Containers revolutionized deployment, but they also introduced new attack surfaces. A Docker container isn't a VM; it shares the host kernel. If a container is compromised, the host can be next.
1. Secure the Supply Chain (Image Scanning)
Vulnerabilities often arrive pre-packaged. Pulling "latest" from Docker Hub is risky. Use tools like Trivy, Clair, or Snyk to scan images for known CVEs (Common Vulnerabilities and Exposures) before they ever reach production. Block builds that fail security checks.
2. Least Privilege (Don't Run as Root)
Tests show over 50% of Docker images run as root by default. This is dangerous. If an attacker breaks out of the container, they have root on your host. Always define a USER instruction in your Dockerfile to run applications as a limited, non-privileged user.
3. Read-Only Filesystems
Immutability is security. Run containers with a read-only root filesystem. Attackers can't install malware or modify system files if the disk is physically unwritable. Use separate, mounted volumes for the specific directories that need write access (like /tmp or /var/log).
4. Network Segmentation
By default, all containers can talk to each other. In a microservices architecture, the "frontend" shouldn't be able to talk to the "payroll" database directly. Use Network Policies (in Kubernetes) or Docker User Defined Networks to strictly whitelist allowed communication paths.
5. Secrets Management
Never bake API keys or passwords into Docker images or environment variables (which can be read via docker inspect). Use dedicated secrets managers like HashiCorp Vault, AWS Secrets Manager, or Kubernetes Secrets to inject credentials only at runtime into memory.