Incorporate Keycloak login into SPA

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We're currently evaluating Keycloak as our SSO solution and while it works for our servlet-based applications there's a question regarding our (React-based) SPAs.

What our designers want: as an example let's say we have an email client spa. The user is in the process of writing an email but then gets distracted. When he returns the SSO session has already timed out and a re-login is required. The user should now be presented with a login form and after login it should be possible to send the email that's still in the SPA's local storage (i.e. re-login without restarting the SPA or losing data).

AFAIK Keycloak doesn't provide an authentication-api (for good reasons) and uses a redirect to the login page and back to the application (as I understand it for mobile apps the system browser would be used). If I'm not mistaken that redirect would then mean the SPA is then reinitialized and thus the data would be lost.

So here's the question: is what our designers want possible to do with Keycloak?

If yes, how would it be done? Directly posting to the login-url that Keycloak is using seems like a bad idea since the tokens would probably not be stored correctly and there might be same-origin policy problems. Would doing it inside an iframe or popup-window work?

single-page-application keycloak reactjs single-sign-on
2 Answers

For someone who comes back to this question,

I think it's better to stick to the best practice for oAuth2/OpenId Connect for SPAs which is currently "Authorization Code Flow" with PKCE.

https://oauth.net/2/pkce/

https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics-13

A normal flow here needs a complete redirect to the auth server and back so your app will completely re-initialize. Or you use check-sso like Sébastien already mentioned with silent mode.

https://github.com/keycloak/keycloak-documentation/blob/master/securing_apps/topics/oidc/javascript-adapter.adoc

You can configure a silent check-sso option. With this feature enabled, your browser won’t do a full redirect to the {project_name} server and back to your application, but this action will be performed in a hidden iframe, so your application resources only need to be loaded and parsed once by the browser when the app is initialized and not again after the redirect back from {project_name} to your app. This is particularly useful in case of SPAs (Single Page Applications).

This way the login will happen in an iframe and the app initializes only once and should preserve state.

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