Container Security: Why Docker Doesn’t Isolate What You Think It Does

Development teams embrace containers for their portability and efficiency, deploying applications in Docker and Kubernetes environments without fully understanding the security implications. Containers provide process isolation, not security isolation. This distinction matters enormously when containers share kernel space with other workloads. Many organisations treat containers as lightweight virtual machines, assuming they provide similar security boundaries. This fundamental misunderstanding leads to dangerous security practices where sensitive workloads run in containers without appropriate additional protections.

Container Isolation Isn’t What It Seems

Containers share the host operating system kernel. A container escape vulnerability allows attackers to break out of container isolation and access the host system or other containers. These escapes happen regularly as researchers discover new kernel exploitation techniques. Default container configurations often run processes as root inside containers. Even though this is container root rather than host root, it provides unnecessary privileges that attackers can exploit. Many container images from public repositories include unnecessary software that expands attack surfaces without providing value. Container registries contain images with known vulnerabilities that organisations deploy without scanning. Public registries host malicious images disguised as legitimate software. Teams pull these images, trusting that popular images must be safe, then wonder how attackers accessed their environments.

Expert Commentary

Name: William Fieldhouse

Title: Director of Aardwolf Security Ltd

Comments: “Container security assessments reveal consistent patterns of misconfiguration. Teams deploy containers with excessive privileges, skip image scanning entirely, and expose orchestration APIs without authentication. The speed of container deployment often bypasses security reviews that would catch these issues in traditional environments.”

Securing Container Environments

Scan container images for vulnerabilities before deployment and continuously monitor running containers. Vulnerabilities discovered after deployment require patching just like traditional systems. Automated scanning in CI/CD pipelines catches known vulnerabilities before containers reach production. Run containers with least privilege by avoiding root users and dropping unnecessary Linux capabilities. Most applications don’t require root access to function properly. Non-root containers significantly reduce the impact of container escapes. Implement network policies that restrict container communication to only necessary connections. Default Kubernetes networking allows all pods to communicate freely. Network policies enforce segmentation that limits lateral movement after initial compromise.

Regular web application penetration testing should include container and orchestration security assessment. Testing identifies misconfigurations in container deployments, Kubernetes RBAC issues, and exposed orchestration APIs that attackers could exploit.

Use minimal base images that include only required software. Large base images containing compilers, debugging tools, and unnecessary packages provide attackers with tools for post-compromise activities. Distroless images or scratch images eliminate this attack surface.

Kubernetes Security Challenges

Kubernetes API server access must be strictly controlled and monitored. Overly permissive RBAC policies grant excessive cluster access that attackers leverage for privilege escalation. Review and minimize service account permissions regularly. Secrets management in Kubernetes requires careful handling. Storing secrets in plain text ConfigMaps or mounting them as environment variables exposes sensitive data. Use proper secrets management solutions that encrypt secrets at rest and in transit.

Working with the best penetration testing company experienced in container security provides validation of your container and Kubernetes configurations. Professional assessment identifies security gaps specific to containerised environments.

Enable audit logging for Kubernetes API server to track all access and modifications. Without comprehensive logging, detecting unauthorised access or configuration changes becomes impossible. Logs must be shipped to secure external storage that attackers can’t modify.

Runtime Security for Containers

Monitor container behaviour at runtime for anomalous activities. Containers should behave predictably; unexpected network connections, file system modifications, or process execution indicates potential compromise. Runtime security tools detect and block malicious activities. Implement admission controllers that enforce security policies before containers deploy. These controllers can reject containers running as root, missing required security labels, or coming from untrusted registries. Policy enforcement at deployment time prevents insecure containers from running. Regularly update base images and redeploy containers to patch vulnerabilities. Container immutability means you can’t patch running containers; you must replace them with updated versions. This requires well-designed deployment pipelines that enable frequent updates without service disruption. Container security requires understanding fundamental differences between containers and virtual machines. Treating containers as secure by default leads to exploitable configurations that attackers leverage regularly. Proper container security combines vulnerability scanning, least privilege configurations, network segmentation, and runtime monitoring to create defence in depth.