policy-as-code – Coding skill for AI agents

Policy as Code

Automate policy enforcement through code using OPA/Rego, Kyverno, Checkov, and CI/CD integration to prevent compliance violations before they reach production.

Enforcement contract

Examples cover selected controls, not full compliance. Terraform plan JSON can

contain secrets: restrict artifact permissions/retention and never upload it as a

public artifact. Planning runs providers/data sources and needs approved account

scope; do not run untrusted PR configuration with production credentials.

opa eval normally exits successfully even when deny contains findings; use a

reviewed aggregate decision or Conftest with all namespaces and fail on parse,

missing input, unknown values and missing required policy bundles. Resolve the

provider version/schema first: ACLs, encryption and tags use different resources.

The S3 versioning resource check cannot detect a bucket with no versioning resource;

add a graph relationship check. Kyverno examples require compatible CRDs, audit

rollout, full container/init/ephemeral coverage and separate digest/signature

verification: banning latest does not make another mutable tag trustworthy.

Open Policy Agent (OPA) Rego Policies

# deny_public_s3.rego - Deny S3 buckets with public access
package terraform.aws.s3

import rego.v1

deny contains msg if {
    resource := input.resource_changes[_]
    resource.type in {"aws_s3_bucket", "aws_s3_bucket_acl"}
    resource.change.after.acl == "public-read"
    msg := sprintf(
        "S3 bucket '%s' has public-read ACL. All buckets must be private. [Policy: no-public-s3]",
        [resource.address]
    )
}

deny contains msg if {
    resource := input.resource_changes[_]
    resource.type in {"aws_s3_bucket", "aws_s3_bucket_acl"}
    resource.change.after.acl == "public-read-write"
    msg := sprintf(
        "S3 bucket '%s' has public-read-write ACL. This is strictly prohibited. [Policy: no-public-s3]",
        [resource.address]
    )
}

# require_encryption.rego - Require encryption on data stores
package terraform.aws.encryption

import rego.v1

deny contains msg if {
    resource := input.resource_changes[_]
    resource.type == "aws_db_instance"
    resource.change.after != null
    resource.change.after.storage_encrypted != true
    msg := sprintf(
        "RDS instance '%s' does not have storage encryption enabled. [Policy: require-rds-encryption]",
        [resource.address]
    )
}

deny contains msg if {
    resource := input.resource_changes[_]
    resource.type == "aws_ebs_volume"
    resource.change.after != null
    resource.change.after.encrypted != true
    msg := sprintf(
        "EBS volume '%s' is not encrypted. [Policy: require-ebs-encryption]",
        [resource.address]
    )
}

# S3 encryption is modeled by a separate Terraform resource in current providers.
# Join aws_s3_bucket_server_side_encryption_configuration to its bucket and inspect
# key/SSE settings; this snippet intentionally makes no S3 coverage claim.

# require_tags.rego - Enforce mandatory tagging
package terraform.aws.tags

import rego.v1

required_tags := {"Environment", "Owner", "CostCenter", "DataClassification"}

deny contains msg if {
    resource := input.resource_changes[_]
    resource.mode == "managed"
    resource.type in {"aws_instance", "aws_db_instance", "aws_ebs_volume", "aws_s3_bucket"}
    resource.change.after != null
    tags := object.get(resource.change.after, "tags_all", {})
    missing := required_tags - {key | value := tags[key]; is_string(value); trim_space(value) != ""}
    count(missing) > 0
    msg := sprintf(
        "Resource '%s' is missing required tags: %v. [Policy: required-tags]",
        [resource.address, missing]
    )
}

Region enforcement must resolve each resource's provider alias/module mapping and

unknown expressions. Reading only provider_config.aws silently misses aliased or

variable-driven regions. Require a normalized resolved region per resource from a

reviewed adapter, and deny unknown mappings; no generic region policy is supplied.

# Evaluate OPA policies against Terraform plan
terraform plan -out=tfplan
terraform show -json tfplan > tfplan.json

# Run OPA evaluation
opa eval \
  --data policies/ \
  --input tfplan.json \
  "data.terraform.aws.s3.deny" \
  --format pretty

# Use conftest for easier CI integration
conftest test tfplan.json --policy policies/ --all-namespaces --output table

Kyverno Kubernetes Policies

# require-labels.yaml - Enforce required labels on all pods
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: require-labels
  annotations:
    policies.kyverno.io/title: Require Labels
    policies.kyverno.io/category: Best Practices
    policies.kyverno.io/severity: medium
spec:
  validationFailureAction: Enforce
  background: true
  rules:
  - name: check-required-labels
    match:
      any:
      - resources:
          kinds:
          - Pod
    validate:
      message: >-
        Labels 'app.kubernetes.io/name', 'app.kubernetes.io/version',
        and 'team' are required on all Pods.
      pattern:
        metadata:
          labels:
            app.kubernetes.io/name: "?*"
            app.kubernetes.io/version: "?*"
            team: "?*"
---
# disallow-privileged.yaml - Block privileged containers
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: disallow-privileged-containers
  annotations:
    policies.kyverno.io/title: Disallow Privileged Containers
    policies.kyverno.io/category: Pod Security
    policies.kyverno.io/severity: high
spec:
  validationFailureAction: Enforce
  background: true
  rules:
  - name: deny-privileged
    match:
      any:
      - resources:
          kinds:
          - Pod
    validate:
      message: "Privileged containers are not allowed."
      pattern:
        spec:
          containers:
          - =(securityContext):
              =(privileged): false
          =(initContainers):
          - =(securityContext):
              =(privileged): false
          =(ephemeralContainers):
          - =(securityContext):
              =(privileged): false
---
# require-resource-limits.yaml - Enforce resource limits
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: require-resource-limits
  annotations:
    policies.kyverno.io/title: Require Resource Limits
    policies.kyverno.io/severity: medium
spec:
  validationFailureAction: Enforce
  background: true
  rules:
  - name: check-resource-limits
    match:
      any:
      - resources:
          kinds:
          - Pod
    validate:
      message: "All containers must have CPU and memory limits defined."
      pattern:
        spec:
          containers:
          - resources:
              limits:
                memory: "?*"
                cpu: "?*"
---
# disallow-latest-tag.yaml - Block usage of 'latest' image tag
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: disallow-latest-tag
  annotations:
    policies.kyverno.io/title: Disallow Latest Tag
    policies.kyverno.io/severity: medium
spec:
  validationFailureAction: Enforce
  background: true
  rules:
  - name: validate-image-tag
    match:
      any:
      - resources:
          kinds:
          - Pod
    validate:
      message: "Images must use a specific tag, not 'latest'."
      pattern:
        spec:
          containers:
          - image: "!*:latest & *:*"
---
# restrict-image-registries.yaml - Allow only approved registries
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: restrict-image-registries
  annotations:
    policies.kyverno.io/title: Restrict Image Registries
    policies.kyverno.io/severity: high
spec:
  validationFailureAction: Enforce
  background: true
  rules:
  - name: validate-registries
    match:
      any:
      - resources:
          kinds:
          - Pod
    validate:
      message: >-
        Images must come from approved registries:
        123456789012.dkr.ecr.us-east-1.amazonaws.com or ghcr.io/your-org.
      pattern:
        spec:
          containers:
          - image: "123456789012.dkr.ecr.*.amazonaws.com/* | ghcr.io/your-org/*"
---
# NetworkPolicy presence cannot be checked by comparing a namespace string to itself.
# Use a reviewed apiCall context with scoped RBAC to list policies, then verify an
# applicable default-deny policy and CNI enforcement. This requires integration tests;
# absence of a real policy query must never be represented as a passing control.

Checkov Custom Checks

# custom_checks/require_s3_versioning.py
from checkov.terraform.checks.resource.base_resource_check import BaseResourceCheck
from checkov.common.models.enums import CheckResult, CheckCategories


class S3Versioning(BaseResourceCheck):
    def __init__(self):
        name = "Ensure S3 bucket has versioning enabled"
        id = "CUSTOM_S3_001"
        supported_resources = ["aws_s3_bucket_versioning"]
        categories = [CheckCategories.BACKUP_AND_RECOVERY]
        sup

… (full content is in the download package)