Single sign-on for Spring Cloud Gateway for K8s

Spring Cloud Gateway for Kubernetes supports authentication and authorization using single sign-on (SSO) with an OpenID identity provider that supports OpenID Connect Discovery protocol.

In addition to using an SSO authentication workflow, you can also set up filters to support:

Configure single sign-on (SSO) for Kubernetes

You can configure Spring Cloud Gateway for Kubernetes to authenticate requests via Single Sign-On (SSO), using an OpenID identity provider.

To configure a Gateway instance to use SSO:

Create a file called sso-credentials.txt, including the following properties:
```
scope=openid,profile,email
client-id={your_client_id}
client-secret={your_client_secret}
issuer-uri={your-issuer-uri}
```
For the client-id, client-secret, and issuer-uri values, use values from your OpenID identity provider. For the scope value, use a list of scopes to include in JWT identity tokens. This list should be based on the scopes allowed by your identity provider.

issuer-uri configuration should follow Spring Security convention, as described in the official Spring Security documentation:

The provider needs to be configured with an issuer-uri which is the URI that the it asserts as its Issuer Identifier. For example, if the issuer-uri provided is https://example.com, then an OpenID Provider Configuration Request will be made to https://example.com/.well-known/openid-configuration. The result is expected to be an OpenID Provider Configuration Response.

Note that only authorization servers supporting OpenID Connect Discovery protocol can be used.

If your authorization server requires Proof Key for Code Exchange (PKCE) as additional verification, add pkce-enabled=true to sso-credentials.txt.
Configure external authorization server to allow redirects back to the gateway. Please refer to your authorization server's documentation and add https://<gateway-external-url-or-ip-address>/login/oauth2/code/sso to the list of allowed redirect URIs.
In the Spring Cloud Gateway for Kubernetes namespace, create a Kubernetes secret using the sso-credentials.txt file created in the previous step:
```
$ kubectl create secret generic my-sso-credentials --from-env-file=./sso-credentials.txt
```
Examine the secret using the kubectl describe command. Verify that the Data column of the secret contains all of the required properties listed above.
Add the SSO secret in the SpringCloudGateway definition in the spec.sso.secret field. In the routes list of the SpringCloudGatewayRouteConfig object, add the setting ssoEnabled: true to each route that must have authenticated access. See the following updated gateway-config.yaml and route-config.yaml files:
```
apiVersion: "tanzu.vmware.com/v1"
kind: SpringCloudGateway
metadata:
  name: my-gateway
spec:
  api:
    serverUrl: https://my-gateway.my-example-domain.com
    title: Animal Rescue APIs
    description: Make and track adoption requests for animals that need to be rescued.
    version: "1.0"
  sso:
    secret: my-sso-credentials
```
```
apiVersion: "tanzu.vmware.com/v1"
kind: SpringCloudGatewayRouteConfig
metadata:
  name: my-gateway-routes
spec:
  routes:
  - uri: https://github.com
    ssoEnabled: true
    predicates:
      - Path=/github/**
    filters:
      - StripPrefix=1
```
With ssoEnabled set to true, the Gateway instance will use SSO for all API routes that are configured to allow authenticated access only.

Important If you add ssoEnabled to the filters arrays it will be ignored. You set it using the boolean fields spec.service.ssoEnabled and/or spec.routes.ssoEnabled as described above. This helps ensure the filter is applied to the Gateway in the correct order.

Apply the updated Gateway and RouteConfig definition file:

$ kubectl apply -f gateway-config.yaml
$ kubectl apply -f route-config.yaml

Update single sign-on credentials

To update the SSO credentials for the gateway:

Update the value in secret (e.g. my-sso-credentials) by deleting the old secret then recreate it again:

$ kubectl delete secret my-sso-credentials
$ kubectl create secret generic my-sso-credentials --from-env-file=./sso-credentials-updated.txt

Alternatively, edit existing secret with new base64 encoded values:

$ echo $NEW_CLIENT_SECRET | base64 | pbcopy
$ kubectl edit secret my-sso-credentials

Rollout restart the gateway statefulset to enforce secret update:
```
kubectl rollout restart statefulset my-gateway
```

Refer to the Okta Tutorial for a tutorial on configuring a gateway with Single Sign-On using Okta for the Animal Rescue demo app.

Single sign-on with self-signed certificates on TAP

If you are using the Tanzu Application Platform (TAP), see the TLS Configuration for configuring single sign-on with self-signed certificates.

Configure single sign-on (SSO) in Standalone Gateway

SSO features are equally supported in a Standalone instance. See SSO Standalone Configuration

OpenAPI security schemes

When SSOEnabled is set to true on any route, two securityScheme (See Swagger Authentication docs) are registered as a component in the OpenAPI spec generated:

AuthBearer to activate a dialog for providing a Bearer Authorization header
OpenId to activate a dialog for getting a token from an OIDC configuration and adding it as a header

And, the schemes are bound to any of those routes. Other routes will not be affected and the scheme will not be applied on them.

Log out

Spring Cloud Gateway for Kubernetes instances provide a default API endpoint to log out of the current SSO session: GET /scg-logout.

If the OIDC provider supports RP-Initiated Logout, the /scg-logout call will also log the user out of the OIDC provider session.

You can redirect the user to another endpoint or url by adding a redirect query parameter to the logout call. For example, a GET call to /scg-logout?redirect=/home will redirect the user to the /home page.