Making the Internet more secure one signed container at a time

Posted by Priya Wadhwa, Jake Sanders, Google Open Source Security Team

With over 16 million pulls per month, Google’s `distroless` base images are widely used and depended on by large projects like Kubernetes and Istio. These minimal images don’t include common tools like shells or package managers, making their attack surface (and download size!) smaller than traditional base images such as `ubuntu` or `alpine`. Even with this additional protection, users could still fall prey to typosquatting attacks, or receive a malicious image if the distroless build process was compromised – making users vulnerable to accidentally using a malicious image instead of the actual distroless image. This problem isn’t unique to distroless images – until now, there just hasn’t been an easy way to verify that images are what they claim to be.

Introducing Cosign

Cosign simplifies signing and verifying container images, aiming to make signatures invisible infrastructure – basically, it takes over the hard part of signing and verifying software for you.

We developed cosign in collaboration with the sigstore project, a Linux Foundation project and a non-profit service that seeks to improve the open source software supply chain by easing the adoption of cryptographic software signing, backed by transparency log technologies.

We’re excited to announce that all of our distroless images are now signed by cosign! This means that all users of distroless can verify that they are indeed using the base image they intended to before kicking off image builds, making distroless images even more trustworthy. In fact, Kubernetes has already begun performing this check in their builds.

As we look to the future, Kubernetes SIG Release's vision is to establish a consumable, introspectable, and secure supply chain for the project. By collaborating with the sigstore maintainers (who are fellow Kubernetes contributors) to integrate signing and transparency into our supply chain, we hope to be an exemplar for standards in the cloud native (and wider) tech industry, said Stephen Augustus, co-chair for Kubernetes SIG Release.

How it works

To start signing distroless we integrated cosign into the distroless CI system, which builds and pushes images via Cloud Build. Signing every distroless image was as easy as adding an additional Cloud Build step to the Cloud Build job responsible for building and pushing the images. This additional step uses the cosign container image and a key pair stored in GCP KMS to sign every distroless image. With this additional signing step, users can now verify that the distroless image they’re running was built in the correct CI environment.



Right now, cosign can be run as an image or as a CLI tool. It supports:

• Hardware and KMS signing
• Bring-your-own PKI
• Our free OIDC PKI (Fulcio)
• Built-in binary transparency and timestamping service (Rekor)

Signing distroless with cosign is just the beginning, and we plan to incorporate other sigstore technologies into distroless to continue to improve it over the next few months. We also can’t wait to integrate sigstore with other critical projects. Stay tuned here for updates! To get started verifying your own distrolesss images, check out the distroless README and to learn more about sigstore, check out sigstore.dev.