Verifying SLSA provenance of Stackable images

Starting with SDP 26.7, all Stackable container images, both product images and operator images, are published with SLSA build provenance attestations.

Provenance is a signed statement describing how an artifact was built: which source repository and commit it was built from, which workflow built it, and on what infrastructure. With it, you can verify that an image you pulled from our registry was really built by Stackable’s CI and it allows you to trace how it was built.

The provenance is generated with the slsa-github-generator container workflow, which meets the requirements of SLSA Level 3: the provenance is created and signed by a trusted builder that is isolated from the project’s own build workflow, so a compromised build job cannot forge it. Just like our SBOM attestations and images, the attestation is signed keylessly via Sigstore (with GitHub Actions as the OIDC identity provider) and published in our OCI registry next to the image.

Verifying provenance with slsa-verifier

The recommended tool for verification is slsa-verifier. It verifies the signature on the provenance and checks that the image was built from the source repository you expect.

slsa-verifier requires the image to be referenced by digest, not by tag. You can obtain the digest of an image with a tool like crane:

crane digest oci.stackable.tech/sdp/kafka:3.9.1-stackable26.7.0

Product images are built from the docker-images repository:

slsa-verifier verify-image \
  oci.stackable.tech/sdp/kafka@sha256:... \
  --source-uri github.com/stackabletech/docker-images

Operator images are built from their respective operator repository:

slsa-verifier verify-image \
  oci.stackable.tech/sdp/kafka-operator@sha256:... \
  --source-uri github.com/stackabletech/kafka-operator

On success, slsa-verifier prints PASSED: SLSA verification passed and exits with code 0. It can also print the full verified provenance with --print-provenance, which is useful if you want to inspect the exact commit and workflow the image was built from.

Verifying provenance with cosign

Alternatively, cosign can verify and extract the provenance attestation. Two things to note:

The certificate identity is the trusted builder workflow of the slsa-github-generator project, not a Stackable workflow, because the provenance is deliberately signed by that isolated builder.
The slsa-github-generator attaches the provenance in the legacy attestation format, so cosign 3 needs the --new-bundle-format=false flag to find it.

cosign verify-attestation --type slsaprovenance \
--new-bundle-format=false \
--certificate-identity-regexp \
'^https://github.com/slsa-framework/slsa-github-generator/.github/workflows/generator_container_slsa3.yml@refs/tags/v.+' \
--certificate-oidc-issuer https://token.actions.githubusercontent.com \
oci.stackable.tech/sdp/kafka@sha256:... \
| jq '.payload' -r | base64 -d | jq '.predicate'

The extracted predicate contains, among other things, the source repository and commit digest under invocation.configSource and the builder identity under builder.id. If you use cosign directly, make sure to check these fields yourself; slsa-verifier does this for you.