# Build attestations 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: ```bash # 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`](/build/building/variables/#buildx_no_default_attestations) environment variable. See [Provenance attestation](/build/metadata/slsa-provenance/) for more details about provenance modes and options. ## Storage BuildKit produces attestations in the [in-toto format](https://github.com/in-toto/attestation), as defined by the [in-toto framework](https://in-toto.io/), 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. ```json { "_type": "https://in-toto.io/Statement/v0.1", "predicateType": "https://spdx.dev/Document", "subject": [ { "name": "pkg:docker//@?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 ", "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)](/build/metadata/attestations/attestation-storage/). ## 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: ```json { "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](/build/exporters/image-registry/#synopsis) 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. [empty descriptor]: https://github.com/opencontainers/image-spec/blob/main/manifest.md#guidance-for-an-empty-descriptor The following example shows an attestation with the OCI artifact format: ```json { "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: - [Provenance](/build/metadata/attestations/slsa-provenance/) - [SBOM](/build/metadata/attestations/sbom/)