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4.3 Enable Block Project-Wide SSH Keys for VM Instances (Automated)​

Profile Applicability​

• Level 1

Description​

Best practices recommend using Instance specific SSH keys instead of using common/shared project-wide SSH keys to access Instances.

Rationale​

Project-wide SSH keys are stored in Compute/Project-meta-data. Project wide SSH keys can be used to login into all the instances within project. Using project-wide SSH keys eases the SSH key management but if compromised, poses the security risk which can impact all the instances within project. It is recommended to use Instance specific SSH keys which can limit the attack surface if the SSH keys are compromised.

Impact​

Users already having Project-wide ssh key pairs and using third party SSH clients will lose access to the impacted Instances. For Project users using gcloud or GCP Console based SSH option, no manual key creation and distribution is required and will be handled by GCE (Google Compute Engine) itself. To access Instance using third party SSH clients Instance specific SSH key pairs need to be created and distributed to the required users.

Audit​

From Console:

  1. Go to the VM instances page by visiting https://console.cloud.google.com/compute/instances. It will list all the instances in your project.

  2. For every instance, click on the name of the instance.

  3. Under SSH Keys, ensure Block project-wide SSH keys is selected.

From Command Line:

  1. List the instances in your project and get details on each instance:
gcloud compute instances list --format=json
  1. Ensure key: block-project-ssh-keys is set to value: 'true'.

Remediation​

From Console:

  1. Go to the VM instances page by visiting: https://console.cloud.google.com/compute/instances. It lists all the instances in your project.

  2. Click the name of the Impacted instance.

  3. Click Edit in the toolbar.

  4. Under SSH Keys, go to the Block project-wide SSH keys checkbox.

  5. To block users with project-wide SSH keys from connecting to this instance, select Block project-wide SSH keys.

  6. Click Save at the bottom of the page.

  7. Repeat steps for every impacted Instance.

From Command Line:

To block project-wide public SSH keys, set the metadata value to TRUE:

gcloud compute instances add-metadata <instance_name> --metadata block-project-ssh-keys=TRUE

References​

https://cloud.google.com/compute/docs/instances/adding-removing-ssh-keys
https://cloud.google.com/sdk/gcloud/reference/topic/formats

Additional Information​

It is not necessary to block project-wide SSH keys with OS Login enabled, as this ignores SSH keys in instance metadata.

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