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Build attestations

Table of contents

Build attestations describe how an image was built, and what it contains. The attestations are created at build-time by BuildKit, and become attached to the final image as metadata.

The purpose of attestations is to make it possible to inspect an image and see where it comes from, who created it and how, and what it contains. This enables you to make informed decisions about how an image impacts the supply chain security of your application. It also enables the use of policy engines for validating images based on policy rules you've defined.

Two types of build attestations are available:

  • Software Bill of Material (SBOM): list of software artifacts that an image contains, or that were used to build the image.
  • Provenance: how an image was built.

Purpose of attestations

The use of open source and third-party packages is more widespread than ever before. Developers share and reuse code because it helps increase productivity, allowing teams to create better products, faster.

Importing and using code created elsewhere without vetting it introduces a severe security risk. Even if you do review the software that you consume, new zero-day vulnerabilities are frequently discovered, requiring development teams take action to remediate them.

Build attestations make it easier to see the contents of an image, and where it comes from. Use attestations to analyze and decide whether to use an image, or to see if images you are already using are exposed to vulnerabilities.

Creating attestations

BuildKit generates the attestations when building the image. Provenance attestations with the mode=min level are added to images by default. The attestation records are wrapped in the in-toto JSON format and attached to the image index in a manifest for the final image.

You can customize attestation behavior using the --provenance and --sbom flags:

# Opt in to SBOM attestations:
docker buildx build --sbom=true .
# Opt in to max-level provenance attestations:
docker buildx build --provenance=mode=max .
# Opt out of provenance attestations:
docker buildx build --provenance=false .

You can also disable default provenance attestations by setting the BUILDX_NO_DEFAULT_ATTESTATIONS environment variable. See Provenance attestation for more details about provenance modes and options.

Storage

BuildKit produces attestations in the in-toto format, as defined by the in-toto framework, a standard supported by the Linux Foundation.

Attestations attach to images as a manifest in the image index. The data records of the attestations are stored as JSON blobs.

Because attestations attach to images as a manifest, it means that you can inspect the attestations for any image in a registry without having to pull the whole image.

All BuildKit exporters support attestations. The local and tar can't save the attestations to an image manifest, since it's outputting a directory of files or a tarball, not an image. Instead, these exporters write the attestations to one or more JSON files in the root directory of the export.

Example

The following example shows a truncated in-toto JSON representation of an SBOM attestation.

{
  "_type": "https://in-toto.io/Statement/v0.1",
  "predicateType": "https://spdx.dev/Document",
  "subject": [
    {
      "name": "pkg:docker/<registry>/<image>@<tag/digest>?platform=<platform>",
      "digest": {
        "sha256": "e8275b2b76280af67e26f068e5d585eb905f8dfd2f1918b3229db98133cb4862"
      }
    }
  ],
  "predicate": {
    "SPDXID": "SPDXRef-DOCUMENT",
    "creationInfo": {
      "created": "2022-12-15T11:47:54.546747383Z",
      "creators": ["Organization: Anchore, Inc", "Tool: syft-v0.60.3"],
      "licenseListVersion": "3.18"
    },
    "dataLicense": "CC0-1.0",
    "documentNamespace": "https://anchore.com/syft/dir/run/src/core-da0f600b-7f0a-4de0-8432-f83703e6bc4f",
    "name": "/run/src/core",
    // list of files that the image contains, e.g.:
    "files": [
      {
        "SPDXID": "SPDXRef-1ac501c94e2f9f81",
        "comment": "layerID: sha256:9b18e9b68314027565b90ff6189d65942c0f7986da80df008b8431276885218e",
        "fileName": "/bin/busybox",
        "licenseConcluded": "NOASSERTION"
      }
    ],
    // list of packages that were identified for this image:
    "packages": [
      {
        "name": "busybox",
        "originator": "Person: Sören Tempel <soeren+alpine@soeren-tempel.net>",
        "sourceInfo": "acquired package info from APK DB: lib/apk/db/installed",
        "versionInfo": "1.35.0-r17",
        "SPDXID": "SPDXRef-980737451f148c56",
        "description": "Size optimized toolbox of many common UNIX utilities",
        "downloadLocation": "https://busybox.net/",
        "licenseConcluded": "GPL-2.0-only",
        "licenseDeclared": "GPL-2.0-only"
        // ...
      }
    ],
    // files-packages relationship
    "relationships": [
      {
        "relatedSpdxElement": "SPDXRef-1ac501c94e2f9f81",
        "relationshipType": "CONTAINS",
        "spdxElementId": "SPDXRef-980737451f148c56"
      },
      ...
    ],
    "spdxVersion": "SPDX-2.2"
  }
}

To deep-dive into the specifics about how attestations are stored, see Image Attestation Storage (BuildKit).

Attestation manifest format

Attestations are stored as manifests, referenced by the image's index. Each attestation manifest refers to a single image manifest (one platform-variant of the image). Attestation manifests contain a single layer, the "value" of the attestation.

The following example shows the structure of an attestation manifest:

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "config": {
    "mediaType": "application/vnd.oci.image.config.v1+json",
    "size": 167,
    "digest": "sha256:916d7437a36dd0e258e64d9c5a373ca5c9618eeb1555e79bd82066e593f9afae"
  },
  "layers": [
    {
      "mediaType": "application/vnd.in-toto+json",
      "size": 1833349,
      "digest": "sha256:3138024b98ed5aa8e3008285a458cd25a987202f2500ce1a9d07d8e1420f5491",
      "annotations": {
        "in-toto.io/predicate-type": "https://spdx.dev/Document"
      }
    }
  ]
}

Attestations as OCI artifacts

You can configure the format of the attestation manifest using the oci-artifact option for the image and registry exporters. If set to true, the structure of the attestation manifest changes as follows:

  • An artifactType field is added to the attestation manifest, with a value of application/vnd.docker.attestation.manifest.v1+json.
  • The config field is an empty descriptor instead of a "dummy" config.
  • A subject field is also added, pointing to the image manifest that the attestation refers to.

The following example shows an attestation with the OCI artifact format:

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "artifactType": "application/vnd.docker.attestation.manifest.v1+json",
  "config": {
    "mediaType": "application/vnd.oci.empty.v1+json",
    "size": 2,
    "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a",
    "data": "e30="
  },
  "layers": [
    {
      "mediaType": "application/vnd.in-toto+json",
      "size": 2208,
      "digest": "sha256:6d2f2c714a6bee3cf9e4d3cb9a966b629efea2dd8556ed81f19bd597b3325286",
      "annotations": {
        "in-toto.io/predicate-type": "https://slsa.dev/provenance/v0.2"
      }
    }
  ],
  "subject": {
    "mediaType": "application/vnd.oci.image.manifest.v1+json",
    "size": 1054,
    "digest": "sha256:bc2046336420a2852ecf915786c20f73c4c1b50d7803aae1fd30c971a7d1cead",
    "platform": {
      "architecture": "amd64",
      "os": "linux"
    }
  }
}

What's next

Learn more about the available attestation types and how to use them: