Using Google Cloud Console:
  1. Go to Kubernetes Engine by visiting Google Cloud Console Kubernetes Engine page
  2. Select the cluster under test and click on each Node pool to bring up the Node pool details page.
Ensure that for each Node pool the Service account is not set to default under the Security heading. To check the permissions allocated to the service account are the minimum required for cluster operation:
  1. Go to IAM by visiting Google Cloud Console IAM page
  2. From the list of Service accounts, ensure each cluster Service account has only the following roles:
    • Logs Writer
    • Monitoring Metric Writer
    • Monitoring Viewer
Using Command line:
To check an existing cluster, first define 4 variables for Node Pool, Cluster Name, Location and Project, and then run the following command:
gcloud container node-pools describe $NODE_POOL --cluster $CLUSTER_NAME -- location $LOCATION --project $PROJECT_ID --format json | jq '.config.serviceAccount'
The output of the above command will return the specific Service account being used for Project access. To check that the permissions allocated to the service account are the minimum required for cluster operation:
gcloud projects get-iam-policy <project_id> \ --flatten="bindings[].members" \ --format='table(bindings.role)' \ --filter="bindings.members:<service_account>"
Review the output to ensure that the service account only has the roles required to run the cluster:
  • roles/logging.logWriter
  • roles/monitoring.metricWriter
  • roles/monitoring.viewer

Remediation

Using Google Cloud Console:
To create a minimally privileged service account:
  1. Go to Service Accounts by visiting: Google Cloud Console Service Accounts page.
  2. Click on CREATE SERVICE ACCOUNT.
  3. Enter Service Account Details.
  4. Click CREATE AND CONTINUE.
  5. Within Service Account permissions add the following roles:
    • Logs Writer
    • Monitoring Metric Writer
    • Monitoring Viewer
  6. Click CONTINUE.
  7. Grant users access to this service account and create keys as required.
  8. Click DONE.
To create a Node pool to use the Service account:
  1. Go to Kubernetes Engine by visiting: Google Cloud Console Kubernetes Engine page.
  2. Click on the cluster name within which the Node pool will be launched.
  3. Click on ADD NODE POOL.
  4. Within the Node Pool details, select the Security subheading, and under Identity defaults, select the minimally privileged service account from the Service Account drop-down.
  5. Click CREATE to launch the Node pool.
    Note
    Note
    The workloads will need to be migrated to the new Node pool, and the old node pools that use the default service account should be deleted to complete the remediation.
Using Command Line:
To create a minimally privileged service account
gcloud iam service-accounts create <node_sa_name> --display-name "GKE Node 
Service Account" 
export NODE_SA_EMAIL=gcloud iam service-accounts list --format='value(email)' 
--filter='displayName:GKE Node Service Account'
Grant the following roles to the service account:
export PROJECT_ID=gcloud config get-value project 
gcloud projects add-iam-policy-binding <project_id> --member 
serviceAccount:<node_sa_email> --role roles/monitoring.metricWriter 
gcloud projects add-iam-policy-binding <project_id> --member 
serviceAccount:<node_sa_email> --role roles/monitoring.viewer 
gcloud projects add-iam-policy-binding <project_id> --member 
serviceAccount:<node_sa_email> --role roles/logging.logWriter
To create a new Node pool using the Service account, run the following command:
gcloud container node-pools create <node_pool> --service- 
account=<sa_name>@<project_id>.iam.gserviceaccount.com-- 
cluster=<cluster_name> --zone <compute_zone>
Note
Note
The workloads will need to be migrated to the new Node pool, and the old node pools that use the default service account should be deleted to complete the remediation.