Guarding Kubernetes From the Threat Landscape: Effective Practices for Container Security

Editor's Note: The following is an article written for and published in DZone's 2024 Trend Report, Kubernetes in the Enterprise: Once Decade-Defining, Now Forging a Future in the SDLC.

Kubernetes is driving the future of cloud computing, but its security challenges require us to adopt a full-scale approach to ensure the safety of our environments. Security is not a one-size-fits-all solution; security is a spectrum, influenced by the specific context in which it is applied. Security professionals in the field rarely declare anything as entirely secure, but always as more or less secure than alternatives. In this article, we are going to present various methods to brace the security of your containers. 

Understanding and Mitigating Container Security Threats

To keep your containerized systems secure, it's important to understand the threats they face. Just like a small leak can sink a ship, even a tiny vulnerability can cause big issues. This section will help you gain a deeper understanding of container security and will provide guidance on how to mitigate the threats that come with it.

Core Principles of Container Security

Attackers often target containers to hijack their compute power — a common example is to gain access for unauthorized cryptocurrency mining. Beyond this, a compromised container can expose sensitive data, including customer information and workload details. In more advanced attacks, the goal is to escape the container and infiltrate the underlying node. If the attacker succeeds, they can move laterally across the cluster, gaining ongoing access to critical resources such as user code, processing power, and valuable data across other nodes.

One particularly dangerous attack method is container escape, where an attacker leverages the fact that containers share the host's kernel. If they gain elevated privileges within a compromised container, they could potentially access data or processes in other containers on the same host. Additionally, the Kubernetes control plane is a prime target. If an attacker compromises one of the control plane components, they can manipulate the entire environment, potentially taking it offline or causing significant disruption. Furthermore, if the etcd database is compromised, attackers could alter or destroy the cluster, steal secrets and credentials, or gather enough information to replicate the application elsewhere. 

Defense in Depth

Maintaining a secure container environment requires a layered strategy that underscores the principle of defense in depth. This approach involves implementing multiple security controls at various levels. By deploying overlapping security measures, you create a system where each layer of defense reinforces the others. This way, even if one security measure is breached, the others continue to protect the environment. 

Figure 1. Defense-in-depth strategy

Understanding the Attack Surface

Part of the security strategy is understanding and managing the attack surface, which encompasses all potential points of exploitation, including container images, runtime, orchestration tools, the host, and network interfaces. Reducing the attack surface means simplifying the system and minimizing unnecessary components, services, and code. By limiting what is running and enforcing strict access controls, you decrease the opportunities for vulnerabilities to exist or be exploited, making the system more secure and harder for attackers to penetrate.

Common Threats and Mitigation Strategies

Let's shift our focus to the everyday threats in container security and discover the tools you can immediately put to work to safeguard your systems.

Vulnerable Container Images

Relying on container images with security vulnerabilities poses significant risks as these vulnerable images often include outdated software or components with publicly known vulnerabilities. A vulnerability, in this context, is essentially a flaw in the code that malicious actors can leverage to trigger harmful outcomes. An example of this is the infamous Heartbleed flaw in the OpenSSL library, which allowed attackers to access sensitive data by exploiting a coding error. When such flaws are present in container images, they create opportunities for attackers to breach systems, leading to potential data theft or service interruptions.

Best practices to secure container images include the following:

• To effectively reduce the attack surface, start by using minimal base imagesthat include only the essential components required for your application. This approach minimizes potential vulnerabilities and limits what an attacker can exploit. 
  • Tools like Docker's FROM scratch or distroless images can help create these minimal environments.
• Understanding and managing container image layers is crucial as each layer can introduce vulnerabilities. By keeping layers minimal and only including what is necessary, you reduce potential attack vectors. 
  • Use multi-stage builds to keep the final image lean and regularly review and update your Dockerfiles to remove unnecessary layers.

It's important to avoid using unverified or outdated images. Unverified images from public repositories may contain malware, backdoors, or other malicious components. Outdated images often have unpatched vulnerabilities that attackers can exploit. To mitigate these risks, always source images from trusted repositories and regularly update them to the latest versions.

Insecure Container Runtime

An insecure container runtime is a critical threat as it can lead to privilege escalation, allowing attackers to gain elevated access within the system. With elevated access, attackers can disrupt services by modifying or terminating critical processes, causing downtime and impacting the availability of essential applications. They can gain full control over the container environment, manipulating configurations to deploy malicious containers or introduce malware, which can be used as a launchpad for further attacks.

Best practices for hardening the container runtime include the following:

• Implementing strict security boundaries and adhering to the principle of least privilege are essential for protecting the container runtime.

  • Containers should be configured to run with only the permissions they need to function, minimizing the potential impact of a security breach. This involves setting up role-based access controls.

• Admission control is a critical aspect of runtime security that involves validating and regulating requests to create or update containers in the cluster. By employing admission controllers, you can enforce security policies and ensure that only compliant and secure container configurations are deployed.

  • This can include checking for the use of approved base images, ensuring that security policies are applied, and verifying that containers are not running as root.

  • Tools like Open Policy Agent (OPA) can be integrated into your Kubernetes environment to provide flexible and powerful admission control capabilities. Here's an example for OPA policy that acts as a gatekeeper, ensuring no container runs with root privileges:

package kubernetes.admission

deny[msg] {
    input.request.kind.kind == "Pod"
    input.request.object.spec.containers[_].securityContext.runAsUser == 0
    msg = "Containers must not run as root."
}

There are a few practices to avoid when securing container runtime:

• If a container running as root is compromised, an attacker can gain root-level access to the host system, potentially leading to a full system takeover.
• When containers have unrestricted access to host resources, like the file system, network, or devices, a compromised container could exploit this access to then tamper with the host system, steal sensitive data, or disrupt other services. 

  • To prevent such scenarios, use tools like seccomp and AppArmor. These tools can restrict the system calls that containers make and enforce specific security policies. 

  • By applying these controls, you can confine containers to their intended operations, protecting the host system from potential breaches or unauthorized activities.

Misconfigured Kubernetes Settings

Misconfigured Kubernetes settings are a significant threat as they expose the cluster to attacks through overly permissive network policies, weak access controls, and poor secrets management: 

• Overly permissive network policies enable attackers to intercept and tamper with data.
• Weak access controls allow unauthorized users to perform administrative tasks, disrupt services, and alter configurations. 
• Poor secrets management exposes sensitive information like API keys and passwords, enabling attackers to escalate privileges.

Best practices for secure Kubernetes configuration are as follows:

• The risk of transmitting sensitive information without protection is that it can be intercepted or tampered with by malicious actors during transit. To mitigate this risk, secure all communication channels with transport layer security (TLS). 

  • Kubernetes offers tools like cert-manager to automate the management and renewal of TLS certificates. This ensures that communication between services remains encrypted and secure, thereby protecting your data from interception or manipulation.

• Network policies control the traffic flow between Pods and services in a Kubernetes cluster. By defining network policies, you can isolate sensitive workloads and reduce the risk of lateral movement in case of a compromise.

  • Use Kubernetes' native NetworkPolicy resource to create rules that enforce your desired network security posture.

On the other hand, it's important to avoid exposing unnecessary application ports. Exposure of ports provides multiple entry points for attackers, making the cluster more vulnerable to exploits. 

CI/CD Security

CI/CD pipelines are granted extensive permissions, ensuring they can interact closely with production systems and manage updates. However, this extensive access also makes CI/CD pipelines a significant security risk. If compromised, attackers can exploit these broad permissions to manipulate deployments, introduce malicious code, gain unauthorized access to critical systems, steal sensitive data, or create backdoors for ongoing access.

There are several best practices to implement when securing CI/CD. The first best practice is ensuring that once a container image is built and deployed, it is immutable. We always want to make sure the Pod is running on exactly what we intended. It also helps in quickly identifying and rolling back to previous stable versions if a security issue arises, maintaining a reliable and predictable deployment process.

Implementing immutable deployments involves several key steps to ensure consistency and security: 

1. Assign unique version tags to each container image build, avoiding mutable tags like "latest," and use Infrastructure-as-Code tools like Terraform or Ansible to maintain consistent setups. 
2. Configure containers with read-only file systems to prevent changes post-deployment.
3. Implement continuous monitoring with tools like Prometheus and runtime security with Falco to help detect and alert to unauthorized changes, maintaining the security and reliability of your deployments.

Another best practice is implementing image vulnerability scanning in CI/CD. Vulnerability scanners meticulously analyze the components of container images, identifying known security flaws that could be exploited. Beyond just examining packages managed by tools like DNF or apt, advanced scanners also inspect additional files added during the build process, such as those introduced through Dockerfile commands like ADD, COPY, or RUN.

It's important to include both third-party and internally created images in these scans as new vulnerabilities are constantly emerging. To guarantee that images are thoroughly scanned for vulnerabilities before deployment, scanning tools like Clair or Trivy can be directly embedded into your CI/CD pipeline.

Do not store sensitive information directly in the source code (e.g., API keys, passwords) as this increases the risk of unauthorized access and data breaches. Use secrets management tools like SOPS, AWS Secrets Manager, or Google Cloud Secret Manager to securely handle and encrypt sensitive information.

Conclusion

Regularly assessing and improving Kubernetes security measures is not just important — it's essential. By implementing the strategies we introduced above, organizations can protect their Kubernetes environments, ensuring that containerized applications are more secure and resilient against challenges. In the future, we anticipate that attackers will develop more sophisticated methods to specifically bypass Kubernetes' built-in security features. As organizations increasingly rely on Kubernetes for critical workloads, attackers will likely invest time in uncovering new vulnerabilities or weaknesses in Kubernetes' security architecture, potentially leading to breaches that are more difficult to detect and mitigate.

The path to a secure Kubernetes environment is clear, and the time to act is now. Prioritize security to safeguard your future.

This is an excerpt from DZone's 2024 Trend Report, Kubernetes in the Enterprise: Once Decade-Defining, Now Forging a Future in the SDLC.

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