Admission controllers for Enhanced Cluster Security

The kube guy
3 min readDec 7, 2023

In our journey through our security series, Here we came across admission controllers, which helps to increase cluster security. This one is an intermediate-level concept that plays a pivotal role in Kubernetes security is the use of Admission Controllers. This blog post aims to demystify Admission Controllers and demonstrate how they can be leveraged to enforce security policies and maintain the integrity of your Kubernetes cluster.

admission controllers

What are Admission Controllers?

Admission Controllers are a set of plugins that intercept requests to the Kubernetes API server prior to persistence of the object, but after the request is authenticated and authorized. They act as gatekeepers, allowing or denying requests based on specific criteria. There are two types of Admission Controllers: Mutating and Validating. Mutating Admission Controllers can modify requests, while Validating Admission Controllers can only approve or reject them.

Importance in Kubernetes Security

Admission Controllers are crucial for enforcing organizational policies and ensuring that clusters adhere to security best practices. They can restrict potentially harmful actions, like the creation of overly privileged containers or the exposure of sensitive data through configurations. For instance, a Validating Admission Controller might reject any deployment that doesn’t meet your organization’s security requirements, like running as a non-root user.

Setting Up Admission Controllers

To set up Admission Controllers in Kubernetes, you need to modify the API server configuration. This involves specifying the controllers in the --enable-admission-plugins flag. Kubernetes comes with several built-in Admission Controllers, like NamespaceLifecycle, LimitRanger, and ServiceAccount, which can be enabled as needed.

Custom Admission Controllers with Webhooks

For more customized control, you can implement your own Admission Controllers using Mutating and Validating Webhooks. These webhooks allow you to define and enforce custom policies. For instance, a Mutating Webhook could automatically inject sidecar containers for logging or monitoring, while a Validating Webhook could enforce that all images come from a trusted container registry.

Example: Implementing a Validating Webhook (100 words) Imagine you want to enforce that all pods in your cluster must have resource limits defined. You can create a Validating Webhook that checks the pod specifications in incoming requests and rejects any pod creation requests that don’t specify these limits.

Best Practices and Considerations

  • Test your Admission Controllers in a non-production environment first.
  • Be cautious with Mutating Admission Controllers, as they can alter resource specifications.
  • Ensure Admission Controllers do not create bottlenecks or significantly delay API responses.
  • Keep your Admission Controllers’ logic as simple and clear as possible to avoid unintended consequences.

Admission Controllers are a powerful tool in the Kubernetes security arsenal. By understanding and utilizing them effectively, you can significantly enhance the security and compliance of your Kubernetes clusters, ensuring that they operate safely and efficiently.

If you’re hearing this concept for the first time, there is no need to panic, and these are used only when you’re working with complex applications. If you’re in beginning stage of learning Kubernetes, go through our previous posts. Also, if you’ve any queries post them in comment section, we are very happy to help you.



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