Container Hardening: Securing your software supply chain

Container adoption has changed how teams build and deploy applications. While Docker containers have become the norm in production environments, they've also created new security challenges that traditional tools weren't designed to handle. Container hardening emerged as a response to these new problems, shifting security from reactive scanning to proactive, automated defense that protects your entire software supply chain.

Most organizations still scan for vulnerabilities and patch after deployment. This reactive approach leaves gaps that attackers love to exploit, especially when containers move from development to production in hours rather than days. Modern container security requires building hardening directly into your images, runtime configurations, and Kubernetes deployments.

This guide covers container hardening, the methods that work, and practical ways to implement them in your DevSecOps workflows. We'll show how secure-by-default approaches can reduce vulnerabilities while making compliance easier and reducing the workload on your security team.

What is container hardening?

Container hardening reduces attack surfaces by removing unnecessary software components, enforcing security controls, and implementing defense-in-depth strategies. This process addresses vulnerabilities at multiple layers: base images, application dependencies, runtime permissions, and orchestration configurations.

Effective hardening combines image-level modifications with runtime security controls. Image hardening involves selecting minimal base images, removing unused binaries, and addressing known vulnerabilities before deployment. In contrast, runtime hardening focuses on issues that appear down the line, such as access control, filesystem restrictions, and capability management within the host system.

Traditional hardening approaches require manual configuration across multiple security tools. Modern platforms like Chainguard provide secure-by-default container images that eliminate the need for manual hardening through automated vulnerability remediation and continuous rebuilds.

Why container hardening matters (more than ever)

As more organizations adopt containers, their attack surface areas expand, as hackers find vulnerabilities such as misconfigured Dockerfiles, outdated software libraries, and weak authentication.

Recent security incidents show how container vulnerabilities spread through software development lifecycles. Vulnerabilities in the base images affect all downstream applications, while configuration errors in Kubernetes deployments expose entire clusters.

Supply chain attacks increasingly target container registries and open source dependencies. Without proper hardening, organizations cannot verify the provenance of their container images or detect tampering during the build process.

Container hardening helps improve security by:

• Reducing exposure to CVEs (Common Vulnerabilities and Exposures, aka cataloged security flaws) and zero-day vulnerabilities

• Simplifying compliance with Center for Internet Security (CIS) benchmarks, STIG requirements, and FedRAMP standards

• Creating trust in software supply chain integrity

• Decreasing security operational overhead

Common container hardening methods

These technical building blocks form the foundation of container security. While manual implementation remains possible, automated tooling and secure-by-default images reduce complexity, human error, and toil.

Minimizing base images and dependencies

Fewer packages directly correlate with fewer CVEs and reduced maintenance overhead. Large base images like Ubuntu or Red Hat contain hundreds of packages that expand attack surfaces unnecessarily. Alpine Linux reduces this risk by including fewer packages.

Even better are minimal images that contain only your application and its runtime dependencies, meaning they don’t come with complete Linux distributions (we call this distroless). They limit package managers, shells, and system utilities that attackers commonly exploit. Chainguard's minimal images are an example of this, shipping with only the essentials and zero known vulnerabilities.

Dropping Linux capabilities

Linux capabilities let you remove specific privileges that containers don't actually need. For example, most containers get permission to reboot the system, access all network interfaces, and modify system time, which they usually don’t need. By default, containers run with many of these overprivileged capabilities. To address this in Docker, you can drop unnecessary capabilities with the –cap-drop and –cap-add flags.

Setting Up Read-Only Filesystems and Volume Controls

Immutable containers prevent runtime modifications that could introduce vulnerabilities or hide malicious activity. Read-only root filesystems force applications to write only to designated volumes. Temporary directories should use memory-backed volumes to maintain immutability while supporting application requirements.

Enforcing image signing and provenance

Cryptographic signing verifies that container images haven't been tampered with as they move through your development pipeline. Sigstore provides open source tools for signing and verifying images without the complexity of managing your own private keys.

SLSA (Supply-chain Levels for Software Artifacts) is a framework that tracks where your images came from and how they were built, like a chain of custody for your containers. This provenance tracking helps prevent attackers from substituting malicious images while simplifying compliance audits.

Why scanning alone falls short

Vulnerability scanning cannot prevent attacks in real time. Because traditional scanners generate alerts after containers deploy, there are windows where vulnerable workloads are already running in production environments before issues are detected and remediated.

Common scanning limitations include:

• Alert fatigue from false positives and low-priority CVEs

• Slow patching cycles that leave vulnerabilities exposed

• Inability to verify supply chain integrity

• Limited effectiveness against configuration errors

Hardened images shift security left in the development lifecycle, preventing vulnerabilities from reaching production systems.

Container hardening best practices

The following principles help you harden containers to prevent bloated images and development-production drift while balancing compliance pressure and security team coordination. Here’s how to balance security requirements with developer productivity.

Follow the principle of least privilege

Running containers as root creates unnecessary privilege escalation risks. If an attacker gains access to your container, root access gives them full administrative privileges that could lead to container escape or host compromise. Accordingly, grant containers only the minimum permissions required for functionality. Tools like AppArmor and SELinux provide mandatory access controls that complement container isolation.

Modern container platforms support user namespaces that map container root to unprivileged host users, providing kernel-level isolation that prevents container breakout scenarios. Create users with only the privileges they need to run.

Configure these systems to restrict filesystem access, network connections, and system calls. Chainguard Images are built to run non-root by default, so if you use them, you don’t need to worry about this configuration step.

Keep containers immutable and reproducible

Maintain a consistent security posture across environments with reproducible builds. Use SLSA provenance and software bills of materials (SBOMs) to document build processes and component origins.

Implement immutable infrastructure through:

• Versioned, tagged images rather than "latest" tags

• Automated rebuilds triggered by dependency updates

• Configuration management through external sources

Solutions like Chainguard deliver reproducible, signed containers as a core feature.

Align with CIS benchmarks and STIGs

CIS benchmarks and STIGs provide hardening baselines for Docker and Kubernetes deployments.

Some recommendations include:

• Enabling Docker Content Trust for image verification

• Configuring resource limits to prevent resource exhaustion

• Implementing network segmentation through firewall rules

• Regular security auditing and compliance validation

Using hardened images makes it dramatically easier to align with benchmarks.

Eliminate embedded secrets from images

Never include API keys, passwords, or certificates in container images. For sensitive data, use external secret management systems and runtime injection.

Kubernetes secrets, HashiCorp Vault, and cloud provider secret managers provide secure alternatives to embedded credentials. Configure CI/CD pipelines to inject secrets at deployment time.

Automate rebuilds and patching processes

Security hardening requires continuous maintenance as new vulnerabilities constantly appear. Automated rebuild processes keep images current with security patches.

Effective workflows:

• Monitor upstream dependencies for security updates

• Trigger rebuilds when CVEs affect base images

• Test updated images before production deployment

• Maintain rollback capabilities for problematic updates

Chainguard automates rebuilds when new CVEs appear, keeping images secure by default.

Approaches to making a container hardening process

To scale security across DevOps teams, you should standardize processes that integrate with existing workflows. Treat security as infrastructure code rather than manual procedures to align with platform engineering approaches.

• Bake Security into your Dockerfiles and Build System: Start with secure base images and implement multi-stage builds. Sign artifacts, generate SBOMs, and use security gates to prevent vulnerable images from reaching production.

• Integrate Hardening into CI/CD Workflows: Use GitHub Actions, Tekton, Jenkins, or GitLab features to automatically scan and sign images as part of your existing pipelines.

• Automate Updates With Minimal Disruption: Set up automated patching and continuous rebuilds when upstream dependencies release patches. Use blue-green deployments to reduce update risks.

• Monitor Runtime Behavior and Policy Violations: Deploy tools like Falco to detect deviations from expected container behavior and identify potential compromises.

• Document and Enforce Policies Organization-Wide: Codify hardening practices in internal standards and use policy-as-code tools like OPA/Gatekeeper for automated enforcement.

Modern container hardening starts with Chainguard

Many teams spend lots of time implementing these practices manually, but secure-by-default images can eliminate much of that work. Chainguard builds hardened container images from scratch to be minimal, verifiable, non-root, signed, and continuously updated.

With Chainguard, you get:

• Signed, SLSA-compliant images with provenance and SBOMs baked in

• Automatic updates triggered by new vulnerabilities with no patch lag

• Purpose-built, minimal, hardened images with zero or near-zero CVEs

• Designed for developer productivity and enterprise compliance

• Easy integration into CI/CD pipelines and DevSecOps workflows

• Customizable with Custom Assembly and extensible with access to our Private APK Repositories

• Helm Charts that help you easily deploy application images

Designed for developer productivity and enterprise compliance, Chainguard frees you up to focus on application security rather than infrastructure hardening.

Talk to an expert to learn how hardened images with Chainguard can improve your security today.