Chainguard Image for cosign

Minimalist Wolfi-based Cosign images for signing and verifying images using Sigstore.

Chainguard Images are regularly-updated, minimal container images with low-to-zero CVEs.

Download this Image

This image is available on cgr.dev:

docker pull cgr.dev/ORGANIZATION/cosign:latest

Be sure to replace the ORGANIZATION placeholder with the name used for your organization's private repository within the Chainguard registry.

Compatibility Notes

Like most other Chainguard Images, the Cosign image has few-to-zero CVEs and does not run as the root user.

Getting Started

You can use the Cosign image to sign and verify container images via Docker or through automated workflows such as GitHub Actions.

Verifying a container image signature

To verify an image signature, use the image to run Cosign's verify command. As of Cosign 2.0, Cosign defaults to using Sigstore's keyless mode, you'll need to also specify the OIDC issuer and signer identity to tell Cosign who you trust for the verification process.

For convenience, you can export those values as environment variables in your shell, and then tell Docker to pass those environment variables into the running Cosign container:

export COSIGN_CERTIFICATE_OIDC_ISSUER=https://token.actions.githubusercontent.com
export COSIGN_CERTIFICATE_IDENTITY=https://github.com/chainguard-images/images/.github/workflows/release.yaml@refs/heads/main

The following example uses the Cosign image to verify the signature of the Cosign image itself:

docker run --rm \
  -e COSIGN_CERTIFICATE_OIDC_ISSUER \
  -e COSIGN_CERTIFICATE_IDENTITY \
  cgr.dev/chainguard/cosign \
  verify cgr.dev/chainguard/cosign

Signing a container image

The following GitHub Action will log into the registry, push an image, and then sign it with Cosign:

on:
  workflow_dispatch:

env:
  IMAGE: ghcr.io/${{ github.repository }}
  DOCKER_CONFIG: .docker-tmp
jobs:
  push-and-sign:
    runs-on: ubuntu-latest
    permissions:
      id-token: write
      packages: write
    steps:
      - name: Log in to registry
        run: |
          set -x
          mkdir -p "${DOCKER_CONFIG}"
          echo '{}' > "${DOCKER_CONFIG}/config.json"
          echo "${{ github.token }}" | docker login \
            -u "${{ github.repository_owner }}" \
            --password-stdin ghcr.io
      - name: Push image with docker
        run: |
          set -x
          docker pull cgr.dev/chainguard/wolfi-base
          docker tag cgr.dev/chainguard/wolfi-base "${IMAGE}"
          docker push "${IMAGE}"
      - name: Sign image with cosign
        run: |
          set -x
          env | grep -v ^HOME= > github-actions.txt
          docker run --rm --env-file=./github-actions.txt \
            -v "${PWD}/${DOCKER_CONFIG}:/tmp/${DOCKER_CONFIG}" \
            -e DOCKER_CONFIG="/tmp/${DOCKER_CONFIG}" \
            cgr.dev/chainguard/cosign \
            sign "${IMAGE}" \
              --yes \
              -a sha=${{ github.sha }} \
              -a run_id=${{ github.run_id }} \
              -a run_attempt=${{ github.run_attempt }}

Attesting to an image's SBOM

Another common use case for Cosign is attesting to an image's SBOM (software bill of materials). The following command creates an attestation of type spdxjson for an image based on the provided SBOM file and image digest:

docker run --rm cgr.dev/chainguard/cosign attest --type spdxjson \
 --predicate example-image.spdx.json \
 sha256:7f95c5aa7e789ad51737e03021586949346f38ced1533ecdcb2ef2b9104137ac

A detailed guide on how to sign an SBOM with Cosign is available on Chainguard Academy.

Verifying an Attestation

Cosign can also be used to verify the identity of the person or entity issuing an attestation. To verify that an attestation was issued by a specific entity, we use the cosign verify-attestation command, specifying the email address of the issuer:

cosign verify-attestation \
 --certificate-oidc-issuer=https://github.com/login/oauth \
 --type https://spdx.dev/Document \
 --certificate-identity=emailaddress@emailprovider.com \
 sha256:7f95c5aa7e789ad51737e03021586949346f38ced1533ecdcb2ef2b9104137ac

If the identity is successfully verified, an initial message similar to the following is printed to stderr:

Verification for user/example-image@sha256:545a731e803b917daf44e292b03b427427f8090c4e6c4a704e4c18d56c38539f --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The code-signing certificate was verified using trusted certificate authority certificates
Certificate subject: <you@domain.com>
Certificate issuer URL: https://github.com/login/oauth

Configuration

The following environment variables are required when verifying a container image signature:

• COSIGN_CERTIFICATE_OIDC_ISSUER: Trusted OIDC issuer, example: https://token.actions.githubusercontent.com
• COSIGN_CERTIFICATE_IDENTITY: Trusted certificate identity, example: https://github.com/chainguard-images/images/.github/workflows/release.yaml@refs/heads/main

Detailed Environment Information

To obtain detailed information about the environment, you can run the cosign env command:

docker run --rm cgr.dev/chainguard/cosign env --show-descriptions=false

You'll get output similar to this, containing the environment variables used to configure Cosign:

ACTIONS_ID_TOKEN_REQUEST_TOKEN=
ACTIONS_ID_TOKEN_REQUEST_URL=
BUILDKITE_AGENT_ACCESS_TOKEN=
BUILDKITE_AGENT_ENDPOINT=
BUILDKITE_AGENT_LOG_LEVEL=
BUILDKITE_JOB_ID=
COSIGN_DOCKER_MEDIA_TYPES=
COSIGN_EXPERIMENTAL=
COSIGN_MAX_ATTACHMENT_SIZE=
COSIGN_PASSWORD=
COSIGN_PKCS11_IGNORE_CERTIFICATE=
COSIGN_PKCS11_MODULE_PATH=
COSIGN_PKCS11_PIN=
COSIGN_REPOSITORY=
GITHUB_HOST=
GITHUB_TOKEN=
GITLAB_HOST=
GITLAB_TOKEN=
GOOGLE_SERVICE_ACCOUNT_NAME=
SIGSTORE_CT_LOG_PUBLIC_KEY_FILE=
SIGSTORE_ID_TOKEN=
SIGSTORE_REKOR_PUBLIC_KEY=
SIGSTORE_ROOT_FILE=
SIGSTORE_TSA_CERTIFICATE_FILE=
SOURCE_DATE_EPOCH=
SPIFFE_ENDPOINT_SOCKET=
COSIGN_CERTIFICATE_OIDC_ISSUER=******
COSIGN_CERTIFICATE_IDENTITY=******

Documentation and Resources

• Chainguard Academy: An Introduction to Cosign

• Blog Post: Chainguard announces new Sigstore Images to bring critical software supply chain tooling to enterprises

• Chainguard Academy: How to Sign an SBOM with Cosign

• Chainguard Academy: Verifying Chainguard Images and Metadata Signatures with Cosign

• Video: Signing and Verifying Container Images With Sigstore Cosign and Kyverno

Contact Support

If you have a Zendesk account (typically set up for you by your Customer Success Manager) you can reach out to Chainguard's Customer Success team through our Zendesk portal.

What are Chainguard Images?

Chainguard Images are a collection of container images designed for security and minimalism.

Many Chainguard Images are distroless; they contain only an open-source application and its runtime dependencies. These images do not even contain a shell or package manager. Chainguard Images are built with Wolfi, our Linux undistro designed to produce container images that meet the requirements of a secure software supply chain.

The main features of Chainguard Images include:

• Minimal design, with no unnecessary software bloat
• Automated nightly builds to ensure Images are completely up-to-date and contain all available security patches
• High quality build-time SBOMs (software bills of materials) attesting the provenance of all artifacts within the Image
• Verifiable signatures provided by Sigstore
• Reproducible builds with Cosign and apko (read more about reproducibility)

-dev Variants

As mentioned previously, Chainguard’s distroless Images have no shell or package manager by default. This is great for security, but sometimes you need these things, especially in builder images. For those cases, most (but not all) Chainguard Images come paired with a -dev variant which does include a shell and package manager.

Although the -dev image variants have similar security features as their distroless versions, such as complete SBOMs and signatures, they feature additional software that is typically not necessary in production environments. The general recommendation is to use the -dev variants only to build the application and then copy all application artifacts into a distroless image, which will result in a final container image that has a minimal attack surface and won’t allow package installations or logins.

That being said, it’s worth noting that -dev variants of Chainguard Images are completely fine to run in production environments. After all, the -dev variants are still more secure than many popular container images based on fully-featured operating systems such as Debian and Ubuntu since they carry less software, follow a more frequent patch cadence, and offer attestations for what they include.

Learn More

To better understand how to work with Chainguard Images, we encourage you to visit Chainguard Academy, our documentation and education platform.