Mutual TLS Authentication

Istio aims to enhance the security of microservices and their communication without requiring service code changes. It is responsible for:

Providing each service with a strong identity that represents its role to enable interoperability across clusters and clouds
Securing service to service communication and end-user to service communication
Providing a key management system to automate key and certificate generation, distribution, rotation, and revocation

Architecture

The diagram below shows Istio's security-related architecture, which includes three primary components: identity, key management, and communication security. This diagram describes how Istio is used to secure the service-to-service communication between service ‘frontend’ running as the service account ‘frontend-team’ and service ‘backend’ running as the service account ‘backend-team’. Istio supports services running on both Kubernetes containers and VM/bare-metal machines.

As illustrated in the diagram, Istio leverages secret volume mount to deliver keys/certs from Citadel to Kubernetes containers. For services running on VM/bare-metal machines, we introduce a node agent, which is a process running on each VM/bare-metal machine. It generates the private key and CSR (certificate signing request) locally, sends CSR to Citadel for signing, and delivers the generated certificate together with the private key to Envoy.

Components

Identity

Istio uses Kubernetes service accounts to identify who runs the service:

A service account in Istio has the format “spiffe://<_domain_>/ns/<_namespace_>/sa/<_serviceaccount_>”.
- domain is currently cluster.local. We will support customization of domain in the near future.
- namespace is the namespace of the Kubernetes service account.
- serviceaccount is the Kubernetes service account name.
A service account is the identity (or role) a workload runs as, which represents that workload's privileges. For systems requiring strong security, the amount of privilege for a workload should not be identified by a random string (i.e., service name, label, etc), or by the binary that is deployed.
- For example, let's say we have a workload pulling data from a multi-tenant database. If Alice ran this workload, she will be able to pull a different set of data than if Bob ran this workload.
Service accounts enable strong security policies by offering the flexibility to identify a machine, a user, a workload, or a group of workloads (different workloads can run as the same service account).
The service account a workload runs as won't change during the lifetime of the workload.
Service account uniqueness can be ensured with domain name constraint

Communication security

Service-to-service communication is tunneled through the client side Envoy and the server side Envoy. End-to-end communication is secured by:

Local TCP connections between the service and Envoy
Mutual TLS connections between proxies
Secure Naming: during the handshake process, the client side Envoy checks that the service account provided by the server side certificate is allowed to run the target service

Key management

Istio 0.2 supports services running on both Kubernetes pods and VM/bare-metal machines. We use different key provisioning mechanisms for each scenario.

For services running on Kubernetes pods, the per-cluster Citadel (acting as Certificate Authority) automates the key & certificate management process. It mainly performs four critical operations:

Generate a SPIFFE key and certificate pair for each service account
Distribute a key and certificate pair to each pod according to the service account
Rotate keys and certificates periodically
Revoke a specific key and certificate pair when necessary

For services running on VM/bare-metal machines, the above four operations are performed by Citadel together with node agents.

Workflow

The Istio Security workflow consists of two phases, deployment and runtime. For the deployment phase, we discuss the two scenarios (i.e., in Kubernetes and VM/bare-metal machines) separately since they are different. Once the key and certificate are deployed, the runtime phase is the same for the two scenarios. We briefly cover the workflow in this section.

Deployment phase (Kubernetes Scenario)

Citadel watches the Kubernetes API Server, creates a SPIFFE key and certificate pair for each of the existing and new service accounts, and sends them to the API Server.
When a pod is created, API Server mounts the key and certificate pair according to the service account using Kubernetes secrets.
Pilot generates the config with proper key and certificate and secure naming information, which defines what service account(s) can run a certain service, and passes it to Envoy.

Deployment phase (VM/bare-metal Machines Scenario)

Citadel creates a gRPC service to take CSR request.
Node agent creates the private key and CSR, sends the CSR to Citadel for signing.
Citadel validates the credentials carried in the CSR, and signs the CSR to generate the certificate.
Node agent puts the certificate received from Citadel and the private key to Envoy.
The above CSR process repeats periodically for rotation.

Runtime phase

The outbound traffic from a client service is rerouted to its local Envoy.
The client side Envoy starts a mutual TLS handshake with the server side Envoy. During the handshake, it also does a secure naming check to verify that the service account presented in the server certificate can run the server service.
The traffic is forwarded to the server side Envoy after mTLS connection is established, which is then forwarded to the server service through local TCP connections.

Best practices

In this section, we provide a few deployment guidelines and then discuss a real-world scenario.

Deployment guidelines

If there are multiple service operators (a.k.a. SREs) deploying different services in a cluster (typically in a medium- or large-size cluster), we recommend creating a separate namespace for each SRE team to isolate their access. For example, you could create a “team1-ns” namespace for team1, and “team2-ns” namespace for team2, such that both teams won't be able to access each other's services.
If Citadel is compromised, all its managed keys and certificates in the cluster may be exposed. We strongly recommend running Citadel on a dedicated namespace (for example, istio-citadel-ns), which only cluster admins have access to.

Example

Let's consider a 3-tier application with three services: photo-frontend, photo-backend, and datastore. Photo-frontend and photo-backend services are managed by the photo SRE team while the datastore service is managed by the datastore SRE team. Photo-frontend can access photo-backend, and photo-backend can access datastore. However, photo-frontend cannot access datastore.

In this scenario, a cluster admin creates 3 namespaces: istio-citadel-ns, photo-ns, and datastore-ns. Admin has access to all namespaces, and each team only has access to its own namespace. The photo SRE team creates 2 service accounts to run photo-frontend and photo-backend respectively in namespace photo-ns. The datastore SRE team creates 1 service account to run the datastore service in namespace datastore-ns. Moreover, we need to enforce the service access control in Istio Mixer such that photo-frontend cannot access datastore.

In this setup, Citadel is able to provide keys and certificates management for all namespaces, and isolate microservice deployments from each other.

Future work

Inter-cluster service-to-service authentication
Powerful authorization mechanisms: ABAC, RBAC, etc
Per-service auth enablement support
Secure Istio components (Mixer, Pilot)
End-user to service authentication using JWT/OAuth2/OpenID_Connect.
Support GCP service account
Unix domain socket for local communication between service and Envoy
Middle proxy support
Pluggable key management component