Container Image Security: Challenges, Scanning in the SDLC, and Best Practices

Container images have become a core component of cloud‑native environments, and their security is critical because vulnerabilities in images directly affect the security of the underlying cloud platform.

Recent data shows a worrying trend: the number of Linux OS vulnerabilities has risen from less than 300 in 2008 to over 2,000 in 2018, high‑severity vulnerabilities have increased from under 2,000 in 2014 to more than 4,000 in 2018, popular images such as nginx , ubuntu , and java contain dozens of flaws, and 40% of respondents admit they do not apply any security measures during the CI stage.

These characteristics—complexity, convenience, and resulting chaos—make image security a difficult and often overlooked problem.

1. Image characteristics

Complexity : Images bundle OS libraries, language runtimes, and custom files; they originate from diverse sources and Dockerfile commands (ADD vs COPY, CMD vs ENTRYPOINT) add further complexity, especially when security is considered.

Convenience : Building an image only requires a Dockerfile, and a single docker run command can turn a static image into a running container, which also leads to chaotic proliferation of images.

Chaos : Anyone can create and manage images, resulting in a massive, unstandardized image ecosystem with no effective governance.

Because of these traits, ensuring image security requires proactive measures, starting with image scanning.

2. Position of image scanning in the SDLC

Image scanning is typically performed after an image is pushed to a registry, as shown in the diagram below, which introduces several drawbacks:

Scanning lag : Vulnerable images may reside in the registry before they are scanned.

Cannot stop CI/CD : If a vulnerable image is detected only after push, the pipeline cannot be halted before deployment.

Resource waste : Storing large numbers of vulnerable images consumes disk space and increases costs.

To address these issues, the scanning step should be moved forward—placed after the image is built but before it is pushed. This allows the pipeline to abort immediately when a scan fails, preventing unsafe images from ever reaching the registry.

Existing scanning tools such as Clair, Anchore, and Trivy can be used for this pre‑push scan; a detailed comparison will be covered in a future article.

3. Image scanning prerequisites

By inserting the scan before the push, the pipeline can either continue to deployment when the image is clean or abort and notify the responsible team when vulnerabilities are found, also enabling additional controls like black‑/whitelists and RBAC.

4. Container image security best practices

4.1 Choose lightweight base images – Prefer minimal distributions (e.g., Alpine) over full‑featured ones (e.g., Ubuntu) to reduce attack surface and image size. An Anchore comparison showed openjdk:8-jdk-alpine has 48 vulnerabilities versus 128 in openjdk:8-jdk , and its size is less than one‑third.

FROM your_base_image
RUN groupadd -r devsecops && useradd -r -g devops devsecops
USER devsecops
... (other Dockerfile steps)

4.2 Run containers as non‑root – Add a dedicated non‑root user in the Dockerfile and use the USER directive to avoid giving the container root privileges on the host.

4.3 Do not embed secrets – Avoid storing tokens, passwords, or SSH keys inside images; use Docker secrets or multi‑stage builds instead.

4.4 Install only required packages – Limit installed packages to those needed at runtime; unnecessary packages increase image size, dependencies, and attack surface.

4.5 Scan Dockerfiles – Use tools like Hadolint to detect insecure Dockerfile patterns. Example:

FROM alpine
RUN apk add busybox-extras

/dev/stdin:1 DL3006 Always tag the version of an image explicitly
/dev/stdin:2 DL3018 Pin versions in apk add. Instead of `apk add
` use `apk add
=
`
/dev/stdin:2 DL3019 Use the `--no-cache` switch to avoid the need to use `--update` and remove `/var/cache/apk/*` when done installing packages

4.6 Avoid unknown or unmaintained images – Prefer official, frequently updated images with clear Dockerfiles; do not rely solely on pull count or star rating.

These practices, combined with pre‑push scanning, form a “prevention‑first, treatment‑combined” strategy for image security.

5. Conclusion

There is no absolute security, but continuous effort is essential. Image scanning helps discover vulnerabilities, while proactive best practices—lightweight bases, non‑root users, secret management, minimal packages, Dockerfile linting, and trusted sources—prevent many issues from arising.