Build-Time Security
Minimal Base Images Minimal base images including Alpine, Distroless, or scratch reduce attack surface by eliminating unnecessary packages and utilities. Smaller images contain fewer vulnerabilities and reduce deployment time. Pinned base image versions prevent unexpected changes from upstream image updates. Version pinning should be combined with automated updates to incorporate security patches. Multi-stage builds separate build dependencies from runtime images, reducing final image size and attack surface. Build tools should not be present in production images. Non-Root Users Containers should run as non-root users to limit impact from container escapes. Non-root users prevent attackers from gaining root privileges on container hosts. User namespaces provide additional isolation by mapping container root to unprivileged host users. User namespace support varies by container runtime and orchestrator. Software Bill of Materials SBOMs document all packages and dependencies in container images, enabling vulnerability tracking and license compliance. SBOM generation should be automated in build pipelines. SBOM formats including SPDX and CycloneDX provide standardized dependency documentation. SBOMs enable rapid identification of affected images when vulnerabilities are disclosed. Image Signing Cryptographic image signing through Cosign or Docker Content Trust provides provable image integrity and provenance. Signed images enable verification that images have not been tampered with. Signature verification at deployment time prevents deployment of unsigned or tampered images. Signing should be integrated into CI/CD pipelines.Registry Security
Private Registries Private container registries provide access control and audit logging for container images. Public registries should be avoided for proprietary images. Registry access should follow least privilege, with separate credentials for push and pull operations. Service accounts for automated systems should have minimal required permissions. Content Trust Content trust ensures that only signed images can be pushed to or pulled from registries. Content trust prevents deployment of unsigned images. Notary provides content trust implementation for Docker registries. Notary signatures should be verified before image deployment. Vulnerability Scanning Vulnerability scanning on image push identifies vulnerabilities before images reach production. Scanning should block pushes of images with critical vulnerabilities. Periodic rescanning of stored images identifies newly disclosed vulnerabilities in existing images. Rescanning enables proactive patching of deployed images. Scan results should integrate with deployment pipelines, preventing deployment of vulnerable images.Admission Control and Policy Enforcement
Signature and Provenance Verification Admission controllers should verify image signatures before allowing pod creation. Unsigned images should be rejected. Provenance verification ensures that images were built by trusted build systems. SLSA (Supply chain Levels for Software Artifacts) framework provides provenance standards. Pod Security Standards Pod Security Admission enforces security policies at pod creation time. Policies should enforce baseline security requirements including non-root users and restricted capabilities. Privileged containers should be disallowed except for specific infrastructure components requiring host access. Privileged containers can escape to container hosts. Host path mounts should be restricted to prevent container access to host filesystems. Host mounts enable container escapes and data exfiltration. Policy as Code Open Policy Agent (OPA), Gatekeeper, or Kyverno provide policy-as-code enforcement for Kubernetes. Policies should be version-controlled and tested. Policies should enforce security requirements including required labels, resource limits, network policies, and security contexts. Policy violations should block pod creation. Capability Restrictions Linux capabilities including CAP_SYS_ADMIN should be dropped from containers. Capabilities enable privilege escalation and container escapes. Read-only root filesystems prevent container modification, limiting attacker capabilities. Writable volumes should be minimized and explicitly declared.Runtime Security
System Call Filtering Seccomp profiles restrict system calls available to containers, preventing exploitation of kernel vulnerabilities. Default seccomp profiles should be applied to all containers. Custom seccomp profiles for specific applications can further restrict system calls. Profile generation tools can create profiles based on observed behavior. AppArmor or SELinux mandatory access control provides additional runtime restrictions. MAC policies should be enforced for all containers. Runtime Detection Falco or eBPF-based runtime detection identifies suspicious container behavior including unexpected process execution, file access, and network connections. Runtime detections should integrate with incident response workflows, enabling rapid investigation and response. Detection rules should be tuned to reduce false positives. Behavioral baselines enable detection of anomalous container behavior. Deviations from baselines may indicate compromises. Network Policies Network policies should default deny all traffic, with explicit allows for required connections. Default deny prevents lateral movement between containers. Network policies should be namespace-scoped, preventing cross-namespace communication except where explicitly required. Service mesh implementations provide additional network security including mutual TLS and fine-grained authorization.Feedback Loops and Continuous Improvement
Deployment Blocking Critical vulnerabilities should block deployment unless explicit exceptions are granted. Exceptions should be time-bounded with compensating controls. Vulnerability severity thresholds should be configurable per environment, with production having strictest requirements. Automated Remediation Automated pull requests to update base images enable rapid patching. Automation should include testing to verify that updates do not break functionality. Image rebuild automation ensures that images are regularly rebuilt with latest patches. Rebuild frequency should balance security with operational overhead. Policy Testing Policy test suites validate that policies correctly allow compliant workloads while blocking non-compliant workloads. Policy tests should run in CI/CD pipelines. Policy changes should be tested in non-production environments before production deployment. Testing prevents policy changes from breaking production workloads. Metrics and Monitoring Image age metrics identify outdated images requiring updates. Old images likely contain known vulnerabilities. Vulnerability metrics including count and severity distribution identify security debt. Trending metrics show whether security posture is improving. Policy violation metrics identify teams or applications with frequent policy violations. Violations may indicate training needs or policy issues.Conclusion
Container security in DevOps requires security controls across build, registry, admission, and runtime stages. Security engineers design container security programs that integrate with DevOps workflows, providing automated security guardrails without blocking developer productivity. Success requires treating container security as continuous process with automated scanning, policy enforcement, and runtime detection. Organizations that invest in container security fundamentals build secure containerized applications while maintaining rapid deployment velocity.References
- Kubernetes Security Best Practices
- NIST SP 800-190 Application Container Security Guide
- Sigstore for Container Signing
- SLSA Supply Chain Security Framework
- CIS Kubernetes Benchmark