Flux2 can reconcile the state of a remote cluster when provided with a kubeconfig with the correct access rights. Kubeconfig files can define commands to be executed to generate on-demand authentication tokens. A malicious user with write access to a Flux source or direct access to the target cluster, could craft a kubeconfig to execute arbitrary code inside the controller’s container.
In multi-tenancy deployments this can also lead to privilege escalation if the controller's service account has elevated permissions.
Impact
Within the affected versions range, one of the permissions set below would be required for the vulnerability to be exploited:
- Direct access to the cluster to create Flux
Kustomization or HelmRelease objects and Kubernetes Secrets.
- Direct access to the cluster to modify existing Kubernetes secrets being used as
kubeconfig in existing Flux Kustomization or HelmRelease objects.
- Direct access to the cluster to modify existing Flux
Kustomization or HelmRelease objects and access to create or modify existing Kubernetes secrets.
- Access rights to make changes to a configured Flux Source (i.e. Git repository).
Patches
This vulnerability was fixed in kustomize-controller v0.23.0 and helm-controller v0.19.0, both included in flux2 v0.29.0. Starting from the fixed versions, both controllers disable the use of command execution from kubeconfig files by default, users have to opt-in by adding the flag --insecure-kubeconfig-exec to the controller’s command arguments. Users are no longer allowed to refer to files in the controller’s filesystem in the kubeconfig files provided for the remote apply feature.
Workarounds
- The functionality can be disabled via Validating Admission webhooks (e.g. OPA Gatekeeper, Kyverno) by restricting users from being able to set the
spec.kubeConfig field in Flux Kustomization and HelmRelease objects.
- Applying restrictive AppArmor and SELinux profiles on the controller’s pod to limit what binaries can be executed.
Credits
The Flux engineering team found and patched this vulnerability.
For more information
If you have any questions or comments about this advisory please open an issue in the flux2 repository.
References
pjbgf Analyst
Flux2 can reconcile the state of a remote cluster when provided with a kubeconfig with the correct access rights.
Kubeconfigfiles can define commands to be executed to generate on-demand authentication tokens. A malicious user with write access to a Flux source or direct access to the target cluster, could craft akubeconfigto execute arbitrary code inside the controller’s container.In multi-tenancy deployments this can also lead to privilege escalation if the controller's service account has elevated permissions.
Impact
Within the affected versions range, one of the permissions set below would be required for the vulnerability to be exploited:
KustomizationorHelmReleaseobjects and Kubernetes Secrets.kubeconfigin existing FluxKustomizationorHelmReleaseobjects.KustomizationorHelmReleaseobjects and access to create or modify existing Kubernetes secrets.Patches
This vulnerability was fixed in kustomize-controller v0.23.0 and helm-controller v0.19.0, both included in flux2 v0.29.0. Starting from the fixed versions, both controllers disable the use of command execution from
kubeconfigfiles by default, users have to opt-in by adding the flag--insecure-kubeconfig-execto the controller’s command arguments. Users are no longer allowed to refer to files in the controller’s filesystem in thekubeconfigfiles provided for the remote apply feature.Workarounds
spec.kubeConfigfield in FluxKustomizationandHelmReleaseobjects.Credits
The Flux engineering team found and patched this vulnerability.
For more information
If you have any questions or comments about this advisory please open an issue in the flux2 repository.
References