# Kubernetes ValidatingWebhookConfiguration

**The original author of this page is** [**Guillaume**](https://www.linkedin.com/in/guillaume-chapela-ab4b9a196)

## Definition

ValidatingWebhookConfiguration is a Kubernetes resource that defines a validating webhook, which is a server-side component that validates incoming Kubernetes API requests against a set of predefined rules and constraints.

## Purpose

The purpose of a ValidatingWebhookConfiguration is to define a validating webhook that will enforce a set of predefined rules and constraints on incoming Kubernetes API requests. The webhook will validate the requests against the rules and constraints defined in the configuration, and will return an error if the request does not conform to the rules.

**Example**

Here is an example of a ValidatingWebhookConfiguration:

```yaml
apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingWebhookConfiguration
metadata:
  name: example-validation-webhook
  namespace: default
webhook:
  name: example-validation-webhook
  clientConfig:
    url: https://example.com/webhook
    serviceAccountName: example-service-account
  rules:
    - apiGroups:
        - ""
      apiVersions:
        - "*"
      operations:
        - CREATE
        - UPDATE
      resources:
        - pods
```

The main difference between a ValidatingWebhookConfiguration and policies :

<figure><img src="../../images/Kyverno.png" alt=""><figcaption><p>Kyverno.png</p></figcaption></figure>

- **ValidatingWebhookConfiguration (VWC)** : A Kubernetes resource that defines a validating webhook, which is a server-side component that validates incoming Kubernetes API requests against a set of predefined rules and constraints.
- **Kyverno ClusterPolicy**: A policy definition that specifies a set of rules and constraints for validating and enforcing Kubernetes resources, such as pods, deployments, and services

## Enumeration

```
$ kubectl get ValidatingWebhookConfiguration
```

### Abusing Kyverno and Gatekeeper VWC

As we can see all operators installed have at least one ValidatingWebHookConfiguration(VWC).

**Kyverno** and **Gatekeeper** are both Kubernetes policy engines that provide a framework for defining and enforcing policies across a cluster.

Exceptions refer to specific rules or conditions that allow a policy to be bypassed or modified under certain circumstances but this is not the only way !

For **kyverno**, as you as there is a validating policy, the webhook `kyverno-resource-validating-webhook-cfg` is populated.

For Gatekeeper, there is `gatekeeper-validating-webhook-configuration` YAML file.

Both come from with default values but the Administrator teams might updated those 2 files.

### Use Case

```bash
$ kubectl get validatingwebhookconfiguration kyverno-resource-validating-webhook-cfg -o yaml
```

Now, identify the following output :

```yaml
namespaceSelector:
  matchExpressions:
    - key: kubernetes.io/metadata.name
      operator: NotIn
      values:
        - default
        - TEST
        - YOYO
        - kube-system
        - MYAPP
```

Here, `kubernetes.io/metadata.name` label refers to the namespace name. Namespaces with names in the `values` list will be excluded from the policy :

Check namespaces existence. Sometimes, due to automation or misconfiguration, some namespaces might have not been created. If you have permission to create namespace, you could create a namespace with a name in the `values` list and policies won't apply your new namespace.

The goal of this attack is to exploit **misconfiguration** inside VWC in order to bypass operators restrictions and then elevate your privileges with other techniques

{{#ref}}
abusing-roles-clusterroles-in-kubernetes/
{{#endref}}

## References

- [https://github.com/open-policy-agent/gatekeeper](https://github.com/open-policy-agent/gatekeeper)
- [https://kyverno.io/](https://kyverno.io/)
- [https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/](https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/)



