Image signing

What it means

Image signing means attaching a digital signature to a container image when your pipeline builds it. Later, anything that wants to run the image can check the signature against a public key or an identity you trust.

A valid signature says the image came from a signer you recognise and has not changed since. It does not say the image is free of flaws. It answers where it came from, not whether it is good.

Why it matters

A registry or a build system can be tampered with. If someone replaces an image with a poisoned one, or a stolen credential pushes an image under a trusted name, an unsigned cluster will run it without question.

Signing is a defence against the kind of break-in described in supply chain attacks. It also gives customers who ask how you know what you deploy a concrete answer.

How it shows up

Tools such as Sigstore's cosign sign images in the pipeline and store the signature beside the image in the registry. Signing alone does nothing. The value comes from enforcement: an admission controller in the cluster rejects images without a valid signature.

Common gaps are signing but never verifying, trusting any signature rather than your pipeline's identity, and signing by tag instead of by digest. Protect the signing keys or use keyless signing tied to your build identity, and pair signing with build provenance.