Flux2 is an open and extensible continuous delivery solution for Kubernetes. Flux2 versions between 0.1.0 and 0.29.0, helm-controller 0.1.0 to v0.19.0, and kustomize-controller 0.1.0 to v0.23.0 are vulnerable to Code Injection via malicious Kubeconfig. In multi-tenancy deployments this can also lead to privilege escalation if the controller's service account has elevated permissions. Workarounds include disabling functionality via Validating Admission webhooks by restricting users from setting the spec.kubeConfig field in Flux Kustomization and HelmRelease objects. Additional mitigations include applying restrictive AppArmor and SELinux profiles on the controller’s pod to limit what binaries can be executed. This vulnerability is fixed in kustomize-controller v0.23.0 and helm-controller v0.19.0, both included in flux2 v0.29.0

Flux controllers within the affected versions range are vulnerable to a denial of service attack. Users that have permissions to change Flux’s objects, either through a Flux source or directly within a cluster, can provide invalid data to fields .spec.interval or .spec.timeout (and structured variations of these fields), causing the entire object type to stop being processed.

The issue has two root causes: a) the Kubernetes type metav1.Duration not being fully compatible with the Go type time.Duration as explained on upstream report; b) lack of validation within Flux to restrict allowed values.

Workarounds

Admission controllers can be employed to restrict the values that can be used for fields .spec.interval and .spec.timeout, however upgrading to the latest versions is still the recommended mitigation.

Credits

This issue was reported by Alexander Block (@codablock) through the Flux security mailing list (as recommended).

For more information

If you have any questions or comments about this advisory:

- Open an issue in any of the affected repositories. - Contact us at the CNCF Flux channel.

References

- https://github.com/kubernetes/apimachinery/issues/131