cis-oke-v180-3.1.1

CIS OKE Benchmark v1.8.0 - Control 3.1.1

Profile Applicability

• Level: 1

Description

If kubelet is running, and if it is using a file-based kubelet-config.json file, ensure that the proxy kubelet-config.json file has permissions of 644 or more restrictive.

Rationale

The kubelet kubelet-config.json file controls various parameters of the kubelet service in the worker node. You should restrict its file permissions to maintain the integrity of the file. The file should be writable by only the administrators on the system.

It is possible to run kubelet with the kubeconfig parameters configured as a Kubernetes ConfigMap instead of a file. In this case, there is no proxy kubelet-config.json file.

Impact

Ensuring that the kubelet-config file permissions are set to 644 or more restrictive significantly strengthens the security posture of the Kubernetes environment by preventing unauthorized modifications. This restricts write access to the kubelet-config file, ensuring only administrators can alter crucial kubelet configurations, thereby reducing the risk of malicious alterations that could compromise the cluster's integrity.

However, this configuration may slightly impact usability, as it limits the ability for non-administrative users to make quick adjustments to the kubelet settings. Administrators will need to balance security needs with operational flexibility, potentially requiring adjustments to workflows for managing kubelet configurations.

Audit Procedure

Method 1

SSH to the worker nodes.

To check to see if the Kubelet Service is running:

sudo systemctl status kubelet

The output should return Active: active (running) since..

Run the following command on each node to find the appropriate kubelet-config.json file:

ps -ef | grep kubelet

The output of the above command should return something similar to --kubeconfig /etc/kubernetes/kubelet-config.json which is the location of the kubelet-config.json file.

Run this command to obtain the kubelet-config.json file permissions:

stat -c %a /etc/kubernetes/kubelet-config.json

The output of the above command gives you the kubelet-config.json file's permissions. Verify that if a file is specified and it exists, the permissions are 644 or more restrictive.

Method 2

Create and Run a Privileged Pod.

You will need to run a pod that is privileged enough to access the host's file system. This can be achieved by deploying a pod that uses the hostPath volume to mount the node's file system into the pod.

Here's an example of a simple pod definition that mounts the root of the host to /host within the pod:

apiVersion: v1
kind: Pod
metadata:
  name: file-check
spec:
  volumes:
    - name: host-root
      hostPath:
        path: /
        type: Directory
  containers:
    - name: nsenter
      image: busybox
      command: ["sleep", "3600"]
      volumeMounts:
        - name: host-root
          mountPath: /host
      securityContext:
        privileged: true

Save this to a file (e.g., file-check-pod.yaml) and create the pod:

kubectl apply -f file-check-pod.yaml

Once the pod is running, you can exec into it to check file permissions on the node:

kubectl exec -it file-check -- sh

Now you are in a shell inside the pod, but you can access the node's file system through the /host directory and check the permission level of the file:

ls -l /host/etc/kubernetes/kubelet-config.json

Verify that if a file is specified and it exists, the permissions are 644 or more restrictive.

Remediation

Run the below command (based on the file location on your system) on the each worker node. For example,

chmod 644 <kubelet-config.json file>

Default Value

See the OKE documentation for the default value.

References

1. https://kubernetes.io/docs/admin/kube-proxy/
2. https://docs.cloud.oracle.com/en-us/iaas/Content/ContEng/Concepts/contengoverview.htm

CIS Controls

MITRE ATT&CK Mappings

Profile

Level 1 (Automated)