K3s Hardening Guides
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.
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 |
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:
- K3s does not modify the host operating system. Any host-level modifications need to be done manually.
- Certain CIS policy controls for
NetworkPolicies
andPodSecurityStandards
(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
- v1.25 and Newer
- v1.24 and Older
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.
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
:
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
- v1.25 and Newer
- v1.24 and Older
K3s v1.25 and newer support Pod Security admission (PSA) for controlling pod security.
You can specify the PSA configuration by setting the defaultPodSecurityAdmissionConfigurationTemplateName
field in the cluster configuration in Rancher:
spec:
defaultPodSecurityAdmissionConfigurationTemplateName: rancher-restricted
The rancher-restricted
template is provided by Rancher to enforce the highly-restrictive Kubernetes upstream Restricted
profile with best practices for pod hardening.
K3s v1.24 and older support Pod Security Policy (PSP) for controlling pod security.
You can enable PSPs by passing the following flags in the cluster configuration in Rancher:
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.
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:
---
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.
---
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.
sudo mkdir -p -m 700 /var/lib/rancher/k3s/server/manifests
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.
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.
sudo mkdir -p -m 700 /var/lib/rancher/k3s/server/manifests
Here is an example of a compliant network policy:
---
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:
---
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
name: default-network-dns-policy
namespace: <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.
---
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
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.
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.
---
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:
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.
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
.
---
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.
#!/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.
- v1.25 and Newer
- v1.24 and Older
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
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.