--- title: K3s Hardening Guide --- This document provides prescriptive guidance for how to harden a K3s cluster intended for production, before provisioning it with Rancher. It outlines the configurations and controls required for Center for Information Security (CIS) Kubernetes benchmark controls. :::note This hardening guide describes how to secure the nodes in your cluster. We recommended that you follow this guide before you install Kubernetes. ::: This hardening guide is intended to be used for K3s clusters and is associated with the following versions of the CIS Kubernetes Benchmark, Kubernetes, and Rancher: | Rancher Version | CIS Benchmark Version | Kubernetes Version | |-----------------|-----------------------|------------------------------| | Rancher v2.7 | Benchmark v1.23 | Kubernetes v1.23 | | Rancher v2.7 | Benchmark v1.24 | Kubernetes v1.24 | | Rancher v2.7 | Benchmark v1.7 | Kubernetes v1.25 up to v1.26 | :::note In Benchmark v1.7, the `--protect-kernel-defaults` (4.2.6) parameter isn't required anymore, and was removed by CIS. ::: For more details on how to evaluate a hardened K3s cluster against the official CIS benchmark, refer to the K3s self-assessment guides for specific Kubernetes and CIS benchmark versions. K3s passes a number of the Kubernetes CIS controls without modification, as it applies several security mitigations by default. There are some notable exceptions to this that require manual intervention to fully comply with the CIS Benchmark: 1. K3s does not modify the host operating system. Any host-level modifications need to be done manually. 2. Certain CIS policy controls for `NetworkPolicies` and `PodSecurityStandards` (`PodSecurityPolicies` on v1.24 and older) restrict cluster functionality. You must opt into having K3s configure these policies. Add the appropriate options to your command-line flags or configuration file (enable admission plugins), and manually apply the appropriate policies. See further for more details. The first section (1.1) of the CIS Benchmark primarily focuses on pod manifest permissions and ownership. Since everything in the distribution is packaged in a single binary, this section does not apply to the core components of K3s. ## Host-level Requirements ### Ensure `protect-kernel-defaults` is set The `protect-kernel-defaults` is no longer required since CIS benchmark 1.7. This is a kubelet flag that will cause the kubelet to exit if the required kernel parameters are unset or are set to values that are different from the kubelet's defaults. The `protect-kernel-defaults` flag can be set in the cluster configuration in Rancher. ```yaml spec: rkeConfig: machineSelectorConfig: - config: protect-kernel-defaults: true ``` ### Set kernel parameters The following `sysctl` configuration is recommended for all nodes type in the cluster. Set the following parameters in `/etc/sysctl.d/90-kubelet.conf`: ```ini vm.panic_on_oom=0 vm.overcommit_memory=1 kernel.panic=10 kernel.panic_on_oops=1 ``` Run `sudo sysctl -p /etc/sysctl.d/90-kubelet.conf` to enable the settings. This configuration needs to be done before setting the kubelet flag, otherwise K3s will fail to start. ## Kubernetes Runtime Requirements The CIS Benchmark runtime requirements center around pod security (via PSP or PSA), network policies and API Server auditing logs. By default, K3s does not include any pod security or network policies. However, K3s ships with a controller that enforces any network policies you create. By default, K3s enables both the `PodSecurity` and `NodeRestriction` admission controllers, among others. ### Pod Security K3s v1.25 and newer support [Pod Security admission (PSA)](https://kubernetes.io/docs/concepts/security/pod-security-admission/) for controlling pod security. You can specify the PSA configuration by setting the `defaultPodSecurityAdmissionConfigurationTemplateName` field in the cluster configuration in Rancher: ```yaml spec: defaultPodSecurityAdmissionConfigurationTemplateName: rancher-restricted ``` The `rancher-restricted` template is provided by Rancher to enforce the highly-restrictive Kubernetes upstream [`Restricted`](https://kubernetes.io/docs/concepts/security/pod-security-standards/#restricted) profile with best practices for pod hardening. K3s v1.24 and older support [Pod Security Policy (PSP)](https://v1-24.docs.kubernetes.io/docs/concepts/security/pod-security-policy/) for controlling pod security. You can enable PSPs by passing the following flags in the cluster configuration in Rancher: ```yaml spec: rkeConfig: machineGlobalConfig: kube-apiserver-arg: - enable-admission-plugins=NodeRestriction,PodSecurityPolicy,ServiceAccount ``` This maintains the `NodeRestriction` plugin and enables the `PodSecurityPolicy`. Once you enable PSPs, you can apply a policy to satisfy the necessary controls described in section 5.2 of the CIS Benchmark. :::note These are manual checks in the CIS Benchmark. The CIS scan flags the results as `warning`, because manual inspection is necessary by the cluster operator. ::: Here is an example of a compliant PSP: ```yaml --- apiVersion: policy/v1beta1 kind: PodSecurityPolicy metadata: name: restricted-psp spec: privileged: false # CIS - 5.2.1 allowPrivilegeEscalation: false # CIS - 5.2.5 requiredDropCapabilities: # CIS - 5.2.7/8/9 - ALL volumes: - 'configMap' - 'emptyDir' - 'projected' - 'secret' - 'downwardAPI' - 'csi' - 'persistentVolumeClaim' - 'ephemeral' hostNetwork: false # CIS - 5.2.4 hostIPC: false # CIS - 5.2.3 hostPID: false # CIS - 5.2.2 runAsUser: rule: 'MustRunAsNonRoot' # CIS - 5.2.6 seLinux: rule: 'RunAsAny' supplementalGroups: rule: 'MustRunAs' ranges: - min: 1 max: 65535 fsGroup: rule: 'MustRunAs' ranges: - min: 1 max: 65535 readOnlyRootFilesystem: false ``` For the example PSP to be effective, we need to create a `ClusterRole` and a `ClusterRoleBinding`. We also need to include a "system unrestricted policy" for system-level pods that require additional privileges, and an additional policy that allows the necessary sysctls for full functionality of ServiceLB. ```yaml --- apiVersion: policy/v1beta1 kind: PodSecurityPolicy metadata: name: restricted-psp spec: privileged: false allowPrivilegeEscalation: false requiredDropCapabilities: - ALL volumes: - 'configMap' - 'emptyDir' - 'projected' - 'secret' - 'downwardAPI' - 'csi' - 'persistentVolumeClaim' - 'ephemeral' hostNetwork: false hostIPC: false hostPID: false runAsUser: rule: 'MustRunAsNonRoot' seLinux: rule: 'RunAsAny' supplementalGroups: rule: 'MustRunAs' ranges: - min: 1 max: 65535 fsGroup: rule: 'MustRunAs' ranges: - min: 1 max: 65535 readOnlyRootFilesystem: false --- apiVersion: policy/v1beta1 kind: PodSecurityPolicy metadata: name: system-unrestricted-psp annotations: seccomp.security.alpha.kubernetes.io/allowedProfileNames: '*' spec: allowPrivilegeEscalation: true allowedCapabilities: - '*' fsGroup: rule: RunAsAny hostIPC: true hostNetwork: true hostPID: true hostPorts: - max: 65535 min: 0 privileged: true runAsUser: rule: RunAsAny seLinux: rule: RunAsAny supplementalGroups: rule: RunAsAny volumes: - '*' --- apiVersion: policy/v1beta1 kind: PodSecurityPolicy metadata: name: svclb-psp annotations: seccomp.security.alpha.kubernetes.io/allowedProfileNames: '*' spec: allowPrivilegeEscalation: false allowedCapabilities: - NET_ADMIN allowedUnsafeSysctls: - net.ipv4.ip_forward - net.ipv6.conf.all.forwarding fsGroup: rule: RunAsAny hostPorts: - max: 65535 min: 0 runAsUser: rule: RunAsAny seLinux: rule: RunAsAny supplementalGroups: rule: RunAsAny --- apiVersion: rbac.authorization.k8s.io/v1 kind: ClusterRole metadata: name: psp:restricted-psp rules: - apiGroups: - policy resources: - podsecuritypolicies verbs: - use resourceNames: - restricted-psp --- apiVersion: rbac.authorization.k8s.io/v1 kind: ClusterRole metadata: name: psp:system-unrestricted-psp rules: - apiGroups: - policy resources: - podsecuritypolicies resourceNames: - system-unrestricted-psp verbs: - use --- apiVersion: rbac.authorization.k8s.io/v1 kind: ClusterRole metadata: name: psp:svclb-psp rules: - apiGroups: - policy resources: - podsecuritypolicies resourceNames: - svclb-psp verbs: - use --- apiVersion: rbac.authorization.k8s.io/v1 kind: ClusterRole metadata: name: psp:svc-local-path-provisioner-psp rules: - apiGroups: - policy resources: - podsecuritypolicies resourceNames: - system-unrestricted-psp verbs: - use --- apiVersion: rbac.authorization.k8s.io/v1 kind: ClusterRole metadata: name: psp:svc-coredns-psp rules: - apiGroups: - policy resources: - podsecuritypolicies resourceNames: - system-unrestricted-psp verbs: - use --- apiVersion: rbac.authorization.k8s.io/v1 kind: ClusterRole metadata: name: psp:svc-cis-operator-psp rules: - apiGroups: - policy resources: - podsecuritypolicies resourceNames: - system-unrestricted-psp verbs: - use --- apiVersion: rbac.authorization.k8s.io/v1 kind: ClusterRoleBinding metadata: name: default:restricted-psp roleRef: apiGroup: rbac.authorization.k8s.io kind: ClusterRole name: psp:restricted-psp subjects: - kind: Group name: system:authenticated apiGroup: rbac.authorization.k8s.io --- apiVersion: rbac.authorization.k8s.io/v1 kind: ClusterRoleBinding metadata: name: system-unrestricted-node-psp-rolebinding roleRef: apiGroup: rbac.authorization.k8s.io kind: ClusterRole name: psp:system-unrestricted-psp subjects: - apiGroup: rbac.authorization.k8s.io kind: Group name: system:nodes --- apiVersion: rbac.authorization.k8s.io/v1 kind: RoleBinding metadata: name: system-unrestricted-svc-acct-psp-rolebinding namespace: kube-system roleRef: apiGroup: rbac.authorization.k8s.io kind: ClusterRole name: psp:system-unrestricted-psp subjects: - apiGroup: rbac.authorization.k8s.io kind: Group name: system:serviceaccounts --- apiVersion: rbac.authorization.k8s.io/v1 kind: RoleBinding metadata: name: svclb-psp-rolebinding namespace: kube-system roleRef: apiGroup: rbac.authorization.k8s.io kind: ClusterRole name: psp:svclb-psp subjects: - kind: ServiceAccount name: svclb --- apiVersion: rbac.authorization.k8s.io/v1 kind: RoleBinding metadata: name: svc-local-path-provisioner-psp-rolebinding namespace: kube-system roleRef: apiGroup: rbac.authorization.k8s.io kind: ClusterRole name: psp:svc-local-path-provisioner-psp subjects: - kind: ServiceAccount name: local-path-provisioner-service-account --- apiVersion: rbac.authorization.k8s.io/v1 kind: RoleBinding metadata: name: svc-coredns-psp-rolebinding namespace: kube-system roleRef: apiGroup: rbac.authorization.k8s.io kind: ClusterRole name: psp:svc-coredns-psp subjects: - kind: ServiceAccount name: coredns --- apiVersion: rbac.authorization.k8s.io/v1 kind: RoleBinding metadata: name: svc-cis-operator-psp-rolebinding namespace: cis-operator-system roleRef: apiGroup: rbac.authorization.k8s.io kind: ClusterRole name: psp:svc-cis-operator-psp subjects: - kind: ServiceAccount name: cis-operator-serviceaccount ``` The policies presented above can be placed in a file named `policy.yaml` in the `/var/lib/rancher/k3s/server/manifests` directory. Both the policy file and the its directory hierarchy must be created before starting K3s. A restrictive access permission is recommended to avoid leaking potential sensitive information. ```shell sudo mkdir -p -m 700 /var/lib/rancher/k3s/server/manifests ``` :::note The critical Kubernetes additions such as CNI, DNS, and Ingress are run as pods in the `kube-system` namespace. Therefore, this namespace has a less restrictive policy, so that these components can run properly. ::: ### Network Policies CIS requires that all namespaces apply a network policy that reasonably limits traffic into namespaces and pods. :::note This is a manual check in the CIS Benchmark. The CIS scan flags the result as a `warning`, because manual inspection is necessary by the cluster operator. ::: The network policies can be placed in the `policy.yaml` file in `/var/lib/rancher/k3s/server/manifests` directory. If the directory was not created as part of the PSP (as described above), it must be created first. ```shell sudo mkdir -p -m 700 /var/lib/rancher/k3s/server/manifests ``` Here is an example of a compliant network policy: ```yaml --- kind: NetworkPolicy apiVersion: networking.k8s.io/v1 metadata: name: intra-namespace namespace: kube-system spec: podSelector: {} ingress: - from: - namespaceSelector: matchLabels: name: kube-system --- kind: NetworkPolicy apiVersion: networking.k8s.io/v1 metadata: name: intra-namespace namespace: default spec: podSelector: {} ingress: - from: - namespaceSelector: matchLabels: name: default --- kind: NetworkPolicy apiVersion: networking.k8s.io/v1 metadata: name: intra-namespace namespace: kube-public spec: podSelector: {} ingress: - from: - namespaceSelector: matchLabels: name: kube-public ``` The active restrictions block DNS unless purposely allowed. Below is a network policy that allows DNS-related traffic: ```yaml --- apiVersion: networking.k8s.io/v1 kind: NetworkPolicy metadata: name: default-network-dns-policy namespace: spec: ingress: - ports: - port: 53 protocol: TCP - port: 53 protocol: UDP podSelector: matchLabels: k8s-app: kube-dns policyTypes: - Ingress ``` The metrics-server and Traefik ingress controller are blocked by default if network policies are not created to allow access. ```yaml --- apiVersion: networking.k8s.io/v1 kind: NetworkPolicy metadata: name: allow-all-metrics-server namespace: kube-system spec: podSelector: matchLabels: k8s-app: metrics-server ingress: - {} policyTypes: - Ingress --- apiVersion: networking.k8s.io/v1 kind: NetworkPolicy metadata: name: allow-all-svclbtraefik-ingress namespace: kube-system spec: podSelector: matchLabels: svccontroller.k3s.cattle.io/svcname: traefik ingress: - {} policyTypes: - Ingress --- apiVersion: networking.k8s.io/v1 kind: NetworkPolicy metadata: name: allow-all-traefik-v121-ingress namespace: kube-system spec: podSelector: matchLabels: app.kubernetes.io/name: traefik ingress: - {} policyTypes: - Ingress ``` :::note You must manage network policies as normal for any additional namespaces you create. ::: ### API Server audit configuration CIS requirements 1.2.22 to 1.2.25 are related to configuring audit logs for the API Server. K3s does not create by default the log directory and audit policy, as auditing requirements are specific to each user's policies and environment. If you need a log directory, it must be created before you start K3s. We recommend a restrictive access permission to avoid leaking sensitive information. ```bash sudo mkdir -p -m 700 /var/lib/rancher/k3s/server/logs ``` The following is a starter audit policy to log request metadata. This policy should be written to a file named `audit.yaml` in the `/var/lib/rancher/k3s/server` directory. Detailed information about policy configuration for the API server can be found in the [official Kubernetes documentation](https://kubernetes.io/docs/tasks/debug/debug-cluster/audit/). ```yaml --- apiVersion: audit.k8s.io/v1 kind: Policy rules: - level: Metadata ``` Further configurations are also needed to pass CIS checks. These are not configured by default in K3s, because they vary based on your environment and needs: - Ensure that the `--audit-log-path` argument is set. - Ensure that the `--audit-log-maxage` argument is set to 30 or as appropriate. - Ensure that the `--audit-log-maxbackup` argument is set to 10 or as appropriate. - Ensure that the `--audit-log-maxsize` argument is set to 100 or as appropriate. Combined, to enable and configure audit logs, add the following lines to the K3s cluster configuration file in Rancher: ```yaml spec: rkeConfig: machineGlobalConfig: kube-apiserver-arg: - audit-policy-file=/var/lib/rancher/k3s/server/audit.yaml # CIS 3.2.1 - audit-log-path=/var/lib/rancher/k3s/server/logs/audit.log # CIS 1.2.18 - audit-log-maxage=30 # CIS 1.2.19 - audit-log-maxbackup=10 # CIS 1.2.20 - audit-log-maxsize=100 # CIS 1.2.21 ``` ### Controller Manager Requirements CIS requirement 1.3.1 checks for garbage collection settings in the Controller Manager. Garbage collection is important to ensure sufficient resource availability and avoid degraded performance and availability. Based on your system resources and tests, choose an appropriate threshold value to activate garbage collection. This can be remediated by setting the following configuration in the K3s cluster file in Rancher. The value below is only an example. The appropriate threshold value is specific to each user's environment. ```yaml spec: rkeConfig: machineGlobalConfig: kube-controller-manager-arg: - terminated-pod-gc-threshold=10 # CIS 1.3.1 ``` ### Configure `default` Service Account Kubernetes provides a `default` service account which is used by cluster workloads where no specific service account is assigned to the pod. Where access to the Kubernetes API from a pod is required, a specific service account should be created for that pod, and rights granted to that service account. For CIS requirement 5.1.5 the `default` service account should be configured such that it does not provide a service account token and does not have any explicit rights assignments. This can be remediated by updating the `automountServiceAccountToken` field to `false` for the `default` service account in each namespace. For `default` service accounts in the built-in namespaces (`kube-system`, `kube-public`, `kube-node-lease`, and `default)`, K3s does not automatically do this. Save the following configuration to a file called `account_update.yaml`. ```yaml --- apiVersion: v1 kind: ServiceAccount metadata: name: default automountServiceAccountToken: false ``` Create a bash script file called `account_update.sh`. Be sure to `chmod +x account_update.sh` so the script has execute permissions. ```shell #!/bin/bash -e for namespace in $(kubectl get namespaces -A -o=jsonpath="{.items[*]['metadata.name']}"); do kubectl patch serviceaccount default -n ${namespace} -p "$(cat account_update.yaml)" done ``` Run the script every time a new service account is added to your cluster. ## Reference Hardened K3s Template Configuration The following reference template configuration is used in Rancher to create a hardened K3s custom cluster based on each CIS control in this guide. This reference does not include other required **cluster configuration** directives, which vary based on your environment. ```yaml apiVersion: provisioning.cattle.io/v1 kind: Cluster metadata: name: # Define cluster name spec: defaultPodSecurityAdmissionConfigurationTemplateName: rancher-restricted enableNetworkPolicy: true kubernetesVersion: # Define K3s version rkeConfig: machineGlobalConfig: kube-apiserver-arg: - enable-admission-plugins=NodeRestriction,ServiceAccount # CIS 1.2.15, 1.2.13 - audit-policy-file=/var/lib/rancher/k3s/server/audit.yaml # CIS 3.2.1 - audit-log-path=/var/lib/rancher/k3s/server/logs/audit.log # CIS 1.2.18 - audit-log-maxage=30 # CIS 1.2.19 - audit-log-maxbackup=10 # CIS 1.2.20 - audit-log-maxsize=100 # CIS 1.2.21 - request-timeout=300s # CIS 1.2.22 - service-account-lookup=true # CIS 1.2.24 kube-controller-manager-arg: - terminated-pod-gc-threshold=10 # CIS 1.3.1 secrets-encryption: true machineSelectorConfig: - config: kubelet-arg: - make-iptables-util-chains=true # CIS 4.2.7 ``` ```yaml apiVersion: provisioning.cattle.io/v1 kind: Cluster metadata: name: # Define cluster name spec: enableNetworkPolicy: true kubernetesVersion: # Define K3s version rkeConfig: machineGlobalConfig: kube-apiserver-arg: - enable-admission-plugins=NodeRestriction,PodSecurityPolicy,ServiceAccount # CIS 1.2.15, 5.2, 1.2.13 - audit-policy-file=/var/lib/rancher/k3s/server/audit.yaml # CIS 3.2.1 - audit-log-path=/var/lib/rancher/k3s/server/logs/audit.log # CIS 1.2.18 - audit-log-maxage=30 # CIS 1.2.19 - audit-log-maxbackup=10 # CIS 1.2.20 - audit-log-maxsize=100 # CIS 1.2.21 - request-timeout=300s # CIS 1.2.22 - service-account-lookup=true # CIS 1.2.24 kube-controller-manager-arg: - terminated-pod-gc-threshold=10 # CIS 1.3.1 secrets-encryption: true machineSelectorConfig: - config: kubelet-arg: - make-iptables-util-chains=true # CIS 4.2.7 protect-kernel-defaults: true # CIS 4.2.6 ``` ## Conclusion If you have followed this guide, your K3s custom cluster provisioned by Rancher will be configured to pass the CIS Kubernetes Benchmark. You can review our K3s self-assessment guides to understand how we verified each of the benchmarks and how you can do the same on your cluster.