Blank page after login using bookmarked authorization URL in IdentityServer4 - identityserver4

Question

We have discovered that our users very often for the first time visits our web application by browsing the direct URL of the OIDC client (https://oidcclienturl.com/), The ASP.NET Core OIDC authentication middleware kicks in and the user gets redirected back to Identityserver 4 login page.
Everything works fine but then they decide to add the (temporary? state, nonce, cookies...) authorization URL as a bookmark in their browser before entering their credentials and continuing back to the web application.
This causes an issue when the user later uses the bookmark in a new session. The login seem to actually work after entering valid user credentials even if the user uses an old authorization URL, but when the user gets redirected back to the web application they end up on a blank page (https://oidcclienturl.com/signin-oidc).
After the blank page have been loaded the user is able to browse the direct URL (https://oidcclienturl.com/) sucessfully and appear as an authentcated user in the web application.
Any ideas whats causing the blank page?
That blank page shouldnt exist, if I understand it correctly its the default callback path of the oidc authentication middleware in ASP.NET Core.

Answer 1

Unfortunately, the real-world problem of users bookmarking the login page isn't handled cleanly by OIDC, which requires the client app to initiate the login flow.
I've addressed this by adding a RegistrationClientId column to my user data table, which is the Identity Server ClientId corresponding to the client app that called IDS when the user account was created. In the client app configuration, we use the custom Properties dictionary to add a URI fragment:
new Client
{
ClientId = "some_client",
ClientName = "Some Client",
ClientUri = "https://localhost:5000",
Properties = new Dictionary<string, string>
{
{ "StartLoginFragment", "/Auth/StartLogin" }
}
// other config omitted
};
When a user logs in, an empty return URL indicates IDS wasn't called by a client app, so we use RegistrationClientId to query IClientStore, then we combine the ClientUri and StartLoginFragment URIs and use the resulting URI to redirect the user back to the client application.
Over in the client application, that endpoint kicks off the OIDC sign-in flow, and since the user is already signed-in on IDS, it comes right back to the correct location in the client app. The controller action looks like this:
[HttpGet]
public async Task StartLogin()
{
await acctsvc.SignOutAsync();
await HttpContext.ChallengeAsync("oidc",
new AuthenticationProperties()
{
RedirectUri = "/"
});
}
The call to SignOutAsync just ensures any client-app signin cookies are cleaned up. It's in our custom account service, but it just runs HttpContext.SignOutAsync on the usual "Cookies" and "oidc" schemes. Normally that would also result in a signout call to IDS, but the redirection by the subsequent ChallengeAsync replaces the pending signout call.
The downside is that the action is an HTTP GET meaning pretty much anyone could theoretically trigger this action. At most it would be an annoyance.
In the special case where your IDS is only handling auth for a single client, you can skip a lot of that -- if they land on the page with no return URL, just send them to your client app start-login endpoint straightaway, before they login.