Using Admission Controllers

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

Parts of the K8S Security Lab series

Container Runetime Security
Advanced Kernel Security
Network Security
Secure Kubernetes Install
User Security
Image Security
  • Part1: Image security Admission Controller
  • Part2: Image security Admission Controller V2
  • Part3: Image security Admission Controller V3
  • Part4: Continuous Image security
  • Part5: trivy-operator 1.0
  • Part6: trivy-operator 2.1: Trivy-operator is now an Admisssion controller too!!!
  • Part7: trivy-operator 2.2: Patch release for Admisssion controller
  • Part8: trivy-operator 2.3: Patch release for Admisssion controller
  • Part8: trivy-operator 2.4: Patch release for Admisssion controller
  • Part8: trivy-operator 2.5: Patch release for Admisssion controller
  • Part9_ Image Signature Verification with Connaisseur
  • Part10: Image Signature Verification with Connaisseur 2.0
  • Part11: Image Signature Verification with Kyverno
  • Part12: How to use imagePullSecrets cluster-wide??
  • Part13: Automatically change registry in pod definition
  • Part14: ArgoCD auto image updater
    Pod Security
    Secret Security
    Monitoring and Observability
    Backup

    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.