Cis Aks V170 3.2.3

---
name: cis-aks-v170-3.2.3
description: "Ensure that the --client-ca-file argument is set as appropriate (Automated)"
category: cis-aks
version: "1.7.0"
author: cyberstrike-official
tags: [cis, aks, kubernetes, azure, kubelet, authentication, client-ca-file, tls]
cis_id: "3.2.3"
cis_benchmark: "CIS Azure Kubernetes Service (AKS) Benchmark v1.7.0"
tech_stack: [kubernetes, azure, aks]
cwe_ids: []
chains_with: []
prerequisites: []
severity_boost: {}
---

# 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:

```bash
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:

```bash
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`

```bash
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:

```json
"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](https://kubernetes.io/docs/tasks/administer-cluster/reconfigure-kubelet/), 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:

```bash
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

## CIS Controls

| Controls Version | Control                                           | IG 1 | IG 2 | IG 3 |
| ---------------- | ------------------------------------------------- | ---- | ---- | ---- |
| v8               | 3.10 Encrypt Sensitive Data in Transit            |      | X    | X    |
| v8               | 6 Access Control Management                       |      |      |      |
| v7               | 14.4 Encrypt All Sensitive Information in Transit |      | X    | X    |