Verify a Docker Hardened Image or chart

もくじ

Docker Hardened Images (DHI) and charts include signed attestations that verify the build process, contents, and security posture. These attestations are available for each image variant and chart and can be verified using cosign or the Docker Scout CLI.

Docker's public key for DHI images and charts is published at:

重要

You must authenticate to the Docker Hardened Image registry (dhi.io) to pull images. To do this, you can use docker login:

$ docker login dhi.io

Verify image attestations with Docker Scout

You can use the Docker Scout CLI to list and retrieve attestations for Docker Hardened Images.

メモ

Before you run docker scout attest commands, ensure any image that you have pulled locally is up to date with the remote image. You can do this by running docker pull. If you don't do this, you may see No attestation found.

Why use Docker Scout instead of cosign directly?

While you can use cosign to verify attestations manually, the Docker Scout CLI offers several key advantages when working with Docker Hardened Images and charts:

  • Purpose-built experience: Docker Scout understands the structure of DHI attestations and naming conventions, so you don't have to construct full digests or URIs manually.

  • Automatic platform resolution: With Scout, you can specify the platform (e.g., --platform linux/amd64), and it automatically verifies the correct image variant. Cosign requires you to look up the digest yourself.

  • Human-readable summaries: Scout returns summaries of attestation contents (e.g., package counts, provenance steps), whereas cosign only returns raw signature validation output.

  • One-step validation: The --verify flag in docker scout attest get validates the attestation and shows the equivalent cosign command, making it easier to understand what's happening behind the scenes.

  • Integrated with Docker Hub and DHI trust model: Docker Scout is tightly integrated with Docker’s attestation infrastructure and public keyring, ensuring compatibility and simplifying verification for users within the Docker ecosystem.

In short, Docker Scout streamlines the verification process and reduces the chances of human error, while still giving you full visibility and the option to fall back to cosign when needed.

List available attestations

To list attestations for a mirrored DHI image:

メモ

If the image exists locally on your device, you must prefix the image name with registry://. For example, use registry://dhi.io/python:3.13 instead of dhi.io/python:3.13.

$ docker scout attest list dhi.io/<image>:<tag>

This command shows all available attestations, including SBOMs, provenance, vulnerability reports, and more.

Retrieve a specific attestation

To retrieve a specific attestation, use the --predicate-type flag with the full predicate type URI:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  dhi.io/<image>:<tag>
メモ

If the image exists locally on your device, you must prefix the image name with registry://. For example, use registry://dhi.io/python:3.13 instead of dhi.io/python:3.13.

For example:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  dhi.io/python:3.13

To retrieve only the predicate body:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  --predicate \
  dhi.io/<image>:<tag>

For example:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  --predicate \
  dhi.io/python:3.13

Validate the attestation with Docker Scout

To validate the attestation using Docker Scout, you can use the --verify flag:

$ docker scout attest get dhi.io/<image>:<tag> \
   --predicate-type https://scout.docker.com/sbom/v0.1 --verify
メモ

If the image exists locally on your device, you must prefix the image name with registry://. For example, use registry://dhi.io/node:20.19-debian12 instead of dhi.io/node:20.19-debian12.

For example, to verify the SBOM attestation for the dhi.io/node:20.19-debian12 image:

$ docker scout attest get dhi.io/node:20.19-debian12 \
   --predicate-type https://scout.docker.com/sbom/v0.1 --verify

Handle missing transparency log entries

When using --verify, you may sometimes see an error like:

ERROR no matching signatures: signature not found in transparency log

This occurs because Docker Hardened Images don't always record attestations in the public Rekor transparency log. In cases where an attestation would contain private user information (for example, your organization's namespace in the image reference), writing it to Rekor would expose that information publicly.

Even if the Rekor entry is missing, the attestation is still signed with Docker's public key and can be verified offline by skipping the Rekor transparency log check.

To skip the transparency log check and validate against Docker's key, use the --skip-tlog flag:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  dhi.io/<image>:<tag> \
  --verify --skip-tlog
メモ

The --skip-tlog flag is only available in Docker Scout CLI version 1.18.2 and later.

If the image exists locally on your device, you must prefix the image name with registry://. For example, use registry://dhi.io/python:3.13 instead of dhi.io/python:3.13.

This is equivalent to using cosign with the --insecure-ignore-tlog=true flag, which validates the signature against Docker's published public key, but ignores the transparency log check.

Show the equivalent cosign command

When using the --verify flag, it also prints the corresponding cosign command to verify the image signature:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  --verify \
  dhi.io/<image>:<tag>
メモ

If the image exists locally on your device, you must prefix the image name with registry://. For example, use registry://dhi.io/python:3.13 instead of dhi.io/python:3.13.

For example:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  --verify \
  dhi.io/python:3.13

If verification succeeds, Docker Scout prints the full cosign verify command.

Example output:

    v SBOM obtained from attestation, 101 packages found
    v Provenance obtained from attestation
    v cosign verify ...
重要

When using cosign, you must first authenticate to both the DHI registry and the Docker Scout registry.

For example:

$ docker login dhi.io
$ docker login registry.scout.docker.com
$ cosign verify ...

Verify Helm chart attestations with Docker Scout

Docker Hardened Image Helm charts include the same comprehensive attestations as container images. The verification process for charts is identical to that for images, using the same Docker Scout CLI commands.

List available chart attestations

To list attestations for a DHI Helm chart:

$ docker scout attest list oci://dhi.io/<chart>:<version>

For example, to list attestations for the Redis HA chart:

$ docker scout attest list oci://dhi.io/redis-ha-chart:0.1.0

This command shows all available chart attestations, including SBOMs, provenance, vulnerability reports, and more.

Retrieve a specific chart attestation

To retrieve a specific attestation from a Helm chart, use the --predicate-type flag with the full predicate type URI:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  oci://dhi.io/<chart>:<version>

For example:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  oci://dhi.io/redis-ha-chart:0.1.0

To retrieve only the predicate body:

$ docker scout attest get \
  --predicate-type https://cyclonedx.org/bom/v1.6 \
  --predicate \
  oci://dhi.io/<chart>:<version>

Validate chart attestations with Docker Scout

To validate a chart attestation using Docker Scout, use the --verify flag:

$ docker scout attest get oci://dhi.io/<chart>:<version> \
   --predicate-type https://scout.docker.com/sbom/v0.1 --verify

For example, to verify the SBOM attestation for the Redis HA chart:

$ docker scout attest get oci://dhi.io/redis-ha-chart:0.1.0 \
   --predicate-type https://scout.docker.com/sbom/v0.1 --verify

The same --skip-tlog flag described in Handle missing transparency log entries can also be used with chart attestations when needed.

Available DHI attestations

See available attestations for a list of attestations available for each DHI image and Helm chart attestations for a list of attestations available for each DHI chart.

Explore attestations on Docker Hub

You can also browse attestations visually when exploring an image variant. The Attestations section lists each available attestation with its:

  • Type (e.g. SBOM, VEX)
  • Predicate type URI
  • Digest reference for use with cosign

These attestations are generated and signed automatically as part of the Docker Hardened Image or chart build process.