Userdata Privacy

Cluster API Provider AWS bootstraps EC2 instances to create and join Kubernetes clusters using instance user data. Because Kubernetes clusters are secured using TLS using multiple Certificate Authorities, these are generated by Cluster API and injected into the user data. It is important to note that without the configuring of host firewalls, processes can retrieve instance userdata from http://169.254.169.254/latest/api/token

Requirements

  • An AMI that includes the AWS CLI
  • AMIs using CloudInit
  • A working /bin/bash shell
  • LFS directory layout (i.e. /etc exists and is readable by CloudInit)

Listed AMIs on 1.16 and up should include the AWS CLI.

How Cluster API secures TLS secrets

In 0.5.x/v1alpha3, by default, Cluster API Provider AWS will use AWS Secrets Manager as a limited-time secret store, storing the userdata using KMS encryption at rest in AWS. The EC2 IMDS userdata will contain a boot script to download the encrypted userdata secret using instance profile permissions, then immediately delete it from AWS Secrets Manager, and then execute it.

To avoid guessing keys in the AWS Secrets Manager key-value store and to prevent collisions, the key is an encoding the Kubernetes namespace, cluster name and instance name, with a random string appended, providing ~256-bits of entropy.

Cluster API Provider AWS also stores the secret ARN in the AWSMachine spec, and will delete the secret if it isn’t already deleted and the machine has registered successfully against the workload cluster API server as a node. Cluster API Provider AWS will also attempt deletion of the secret if the AWSMachine is otherwise deleted or the EC2 instance is terminated or failed.

This method is only compatible with operating systems and distributions using cloud-init. If you are using a different bootstrap process, you will need to co-ordinate this externally and set the following in the specification of the AWSMachine types to disable the use of a cloud-init boothook:

cloudInit:
  insecureSkipSecretsManager: true

Troubleshooting

Script errors

cloud-init does not print boothook script errors to the systemd journal. Logs for the script, if it errored can be found in /var/log/cloud-init-output.log

Warning messages

Because cloud-init will attempt to read the final file at start, cloud-init will always print a /etc/secret-userdata.txt cannot be found message. This can be safely ignored.

Secrets manager console

The AWS secrets manager console should show secrets being created and deleted, with a lifetime of around a minute. No plaintext secret data will appear in the console as Cluster API Provider AWS stores the userdata as fragments of a gzipped data stream.