KubeWarden's AdmissionPolicy and AdmissionPolicyGroup policies can be used to alter PolicyReport resources

Impact

By design, AdmissionPolicy and AdmissionPolicyGroup can evaluate only namespaced resources. The resources to be evaluated are determined by the rules provided by the user when defining the policy. There might be Kubernetes namespaced resources that should not be validated by AdmissionPolicy and by the AdmissionPolicyGroup policies because of their sensitive nature. For example, PolicyReport are namespaced resources that contain the list of non compliant objects found inside of a namespace. See this section of Kubewarden’s documentation for more details about PolicyReport resources. An attacker can use either an AdmissionPolicy or an AdmissionPolicyGroup to prevent the creation and update of PolicyReport objects to hide non-compliant resources. Moreover, the same attacker might use a mutating AdmissionPolicy to alter the contents of the PolicyReport created inside of the namespace.

Patches

Starting from the 1.21.0 release, the validation rules applied to AdmissionPolicy and AdmissionPolicyGroup have been tightened to prevent them from validating sensitive types of namespaced resources. The new validation will also restrict the usage of wildcards when defining apiGroups and resources rules for AdmissionPolicy and AdmissionPolicyGroup objects.

Workarounds

On clusters running Kubewarden < 1.21.0, the following Kubewarden policy can be applied to prevent the creation of AdmissionPolicy and AdmissionPolicyGroup resources that interact with PolicyReport resources:

apiVersion: policies.kubewarden.io/v1
kind: ClusterAdmissionPolicy
metadata:
  name: "deny-interaction-with-policyreport"
spec:
  module: registry://ghcr.io/kubewarden/policies/cel-policy:latest
  settings:
    variables:
      - name: hasWildcardInsideOfApiGroup
        expression: "object.spec.rules.exists(r, r.apiGroups.exists(ag, ag == '*'))"
      - name: hasWildcardInsideOfResources
        expression: "object.spec.rules.exists(r, r.resources.exists(ag, ag == '*' || ag == '*/*' || ag == 'policyreports/*'))"
      - name: dealsWithPolicyReportApiGroup
        expression: "object.spec.rules.exists(r, r.apiGroups.exists(ag, ag == 'wgpolicyk8s.io'))"
      - name: dealsWithPolicyReportResource
        expression: "object.spec.rules.exists(r, r.resources.exists(ag, ag == 'policyreports' || ag == 'policyreports/'))"
      - name: isPendingDeletion
        expression: "has(object.metadata.deletionTimestamp)"
    validations:
      - expression: |
          !( variables.hasWildcardInsideOfApiGroup ||
             variables.hasWildcardInsideOfResources ||
             variables.dealsWithPolicyReportResource ||
             variables.dealsWithPolicyReportApiGroup
          ) || variables.isPendingDeletion
        message: "cannot target PolicyReport resources or use wildcards in apiGroups or resources"
  rules:
    - apiGroups: ["policies.kubewarden.io"]
      apiVersions: ["v1"]
      operations: ["CREATE", "UPDATE"]
      resources: ["admissionpolicies", "admissionpolicygroups"]
  mutating: false
  backgroundAudit: true