As of today, my general answer is _"no, at least not out of the box"_. And in general my advice is to redesign and relocate the policy enforcement point (PEP).
However, since this question popped up so often, I wrote this simple authenticator to support this functionality.
In this mode, the authenticator uses client resources, permissions and policies to restrict authentication.
This mode only works for confidential OIDC clients with authorization enabled.
It works like this:
* The authenticator checks whether a client defines a resource name `Keycloak Client Resource`
* If it does, the authenticator checks whether policies and permission evaluate to `PERMIT`
* If it does, the authenticator returns success (i.e. authentication is successful)
* If it does not, the authenticator returns failure (i.e. authentication is unsuccessful)
* If it does not, the authenticator returns success (i.e. authentication is successful).
This means that you can enable the authenticator on a per-client basis by adding a resource named `Keycloak Client Resource` to your client.
A client with that resource has the authenticator enabled. Users will only be able to authenticate to such a client if the associated policies and permission permit access.
Download a release (*.jar file) that works with your Keycloak version from the [list of releases](https://github.com/sventorben/keycloak-restrict-client-auth/releases).
Create a Wildfly module and deploy it to your Keycloak instance. For details please refer to the [official documentation](https://www.keycloak.org/docs/latest/server_development/#register-a-provider-using-modules).
For Docker-based setups mount or copy the jar to `/opt/jboss/keycloak/providers`. You may want to check [docker-compose.yml](docker-compose.yml) as an example.
Packages are being released to GitHub Packages. You find the coordinates [here](https://github.com/sventorben/keycloak-restrict-client-auth/packages/779937/versions)!
It may happen that I remove older packages without prior notice, because the storage is limited on the free tier.
> The authenticator needs a user identity to check whether the user has the desired role or not. Hence, ensure that you have steps/executions in your flow prior to this authenticator that can ensure user's identity.
3) Afterwards, no user can authenticate to this client. To allow a user to authenticate, assign the role `restricted-access` to the user. You may do so either by assigning the role to the user directly or via groups or combined roles.
You do not like the role name `restricted-access` or you do have some kind of naming conventions in place? You can change the role name globally by configuring the provider.
For details, please refer to [Manage Subsystem Configuration](https://www.keycloak.org/docs/latest/server_installation/index.html#manage-subsystem-configuration) section in the server installation guide.
For details on SPI and provider configuration in Keycloak.X, please refer to [SPIs and Provider Configuration](https://github.com/keycloak/keycloak-community/blob/master/design/keycloak.x/configuration.md#spis-and-provider-configuration) section in the community guide.
5) Set the `Name` and `Display name` to `Keycloak Client Resource` and keep the other fields blank
6) Save the resource
7) Click `Create Permission` to add permissions and policies (see [Authorization Services Guide](https://www.keycloak.org/docs/latest/authorization_services/#_permission_overview) for details)
8) Afterwards, no user can authenticate to this client unless permissions have been granted by configured policies.
### Using a custom error message
If a user tries to log in via a browser-based flow and access gets denied by the authenticator, a custom error message can be displayed.
In the flow choose the `Actions` button and then choose `Config`. You will see the following configuration screen.
You can directly define a particular message or use a property, which will be used for mapping the error message. If you choose a property, the property will be looked up from your custom theme's `messages*.properties` files and therefore supports internationalization.
restricted-access.denied=Access denied. User is missing required role 'restricted-access'
# messages_de.properties
restricted-access.denied=Zugriff verweigert. Dem Benutzer fehlt die notwendige Rolle 'restricted-access'.
```
If the field is left blank, default property `access-denied` is used. In this case you do not need a custom theme, since this property comes with Keycloak out of the box.
For details on how to add custom messages to Keycloak, please refer to section [Messages and Internationalization](https://www.keycloak.org/docs/latest/server_development/#messages) in the server developer guide.
Keycloak SPIs are quite stable. So, there is a high chance this authenticator will work with other versions, too. Check the details of latest [build results](https://github.com/sventorben/keycloak-restrict-client-auth/actions/workflows/buildAndTest.yml) for an overview or simply give it a try.
Authenticator version `X.b.c` is compiled against Keycloak version `X.y.z`. For example, version `12.3.1` will be compiled against Keycloak version `12.y.z`.
I do not guarantee what version `a.b` or `y.z` will be. Neither do I backport features to older version, nor maintain any older versions of this authenticator. If you need the latest features or bugfixes for an older version, please fork this project or update your Keycloak instance. I recommend doing the latter on regular basis anyways.
### Why not use "Allow/Deny Access" authenticators with conditions?
With Keycloak 13 two new authenticators have been added, namely `Allow Access` and `Deny Access`. Together with `Condition - User Role` authenticator authentication may be restricted in a similar way with out-of-the-box features. So, the question is why not use that and override authentication flows on a per client basis?
Here are some reasons/thoughts
* It is not really flexible. Since `Condition - User Role` only allows for checking one concrete (realm or client-specific) role, a very complex flow handling all clients, or a totally separate flow for each individual client would be needed.
* It simply does not work well with federated authentication (ie. identity provider redirects), since there is no way to configure client specific behaviour for `First login flow` or `Post login flows`. In other words, there is no feature like `Authentication flow overrides` at an IdP level. Hence, the same flow will be used for all clients. As said before, this becomes very complicated.
