Securing Your Software Supply Chain with Policy as Code

High-profile attacks like SolarWinds and Log4j have proven that the software supply chain is a primary target for adversaries. Securing your own code is no longer enough; you must also secure the entire lifecycle of dependencies, build processes, and deployment pipelines that bring your software to production.

Policy as Code provides the automated, enforceable guardrails necessary to secure this complex chain of trust. This guide will show you how to apply policies at each stage to create a verifiably secure software supply chain.

The Four Pillars of Supply Chain Security (Based on SLSA)

We can frame our policy enforcement around the core principles of the SLSA framework (Supply-chain Levels for Software Artifacts), which focuses on ensuring the integrity of your software at every step.

Practical Policies for Each Pillar

Pillar 1: Source Integrity Policy

You can enforce source integrity rules directly in your version control system. This example uses a GitHub Actions workflow to check for signed commits.

name: Source Integrity Check
on:
  pull_request:
    types: [opened, synchronize, reopened]

jobs:
  verify-commit-signatures:
    runs-on: ubuntu-latest
    steps:
      - name: Verify all commits are signed
        uses: a-b-r-o-w-n/verify-commit-signatures@v1
        with:
          token: {${ secrets.GITHUB_TOKEN }}

Pillar 2: Build Integrity Policy

This OPA/Rego policy ensures that a CI/CD pipeline definition only uses approved, hardened build runners.

package cicd.build_integrity

# Deny if the build job is not using an approved runner
deny[msg] {
    job := input.jobs[_]
    not startswith(job.runner, "secure-hardened-runner-")
    msg := sprintf("Job '%v' is using an insecure runner '%v'. Only 'secure-hardened-runner-*' is allowed.", [
        job.name,
        job.runner
    ])
}

Pillar 3: Dependency Integrity Policy

This policy inspects a CycloneDX Software Bill of Materials (SBOM) to block any dependencies with `Critical` or `High` severity vulnerabilities.

package sbom.vulnerability_check

# Deny if any component has a critical or high severity vulnerability
deny[msg] {
    component := input.sbom.components[_]
    vulnerability := component.vulnerabilities[_]
    is_severe_vulnerability(vulnerability)
    
    msg := sprintf("Blocked dependency '%v': Contains a '%v' severity vulnerability (%v).", [
        component.name,
        vulnerability.severity,
        vulnerability.id
    ])
}

is_severe_vulnerability(vulnerability) {
    vulnerability.severity == "Critical"
}

is_severe_vulnerability(vulnerability) {
    vulnerability.severity == "High"
}

Pillar 4: Deployment Integrity Policy

This Kyverno policy, for Kubernetes, ensures that no container image can be deployed unless it has been cryptographically signed by your trusted CI/CD system using Cosign/Sigstore.

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: enforce-signed-images
spec:
  validationFailureAction: Enforce
  background: false
  rules:
    - name: check-image-signature
      match:
        any:
        - resources:
            kinds:
              - Pod
      verifyImages:
      - imageReferences:
        - "ghcr.io/your-org/*" # Match all images from your container registry
        attestors:
        - count: 1
          entries:
          - keys:
              # The public key of your CI/CD pipeline's signing identity
              publicKey: |
                -----BEGIN PUBLIC KEY-----
                MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE...
                -----END PUBLIC KEY-----

Conclusion: Building a Verifiable Chain of Trust

Securing the software supply chain requires a defense-in-depth approach. By using Policy as Code to enforce integrity checks at every stage—source, build, dependencies, and deployment—you move from a model of implicit trust to one of explicit, automated verification. This not only enhances your security posture but also creates an auditable, compliant, and trustworthy path from code to cloud.