Secure the Cluster

This is an article from DZone's 2023 Kubernetes in the Enterprise Trend Report.

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Kubernetes security is essential in today's digital landscape. With the increasing adoption of containerization and microservices, Kubernetes has become the go-to solution for orchestrating and managing containers. However, this also means that it has become a target for attackers, making Kubernetes security a top priority. The dynamic and complex nature of Kubernetes requires a proactive and comprehensive approach to security. This involves securing the Kubernetes cluster itself, the workloads running on it, and the entire CI/CD pipeline. It's important to ensure secure configurations, enforce least privilege access, isolate workloads, scan for vulnerabilities regularly, and encrypt sensitive data. 

This article will serve as a comprehensive guide to Kubernetes security, aimed at helping developers protect their applications and data.

Important Kubernetes Security Considerations 

Before diving into key security considerations, it's crucial to understand the architecture. In Kubernetes, the control plane communicates with nodes via the Kubernetes API, which the API server exposes. Nodes use the kubelet to report back to the control plane and communicate with etcd to read configuration details or write new values. 

Kubernetes follows a client-server architecture with two main types of servers: the control plane and the nodes. 

Figure 1: Kubernetes cluster with security controls

Control Plane

The control plane (formerly known as the master node) is responsible for managing the Kubernetes cluster. It is the entry point for all administrative tasks. Components of the control plane include the API server, controller manager and scheduler, and etcd. 

Important security controls for each are as follows: 

• API server (kube-apiserver) – Use role-based access control (RBAC) to limit who can access the API server and what actions they can perform. Enable audit logs to track and analyze every request made to the API server. Use transport layer security (TLS) for all API server traffic.
• Controller manager (kube-controller-manager) and scheduler (kube-scheduler) – These components should only be accessible by administrators. Use TLS for connections and ensure they are only accessible over the local network.
• etcd – This is one of the most critical components from a security perspective, as it stores all cluster data. It should be accessible only by the API server. Protect it with strong access controls and encryption, both in transit and at rest.

Nodes

Nodes (formerly known as worker nodes) run the actual workloads. Each node contains the necessary services to manage networking between containers, communicate with the control plane, and assign resources to containers. Components of a node include the kubelet, kube-proxy, and container runtime. 

Below are security controls to consider: 

• kubelet – The kubelet can be a potential attack surface. Limit API access to the kubelet and use TLS for connections.
• kube-proxy – This component should be secured by ensuring it can only be accessed by the control plane components.
• Container runtime – Ensure you're using secure, up-to-date container images. Regularly scan images for vulnerabilities. Use Pod Security Admission to limit a container's access to resources.

Pods

Pods are the smallest deployable units of computing that you can create and manage in Kubernetes. A Pod encapsulates an application's container (or multiple containers), storage resources, a unique network IP, and options that govern how the container(s) should run. 

The following are security controls to consider: 

• Use namespaces to isolate your Pods from each other.
• Implement network policies to control which Pods can communicate with each other.
• Limit the privileges of a Pod to only what it needs to function.
• Use Kubernetes Secrets to manage sensitive information that Pods need to access.
• Ensure your application code is secure. Even the most secure Kubernetes configuration can't protect against application-level vulnerabilities.

General Guidelines for Kubernetes Security

Let's review some general guidelines for Kubernetes security.

Kubernetes Hardening

Kubernetes hardening involves implementing robust security measures — including access control, network policies, audit logging, and regular updates — to enhance the resilience and protection of Kubernetes clusters against potential threats and vulnerabilities. 

• RBAC – Implement RBAC to regulate access to your Kubernetes API. Assign the least privilege necessary to users, groups, and service accounts. Kubernetes itself provides RBAC as a built-in mechanism for access control.
• Network policies – Define network policies to dictate which Pods can communicate with each other. This acts as a basic firewall for your Pods. You can use Project Calico or Cilium for implementing network policies.
• etcd security – Configure etcd peer-to-peer communication and client-to-server communication with mutual TLS. Enable etcd's built-in authentication and RBAC support.
• Audit logging – Enable audit logging in the API server using the --audit-log-path flag. Define your audit policy to record the necessary level of detail. Fluentd or Fluent Bit are often used for processing Kubernetes audit logs.  
• Update and patch – Regularly apply patches and updates to your Kubernetes components to protect against known vulnerabilities using the Kubernetes built-in mechanism.
• Admission controllers – Admission controllers are built-in plugins that help govern how the cluster is used. Enable specific admission controllers like AlwaysPullImages to ensure images are always pulled from the registry, and DenyEscalatingExec to prevent granting a Pod more privileges than its parent.

DevSecOps and Kubernetes Security

DevSecOps aims to integrate security practices into the DevOps process. It involves introducing security earlier in the lifecycle of application development, rather than relying on end-stage security measures. In a Kubernetes environment, DevSecOps could involve: 

• Secure CI/CD pipelines – CI/CD pipelines are commonly used in Kubernetes deployments. A DevSecOps approach ensures that these pipelines are secure and free of vulnerabilities by integrating security checks and tests at every step of the pipeline. Use practices like static code analysis, dynamic analysis, and dependency checks at the coding and building stages.
• Configuration management – Kubernetes configurations can be complex, and misconfigurations can lead to security vulnerabilities. DevSecOps practices involve managing and reviewing these configurations continuously to ensure security. Use automated configuration management tools, like Ansible or Terraform, to ensure consistent and secure configurations. Regularly audit and update configurations as necessary.
• Image scanning – Container images used in Kubernetes should be scanned for vulnerabilities as part of the CI/CD pipeline. This is a key DevSecOps practice. Use open-source tools like Clair or Grype to regularly scan your container images for known vulnerabilities.
• Runtime security – DevSecOps also involves monitoring and securing the application when it's running in the Kubernetes environment. Implement runtime security through tools that can detect anomalous behavior.

Supply Chain Security and Kubernetes Security

Supply chain security involves securing the software supply chain — from the components used to build your software to the infrastructure and processes used to build and deploy it. 

In a Kubernetes environment, supply chain security could involve: 

• Image assurance – Ensuring that the container images you're using in your Kubernetes deployments are from trusted sources, not tampered with, and free of known vulnerabilities is critical. Use Docker Content Trust or Notary to sign your images and verify signatures before deployment. Use private registries like Harbor or Quay, and secure them using TLS and access controls.
• Dependency management – Kubernetes applications will likely depend on external libraries and components. It's important to ensure these dependencies are secure and up to date. Regularly audit your dependencies for vulnerabilities using tools like OWASP Dependency-Check.
• Secure build processes – The tools and processes used to build your application and create your container images need to be secure. This could involve securing your CI/CD pipelines and using signed images. Use DevOps tools like Jenkins or CircleCI to ensure they are properly secured and updated.
• Secrets management – Safely manage sensitive information such as API keys, passwords, and certificates. Use Kubernetes Secrets or external Secret management tools to store and distribute Secrets securely.

Governance

Governance in Kubernetes security ensures the implementation of policies, access controls, and best practices, fostering a secure ecosystem for managing containerized applications and safeguarding sensitive data within Kubernetes clusters. 

• Policy review – Regularly review and update your security policies to keep them aligned with the latest security best practices and compliance requirements. Tools like kube-score or kube-bench (Go application that checks whether Kubernetes is deployed securely) can be used to assess how well security policies are being followed.
• Documentation – Document all security procedures and ensure your team is aware of them. Use a centralized, version-controlled repository like GitHub for your documentation.
• Compliance audit – Regularly audit your cluster for compliance with your security policies. Use tools like kube-bench or kube-score for automated checks.
• Namespaces – Use Kubernetes' built-in namespaces to segregate different projects or teams. Apply RBAC and network policies at the namespace level to enforce access and communication restrictions.
• Collaborative vendor security – For third-party services or vendors within your Kubernetes ecosystem, ensure they adhere to robust security practices. Regularly review and validate security protocols to maintain a secure supply chain.  

Other Considerations

In addition to fundamental security practices, several advanced considerations are vital for a robust Kubernetes security strategy: 

• Monitoring – Use a comprehensive monitoring solution like Prometheus or Grafana to monitor your cluster. Set up alerts for any signs of suspicious activity.
• Incident response – Have an incident response plan in place. This should include steps for identifying, isolating, and mitigating security incidents. The ELK (Elasticsearch, Logstash, Kibana) or EFK (Elasticsearch, Fluentd, Kibana) stacks can be used for log management and analysis during incident response.
• Backup – Regularly back up your etcd data using etcd's built-in snapshot feature.
• Resource quotas – Use resource quotas and limit ranges to prevent any one application from consuming too many cluster resources.
• Service mesh – Consider using a service mesh for additional security and observability features. This can provide mutual TLS, fine-grained traffic control, and detailed telemetry data. Istio and Linkerd are popular open-source service mesh implementations.

Conclusion 

Securing Kubernetes is not a one-time effort. As Kubernetes and its ecosystem continue to evolve, so do its security threats. Because of this, it's important to continuously monitor and adapt your security practices. Conducting regular security audits, staying updated with the latest Kubernetes version, and training your team on Kubernetes security are all crucial. Moreover, a strong security culture is key. Everyone involved in the Kubernetes lifecycle — from developers to operators — should be aware of the security best practices and their responsibilities. Security should be a shared responsibility across the organization. 

To summarize, Kubernetes security is essential and requires a continuous, proactive approach. By combining robust security practices with a strong security culture, organizations can leverage Kubernetes' benefits while minimizing security risks.

This is an article from DZone's 2023 Kubernetes in the Enterprise Trend Report.

For more:

Read the Report