Kubernetes NetworkPolicies restrict pod-to-pod and pod-to-external traffic

Without NetworkPolicies, any compromised pod in your cluster can initiate connections to any other pod or external service — an attacker who owns the frontend container can directly query your database, scrape environment variables from other pods, or exfiltrate data to an external server. NIST 800-53 SC-7 and AC-4 require network segmentation and information flow enforcement. CIS Kubernetes 5.3.2 mandates network policies. Default Kubernetes networking is flat — every pod can reach every other pod by default, which is the opposite of least-privilege.

Deploy default-deny NetworkPolicies in every application namespace, then explicitly allow required communication paths. In k8s/network-policies.yaml:

---
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: default-deny-all
  namespace: production
spec:
  podSelector: {}
  policyTypes:
    - Ingress
    - Egress
---
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-api-to-db
  namespace: production
spec:
  podSelector:
    matchLabels:
      role: database
  policyTypes:
    - Ingress
  ingress:
    - from:
        - podSelector:
            matchLabels:
              role: api
      ports:
        - protocol: TCP
          port: 5432

Apply the deny-all policy first, then layer explicit allow rules — not the reverse.

• ID: network-policies

• Severity: high

• What to look for: Count all NetworkPolicy resources across all Kubernetes namespaces and manifest directories. For each namespace, check whether a default-deny NetworkPolicy exists for both Ingress and Egress. Enumerate the policyTypes covered by each NetworkPolicy and verify at least 1 NetworkPolicy per namespace enforces default-deny.

• Pass criteria: NetworkPolicies are defined with default-deny rules in every namespace that runs application pods. Both Ingress and Egress traffic are restricted to specific pods and ports via explicit allow rules. At least 1 default-deny NetworkPolicy exists per namespace. Report: "X NetworkPolicies found across Y namespaces."

• Fail criteria: No NetworkPolicy resources found, or policies are present but overly permissive (allow all traffic, or missing either ingress or egress policyTypes).

• Skip (N/A) when: The project does not use Kubernetes, or runs on a non-CNI platform that does not support NetworkPolicies.

• Cross-reference: The egress-restricted check further verifies that egress is not open to 0.0.0.0/0.

• Detail on fail: Quote the namespace and policy gap. Example: "No NetworkPolicy found in namespace 'production'. Pod-to-pod traffic is unrestricted." or "NetworkPolicy exists but allows traffic from all namespaces via podSelector: {}."

• Remediation: Implement default-deny NetworkPolicies and explicitly allow required traffic:

  # Default deny all ingress
  apiVersion: networking.k8s.io/v1
  kind: NetworkPolicy
  metadata:
    name: default-deny-ingress
  spec:
    podSelector: {}
    policyTypes:
      - Ingress
  
  # Allow traffic from app to database
  apiVersion: networking.k8s.io/v1
  kind: NetworkPolicy
  metadata:
    name: allow-app-to-db
  spec:
    podSelector:
      matchLabels:
        role: database
    policyTypes:
      - Ingress
    ingress:
      - from:
          - podSelector:
              matchLabels:
                role: app
        ports:
          - protocol: TCP
            port: 5432