What is Snyk Container and how does it scan container images?

Think of a container image as a layered cake. The bottom layer is the base image (like Alpine Linux or Debian), the middle layers are your system packages and runtime installations, and the top layer is your application code. Snyk Container inspects every layer of the cake to find ingredients that have gone bad, and it can even recommend a different base layer that has fewer problems to begin with.

Snyk Container works by pulling or reading a container image and analyzing its contents layer by layer. It identifies the operating system distribution (Alpine, Debian, Ubuntu, RHEL, etc.) and enumerates all installed OS packages with their exact versions. For example, scanning the payments-api image built on node:18-bullseye, Snyk might find that the Debian Bullseye base contains a vulnerable version of libssl3 and a vulnerable version of curl. Beyond OS packages, Snyk Container also detects application-level dependencies installed in the image, such as npm packages in a Node.js image or pip packages in a Python image, providing a more complete vulnerability picture.

The scanning process works locally through the CLI with 'snyk container test image:tag' or through integrations with container registries like Docker Hub, Amazon ECR, Google Container Registry, Azure Container Registry, and others. When you connect a registry to Snyk, it monitors your images continuously and alerts you when new vulnerabilities are disclosed that affect packages in your stored images. The CLI can scan images from any registry you have access to, local images in your Docker daemon, or even image archives (tar files). Snyk identifies the base image used and provides a critical recommendation: upgrade to a newer tag or switch to a slimmer variant with fewer installed packages and thus fewer potential vulnerabilities.

In production, the user-auth-service team might integrate Snyk Container into their CI pipeline so that every Docker build is scanned before the image is pushed to ECR. If the scan finds a critical vulnerability in the base image, the build fails with a clear message explaining which package is affected and what base image upgrade would resolve it. Snyk might recommend switching from node:18-bullseye to node:18-bookworm-slim, which has 40 fewer vulnerable packages because it ships with updated system libraries and excludes unnecessary tools. The inventory-sync team might configure Snyk to monitor their ECR repository directly, receiving Slack notifications when a new CVE is published that affects any image in production.

A detail beginners overlook is the difference between base image vulnerabilities and application vulnerabilities within a container. A vulnerability in libcurl installed by the base image is an OS-level concern, while a vulnerability in the lodash npm package installed by your Dockerfile COPY and npm install is an application-level concern. Snyk Container reports both, but the remediation paths are different: OS-level issues require base image upgrades, while application-level issues require dependency version changes in your code.