CVE-2022-39272: Denial of Service Vulnerability in Flux Prior to Version .35.

Flux is an open source and extensible continuous delivery solution for Kubernetes, widely used by many developers for managing deployments in a GitOps-centered manner. However, a recent Denial of Service (DoS) vulnerability has been discovered in versions prior to .35.. The vulnerability (CVE-2022-39272) allows users with permissions to change Flux's objects, either through a Flux source or directly within a cluster, to provide invalid data to the fields .spec.interval or .spec.timeout (and structured variations of these fields), which in turn results in the entire object type to stop being processed. This can cause significant disruptions to the overall functioning of the Flux-based Kubernetes environment.

To address this issue, the Flux maintainer team has released a security patch in version .35., which is the recommended mitigation. As an alternative workaround, one can also employ Admission Controllers to restrict the allowed values for the aforementioned fields.

Vulnerability Details

The vulnerability CVE-2022-39272, a Denial of Service (DoS) type, can be exploited by any user with sufficient permissions to modify the affected fields. An example of such an action is shown in the following code snippet where the .spec.interval field is set with an invalid value:

apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
kind: Kustomization
metadata:
  name: my-kustomization
spec:
  sourceRef:
    kind: GitRepository
    name: my-repo
  interval: invalid_interval_value

This can cause the Flux reconciler to stop processing the entire Kustomization objects, severely impacting the management of the Kubernetes environment.

Suggested Mitigation

The official mitigation for this vulnerability is to upgrade Flux to version .35. or later. This patched version handles the invalid values introduced to the affected fields and prevents the DoS condition from occurring. To upgrade to the patched version, follow the instructions provided in the Flux documentation.

Alternative Workaround

An alternative workaround for this vulnerability is to use Kubernetes Admission Controllers, specifically ValidatingAdmissionWebhooks, to restrict the values that can be used for the fields .spec.interval and .spec.timeout.

Here is an example of a simple Admission Controller configuration for validating the interval field:

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingWebhookConfiguration
metadata:
  name: my-kustomization-validation
webhooks:
- name: kustomization-validation.my-webhook.example
  rules:
  - apiGroups:
    - kustomize.toolkit.fluxcd.io
    apiVersions:
    - v1beta1
    operations:
    - CREATE
    - UPDATE
    resources:
    - kustomizations
  clientConfig:
    service:
      namespace: my-webhook-namespace
      name: my-webhook-service
  admissionReviewVersions:
  - v1
  sideEffects: None
  timeoutSeconds: 5

You will need to implement a validation webhook that checks the values of .spec.interval and .spec.timeout fields, and rejects any invalid values to prevent exploitation of the vulnerability.

References

- Flux Changelog: https://github.com/fluxcd/flux2/blob/main/CHANGELOG.md
- Flux CVE-2022-39272 Details: https://github.com/fluxcd/flux2/security/advisories/GHSA-4g66-xr2h-7jw2
- Flux Installation and Upgrade Guide: https://fluxcd.io/docs/installation/#upgrading-flux
- Kubernetes Admission Controller: https://kubernetes.io/docs/reference/access-authn-authz/admission-controllers/

In conclusion, Flux users are strongly advised to upgrade to version .35. or later to protect their environments from the CVE-2022-39272 vulnerability. While using Admission Controllers can provide a viable alternative, upgrading to the latest version remains the recommended approach towards achieving better security and overall performance.

Timeline

Published on: 10/22/2022 00:15:00 UTC
Last modified on: 10/24/2022 16:51:00 UTC