Phase 30 — Cosign + SBOM — Supply Chain Image Signing
Supply chain attacks target software before it reaches your cluster — a tampered image pushed to a registry, a compromised CI runner, or a registry impersonation. Cosign signs every image with a cryptographic certificate proving which CI run built it, and syft generates a Software Bill of Materials (SBOM) — a complete dependency inventory attached to the image digest.
Architecture
GitHub Actions CI ghcr.io Cluster
───────────────── ────────────────────────────── ─────────────────────
1. Build image platform-demo:<sha> harbor.10.0.0.200.nip.io
2. Push to ghcr.io ──▶ platform-demo:<sha>-cosign ──▶ ghcr proxy cache
3. syft SBOM platform-demo:<sha>-sbom kubelet pulls
4. cosign sign │ Gatekeeper warns
5. cosign attach sbom ──────────┘ on non-Harbor images
(OCI referrer)
Why keyless signing?
Traditional key-pair signing creates a long-lived private key — a secret that must be rotated, stored securely, and is valuable to attackers. Keyless signing uses the GitHub Actions OIDC token as the signer identity:
GitHub Actions job
→ requests OIDC token (audience: sigstore)
→ presents token to Sigstore Fulcio CA
→ Fulcio issues a short-lived (10-minute) certificate
→ cosign signs the image with that certificate
→ certificate + signature written to Sigstore Rekor transparency log
The resulting identity is:
Subject: https://github.com/andrelair-platform/platform-demo/.github/workflows/ci.yml@refs/heads/main
Issuer: https://token.actions.githubusercontent.com
No private key to rotate. No secret to leak. Provenance is bound to the exact workflow file + branch.
CI Pipeline Changes
Three steps added to the build-and-push job in .github/workflows/ci.yml after the Build and push step:
permissions:
contents: read
packages: write
id-token: write # request GitHub OIDC token for Cosign
# After Build and push:
- name: Install Cosign
uses: sigstore/cosign-installer@v3
- name: Install syft
uses: anchore/sbom-action/download-syft@v0
- name: Generate SBOM (CycloneDX JSON)
run: |
syft ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${{ steps.build.outputs.digest }} \
-o cyclonedx-json=sbom.json
- name: Sign image (keyless — GitHub OIDC → Sigstore Fulcio)
run: |
cosign sign --yes \
${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${{ steps.build.outputs.digest }}
- name: Attach SBOM to image as OCI referrer
run: |
cosign attach sbom \
--sbom sbom.json \
--type cyclonedx \
${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${{ steps.build.outputs.digest }}
Signing and SBOM generation happen by digest, not by tag. Digests are immutable — a tag can be overwritten, a sha256:... cannot.
Verify a Signature
After the CI run completes, the signature is in the Sigstore Rekor transparency log:
tlog entry created with index: 1900056177
Pushing signature to: ghcr.io/andrelair-platform/platform-demo
Verify from any machine with cosign installed (requires read:packages on the GHCR token):
# Authenticate cosign to ghcr.io
echo $GHCR_PAT | cosign login ghcr.io --username AndreLiar --password-stdin
# Verify the signed image
cosign verify \
--certificate-identity-regexp="https://github.com/andrelair-platform/platform-demo" \
--certificate-oidc-issuer="https://token.actions.githubusercontent.com" \
ghcr.io/andrelair-platform/platform-demo:dc9b66b
# Expected output:
# Verification for ghcr.io/andrelair-platform/platform-demo:dc9b66b --
# The following checks were performed on each of these signatures:
# - The cosign claims were validated
# - Existence of the claims in the transparency log was verified offline
# - The code-signing certificate claims were validated
#
# [{"critical":{"identity":{"docker-reference":"ghcr.io/andrelair-platform/platform-demo"},
# "image":{"docker-manifest-digest":"sha256:535cfe2434b13c1659187252039432b701dd4b83a9301b7a28624b9d5a10ee92"},
# "type":"cosign container image signature"},
# "optional":{"1.3.6.1.4.1.57264.1.1":"https://token.actions.githubusercontent.com",
# "1.3.6.1.4.1.57264.1.2":"push",
# "1.3.6.1.4.1.57264.1.3":"dc9b66be821792b064597ad9d7c1845b5d2571aa",
# "1.3.6.1.4.1.57264.1.4":"CI",
# "1.3.6.1.4.1.57264.1.5":"andrelair-platform/platform-demo",
# "1.3.6.1.4.1.57264.1.6":"refs/heads/main"}}]
1.3.6.1.4.1.57264.1.* are the Sigstore Fulcio OID extensions — they embed the GitHub Actions context (workflow, run, repo, ref) into the certificate.
Search Rekor transparency log directly
The signature is in the public Rekor log regardless of registry visibility:
# Search by image digest (no registry auth needed)
rekor-cli search --sha sha256:535cfe2434b13c1659187252039432b701dd4b83a9301b7a28624b9d5a10ee92
# Or view the entry directly
rekor-cli get --log-index 1900056177
Verify SBOM attachment
cosign download sbom \
ghcr.io/andrelair-platform/platform-demo:dc9b66b \
| jq '{bomFormat, specVersion, metadata: .metadata.component}'
SBOM — What It Contains
A CycloneDX SBOM generated by syft for the platform-demo Go binary lists every dependency by name, version, type, and PURL:
{
"bomFormat": "CycloneDX",
"specVersion": "1.6",
"metadata": {
"component": {
"name": "ghcr.io/andrelair-platform/platform-demo",
"type": "container"
}
},
"components": [
{
"name": "github.com/gofiber/fiber/v2",
"version": "v2.52.6",
"purl": "pkg:golang/github.com/gofiber/fiber/v2@v2.52.6",
"type": "library"
},
{
"name": "github.com/prometheus/client_golang",
"version": "v1.22.0",
"purl": "pkg:golang/github.com/prometheus/client_golang@v1.22.0",
"type": "library"
}
// ... all transitive Go module dependencies
]
}
The SBOM is stored in the registry alongside the image. Any future vulnerability scanner can query it without re-pulling the full image layers.
Gatekeeper — Allowed Registries Policy
A fourth Gatekeeper constraint was added: K8sAllowedRegistries. It requires all pod images to use the harbor.10.0.0.200.nip.io/ prefix — enforcing that every workload goes through Harbor's proxy cache (the Trivy scanning chokepoint).
ConstraintTemplate
# manifests/gatekeeper-policies/03-ct-allowed-registries.yaml
apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
name: k8sallowedregistries
spec:
crd:
spec:
names:
kind: K8sAllowedRegistries
validation:
openAPIV3Schema:
type: object
properties:
registries:
type: array
items:
type: string
targets:
- target: admission.k8s.gatekeeper.sh
rego: |
package k8sallowedregistries
containers[c] {
c := input.review.object.spec.containers[_]
}
containers[c] {
c := input.review.object.spec.template.spec.containers[_]
}
containers[c] {
c := input.review.object.spec.initContainers[_]
}
containers[c] {
c := input.review.object.spec.template.spec.initContainers[_]
}
violation[{"msg": msg}] {
container := containers[_]
not startswith_allowed(container.image)
msg := sprintf("Container '%v' uses image '%v' from a disallowed registry. Allowed prefixes: %v",
[container.name, container.image, input.parameters.registries])
}
startswith_allowed(image) {
registry := input.parameters.registries[_]
startswith(image, registry)
}
Constraint (warn mode)
# manifests/gatekeeper-policies/13-constraint-allowed-registries.yaml
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sAllowedRegistries
metadata:
name: allowed-registries
spec:
enforcementAction: warn
match:
kinds:
- apiGroups: [""]
kinds: ["Pod"]
- apiGroups: ["apps"]
kinds: ["Deployment", "StatefulSet", "DaemonSet"]
namespaceSelector:
matchExpressions:
- key: kubernetes.io/metadata.name
operator: NotIn
values:
- kube-system
- kube-public
- kube-node-lease
- gatekeeper-system
- metallb-system
- longhorn-system
- nfs-provisioner
- cert-manager
- ingress-nginx
- chaos-mesh
- velero
- falco
parameters:
registries:
- "harbor.10.0.0.200.nip.io/"
- "harbor.devandre.sbs/"
Audit results
kubectl --context minicloud get k8sallowedregistries allowed-registries \
-o jsonpath='{.status.totalViolations}'
# 116
116 violations — all in warn mode. The violations are expected: Helm-installed workloads use their upstream registry URLs (quay.io, ghcr.io, docker.io, etc.) by default rather than Harbor proxy-cache prefixes.
Compliant workloads (0 violations):
gitops-demo/platform-demo— usesharbor.10.0.0.200.nip.io/ghcr/andrelair-platform/platform-demo:...✅
Violation examples:
argocd/ quay.io/argoproj/argocd:v3.4.1
monitoring/ docker.io/grafana/grafana:13.0.1
monitoring/ quay.io/prometheus-operator/prometheus-operator:v0.90.1
keda/ ghcr.io/kedacore/keda:2.19.0
vault/ hashicorp/vault:2.0.2 ← Vault Agent sidecar (injected)
backstage/ docker.io/bitnamilegacy/postgresql:15.4.0-debian-11-r10
:::note Vault Agent sidecar
The hashicorp/vault:2.0.2 violation comes from the Vault Agent Injector — it mutates pods to inject a sidecar using its own configured image, bypassing our GitOps manifests. Fixing this requires overriding the sidecar image in the Vault Helm values:
injector:
agentImage:
repository: harbor.10.0.0.200.nip.io/docker-hub/hashicorp/vault
tag: "2.0.2"
:::
Path to zero violations
Migrate each namespace's Helm values to use Harbor proxy-cache URLs:
# docker.io images → harbor.10.0.0.200.nip.io/docker-hub/<image>
# ghcr.io images → harbor.10.0.0.200.nip.io/ghcr/<image>
# quay.io images → harbor.10.0.0.200.nip.io/quay/<image>
# k8s registry → harbor.10.0.0.200.nip.io/k8s-registry/<image>
Once all workloads use Harbor, flip enforcementAction: deny.
Full Supply Chain Security Chain
Source code
│
▼
GHAS (Phase 13)
├── CodeQL SAST — finds injection, RCE, SQL injection
├── Dependabot SCA — CVEs in Go modules
└── Secret scanning — blocks committed credentials
│
▼
GitHub Actions CI
├── go test ./... — unit tests
├── docker build — reproducible Containerfile build
├── docker push → ghcr.io
├── syft SBOM — CycloneDX dependency inventory
├── cosign sign — GitHub OIDC → Sigstore Fulcio (this phase)
└── cosign attach sbom — OCI referrer artifact
│
▼
Harbor (Phases 7, 16)
├── Trivy scan on push — blocks CRITICAL CVEs
├── Proxy cache — mirrors ghcr.io, docker.io, quay.io
├── Cosign signature stored alongside image
└── SBOM accessible via cosign download sbom
│
▼
Cluster admission (Phase 27 + this phase)
├── K8sBlockLatestTag — blocks :latest images
├── K8sNoPrivilegedContainers — blocks privileged pods
├── K8sRequireResourceLimits — blocks uncapped containers
└── K8sAllowedRegistries (warn) — flags non-Harbor images
│
▼
Runtime (Phase 28)
└── Falco — detects unexpected syscalls, privilege escalation
Verification Commands
# List all 4 Gatekeeper constraints
kubectl --context minicloud get constraints
# Check AllowedRegistries violation count
kubectl --context minicloud get k8sallowedregistries allowed-registries \
-o jsonpath='{.status.totalViolations}'
# Verify cosign signature (run after the CI push completes)
cosign verify \
--certificate-identity-regexp="https://github.com/andrelair-platform/platform-demo" \
--certificate-oidc-issuer="https://token.actions.githubusercontent.com" \
ghcr.io/andrelair-platform/platform-demo:latest
# Download and inspect SBOM
cosign download sbom ghcr.io/andrelair-platform/platform-demo:latest \
| jq '.components | length' # total dependencies
Done When
✔ CI workflow: cosign-installer@v3 + syft installed in build-and-push job
✔ syft generates CycloneDX JSON SBOM from image digest
✔ cosign signs image by digest (keyless — GitHub OIDC → Sigstore Fulcio)
✔ cosign attach sbom attaches SBOM as OCI referrer on ghcr.io
✔ K8sAllowedRegistries ConstraintTemplate deployed (k8sallowedregistries)
✔ allowed-registries Constraint active (enforcementAction: warn, 116 violations audited)
✔ platform-demo in gitops-demo namespace: 0 AllowedRegistries violations (compliant)
✔ Full supply chain documented: GHAS → Cosign/SBOM → Harbor Trivy → Gatekeeper → Falco