Vulnerability Discussion

Admission controllers intercept requests to the Kubernetes API before an object is instantiated. Enabling the admissions webhook allows for Kubernetes to apply policies against objects that are to be created, read, updated or deleted.

Admissions controllers can be used for:
- Prevent pod’s ability to run privileged containers
- Prevent pod’s ability to use privileged escalation
- Controlling pod’s access to volume types
- Controlling pod’s access to host file system
- Controlling pod’s usage of host networking objects and configuration

Satisfies: SRG-APP-000340-CTR-000770, SRG-APP-000342-CTR-000775

Check

On each controlplane node, retrieve the "pod-security-admission-config-file" value from the RKE2 config file (/etc/rancher/rke2/config.yaml). For example:

pod-security-admission-config-file: /etc/rancher/rke2/rke2-pss-custom.yaml

Validate that the file referenced by "pod-security-admission-config-file" exists and the default configuration settings match the following:

defaults:
audit: restricted
audit-version: latest
enforce: restricted
enforce-version: latest
warn: restricted
warn-version: latest

If "pod-security-admission-config-file" is not set, the file does not exist, or the configuration file differs from the above, this is a finding.

Fix

On each Control Plane node, create the file "/etc/rancher/rke2/rke2-pss-custom.yaml" and add the following content:

apiVersion: apiserver.config.k8s.io/v1
kind: AdmissionConfiguration
plugins:
- name: PodSecurity
configuration:
apiVersion: pod-security.admission.config.k8s.io/v1beta1
kind: PodSecurityConfiguration
defaults:
enforce: "restricted"
enforce-version: "latest"
audit: "restricted"
audit-version: "latest"
warn: "restricted"
warn-version: "latest"
exemptions:
usernames: []
runtimeClasses: []
namespaces: [kube-system, cis-operator-system, tigera-operator]

Verify the namespace exemptions contain only namespaces requiring access to capabilities outside of the restricted settings above.

Once the file is created, add the following to the RKE2 config file (/etc/rancher/rke2/config.yaml):

pod-security-admission-config-file: /etc/rancher/rke2/rke2-pss-custom.yaml

Once the "pod-security-admission-config-file" has been added, restart the Control Plane nodes with:

systemctl restart rke2-server