Implementing Zero Trust with Istio Authorization Policies

The concept of a secure network perimeter is becoming obsolete. In a modern, cloud-native world, we must adopt a Zero Trust security model, which assumes that threats can exist both inside and outside the network.

The core principle is simple: never trust, always verify.

Istio, a powerful open-source service mesh, provides the tools to implement Zero Trust principles at the service level. Using Istio’s AuthorizationPolicy, you can define fine-grained access control between workloads deciding who can talk to whom, which methods are allowed, and under what identity.

Think of it like the compartmentalized hull of a ship: if one section is breached, watertight doors seal it off to prevent the entire vessel from flooding. In Istio, each service becomes its own compartment, or micro-perimeter.

Scenario Overview

In this walkthrough, you’ll:

• Start a Minikube cluster with Istio installed,
• Deploy two BusyBox pods starboard and portside,
• Apply a default-deny policy,
• Then explicitly allow only one-way communication from starboard to portside.

This setup models a Zero Trust ship where only one compartment can talk to another through a sealed hatch.

Step 0: Start Minikube

First, start a local Kubernetes cluster with enough resources for Istio:

Check that it’s running:

You should see your Minikube node in Ready state.

Step 1: Install Istio

Download and install Istio (v1.22+ recommended):

Install the Istio control plane using the demo profile:

Label the default namespace for automatic sidecar injection:

Step 2: Deploy Two BusyBox Pods

Deploy your “ship compartments” starboard and portside:

Expose portside as a service so it has a stable address:

Verify both are running:

You should see starboard and portside pods up and running.

Step 3: Enforce a Namespace-Wide Default Deny Policy

By default, Istio allows all traffic between workloads. To enforce Zero Trust, we start by denying everything.

Create a file called deny-all.yaml:

Apply it:

What This Does

• Applies to all workloads in the default namespace.
• Since there are no rules, it denies every request by default.
• This sets your Zero Trust baseline. Nothing talks until explicitly allowed.

You can test it:

Expected output: wget: error getting response: Connection refused

Step 4: Allow Starboard to Communicate with Portside

Now, let’s open one “watertight door”: only the starboard service account can make GET requests to portside.

Create allow-starboard-to-portside.yaml:

Apply it:

Step 5: Test Connectivity

Run a request from starboard to portside:

You should now see a 200 OK response (or an empty connection, but no error).

Now test in the opposite direction (portside to starboard):

Expected output: wget: error getting response: Connection refused

Only one-way access is allowed our “ship compartment” logic is working.

Step 6: Verify the Active Policies

Check which AuthorizationPolicies are active:

Step 7: Clean Up

When you’re done experimenting:

Key Takeaways

This pattern deny everything, allow by necessity is the foundation of Zero Trust in Istio.

In summary:

• Start with a namespace-wide default-deny baseline.
• Add explicit ALLOW policies for legitimate paths.
• Treat each service as a compartment strong boundaries, minimal trust.

This not only enforces Zero Trust but also builds a clear audit trail of every permitted communication path. Configuration, not hope, becomes your security model.