sigstore-cosign-supply-chain-review

# Sigstore Cosign Supply Chain Review ## Purpose Review Cosign image signing verification, Kyverno imageVerify admission policy, SBOM and SLSA provenance attestations, Rekor transparency log posture, and keyless vs key-based signing configuration against supply chain integrity, SLSA level claims, and Kubernetes admission-time enforcement. Sigstore's security model depends entirely on the identity constraints baked into admission policy — an imageVerify rule with no issuer or subject constraint is functionally equivalent to no verification at all. ## Lean operating rules - Prefer live evidence (`cosign verify`, `kubectl get clusterpolicies`, `cosign verify-attestation`) when the active client exposes it; otherwise fall back to official Sigstore documentation and sanitized YAML from the user. - Separate confirmed facts from inference. If Kyverno policy state, Rekor log inclusion, or provenance attestation presence was not directly queried, say so. - Treat a Kyverno imageVerify policy missing both `issuer` and `subject` constraints as a critical finding — any Sigstore-signed image from any identity passes. - Treat `exclude` rules in imageVerify that match broad glob patterns (`*` or `registry.io/*`) as a high finding — third-party images bypass verification. - Treat SLSA L2+ claimed but no SLSA provenance attestation verifiable via `slsa-verifier` as a high finding. - Treat long-lived Cosign keypairs stored as CI secrets as a high finding — keyless OIDC Workload Identity is the preferred pattern. - Treat `COSIGN_NO_TLOG=1` on non-private-Rekor setups as a medium finding — public transparency is disabled without a private transparency alternative. - Keep the answer scoped, evidence-labeled, and explicit about what was not queried. ## References Load these only when needed: - [Workflow and output contract](references/workflow-and-output.md) ## Response minimum Return, at minimum: - the scoped target (image, imageVerify policy, CI pipeline signing step, or SLSA level claim) and evidence level, - the signing identity (keyless OIDC via Fulcio, long-lived key, or unverified), - the admission enforcement posture (Kyverno imageVerify, policy-controller, or none), - the attestation inventory (SBOM present/absent, SLSA provenance present/absent), - the Rekor transparency posture (public log, private log, or disabled), - the safest next actions and any assumptions or blockers.