I remember some discussions about this on the ML but I couldn't find a concluding answer.

I have a scenario where I need users from a realm "B" to be able to use an application
that lives in realm "A".

In the concrete use case I have a "B-user" registered in realm "B" that needs to access
an application X from realm "A".
"B-user" is already authenticated in keycloak and accesses the application X in realm "A".
Since the user is not authenticated with realm "A" the user gets redirected to realm "A"s login.

Now I want to make it possible to login the "B-user"either transparently or by clicking on a link 
"login with B" such that he can use application X.

Note that I want to avoid showing B's login.

Is this possible at all?

I thought that this might be possible by defining a Keycloak Identity provider for realm B.

In order to test this I did the following:

I created two realms A and B - each with it's own realm user A-user and B-user respectively
then I defined a new identity provider of type Keycloak OpenID Connect (keycloak-oidc) with the following settings:

                            Alias: Realm B
                       Enabled: On
Authenticate by default: On
           First Login Flow: first broker login
            Post Login flow: --empty--
       Authorization URL: http://localhost:8081/auth/realms/b/protocol/openid-connect/auth
                  Token URL: http://localhost:8081/auth/realms/b/protocol/openid-connect/token
                 Logout URL: http://localhost:8081/auth/realms/b/protocol/openid-connect/logout
             User Info URL: http://localhost:8081/auth/realms/b/protocol/openid-connect/userinfo
                      Client ID: account (account application in realm A)
               Client Secret: fa0c8747-8ea5-43f0-acbd-fea47ad6bab8 (account application in realm A)

In "Mappers" I defined a "user-role-mapper" as a "Hardcoded Role" with "account.view-profile".

As an example app I use the account client that exists in both realms.

Now I login to realm-b and access the account app:
http://localhost:8081/auth/realms/b/account

If I now browse to:
http://localhost:8081/auth/realms/a/account

I get a redirect to:
http://localhost:8081/auth/realms/b/protocol/openid-connect/auth?scope=openid&state=xvB9nevhQp6IhPJzN7-XfRwUI1250UINM-VvegnpNB0.44090b97-e6a2-448d-b453-60d967265cb4&response_type=code&client_id=account&redirect_uri=http%3A%2F%2Flocalhost%3A8081%2Fauth%2Frealms%2Fa%2Fbroker%2FB%2Fendpoint

which results in a page indicating:

    We're sorry ...

    Invalid parameter: redirect_uri

    « Back to Application

Back to application points to "http://localhost:8081/auth/realms/b/account"
Did I do anything wrong here? Why is the redirect_uri invalid?

Cheers,
Thomas