The problem with storing state in a request parameter is that the request URL can get too large (over the common limit of 2000 characters). The OpenID Connect authentication handler provided by ASP.NET Core utilizes this feature of the protocol, and that is how it implements the returnUrl feature mentioned above. Many protocols, including OpenID Connect, allow passing some sort of state as a parameter as part of the request, and the identity provider will return that state on the response. This means that state is captured prior to leaving the client and preserved until the user has returned to the client application. When redirecting to an external provider for sign-in, frequently state from the client application must be round-tripped. State, URL length, and ISecureDataFormat ¶

Issuer // use externalProvider and externalUserId to find your user, or provision a new user Value var externalProvider = userIdClaim. Succeeded != true ) var externalUserId = userIdClaim. ExternalCookieAuthenticationScheme ) if ( result ?. AuthenticateAsync ( IdentityServerConstants. read external identity from the temporary cookie var result = await HttpContext. Custom Token Request Validation and Issuance.Sign-out of External Identity Providers.State, URL length, and ISecureDataFormat.Handling the callback and signing in the user.Adding authentication handlers for external providers.Sign-in with External Identity Providers.Using EntityFramework Core for configuration and operational data.Interactive Applications with ASP.NET Core.Protecting an API using Client Credentials.

External id validation check