Container Image Signing & SLSA Provenance Verification with Sigstore Cosign

Supply chain security is a critical concern for enterprise software applications. Container registries like Docker Hub and GitHub Container Registry (GHCR) store execution binaries that can be compromised if attackers gain access to CI/CD credentials or registry permissions. To address this, Sigstore Cosign provides a solution for signing OCI container images in the CI/CD pipeline without the need to manage private keys.

Cosign uses OIDC (OpenID Connect) identity tokens from Fulcio Certificate Authority to sign container images, ensuring the integrity of the images without the risk of key exposure. The signed images are recorded in the public Rekor transparency log, creating an immutable record that prevents tampering. This transparent logging is crucial for verifying the provenance of container images and ensuring they have not been altered.

In addition to image signing, SLSA (Supply-chain Levels for Software Artifacts) provides a set of guidelines for attesting the build process of software artifacts. Cosign can attach cryptographically signed SLSA build provenance attestations to container images, further enhancing the supply chain's security. This process involves generating and attaching a Software Bill of Materials (SBOM) that includes detailed information about all components used in the build process.

To enforce the use of signed images, Kyverno, an open-source policy engine for Kubernetes, can be utilized. Kyverno policies can be configured to block the deployment of un-signed or non-compliant container images in Kubernetes clusters. By integrating Kyverno with Cosign, Kubernetes clusters can automatically verify that all running containers have valid signatures, ensuring that only trusted images are executed.

This comprehensive approach combines keyless signing with supply chain provenance verification, creating a robust defense against supply chain attacks. By implementing automated monitoring and continuous verification, organizations can ensure that their containerized environments remain secure and compliant with industry standards.