Your pipeline signs every container image. Is your supply chain secure?

What the interviewer is testing

Whether you know the gap between signing and provenance, which is what the significant supply-chain compromises exploited.

The gap

Signing proves the artifact has not been altered since signing. It says nothing about whether what was signed is what you intended.

The major compromises of recent years worked precisely here: the malicious artifact was legitimately signed, because the attacker compromised the build process rather than the signing key. Every verification passed.

What closes it

Provenance attestation. The build system produces a signed statement recording the source repository and commit, the build definition, the inputs consumed, and the identity of the builder.

Verification of the claim's content at admission, not just the signature: this image must come from this repository, built by this pipeline, from a protected branch. That is what turns provenance into a control.

Non-forgeable generation. The provenance must be produced by the build service, not by the build itself — otherwise a compromised build step can assert whatever it likes. This is the distinction the SLSA levels formalise, and it is the one that matters most.

The step most estates skip

Verification at deployment. Generating attestations that nothing checks is a cryptographic paper trail with no control attached — and it is common, because generating them is a pipeline change while verifying them means being willing to block a deployment.

The admission policy is the control. Everything upstream is evidence for it.

What else the supply chain needs

Pinned dependencies with a committed lockfile. Reproducible builds, or the attestation cannot be independently checked. Scoped, short-lived build credentials, since a CI system with deployment credentials is production access with weaker controls. And an SBOM per artifact, stored and queryable across the estate.

What a strong answer adds

Protecting the build environment itself as the highest-value control: ephemeral, isolated runners with no persistent state between builds, no network access after dependency resolution, and no shared credentials. Most of the attestation apparatus assumes the builder is trustworthy.

Common weak answers

Confirming security based on signing. Focusing on key storage, which protects against a different threat.