Enforcing CIS Benchmarks with Policy-as-Code
A practical guide to translating and automating CIS security standards for cloud and container environments using policy-as-code.
📋 Prerequisites
- Understanding of cybersecurity fundamentals and CIS Benchmarks.
- Experience with a policy-as-code engine like OPA Rego.
- Knowledge of cloud infrastructure (AWS) and containers (Docker/Kubernetes).
- Familiarity with compliance frameworks and security standards.
What You'll Learn
🏷️ Topics Covered
What Are CIS Benchmarks?
The Center for Internet Security (CIS) Benchmarks are globally recognized security configuration guidelines. They provide prescriptive, consensus-driven best practices for securing over 100 different technologies, from operating systems and cloud services to network devices. Automating CIS Benchmark compliance with policy-as-code ensures a consistent security posture and dramatically reduces the manual effort of compliance validation.
🎯 Purpose
To provide actionable security configuration recommendations that reduce vulnerabilities, establish secure baselines, and support audit requirements.
📊 Structure
Benchmarks are organized into logical sections with specific, numbered controls, implementation profiles (Level 1, Level 2), and detailed audit and remediation guidance.
🔄 Process
Developed through a community consensus process involving volunteer security experts from around the world, ensuring they are practical and address emerging threats.
How to Translate CIS Controls into Automated Policies
Translating a CIS Benchmark control into an automated policy is a systematic process. It involves understanding the control's intent, identifying the data needed for validation, and then writing the corresponding policy logic.
Analyze the Control
Read the full CIS control description, rationale, and audit procedure. Understand the specific security risk it's designed to mitigate.
Identify the Data Source
Determine which configuration file, API response, or runtime state contains the information needed to verify the control. For Terraform, this is the plan; for Kubernetes, it's the admission request.
Implement the Policy Logic
Write the policy (e.g., in Rego) to check the data source against the control's requirements. The policy should return a clear violation message if the configuration is non-compliant.
Test and Validate
Test the policy against both compliant and non-compliant configurations to ensure it behaves as expected and doesn't produce false positives.
From Control to Code: Practical Examples
Let's see this process in action by translating two common CIS controls into OPA Rego policies.
AWS CIS Control 2.1.1: Ensure S3 Bucket Logging is Enabled
This policy checks a Terraform plan to verify that any new S3 bucket has server access logging configured, a key requirement for auditing and security analysis.
{`package terraform.cis
# CIS AWS Foundations Benchmark v1.4.0, Control 2.1.1
# "Ensure CloudTrail log file validation is enabled"
# This policy checks a more common related control: S3 bucket access logging.
deny[msg] {
# Find all S3 buckets being created
s3_bucket := input.resource_changes[_]
s3_bucket.type == "aws_s3_bucket"
s3_bucket.change.actions[0] == "create"
# Check if the 'logging' block is missing or empty
not s3_bucket.change.after.logging
# If so, generate a violation message
msg := sprintf("S3 bucket '%s' does not have server access logging enabled.", [s3_bucket.name])
}`}Docker CIS Control 5.1: Do Not Run Containers as Root
This policy is for Kubernetes admission control. It inspects incoming Pod specifications and denies any container that is configured to run as the root user, a critical security best practice.
{`package kubernetes.cis
# CIS Docker Benchmark v1.3.1, Control 5.1
# Ensure that containers are not run with the root user
deny[msg] {
# Check all containers in the incoming pod spec
container := input.request.object.spec.containers[_]
# The 'runAsUser' field is either not set, or set to 0 (root)
not container.securityContext.runAsUser
msg := sprintf("Container '%s' must not run as root. Set securityContext.runAsUser to a non-zero value.", [container.name])
}
deny[msg] {
container := input.request.object.spec.containers[_]
container.securityContext.runAsUser == 0
msg := sprintf("Container '%s' must not run as root (runAsUser is 0). Set securityContext.runAsUser to a non-zero value.", [container.name])
}`}Implementation Best Practices
💡 Key Takeaways
- Start with Priority Controls: Focus on high-impact, high-risk CIS controls first (like public access and root account usage) rather than trying to implement everything at once.
- Use Official Tools for Validation: When possible, compare your policy results against official tools like CIS-CAT Pro or open-source scanners like Prowler and kube-bench to ensure accuracy.
- Integrate into CI/CD: The most effective way to enforce CIS benchmarks is by "shifting left" and integrating policy checks directly into your CI/CD pipelines (e.g., GitHub Actions).
- Implement Gradual Rollout: Deploy policies in a "monitor-only" mode first to assess their impact. Then, move to active enforcement, starting with non-production environments.
- Manage Exceptions: Implement a formal process for documenting, reviewing, and approving temporary or permanent exceptions to CIS controls when a business need arises.
- Continuous Monitoring: Don't rely solely on pre-deployment checks. Use policy-as-code to continuously monitor your runtime environments for configuration drift that violates CIS benchmarks.