K8s API Security: Preventing Cluster Breaches

TL;DR

Yes, a compromised K8s API endpoint can lead to full cluster compromise. Attacks have happened and continue to be a risk. The key is strong authentication/authorisation (RBAC), network policies, regular security audits, and keeping your Kubernetes components up-to-date.

Understanding the Risk

Kubernetes API server is the central control point for your cluster. If an attacker gains access to it with sufficient privileges, they can:

• Create/modify deployments, pods, services
• Access sensitive data (secrets, configmaps)
• Escalate privileges
• Potentially gain control of the underlying nodes.

The API server often exposes endpoints that are accessible from outside the cluster, making them a prime target.

How Attacks Happen

1. Weak Authentication/Authorisation: Using default credentials or overly permissive RBAC roles.
2. Exploiting Vulnerabilities in API Server: Unpatched vulnerabilities can allow attackers to bypass authentication or execute arbitrary code.
3. Compromised Service Accounts: If a service account has excessive permissions, an attacker who compromises the pod using that account can escalate privileges.
4. Network Exposure: Exposing the API server directly to the internet without proper protection (e.g., firewalls, VPNs).
5. Supply Chain Attacks: Compromised container images or Kubernetes components.

Real-World Examples

• CloudHops Attack (2020): Exploited a misconfigured Kubernetes dashboard to gain access to sensitive data and potentially control the cluster.
• CaptainHook: A backdoor that compromised multiple K8s clusters by injecting malicious code into container images during build time.
• Various attacks targeting kubelet API: Vulnerabilities in kubelet have been exploited to gain node access.

Preventing Cluster Breaches: Step-by-Step Guide

1. 1. Strong Authentication & Authorisation (RBAC):
   • Use strong passwords and multi-factor authentication for all user accounts.
   • Implement Role-Based Access Control (RBAC) to restrict access based on the principle of least privilege. Avoid cluster-admin roles where possible.
   • Regularly review RBAC configurations to ensure they are still appropriate.
2. 2. Network Policies:
   • Implement network policies to control traffic flow within the cluster. Restrict access to the API server from unnecessary sources.
   • Use a network policy engine (e.g., Calico, Cilium) to enforce these policies.

   kubectl apply -f network-policy.yaml

3. 3. Secure the API Server:
   • Enable TLS encryption for all communication with the API server.
   • Restrict access to the API server using firewalls and VPNs.
   • Audit logging: Enable detailed audit logs to track API server activity. Monitor these logs for suspicious behaviour.
4. 4. Keep Kubernetes Components Up-to-Date:
   • Regularly update Kubernetes components (kubelet, kube-apiserver, kubectl) to the latest versions to patch security vulnerabilities.
   • Automate updates where possible.
5. 5. Secure Service Accounts:
   • Avoid using default service accounts whenever possible.
   • Create dedicated service accounts with minimal permissions for each application.
   • Use token review API to limit the lifetime of service account tokens.
6. 6. Container Image Security:
   • Scan container images for vulnerabilities before deploying them. Use tools like Trivy or Clair.
   • Use a trusted base image and keep it up-to-date.
   • Implement image signing to ensure the integrity of your images.
7. 7. Regular Security Audits:
   • Conduct regular security audits of your Kubernetes cluster to identify and address potential vulnerabilities.
   • Use tools like kube-bench to check for compliance with CIS benchmarks.

Monitoring & Alerting

Implement monitoring and alerting systems to detect suspicious activity in your cluster. Monitor API server logs, network traffic, and resource usage.