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Image Scanner Operator

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Image Scanner is a Kubernetes Operator supporting detection of vulnerabilities in running container images.

Kubernetes clusters run containers created from a great diversity of container images with several origins. Some images are custom for an in-house application, while others are pulled from internal/external registries. It is considered best practice to scan images in container image build pipelines, to provide developers with early feedback on potential vulnerabilities.

While this is good, it is not enough:

Software vulnerabilities are typically detected after the software is available for use, and some applications will typically "just run" in their runtime environment - with little/no maintenance. And some users might not use pipelines at all or use pipelines without image scanning enabled.

Running applications on container images with vulnerabilities might represent an unacceptable threat. To mend this, we want a mechanism to identify vulnerabilities in running container images. This could then be used by developers, system administrators, platform administrators and security officers to take action when vulnerabilities are detected.

Actions could for instance be:

  • upgrade to a patched version of the vulnerable software component
  • conclude that the vulnerability is not relevant
  • conclude that the vulnerability represents an acceptable risk
  • shut down the application

There seem to be quite a lot of companies/communities providing similar software. Some key features of this operator are:

  • Can be configured to scan container images for any Kubernetes Workload, including custom resources. The only requirement is that the resource MUST own Pods - either directly or indirectly.
  • Container images are scanned based on immutable image sha256 digests obtained from Container Runtime Interface (CRI), not from potentially mutable image tags.
  • To avoid unnecessary image pulls, the image scan workload is preferred scheduled on the same node(s) as the workload to be scanned.
  • Since the image to scan should already be present in the node container registry, the image scan workload does not have to bother with image pull secrets for private images, exploiting a Kubernetes "bug".

Description

// TODO(user): An in-depth paragraph about your project and overview of use

Custom resources

The Image Scanner operator currently defines a single user-facing Custom Resource Definition (CRD), ContainerImageScan (CIS), that represents the Kubernetes API for runtime image scanning of workload container images. See stas_v1alpha1_containerimagescan.yaml for a (simplified) example of a CIS resource.

The CIS resource .spec specifies the container image to scan and some additional workload metadata, and the image scan result is added/updated in .status by the ContainerImageScan controller.

CIS resources should not be edited by standard users, as the Workload controller will create CIS resources from running pods. And the standard Kubernetes garbage collector deletes the obsolete CIS resources when the owning pods are gone.

A user can influence the image scanning process by adding annotations to pods. The set of annotations is currently limited, but more might be added in the future:

Pod annotation key Default value Description
image-scanner.statnett.no/ignore-unfixed "false" If set to "true", the Image Scanner will ignore any detected vulnerability that can't be fix by updating package(s)

Supported features

  • Namespaced container image scan API (custom resource): ContainerImageScan (CIS)
  • A CIS resource contains details and a summary of detected vulnerabilities
  • Users can identify the owning/controlling resource (workload) of the scanned container image
  • All container images are rescanned regularly with configurable interval
  • Provides vulnerability summary metrics from CIS to enable dashboards/alerts
  • Any user with access to a namespace is allowed to view CIS
  • Cluster-scoped operator that operates in configured namespaces with an include/block list of namespaces that should be scanned
  • Supports any type of workload (also CRDs) by configuration
  • Scanning workload images from private/authenticated image registries

Future improvements

  • Push-based feedback to stakeholders
  • Enable users to ignore/suppress vulnerabilities
  • Perform actions from detected vulnerabilities

Eschewed Features

  • Produce metrics for vulnerability details
  • Historical container image scans
  • Helm chart installation

Getting Started

You’ll need a Kubernetes cluster to run the Image Scanner. You can use KIND or k3s to get a local cluster for testing.

We currently only support installation using Kustomize, either as a standalone tool, using the kustomize (-k) feature in recent versions of kubectl or any GitOps tool with support for Kustomize.

Install

Since you probably want to adjust the default configuration (and/or have multiple clusters), we suggest you start by creating a kustomize overlay using the Image Scanner default kustomization as a remote directory base. Your initial kustomization.yaml could be as simple as:

resources:
  - https://github.com/statnett/image-scanner-operator?ref=v0.6.15

If you have multiple clusters, you should create one variant overlay per cluster.

To install (or update) the operator into your cluster run:

kubectl apply --server-side -k <overlay-directory>

Configure

The Image Scanner operator is highly configurable and supports numerous flags with corresponding environment variables. To get an overview over all that can be configured, the easiest is to use the CLI:

docker run ghcr.io/statnett/image-scanner-operator --help

You can also use environment variables for configuration, but a flag takes precedence over the corresponding environment variable. Environment variable names can be deduced from flags by upper-casing and replacing the - delimiter with _.

Since we use kustomize to install the operator, the easiest is to customize the environment variables provided from the ConfigMap in the default Image Scanner configuration using a configMapGenerator.

configMapGenerator:
  - name: image-scanner-config
    behavior: merge
    literals:
      - CIS_METRICS_LABELS=app.kubernetes.io/name
      - SCAN_INTERVAL=24h

This example will override the default configuration to:

  • add metric labels with values obtained from app.kubernetes.io/name Pod labels
  • rescan workload images with an interval of 24 hours

Upgrade

At this early stage, we might introduce breaking changes. But when we do, the breaking changes should be highlighted in the changelog and release notes.

We might also do breaking changes in the CRD(s), without adding the complexity of conversion webhooks. So if you experience any issue when upgrading to a newer version, please try to reinstall the operator as a first step.

Uninstall

To uninstall the operator, just use kubectl to delete all resources produced by the overlay used when installing the operator:

kubectl delete -k --ignore-not-found=true <overlay-directory>

How it works

This project aims to follow the Kubernetes Operator pattern.

It uses Controllers which provides a reconcile function responsible for synchronizing resources until the desired state is reached.

Image Scanner consists of three controllers that coordinates scanning of running container images as illustrated in the diagrams below.

The container image scan Kubernetes API is materialized by the ContainerImageScan custom resources providing an eventually consistent image scanning result in its status.

The actual vulnerability scan of a container image and the vulnerability database is provided by an external service to the operator.

Using a simple Pod, with a single container, as an example:

  1. Create a ContainerImageScan when the immutable image reference is available in the Pod status.
  2. Create a scan Job from immutable image reference in the ContainerImageScan spec.
  3. When a scan Job is completed, read the scan result from pod log of the scan Job, and update the ContainerImageScan status.
  4. When the Pod is deleted, the ContainerImageScan is garbage collected.

Image scanner component diagram) Scan image sequence diagram

Contributing

We would love your feedback on any aspect of the Image Scanner! Feel free to open issues for things you think can be improved. Or/and open a PR (better) to show how we can improve.

See Contributing for information about setting up your local development environment, and the contribution workflow expected.

Please ensure that you are following our Code Of Conduct during any interaction with the community.

License

Licensed under the MIT License.