A base image is the starting container image layer that all later application files, libraries, and settings are added to. It is the foundation for a Dockerfile or similar build process, and it often determines the operating system, package manager, and default tools inside the final image.
Base images matter in security because any weakness in them is inherited by every derived image. If a base image contains outdated packages, unnecessary utilities, or a vulnerable runtime, those risks can spread across many services. Attackers may also target popular base images through supply-chain compromise, knowing that a poisoned foundation can affect downstream builds. Defenders reduce this risk by choosing minimal, trusted images, pinning versions or digests, scanning base layers before use, and rebuilding regularly so patches flow into dependent images. In container registries and CI/CD pipelines, the base image is often the first trust decision in the release chain.