Stop Trusting Docker Images Blindly: Secure Your Container Supply Chain with Sigstore & Cosign

Introduction

Container image signing has become a cornerstone of modern software supply chain security. Vulnerability scanning alone cannot prove who built an image or whether it was modified after publication. Sigstore and Cosign provide cryptographic signing and verification that help establish trust.

Why Image Signing Matters

Attackers increasingly target CI/CD pipelines and container registries. A signed image allows consumers to verify authenticity before deployment, reducing the risk of running tampered artifacts.

How Sigstore Works

Sigstore includes Cosign for signing, Rekor for a transparency log, and Fulcio for issuing short-lived certificates used in keyless signing. Together they provide verifiable provenance with a simpler developer experience.

Hands-on

Build and push an image, sign it with cosign sign, then verify it with cosign verify. Integrate verification into CI/CD and Kubernetes admission policies so unsigned images are rejected.

Step-by-Step Sigstore & Cosign Demo

Prerequisites Docker, Cosign, and access to a container registry.

1. Build docker build -t ghcr.io//sigstore-demo:v1 .

2. Push docker push ghcr.io//sigstore-demo:v1

3. Sign cosign sign ghcr.io//sigstore-demo:v1

4. Verify cosign verify ghcr.io//sigstore-demo:v1

5. Generate SBOM syft IMAGE -o spdx-json > sbom.json ; cosign attach sbom --sbom sbom.json IMAGE

6. Scan trivy image IMAGE

7. CI/CD Verify signatures before deployment.

8. Kubernetes Use Kyverno verifyImages policy to enforce signed images.

Best Practices

Use keyless signing where possible, generate SBOMs, scan images with Trivy or Docker Scout, enforce signature verification in Kubernetes, and continuously monitor runtime behavior.

Conclusion

Image signing is a foundational control for zero-trust container security. Combining Cosign with SBOMs, scanning, policy enforcement, and runtime detection creates a resilient DevSecOps pipeline.