: Admission control with Open Policy Agent

Admission control with Open Policy Agent

Table of Contents

Admission control with Open Policy Agent

Prisma Cloud provides a dynamic admission controller for Kubernetes and OpenShift that is built on the Open Policy Agent (OPA). In Console, you can manage and compose rules in Rego, which is OPA’s native query language. Rules can allow or deny (alert or block) pods. Console pushes your policies to Defender, which enforces them. Decisions made by the system are logged.
There is currently no support for Windows.

Open Policy Agent

The Open Policy Agent is an open source, general-purpose policy engine that lets you consolidate policy enforcement in a single place. OPA can enforce policies in microservices, Kubernetes clusters, CI/CD pipelines, API gateways, and so on. OPA provides a high-level declarative language called Rego, which lets you specify policy as code. The OPA APIs let you offload policy decision-making from your software.
OPA decouples policy decision-making from policy enforcement. When your software needs to make policy decisions, it queries OPA and supplies structured data, such as JSON, as input. The data can be inspected and transformed using OPA’s native query language Rego. OPA generates policy decisions by evaluating the query input and against policies and data.
Prisma Cloud operationalizes OPA by:
  • Extending Console to manage and compose policies in Rego.
  • Integrating OPA’s decision-making library into Defender.
  • Connecting Defender’s enforcement capabilities to OPA’s decisions.

Admission webhook

An admission controller is code that intercepts requests to the API server for creating objects. There are two types of admission controllers: built-in and dynamic. Prisma Cloud implements a dynamic admission controller.
Dynamic admission controllers are built as webhooks. After registering to intercept admission requests, they assess requests against policy, and then accept or reject those requests. In Kubernetes terms, these are known as validating admission webhooks.
The Prisma Cloud validating admission webhook handles the API server’s AdmissionReview requests, and returns decisions in an AdmissionReview object. When configuring Prisma Cloud, you’ll create a ValidatingWebookConfiguration object, which sets up the Defender service to intercept all create, update, and connect calls to the API server.
The default ValidatingWebookConfiguration provided here sets failurePolicy to Ignore. The failure policy specifies how your cluster handles unrecognized errors and timeout errors from the admission webhook. When set to Ignore, the API request is allowed to continue.

Configuring the webhook

Configure the API server to route AdmissionReview requests to Prisma Cloud.
  • You have a running instance of Prisma Cloud Compute Console.
  • You have a Kubernetes cluster. Minimum supported version is v1.16.
  • Defender has been deployed to your cluster as a DaemonSet. In Console, you can verify Defenders are running and connected under
    Manage > Defenders > Manage
  1. Go to
    Defend > Access > Admission
  2. Enable admission control.
  3. Click the
    Admission controller
    link and
    Copy configuration
    and save it to a file. Nme this file webhook.yaml.
    If the Defender CA has been rotated and the old certificate still hasn’t expired, you may have Defenders using an old certificate. For daemonset which its Defenders are using an old certificate, you need to retrieve the old Defender CA certificate from the daemonset yaml file you deployed this daemonset with.
    Search for defender-ca.pem within the daemonset yaml, copy its content, then paste it to replace the content of the caBundle field of the webhook. If defender-ca.pem doesn’t exist in the daemonset yaml, use the content of the ca.pem field.
    If you don’t have the yaml file you used to deploy the daemonset, you can retrieve the old CA bundle from the Console certificates folder under old-defender-ca.pem.
    To identify whether your Defenders are using an old certificate, see Console-Defender communication certificates.
  4. Create the webhook configuration object.
    $ kubectl apply -f webhook.yaml
    After creating the object, the Kubernetes API server directs AdmissionReview requests to Defender.

Validating your setup

Validate that your webhook has been properly set up with one of the predefined admission rules.
The order in which the rules appear is the order in which they are evaluated. Higher rules take precedence over lower rules. Rules can be reordered. Use the hamburger icon to drag and drop rules into the right place.
Notice that the processing of rules stops at the first match. To make sure the severe action will be taken in a case of more than one rule match, place the rules with action "Block" first.
  1. Navigate to
    Defend > Access > Admission
    and verify there exist default admission rules and they are all enabled by default.
  2. Create the following YAML file to test the
    Twistlock Labs - CIS - Privileged pod created
    1. Create the following YAML file:
      apiVersion: v1 kind: Pod metadata: name: nginx labels: app: nginx spec: containers: - name: nginx image: nginx ports: - containerPort: 80 securityContext: privileged: true
  3. Create the privileged pod.
    $ kubectl apply -f priv-pod.yaml
  4. Verify an audit is created under
    Monitor > Events > Admission Audits
  5. Clean up. Delete the pod.
    kubectl delete -f priv-pod.yaml

Creating custom admission rules

Use Rego syntax to create custom rules. To learn more about the syntax, review the predefined rules that ship with Prisma Cloud. Rules scripts are based on the admission review input JSON structure. For more information, see: https://github.com/kubernetes/api/blob/master/admission/v1beta1/types.go.


The following examples should give you some ideas about how you can create your own policies by using the Rego language.
Do not allow new namespaces to be created:
match[{"msg": msg}] { input.request.operation == "CREATE" input.request.kind.kind == "Namespace" msg := "It's not allowed to create new namespace!" }
Do not allow a specific image (for example nginx) in new pods:
match[{"msg": msg}] { input.request.operation == "CREATE" input.request.kind.kind == "Pod" input.request.resource.resource == "pods" input.request.object.spec.containers[_].image == "nginx" msg := "It's not allowed to use the nginx Image!" }
Do not allow new pods to expose TCP port 80:
match[{"msg": msg}] { input.request.operation == "CREATE" input.request.kind.kind == "Pod" input.request.resource.resource == "pods" input.request.object.spec.containers[_].ports[_].containerPort == 80 msg := "It's not allowed to use port 80 (HTTP) with a Pod configuration!" }
Control the scope of your the policy rules by checking the object’s metadata, such as namespace or labels.
Do not allow new pods in namespace sock-shop without the owner label:
match[{"msg": msg}] { input.request.operation == "CREATE" input.request.kind.kind == "Pod" input.request.resource.resource == "pods" input.request.object.metadata.namespace == "sock-shop" not input.request.metadata.labels.owner msg := "Pod in namespace sock-shop is missing the owner label" }

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