Cis Aks V180 3.1.1

3.1.1 Ensure that the kubeconfig file permissions are set to 644 or more restrictive (Automated)

Profile Applicability

• Level 1

Description

If kubelet is running, and if it is configured by a kubeconfig file, ensure that the proxy kubeconfig file has permissions of 644 or more restrictive.

Rationale

The kubelet kubeconfig 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 kubeconfig file.

Impact

None.

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 kubeconfig file:

ps -ef | grep kubelet

The output of the above command should return something similar to --kubeconfig /var/lib/kubelet/kubeconfig which is the location of the kubeconfig file.

Run this command to obtain the kubeconfig file permissions:

stat -c %a /var/lib/kubelet/kubeconfig

The output of the above command gives you the kubeconfig 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/var/lib/kubelet/kubeconfig

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 <kubeconfig file>

Default Value

See the Azure AKS documentation for the default value.

References

1. https://kubernetes.io/docs/admin/kube-proxy/
2. https://docs.microsoft.com/security/benchmark/azure/security-controls-v2-posture-vulnerability-management#pv-3-establish-secure-configurations-for-compute-resources

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