Verifying Image Signatures during Deployment

In this lesson, you will learn how to automate the validation of container image signatures at the point of deployment to prevent unauthorized or tampered code from running in your cluster.

The Role of Admission Controllers

Previously, we learned how to sign images using Cosign. However, signing an image is only half the battle; if your cluster blindly pulls any image from a registry, a malicious actor could bypass your security by pushing an unsigned image. To solve this, we use an Admission Controller.

Think of an Admission Controller as a security guard at the gate of your Kubernetes cluster. Every time a request is made to create a Pod, the request must pass through the Admission Controller. By configuring a tool like Kyverno or Policy Agent, we can instruct the cluster to intercept the "Create Pod" request, inspect the image's cryptographic signature, and compare it against our trusted public key.

Configuring Enforcement

When you implement an image verification policy, the controller performs a handshake with the image registry. If the signature is missing, invalid, or signed by an untrusted key, the Admission Controller rejects the request entirely. The Pod will never reach the Pending state, effectively stopping the attack before it touches your infrastructure.

Here is a conceptual example of a validation rule you might apply to your cluster:

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: verify-image-signatures
spec:
  validationFailureAction: Enforce
  rules:
    - name: check-signature
      match:
        resources:
          kinds:
            - Pod
      verifyImages:
        - imageReferences:
            - "my-registry.com/my-app:*"
          attestors:
            - entries:
                - keys:
                    publicKeys: |-
                      -----BEGIN PUBLIC KEY-----
                      (Your trusted public key here)
                      -----END PUBLIC KEY-----

By setting validationFailureAction to Enforce, you ensure that the cluster strictly rejects any deployment that fails the signature check. This creates a "secure-by-default" environment where only verified artifacts are allowed to exist in your production ecosystem.

Key Takeaway: Admission controllers act as the final enforcement layer that blocks the execution of any container image that lacks a verifiable, trusted cryptographic signature.