Software Supply Chain Defense: SLSA L3, Cosign & Kyverno

The Software Supply Chain Threat

Modern software delivery pipelines are prime targets for nation-state adversaries. Attacking the build system or package repository allows adversaries to inject malicious code into trusted downstream artifacts (e.g. SolarWinds, Codecov).

SLSA Framework (Supply-chain Levels for Software Artifacts)

• SLSA Level 1: Documented build process with provenance generated.
• SLSA Level 2: Hosted build platform with authenticated provenance signatures.
• SLSA Level 3: Isolated, hermetic build environments preventing external tampering during compilation, with tamper-evident cryptographic provenance.

Keyless Signing with Sigstore Cosign

Instead of managing private GPG keys, Sigstore Cosign uses keyless signing:

1. The CI workflow initiates an OIDC handshake with Fulcio CA.
2. Fulcio issues a short-lived (10-minute) X.509 certificate binding the builder's identity to an ephemeral key pair.
3. Cosign signs the container image and records the signature in the Rekor immutable transparency log.
4. Kubernetes Kyverno admission controllers verify the signature before any container is permitted to run.

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: verify-signed-images
spec:
  validationFailureAction: Enforce
  rules:
    - name: verify-sigstore-signature
      match:
        resources:
          kinds:
            - Pod
      verifyImages:
        - imageReferences:
            - "ghcr.io/tiny-cto/*"
          attestors:
            - entries:
                - keyless:
                    issuer: "https://token.actions.githubusercontent.com"
                    subject: "https://github.com/tiny-cto/tinycto-tv/.github/workflows/*"