Using Admission Controllers

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In this post I will show you how you can use Admission Controllers.

Parst of the K8S Security series

What is an Admission Controller

An admission controller is a piece of code that intercepts requests to the Kubernetes API server prior to persistence of the object, but after the request is authenticated and authorized. […] Admission controllers may be “validating”, “mutating”, or both. Mutating controllers may modify the objects they admit; validating controllers may not. […] If any of the controllers in either phase reject the request, the entire request is rejected immediately and an error is returned to the end-user. (Source: Kubernetes Website )

In a nutshell, Kubernetes admission controllers are plugins that govern and enforce how the cluster is used. They can be thought of as a gatekeeper that intercepts (authenticated) API requests and may change the request object or deny the request altogether.

How do I turn on an admission controller?

A list of previously implemented controllers comes with Kubernetes, or you can write your own. To do so you must enable them in the kube-apiserver The Kubernetes API server flag enable-admission-plugins takes a comma-delimited list of admission control plugins to invoke prior to modifying objects in the cluster. For example, the following command line enables the NamespaceLifecycle and the LimitRanger admission control plugins:

kube-apiserver --enable-admission-plugins=NamespaceLifecycle,LimitRanger ...

What is an admission webhook?

There are two special admission controllers in the list included in the Kubernetes apiserver: MutatingAdmissionWebhook and ValidatingAdmissionWebhook. These are special admission controllers that send admission requests to external HTTP callbacks and receive admission responses. If these two admission controllers are enabled, a Kubernetes administrator can create and configure an admission webhook in the cluster.

Example image

Validating webhooks can reject a request, but they cannot modify the object they are receiving in the admission request, while mutating webhooks can modify objects by creating a patch that will be sent back in the admission response. If a webhook rejects a request, an error is returned to the end-user.

Why do I need admission controllers?

Security: Admission controllers can increase security by mandating a reasonable security baseline across an entire namespace or cluster. The built-in PodSecurityPolicy admission controller is perhaps the most prominent example; it can be used for disallowing containers from running as root or making sure the container’s root filesystem is always mounted read-only, for example. Further use cases that can be realized by custom, webhook-based admission controllers include:

  • Allow pulling images only from specific registries known to the enterprise, while denying unknown image registries.
  • Reject deployments that do not meet security standards. For example, containers using the privileged flag can circumvent a lot of security checks. This risk could be mitigated by a webhook-based admission controller that either rejects such deployments (validating) or overrides the privileged flag, setting it to false.