Better External Authorization

AuthorizationPolicy now supports CUSTOM action to delegate the authorization to external system.

Feb 9, 2021 | By Yangmin Zhu - Google

Background

Istio’s authorization policy provides access control for services in the mesh. It is fast, powerful and a widely used feature. We have made continuous improvements to make policy more flexible since its first release in Istio 1.4, including the DENY action, exclusion semantics, X-Forwarded-For header support, nested JWT claim support and more. These features improve the flexibility of the authorization policy, but there are still many use cases that cannot be supported with this model, for example:

Solution

In Istio 1.9, we have implemented extensibility into authorization policy by introducing a CUSTOM action, which allows you to delegate the access control decision to an external authorization service.

The CUSTOM action allows you to integrate Istio with an external authorization system that implements its own custom authorization logic. The following diagram shows the high level architecture of this integration:

External Authorization Architecture
External Authorization Architecture

At configuration time, the mesh admin configures an authorization policy with a CUSTOM action to enable the external authorization on a proxy (either gateway or sidecar). The admin should verify the external auth service is up and running.

At runtime,

  1. A request is intercepted by the proxy, and the proxy will send check requests to the external auth service, as configured by the user in the authorization policy.

  2. The external auth service will make the decision whether to allow it or not.

  3. If allowed, the request will continue and will be enforced by any local authorization defined by ALLOW/DENY action.

  4. If denied, the request will be rejected immediately.

Let’s look at an example authorization policy with the CUSTOM action:

apiVersion: security.istio.io/v1beta1
kind: AuthorizationPolicy
metadata:
  name: ext-authz
  namespace: istio-system
spec:
  # The selector applies to the ingress gateway in the istio-system namespace.
  selector:
    matchLabels:
      app: istio-ingressgateway
  # The action "CUSTOM" delegates the access control to an external authorizer, this is different from
  # the ALLOW/DENY action that enforces the access control right inside the proxy.
  action: CUSTOM
  # The provider specifies the name of the external authorizer defined in the meshconfig, which tells where and how to
  # talk to the external auth service. We will cover this more later.
  provider:
    name: "my-ext-authz-service"
  # The rule specifies that the access control is triggered only if the request path has the prefix "/admin/".
  # This allows you to easily enable or disable the external authorization based on the requests, avoiding the external
  # check request if it is not needed.
  rules:
  - to:
    - operation:
        paths: ["/admin/*"]

It refers to a provider called my-ext-authz-service which is defined in the mesh config:

extensionProviders:
# The name "my-ext-authz-service" is referred to by the authorization policy in its provider field.
- name: "my-ext-authz-service"
  # The "envoyExtAuthzGrpc" field specifies the type of the external authorization service is implemented by the Envoy
  # ext-authz filter gRPC API. The other supported type is the Envoy ext-authz filter HTTP API.
  # See more in https://www.envoyproxy.io/docs/envoy/v1.16.2/intro/arch_overview/security/ext_authz_filter.
  envoyExtAuthzGrpc:
    # The service and port specifies the address of the external auth service, "ext-authz.istio-system.svc.cluster.local"
    # means the service is deployed in the mesh. It can also be defined out of the mesh or even inside the pod as a separate
    # container.
    service: "ext-authz.istio-system.svc.cluster.local"
    port: 9000

The authorization policy of CUSTOM action enables the external authorization in runtime, it could be configured to trigger the external authorization conditionally based on the request using the same rule that you have already been using with other actions.

The external authorization service is currently defined in the meshconfig API and referred to by its name. It could be deployed in the mesh with or without proxy. If with the proxy, you could further use PeerAuthentication to enable mTLS between the proxy and your external authorization service.

The CUSTOM action is currently in the experimental stage; the API might change in a non-backward compatible way based on user feedback. The rule currently does not support authentication related fields (e.g. source principal or JWT claim) and only one provider is allowed for a given workload, but you can still use different providers on different workloads.

For more information, please see the Better External Authorization design doc.

Example with OPA

In this section, we will demonstrate using the CUSTOM action with the Open Policy Agent as the external authorizer on the ingress gateway. We will conditionally enable the external authorization on all paths except /ip.

You can also refer to the external authorization task for a more basic introduction that uses a sample ext-authz server.

Create the example OPA policy

Run the following command create an OPA policy that allows the request if the prefix of the path is matched with the claim “path” (base64 encoded) in the JWT token:

$ cat > policy.rego <<EOF
package envoy.authz

import input.attributes.request.http as http_request

default allow = false

token = {"valid": valid, "payload": payload} {
    [_, encoded] := split(http_request.headers.authorization, " ")
    [valid, _, payload] := io.jwt.decode_verify(encoded, {"secret": "secret"})
}

allow {
    is_token_valid
    action_allowed
}

is_token_valid {
  token.valid
  now := time.now_ns() / 1000000000
  token.payload.nbf <= now
  now < token.payload.exp
}

action_allowed {
  startswith(http_request.path, base64url.decode(token.payload.path))
}
EOF
$ kubectl create secret generic opa-policy --from-file policy.rego

Deploy httpbin and OPA

Enable the sidecar injection:

$ kubectl label ns default istio-injection=enabled

Run the following command to deploy the example application httpbin and OPA. The OPA could be deployed either as a separate container in the httpbin pod or completely in a separate pod:

$ kubectl apply -f - <<EOF
apiVersion: v1
kind: Service
metadata:
  name: httpbin-with-opa
  labels:
    app: httpbin-with-opa
    service: httpbin-with-opa
spec:
  ports:
  - name: http
    port: 8000
    targetPort: 80
  selector:
    app: httpbin-with-opa
---
# Define the service entry for the local OPA service on port 9191.
apiVersion: networking.istio.io/v1alpha3
kind: ServiceEntry
metadata:
  name: local-opa-grpc
spec:
  hosts:
  - "local-opa-grpc.local"
  endpoints:
  - address: "127.0.0.1"
  ports:
  - name: grpc
    number: 9191
    protocol: GRPC
  resolution: STATIC
---
kind: Deployment
apiVersion: apps/v1
metadata:
  name: httpbin-with-opa
  labels:
    app: httpbin-with-opa
spec:
  replicas: 1
  selector:
    matchLabels:
      app: httpbin-with-opa
  template:
    metadata:
      labels:
        app: httpbin-with-opa
    spec:
      containers:
        - image: docker.io/kennethreitz/httpbin
          imagePullPolicy: IfNotPresent
          name: httpbin
          ports:
          - containerPort: 80
        - name: opa
          image: openpolicyagent/opa:latest-envoy
          securityContext:
            runAsUser: 1111
          volumeMounts:
          - readOnly: true
            mountPath: /policy
            name: opa-policy
          args:
          - "run"
          - "--server"
          - "--addr=localhost:8181"
          - "--diagnostic-addr=0.0.0.0:8282"
          - "--set=plugins.envoy_ext_authz_grpc.addr=:9191"
          - "--set=plugins.envoy_ext_authz_grpc.query=data.envoy.authz.allow"
          - "--set=decision_logs.console=true"
          - "--ignore=.*"
          - "/policy/policy.rego"
          livenessProbe:
            httpGet:
              path: /health?plugins
              scheme: HTTP
              port: 8282
            initialDelaySeconds: 5
            periodSeconds: 5
          readinessProbe:
            httpGet:
              path: /health?plugins
              scheme: HTTP
              port: 8282
            initialDelaySeconds: 5
            periodSeconds: 5
      volumes:
        - name: proxy-config
          configMap:
            name: proxy-config
        - name: opa-policy
          secret:
            secretName: opa-policy
EOF

Define external authorizer

Run the following command to edit the meshconfig:

$ kubectl edit configmap istio -n istio-system

Add the following extensionProviders to the meshconfig:

apiVersion: v1
data:
  mesh: |-
    # Add the following contents:
    extensionProviders:
    - name: "opa.local"
      envoyExtAuthzGrpc:
        service: "local-opa-grpc.local"
        port: "9191"

Create an AuthorizationPolicy with a CUSTOM action

Run the following command to create the authorization policy that enables the external authorization on all paths except /ip:

$ kubectl apply -f - <<EOF
apiVersion: security.istio.io/v1beta1
kind: AuthorizationPolicy
metadata:
  name: httpbin-opa
spec:
  selector:
    matchLabels:
      app: httpbin-with-opa
  action: CUSTOM
  provider:
    name: "opa.local"
  rules:
  - to:
    - operation:
        notPaths: ["/ip"]
EOF

Test the OPA policy

  1. Create a client pod to send the request:

    Zip
    $ kubectl apply -f @samples/sleep/sleep.yaml@
$ export SLEEP_POD=$(kubectl get pod -l app=sleep -o jsonpath={.items..metadata.name})

  2. Use a test JWT token signed by the OPA:

    $ export TOKEN_PATH_HEADERS="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJwYXRoIjoiTDJobFlXUmxjbk09IiwibmJmIjoxNTAwMDAwMDAwLCJleHAiOjE5MDAwMDAwMDB9.9yl8LcZdq-5UpNLm0Hn0nnoBHXXAnK4e8RSl9vn6l98"

    The test JWT token has the following claims:

    {
  "path": "L2hlYWRlcnM=",
  "nbf": 1500000000,
  "exp": 1900000000
}

    The path claim has value L2hlYWRlcnM= which is the base64 encode of /headers.

  3. Send a request to path /headers without a token. This should be rejected with 403 because there is no JWT token:

    $ kubectl exec ${SLEEP_POD} -c sleep  -- curl http://httpbin-with-opa:8000/headers -s -o /dev/null -w "%{http_code}\n"
403

  4. Send a request to path /get with a valid token. This should be rejected with 403 because the path /get is not matched with the token /headers:

    $ kubectl exec ${SLEEP_POD} -c sleep  -- curl http://httpbin-with-opa:8000/get -H "Authorization: Bearer $TOKEN_PATH_HEADERS" -s -o /dev/null -w "%{http_code}\n"
403

  5. Send a request to path /headers with valid token. This should be allowed with 200 because the path is matched with the token:

    $ kubectl exec ${SLEEP_POD} -c sleep  -- curl http://httpbin-with-opa:8000/headers -H "Authorization: Bearer $TOKEN_PATH_HEADERS" -s -o /dev/null -w "%{http_code}\n"
200

  6. Send request to path /ip without token. This should be allowed with 200 because the path /ip is excluded from authorization:

    $ kubectl exec ${SLEEP_POD} -c sleep  -- curl http://httpbin-with-opa:8000/ip -s -o /dev/null -w "%{http_code}\n"
200

  7. Check the proxy and OPA logs to confirm the result.

Summary

In Istio 1.9, the CUSTOM action in the authorization policy allows you to easily integrate Istio with any external authorization system with the following benefits:

We’re working to promote this feature to a more stable stage in following versions and welcome your feedback at discuss.istio.io.

Acknowledgements

Thanks to Craig Box, Christian Posta and Limin Wang for reviewing drafts of this blog.