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Use Cosign to sign the image your CI pipeline builds, verify both the signature and the authorized signer, then use Open Policy Agent (OPA) Gatekeeper to test and enforce deployment rules in Kubernetes. These are separate controls: a valid signature is evidence that an identity signed an image, not proof that the identity is allowed to publish that image for your workload.
What do Cosign and OPA each protect?
Cosign supplies cryptographic evidence about an image. Gatekeeper evaluates Kubernetes resources against policy. Together, they can check the artifact before deployment and make admission decisions when a workload is submitted to a cluster.
- Signature: Establishes that the signed artifact was signed and that its signature verifies.
- Signer authorization: Determines whether the verified signer identity and identity issuer are trusted for your organization, workflow, repository, or release.
- Deployment policy: Determines whether a Kubernetes resource is acceptable, including whether its image reference and signature evidence meet your rules.
Do not treat these controls as interchangeable. An image can have a valid signature from an identity your organization should not trust. A digest-pinned reference can identify exact image content without establishing who built or signed it.
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- Build and publish: Have CI build the container image and publish it to an OCI-compatible registry. Carry the image digest into later steps so they refer to the exact artifact, rather than relying on a mutable tag as its identity.
- Sign in CI: Sign the published image with Cosign. Sigstore’s GitHub Actions CI example uses OIDC and grants the workflow
id-token: write. Treat that as a workflow permission with security implications: use the intended workflow identity, not an unrestricted identity. - Verify with explicit trust constraints: Verify the image with Cosign while requiring the expected certificate identity and OIDC issuer. Cosign’s verification documentation says the signature payload includes the image digest and that the digest is checked by default.
- Test policy before rollout: Run Gatekeeper’s
gatorCLI against representative Kubernetes objects and the policy you intend to deploy. Gatekeeper documentation also describes packaging policy files as OCI artifacts. - Enforce at admission and audit existing resources: Deploy the policy through Gatekeeper and choose an enforcement mode deliberately. Start with a non-blocking mode if you need to assess existing violations, then move to denial after reviewing policy behavior and exceptions.
This separates artifact identity, signature verification, and admission enforcement. The image digest ties the signature to content; signer and issuer constraints define whose signature counts; Gatekeeper applies the deployment decision.
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How does keyless Cosign signing establish identity?
Sigstore describes keyless signing as identity-based signing. The CI workflow obtains an OIDC identity; Fulcio issues a short-lived certificate that binds an ephemeral key to that identity; and the signing event is recorded in Rekor’s transparency log. This approach reduces the need to distribute and retain long-lived signing keys, but it does not remove the need to define who is trusted.
In practice, the verifier’s expected identity and issuer are central to the trust decision. Avoid copying wildcard identity examples into a production rule. Constrain verification to the identity and issuer associated with the workflow your organization intends to authorize, and maintain those values as workflow identities change.
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Cosign also supports self-managed signing keys. That is an alternative signing model, not a reason to skip verification policy: whichever model you use, define which key or identity is authorized and how verification obtains that expectation.
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- Expected signer identity: Match the identity that should have produced the signature, such as the intended CI workflow identity.
- OIDC issuer: Match the issuer that authenticated that identity. Identity without the expected issuer is not a complete trust constraint.
- Image digest: Verify the specific content being deployed. Cosign checks the digest by default; do not disable claim checking unless there is a documented reason and an alternative claim-validation method.
A successful signature check means the configured verification conditions passed. It does not independently establish that the build was safe, that every dependency was acceptable, or that the image should be deployed in every environment. Those questions belong in additional policy and, where needed, attestations or other controls.
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How do Gatekeeper policies test and enforce image rules?
Gatekeeper uses OPA’s Constraint Framework. A ConstraintTemplate defines a reusable policy type, and a Constraint supplies the policy configuration applied to matching Kubernetes resources. Gatekeeper can evaluate resources during admission and audit resources already in the cluster.
Test policy changes before deployment
Use gator to check representative Kubernetes objects against your policies before applying them to a cluster. Include objects that should pass, objects that should fail, and known exception cases. This helps surface mistakes in policy logic and scope before an admission rule affects workloads. Gatekeeper’s documentation describes bundling policy files as OCI artifacts, which can support consistent distribution of policy alongside versioned delivery workflows.
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Roll out enforcement deliberately
Gatekeeper supports denial, warning, and dry-run behavior for admission validation, as well as audit reporting for existing violations. A staged rollout lets teams observe what a rule would affect, review exceptions, and then decide whether to block requests. The exact enforcement choice should reflect your cluster’s availability requirements and how your organization handles policy-controller failures and exceptions.
Use digest requirements for immutability, not provenance
The Gatekeeper Library’s Image Digests constraint requires image references to include a digest. This helps ensure that a reference identifies specific image content instead of resolving only through a mutable tag. It does not prove that the image came from an authorized builder or signer. Pair digest requirements with signature verification and identity authorization when trusted origin is part of the deployment decision. The cited Gatekeeper Library example was crawled seven months before the documentation check on October 7, 2026, so confirm its current form against the version you deploy.
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Should policy run in Gatekeeper or a managed Kubernetes integration?
Gatekeeper and managed Kubernetes policy integrations are both deployment patterns to evaluate. The available documentation does not establish a general winner on cost, latency, staffing, or security, so choose based on operational ownership and the controls your environment requires.
- Controller operations: Identify who upgrades and operates the policy controller and who responds if it fails.
- Verification location: Decide whether verification happens in CI, at Kubernetes admission, or at both points. CI checks can catch problems before deployment; admission enforcement can evaluate requests reaching the cluster.
- Identity maintenance: Establish how expected signer identities and issuers are constrained, reviewed, and updated as CI workflows evolve.
- Policy lifecycle: Determine how tests run before release and how policy artifacts are versioned and distributed consistently.
- Enforcement and exceptions: Plan the progression through audit, warning, dry-run, or denial, and define how exceptions are approved and expired.
- Failure behavior: Decide the acceptable effect of fail-open or fail-closed behavior, including availability and incident response requirements.
What does this architecture not guarantee?
A signature proves neither that an image is vulnerability-free nor that its build process meets every organizational requirement. Identity restrictions answer who is trusted to sign; policy determines what evidence and resource conditions are required for deployment. If a decision depends on additional build properties, use suitable attestations and policies that inspect those claims. Cosign documents attaching attestations with cosign attest and verifying them with a policy through cosign verify-attestation.
Keep the distinction clear in policy design: signatures provide signing evidence, attestations can carry additional claims, and Gatekeeper decides whether the Kubernetes resource satisfies the rules you have configured. None of those checks should be silently substituted for another.
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Which documentation should teams validate before implementation?
Implementation details can change, especially workflow action versions and product features. The Sigstore and Gatekeeper documentation checked on October 7, 2026, supports the architecture described here, but teams should confirm current commands, configuration, and compatibility for their installed versions. Relevant official documents include Sigstore’s CI example and keyless signing quickstart, Cosign’s image verification and container signing guides, Gatekeeper’s gator and policy how-to documentation, and the Gatekeeper Library’s Image Digests constraint example.
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