oidc-webhook-authenticator

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oidc-webhook-authenticator

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Kubernetes Webhook Authenticator that allows for dynamic registration of OpenID Connect providers

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# OpenID Connect Webhook Authenticator for Kubernetes
[![REUSE status](https://api.reuse.software/badge/github.com/gardener/oidc-webhook-authenticator)](https://api.reuse.software/info/github.com/gardener/oidc-webhook-authenticator)

## Table of content

- [OpenID Connect Webhook Authenticator for Kubernetes](#openid-connect-webhook-authenticator-for-kubernetes)
  - [Table of content](#table-of-content)
  - [Overview](#overview)
  - [Background](#background)
  - [Use cases](#use-cases)
  - [How it works](#how-it-works)
    - [Registration of a new OpenID Connect provider](#registration-of-a-new-openid-connect-provider)
    - [End-user authentication via new OpenIDConnect IDP](#end-user-authentication-via-new-openidconnect-idp)
  - [Docker images](#docker-images)
  - [Local development](#local-development)

## Overview

The OpenID Connect Webhook Authenticator allows Kubernetes cluster administrators to dynamically register new OpenID Connect providers in their clusters to use for kube-apiserver authentication.

> Note: This repository still in `alpha` stage and in active development. It should not be used in production. The API can change without any backwards compatibility.

## Background

In Kubernetes, only a single OpenID Connect authenticator can be used for end-users to authenticate.

To workaround this limitations, a [Webhook Token Authentication](https://kubernetes.io/docs/reference/access-authn-authz/authentication/#webhook-token-authentication) can be configured. The Kube APIServer then sends the [Bearer Tokens (id_token)](https://tools.ietf.org/html/rfc6750#section-2.1) to an external webhook for validation:

```json
{
  "apiVersion": "authentication.k8s.io/v1beta1",
  "kind": "TokenReview",
  "spec": {
    "token": "(BEARERTOKEN)"
  }
}
```

Where upon verification, the remote webhook returns the identity of the user (if authentication succeeds):

```json
{
  "apiVersion": "authentication.k8s.io/v1beta1",
  "kind": "TokenReview",
  "status": {
    "authenticated": true,
    "user": {
      "username": "janedoe@example.com",
      "uid": "42",
      "groups": [
        "developers",
        "qa"
      ],
      "extra": {
        "extrafield1": [
          "extravalue1",
          "extravalue2"
        ]
      }
    }
  }
}
```

This repository is the out-of tree implementation of [Dynamic Authentication KEP](https://github.com/kubernetes/enhancements/pull/1689).

## Use cases

- Establish trust relationship between different Kubernetes clusters using [Service Account Token Volume Projection](https://kubernetes.io/docs/tasks/configure-pod-container/configure-service-account/#service-account-token-volume-projection) and [Service Account Issuer Discovery](https://kubernetes.io/docs/tasks/configure-pod-container/configure-service-account/#service-account-issuer-discovery).
- Offer cluster admins the option to dynamically allow users from other OIDC IDPs to authenticate against their `kube-apiserver`.

## How it works

This webhook is a Kubernetes controller that acts on `OpenIDConnect` resources e.g:

```yaml
apiVersion: authentication.gardener.cloud/v1alpha1
kind: OpenIDConnect
metadata:
  name: foo
spec:
  issuerURL: https://foo.bar
  audiences:
  - some-client-id
  usernameClaim: email
  usernamePrefix: "test-"
  groupsClaim: groups
  groupsPrefix: "baz-"
  supportedSigningAlgs:
  - RS256
  requiredClaims:
    baz: bar
  caBundle: LS0tLS1CRUdJTiBDRVJU...base64-encoded CA certs for issuerURL.
```

> Note: The fields in the specification corresponds to the [kube-apiserver OIDC flags](https://kubernetes.io/docs/reference/access-authn-authz/authentication/#configuring-the-api-server).

### Registration of a new OpenID Connect provider

The flow is following:

1. Admin adds a new `OpenIDConnect` to the cluster.
1. The webhook controller watches for changes on this resource and does [OIDC discovery](https://openid.net/specs/openid-connect-discovery-1_0.html). The [OIDC provider's  configuration](https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderConfigurationResponse) has to be accessible under the `spec.issuerURL` with a [well-known path (.well-known/openid-configuration)](https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderConfig).
1. The webhook controller uses the `jwks_uri` obtained from the OIDC providers configuration, to fetch the OIDC provider's public keys from that endpoint.
1. The webhook controller uses those keys, issuer, client_id and other settings to add OIDC authenticator to a in-memory list of [Token Authenticators](https://pkg.go.dev/k8s.io/apiserver/pkg/authentication/authenticator?tab=doc#Token).

An overview of the controller:

![alt text](https://raw.githubusercontent.com/gardener/oidc-webhook-authenticator/master/docs/registration.svg "Authentication with OIDC webhook")

### End-user authentication via new OpenIDConnect IDP

When a user wants to authenticate to the `kube-apiserver` via this new Custom OpenIDConnect IDP:

1. The user authenticates in Custom IDP.
1. `id_token` is obtained from Custom IDP (e.g. `ddeewfwef...`).
1. The user uses `id_token` to perform an API call to Kube APIServer.
1. As the `id_token` is not matched by any build-in or configured authenticators in the Kube APIServer, it is send to OpenID Connect Webhook Authenticator for validation.

```json
    {
      "TokenReview": {
        "kind": "TokenReview",
        "apiVersion": "authentication.k8s.io/v1",
        "spec": {
          "token": "ddeewfwef..."
        }
      }
    }
    ```

1. The webhook then iterates over all registered `OpenIDConnect` Token authenticators and tries to validate the token.
1. Upon a successful validation it returns the `TokenReview` with user, groups and extra parameters:

```json
    {
      "TokenReview": {
        "kind": "TokenReview",
        "apiVersion": "authentication.k8s.io/v1",
        "spec": {
          "token": "ddeewfwef..."
        },
        "status": {
          "authenticated": true,
          "user": {
            "username": "test-admin@example.com",
            "groups": [
              "test-some-group"
            ],
            "extra": {
              "gardener.cloud/authenticator/name": [
                "gardener"
              ],
              "gardener.cloud/authenticator/uid": [
                "e5062528-e5a4-4b97-ad83-614d015b0979"
              ]
            }
          }
        }
      }
    }
    ```

It adds the following extra information, that can be used by custom authorizers later on:

- `gardener.cloud/authenticator/name` contains the name of the `OpenIDConnect` authenticator which was used.
- `gardener.cloud/authenticator/uid` contains the UID of the `OpenIDConnect` authenticator which was used.

## Docker images

Docker images are available [here](https://console.cloud.google.com/gcr/images/gardener-project/EU/gardener/oidc-webhook-authenticator) or you can choose to pull the latest pre-release version with the following command:

```text
docker pull europe-docker.pkg.dev/gardener-project/public/gardener/oidc-webhook-authenticator:latest
```

## Local development

For this setup the following components are needed:

- [minikube](https://minikube.sigs.k8s.io/docs/start/)

The API server is started with `--authentication-token-webhook-config-file` with `kubeconfig` file pointing to the Webhook.

```shell
mkdir -p ~/.minikube/files/var/lib/minikube/certs
cp config/samples/minikube-webhook-kubeconfig.yaml ~/.minikube/files/var/lib/minikube/certs/minikube-webhook-kubeconfig.yaml

minikube start \
  --extra-config=apiserver.authentication-token-webhook-config-file=/var/lib/minikube/certs/minikube-webhook-kubeconfig.yaml \
  --extra-config=apiserver.authentication-token-webhook-cache-ttl=10s
```

To allow easy communication between the `kube-apiserver` and the `oidc-webhook-authenticator` minikube IP is added as `control-plane.minikube.internal` in `/etc/hosts`

```shell
sudo sed -ie '/control-plane.minikube.internal/d' /etc/hosts
echo "$(minikube ip) control-plane.minikube.internal" | sudo tee -a /etc/hosts
```

Add the CRD:

```shell
kubectl apply -f config/crd/bases/authentication.gardener.cloud_openidconnects.yaml
```

Build the image, so it's accessible by `minikube`:

```shell
minikube image build -t oidc-webhook-authenticator .
```

Deploy the oidc webhook authenticator.

```shell
kubectl apply -f config/samples/deployment.yaml
```

Create an `OpenIDConnect` resource configured with your identity provider's settings (see an example [here](./config/samples/authentication_v1alpha1_openidconnect.yaml)). Get a token from your identity provider. You can now authenticate against the minikube cluster.

```bash
curl -k -H "Authorization: Bearer $MY_TOKEN" $(k config view -o=jsonpath="{.clusters[?(@.name=='minikube')].cluster.server}")
```

Alternatively you can also use a token kubeconfig or the [kubelogin](https://github.com/int128/kubelogin) plugin and configure an OIDC kubeconfig.

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