cis-aks-v170-3.2.3

3.2.3 Ensure that the --client-ca-file argument is set as appropriate (Automated)

Profile Applicability

• Level 1

Description

Enable Kubelet authentication using certificates.

Rationale

The connections from the apiserver to the kubelet are used for fetching logs for pods, attaching (through kubectl) to running pods, and using the kubelet's port-forwarding functionality. These connections terminate at the kubelet's HTTPS endpoint. By default, the apiserver does not verify the kubelet's serving certificate, which makes the connection subject to man-in-the-middle attacks, and unsafe to run over untrusted and/or public networks. Enabling Kubelet certificate authentication ensures that the apiserver could authenticate the Kubelet before submitting any requests.

Impact

You require TLS to be configured on apiserver as well as kubelets.

Audit Procedure

Audit Method 1:

If using a Kubelet configuration file, check that there is an entry for "x509": {"clientCAFile:" set to the location of the client certificate authority file.

First, SSH to the relevant node:

Run the following command on each node to find the appropriate Kubelet config file:

ps -ef | grep kubelet

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

Open the Kubelet config file:

sudo more /etc/kubernetes/kubelet/kubelet-config.json

Verify that the "x509": {"clientCAFile:" argument exists and is set to the location of the client certificate authority file.

If the "x509": {"clientCAFile:" argument is not present, check that there is a Kubelet config file specified by --config, and that the file sets "authentication": { "x509": {"clientCAFile:" to the location of the client certificate authority file.

Audit Method 2:

If using the api configz endpoint consider searching for the status of authentication..x509":("clientCAFile":"/etc/kubernetes/pki/ca.crt by extracting the live configuration from the nodes running kubelet.

Set the local proxy port and the following variables and provide proxy port number and node name; HOSTNAME_PORT="localhost-and-port-number" NODE_NAME="The-Name-Of-Node-To-Extract-Configuration" from the output of kubectl get nodes

kubectl proxy --port=8001 &

export HOSTNAME_PORT=localhost:8001 (example host and port number)
export NODE_NAME=ip-192.168.31.226.aks.internal (example node name from "kubectl get nodes")

curl -sSL "http://${HOSTNAME_PORT}/api/v1/nodes/${NODE_NAME}/proxy/configz"

Remediation

Remediation Method 1:

If modifying the Kubelet config file, edit the kubelet-config.json file /etc/kubernetes/kubelet/kubelet-config.json and set the below parameter to false:

"authentication": { "x509": {"clientCAFile:" to the location of the client CA file.

Remediation Method 2:

If using executable arguments, edit the kubelet service file /etc/systemd/system/kubelet.service.d/10-kubelet-args.conf on each worker node and add the below parameter at the end of the KUBELET_ARGS variable string.

--client-ca-file=<path/to/client-ca-file>

Remediation Method 3:

If using the api configz endpoint consider searching for the status of "authentication.*x509":("clientCAFile":"/etc/kubernetes/pki/ca.crt" by extracting the live configuration from the nodes running kubelet.

**See detailed step-by-step configmap procedures in Reconfigure a Node's Kubelet in a Live Cluster, and then rerun the curl statement from audit process to check for kubelet configuration changes.

For all three remediations: Based on your system, restart the kubelet service and check status:

systemctl daemon-reload
systemctl restart kubelet.service
systemctl status kubelet -l

Default Value

See the Azure AKS documentation for the default value.

References

1. https://kubernetes.io/docs/admin/kubelet/
2. https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet-authentication-authorization/
3. https://kubernetes.io/docs/tasks/administer-cluster/reconfigure-kubelet/
4. https://docs.microsoft.com/security/benchmark/azure/security-controls-v2-data-protection#dp-4-encrypt-sensitive-information-in-transit

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