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Cluster Hardening

If you want to improve the security on your cluster and make it compliant with the CIS Benchmarks, here you can find a configuration to harden your kubernetes installation.

To apply the hardening configuration, create a file (eg. hardening.yaml) and paste the content of the following code snippet into that.

Minimum Requirements

The kubernetes version should be at least v1.23.6 to have all the most recent security features (eg. the new PodSecurity admission plugin, etc).

N.B. Some of these configurations have just been added to kubespray, so ensure that you have the latest version to make it works properly. Also, ensure that other configurations doesn't override these.

hardening.yaml:

# Hardening
---

## kube-apiserver
authorization_modes: ['Node','RBAC']
# AppArmor-based OS
#kube_apiserver_feature_gates: ['AppArmor=true']
kube_apiserver_request_timeout: 120s
kube_apiserver_service_account_lookup: true

# enable kubernetes audit
kubernetes_audit: true
audit_log_path: "/var/log/kube-apiserver-log.json"
audit_log_maxage: 30
audit_log_maxbackups: 10
audit_log_maxsize: 100

tls_min_version: VersionTLS12
tls_cipher_suites:
  - TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
  - TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
  - TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305

# enable encryption at rest
kube_encrypt_secret_data: true
kube_encryption_resources: [secrets]
kube_encryption_algorithm: "secretbox"

kube_apiserver_enable_admission_plugins: ['EventRateLimit,AlwaysPullImages,ServiceAccount,NamespaceLifecycle,NodeRestriction,LimitRanger,ResourceQuota,MutatingAdmissionWebhook,ValidatingAdmissionWebhook,PodNodeSelector,PodSecurity']
kube_apiserver_admission_control_config_file: true
# EventRateLimit plugin configuration
kube_apiserver_admission_event_rate_limits:
  limit_1:
    type: Namespace
    qps: 50
    burst: 100
    cache_size: 2000
  limit_2:
    type: User
    qps: 50
    burst: 100
kube_profiling: false

## kube-controller-manager
kube_controller_manager_bind_address: 127.0.0.1
kube_controller_terminated_pod_gc_threshold: 50
# AppArmor-based OS
#kube_controller_feature_gates: ["RotateKubeletServerCertificate=true","AppArmor=true"]
kube_controller_feature_gates: ["RotateKubeletServerCertificate=true"]

## kube-scheduler
kube_scheduler_bind_address: 127.0.0.1
kube_kubeadm_scheduler_extra_args:
  profiling: false
# AppArmor-based OS
#kube_scheduler_feature_gates: ["AppArmor=true"]

## etcd
etcd_deployment_type: kubeadm

## kubelet
kubelet_authorization_mode_webhook: true
kubelet_authentication_token_webhook: true
kube_read_only_port: 0
kubelet_rotate_server_certificates: true
kubelet_protect_kernel_defaults: true
kubelet_event_record_qps: 1
kubelet_rotate_certificates: true
kubelet_streaming_connection_idle_timeout: "5m"
kubelet_make_iptables_util_chains: true
kubelet_feature_gates: ["RotateKubeletServerCertificate=true","SeccompDefault=true"]
kubelet_seccomp_default: true

# additional configurations
kube_owner: root
kube_cert_group: root

Let's take a deep look to the resultant kubernetes configuration:

  • The anonymous-auth (on kube-apiserver) is set to true by default. This is fine, because it is considered safe if you enable RBAC for the authorization-mode.
  • The enable-admission-plugins has not the PodSecurityPolicy admission plugin. This because it is going to be definitely removed from kubernetes v1.25. For this reason we decided to set the newest PodSecurity (for more details, please take a look here: https://kubernetes.io/docs/concepts/security/pod-security-admission/). Then, we set the EventRateLimit plugin, providing additional configuration files (that are automatically created under the hood and mounted inside the kube-apiserver container) to make it work.
  • The encryption-provider-config provide encryption at rest. This means that the kube-apiserver encrypt data that is going to be stored before they reach etcd. So the data is completely unreadable from etcd (in case an attacker is able to exploit this).
  • The rotateCertificates in KubeletConfiguration is set to true along with serverTLSBootstrap. This could be used in alternative to tlsCertFile and tlsPrivateKeyFile parameters. Additionally it automatically generates certificates by itself, but you need to manually approve them or at least using an operator to do this (for more details, please take a look here: https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet-tls-bootstrapping/).
  • If you are installing kubernetes in an AppArmor-based OS (eg. Debian/Ubuntu) you can enable the AppArmor feature gate uncommenting the lines with the comment # AppArmor-based OS on top.

Once you have the file properly filled, you can run the Ansible command to start the installation:

ansible-playbook -v cluster.yml \
        -i inventory.ini \
        -b --become-user=root \
        --private-key ~/.ssh/id_ecdsa \
        -e "@vars.yaml" \
        -e "@hardening.yaml"

N.B. The vars.yaml contains our general cluster information (SANs, load balancer, dns, etc..) and hardening.yaml is the file described above.

Once completed the cluster deployment, don't forget to approve the generated certificates (check them with kubectl get csr, approve with kubectl certificate approve <csr_name>). This action is necessary because the secureTLSBootstrap option and RotateKubeletServerCertificate feature gate for kubelet are enabled (CIS 4.2.11, 4.2.12).