Kubernetes Security Hardening

Overview

Run a Pod with a restrictive securityContext, enable Pod Security Admission labels on a namespace, and draft a default-deny NetworkPolicy pattern.

Defence in depth: RBAC + securityContext + Pod Security Admission (PSA) + NetworkPolicy + signed/scanned images + Secrets hygiene.

This is a core tutorial in Module 10 · Security of the REBASH Academy Kubernetes for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites

• RBAC and Kubernetes Security Basics

Learning Objectives

By the end of this tutorial, you will be able to:

• Set runAsNonRoot, drop capabilities
• Label ns for PSA (restricted/baseline)
• Explain NetworkPolicy default-deny
• List image policy controls

Architecture

This topic’s control points and relationships are shown below.

Theory

What it is

Hardening layers controls beyond basic RBAC. securityContext settings run containers as non-root, drop Linux capabilities, and disallow privilege escalation. Pod Security Admission (PSA) enforces baseline/restricted profiles per namespace. NetworkPolicies restrict Pod-to-Pod and egress traffic (enforced by the CNI). Image hygiene — digests, scanning, admission policies — reduces supply-chain risk. Secrets encryption and audit logging complete the picture.

Why it matters

A cluster with open RBAC but privileged Pods and unrestricted east-west traffic is one CVE away from lateral movement. CKS-oriented DevOps treats defence in depth as normal: identity, workload, network, and supply chain each fail closed where practical.

How it works (mental model)

1. Workload: Pod/container securityContext + PSA labels (enforce=restricted when apps allow).
2. Network: default-deny NetworkPolicy, then allow only needed ingress/egress.
3. Identity: dedicated SAs, no unnecessary secret mounts, short-lived tokens where possible.
4. Supply chain: pin digests, scan in CI, optional admission (Kyverno/OPA Gatekeeper) to block :latest or unsigned images.
5. Controllers still reconcile — hardening constrains what may run, not the reconcile loop itself.

PSA replaces the older PodSecurityPolicy API; learn labels, not PSP.

Key concepts / comparisons

Control	Layer
RBAC	API access
securityContext	Process privileges
PSA	Admission of Pod specs
NetworkPolicy	East-west / egress
Image policy	What may be pulled/run

PSA level	Strictness
privileged	No restriction
baseline	Minimally opinionated
restricted	Hardened defaults

Common pitfalls

• Labelling enforce=restricted without fixing images that require root — mass Pending/reject.
• Writing NetworkPolicies when the CNI does not enforce them — false sense of safety.
• Dropping ALL capabilities but adding back dangerous ones casually.
• Leaving hostNetwork / hostPID enabled on ordinary apps.
• Scanning images once and never again — rebuild pipelines must re-scan.

Hands-on Lab

Create a workspace for this tutorial.

mkdir -p ~/rebash-k8s/module-10-hard && cd ~/rebash-k8s/module-10-hard

Focus: Apply a Pod Security Context and non-root container settings

Step 1 – Deploy a hardened Pod

kubectl create namespace rebash-lab
cat > hardened.yaml <<'EOF'
apiVersion: v1
kind: Pod
metadata:
  name: hardened
  namespace: rebash-lab
spec:
  securityContext:
    runAsNonRoot: true
    runAsUser: 1000
    seccompProfile:
      type: RuntimeDefault
  containers:
  - name: app
    image: busybox:1.36
    command: ["sh", "-c", "id; sleep 3600"]
    securityContext:
      allowPrivilegeEscalation: false
      readOnlyRootFilesystem: true
      capabilities:
drop: ["ALL"]
EOF
kubectl apply -f hardened.yaml
kubectl -n rebash-lab wait --for=condition=Ready pod/hardened --timeout=60s

Step 2 – Verify identity and security fields

kubectl -n rebash-lab exec hardened -- id
kubectl -n rebash-lab get pod hardened -o jsonpath='{.spec.securityContext}{"
"}'
kubectl -n rebash-lab get pod hardened -o jsonpath='{.spec.containers[0].securityContext}{"
"}'

Final step – Cleanup note

kubectl delete namespace rebash-lab --ignore-not-found
# Workspace kept for notes; remove with: rm -rf "$(pwd)" when finished

Validation

• Lab commands run under ~/rebash-k8s/module-10-hard/
• You can explain each Theory section in your own words
• You used modern tooling where it applies to this topic
• You can describe one production failure mode for this topic

Code Walkthrough

Production practice for Kubernetes Security Hardening always combines:

1. Inspect before you change (status, plan, logs, dry-run)
2. Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
3. Capture evidence (command output, pipeline logs) for handovers
4. Prefer current tools and APIs over legacy shortcuts
5. Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations

• Treat credentials and tokens for kubernetes as privileged — never commit them
• Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
• Validate blast radius before apply/deploy/delete operations
• Restrict who can approve production changes
• Collect audit logs; limit who can read sensitive traces

Common Mistakes

Labelling enforce=restricted without fixing images that require root — mass Pending/reje

Validate assumptions against the Theory section and official docs before changing production.

Writing NetworkPolicies when the CNI does not enforce them — false sense of safety.

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices

• Encode Kubernetes Security Hardening changes as code and review them in pull requests
• Pin versions (images, modules, actions, provider plugins)
• Separate environments with clear promotion gates
• Alert on symptoms with runbooks attached
• Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting

Symptom	Likely cause	Fix
Auth / permission denied	Wrong identity, policy, or scope	Check caller identity, roles, and least-privilege policies
Timeout / no route	Network, DNS, security group, or endpoint	Trace path, DNS, and allow-lists before retrying
Drift / unexpected plan	Manual change or wrong state/workspace	Reconcile desired vs actual; avoid click-ops on managed resources
Pipeline/job red	Flaky step, cache, or missing secret	Read failing step logs; bisect recent workflow/config changes
Cost spike	Idle load balancer, NAT, oversized compute	Inventory billable resources; stop/delete labs promptly

Summary

Kubernetes Security Hardening is essential for Cloud and DevOps engineers working with kubernetes. Practise the lab until the inspection and change path is muscle memory, then continue the track.

Interview Questions

1. What does a Pod securityContext control?
2. Why drop Linux capabilities and disable privilege escalation?
3. What is the purpose of readOnlyRootFilesystem?
4. How do admission policies complement runtime securityContext settings?
5. What is a practical hardening checklist for a typical web Deployment?

Sample answer — question 2

Dropping capabilities and setting allowPrivilegeEscalation false reduce the blast radius if a process is compromised, preventing easy root or capability grabs inside the container.

Sample answer — question 4

Admission policies enforce organisational baselines so individual manifests cannot opt into privileged mode. Runtime settings protect each Pod; admission makes the standard mandatory.

Related Tutorials

• Course overview
• Kubernetes Networking Deep Dive

References

• Pod Security Admission · Network Policies