Allowlist-based vulnerability scanning with Container Analysis

This tutorial describes how to build and use Kritis Signer with Container Analysis to check for vulnerabilities in container images built with Cloud Build. Kritis Signer checks identified vulnerabilities against a vulnerability policy. The Cloud Build build step fails if an identified vulnerability violates the vulnerability policy.


When securing your container-based software supply chain, it can be critically important to prevent a vulnerable container image from being deployed. Container Analysis provides vulnerability scanning services for container images. Kritis Signer is an open-source custom builder for Cloud Build that runs as a build step and uses Container Analysis to identify vulnerabilities in a container image. It then checks those vulnerabilities against a vulnerability policy. If any identified vulnerabilities violate the Kritis Signer vulnerability policy, the build fails. In the context of Binary Authorization, you can add a Kritis Signer build step to a container image build pipeline so that a policy violation would fail the build before an attestation is created.

In this tutorial you:

  1. Clone the Kritis repository.
  2. Build the Kritis Signer custom builder that scans for vulnerabilities with Container Analysis.
  3. View a vulnerability policy.
  4. Submit sample builds to Cloud Build that build a container image and check it for vulnerabilities.

    • The first sample container image build, cloudbuild-bad.yaml, is a failure case. In this case, Kritis Signer finds vulnerabilities in container image that violates the vulnerability policy. The build step fails, terminating the build pipeline.

    • The second sample container image build, cloudbuild-good.yaml, is a success case. In this case, Kritis Signer should not find a vulnerability in container image that violates your vulnerability policy. The build step should succeed.

Products used in this build

This tutorial uses the following Google Cloud and open source products:

  • Cloud Build is a Google Cloud product that executes your builds on Google Cloud infrastructure. Cloud Build executes your build as a series of build steps, where each build step is run in a Docker container. In this tutorial, the Kritis Signer is built into a Cloud Build custom builder that can be used thereafter in your build pipelines. Cloud Build is also used to build the example success and failure cases, demonstrating how to include vulnerability scanning into your build pipelines.

  • Kritis is an open source solution for securing your software supply chain for Kubernetes applications. Kritis Signer is a Cloud Build custom builder that checks vulnerabilities identified by Container Analysis against a vulnerability policy.

  • Container Registry is a Google Cloud private container image registry that runs on Google Cloud. In this tutorial, the Kritis Signer custom builder and the sample container images are stored in Container Registry.

  • Container Analysis is a Google Cloud product that provides vulnerability scanning and metadata storage for software artifacts. The service performs vulnerability scans on built software artifacts, such as the images in Container Registry, then stores the resulting metadata and makes it available for consumption through an API. In this tutorial, Kritis Signer calls Container Analysis to identify vulnerabilities in container images.

Set up

In this section, you perform a one-time setup of the system. This includes the following steps:

  1. Set up your environment.
  2. Enable APIs.
  3. Set permissions on the service account Cloud Build uses to read from Container Analysis.
  4. Clone the Kritis repo.
  5. Build the Kritis Signer Cloud Build custom builder.
  6. Edit a vulnerability policy.

In the next section, you execute sample build pipelines with Cloud Build.

Set up your environment

First, set PROJECT_ID to your Google Cloud project. This project is used for the entire tutorial.


Where PROJECT-ID is the ID of the Google Cloud project.

export PROJECT_NUMBER=$(gcloud projects list --filter="${PROJECT_ID}" --format="value(PROJECT_NUMBER)")

Enable APIs

gcloud --project=$PROJECT_ID services enable
gcloud --project=$PROJECT_ID services enable
gcloud --project=$PROJECT_ID services enable
gcloud --project=$PROJECT_ID services enable

Set up permissions

Run the following command to give the Cloud Build service account read access to vulnerability information from Container Analysis.

gcloud projects add-iam-policy-binding $PROJECT_ID --member serviceAccount:$ --role roles/containeranalysis.notes.occurrences.viewer

Create the Kritis Signer custom builder

Run the following commands to get the code and configuration files used in this tutorial.

  1. Clone the Kritis repo

    git clone --branch policy-check-v1.0.0

    This repository contains the following:

    • Open source for Kritis.
    • Build configuration files used by Cloud Build to build the Kritis Signer custom builder.
    • An example vulnerability policy.
    • Example Cloud Build build configuration files that demonstrate Kritis Signer vulnerability scanning in a build pipeline.
  2. Navigate into the /kritis directory:

    cd kritis
  3. Submit the Kritis Signer custom builder to your project

    This one-time step builds a custom builder that is used in your vulnerability pipelines.

    gcloud --project=$PROJECT_ID builds submit . --config deploy/kritis-signer/cloudbuild.yaml

Edit the vulnerability signing policy

This section describes the vulnerability policy Kritis Signer enforces. Edit this policy to determine allowable severity levels and list specific allowable vulnerabilities.

To view the vulnerability policy:

cat samples/policy-check/policy.yaml

It should look like this:

kind: VulnzSigningPolicy
  name: my-vsp
    maximumFixableSeverity: MEDIUM
    maximumUnfixableSeverity: MEDIUM
    - projects/goog-vulnz/notes/CVE-2020-10543
    - projects/goog-vulnz/notes/CVE-2020-10878
    - projects/goog-vulnz/notes/CVE-2020-14155


  • allowlistCVEs is list of vulnerabilities. Note that each entry must be an exact match of the note name.
  • maximumUnfixableSeverity and maximumFixableSeverity each of these may be one of:

    • HIGH
    • MEDIUM
    • LOW
    • BLOCK_ALL - Fails if any vulnerability is identified
    • ALLOW_ALL - Always succeeds

Test the Kritis Signer custom builder

In this section, you submit example builds to Cloud Build.

In the following steps, two container image builds are submitted to Cloud Build:

  • Failure case: The first build creates a container image that contains a vulnerability that triggers a vulnerability policy violation. This build should fail.
  • Success case: The second build creates a container image that contains vulnerabilities that should be allowed by the vulnerability policy. This build should succeed.

Both example build configuration files contain the following steps:

  1. The build step builds a Docker container image.
  2. The push step pushes the newly built container image to Container Registry.
  3. The vulnsign step analyzes the container image by:
    1. Waiting for Container Analysis to return vulnerability findings on the newly built container image.
    2. Checking the findings against the vulnerability policy.
    3. Succeeding or failing based on policy compliance.

Submit the failure-case sample build

This example builds a container image based on Debian 9. At present, the container has a vulnerability that is not allowed by the vulnerability policy described above. The Kritis Signer custom builder will fail the vulnsign build step.

To view the contents of the build file, enter the following command:

cat samples/policy-check/cloudbuild-bad.yaml

Run the build pipeline

gcloud  --project=$PROJECT_ID builds submit --config=samples/policy-check/cloudbuild-bad.yaml samples/policy-check

You should see something like the following:

ERROR: (gcloud.builds.submit) build [BUILD-ID] completed with status "FAILURE"

Note the [BUILD-ID] value and set the $BUILD_ID environment variable for the next step.


Alternatively, you can find the build ID of your last build by running the following command:

gcloud builds list --limit 1

The build ID is the first returned field. Alter the --limit value to see more builds, or remove the flag altogether to view all builds.

Verify the result

gsutil cat gs://${PROJECT_NUMBER}${BUILD_ID}.txt | grep "does not pass VulnzSigningPolicy"

Submit the success-case sample build

To submit the success-case sample build to Cloud Build, do the following:

gcloud --project=$PROJECT_ID builds submit --config=samples/policy-check/cloudbuild-good.yaml samples/policy-check

Again, make sure to note the build ID and store it in $BUILD_ID environment variable:


Finally, to verify the result, enter the following command:

gcloud --project=$PROJECT_ID builds describe $BUILD_ID | grep status

What's next