AddOpenIdConnect and Refresh Tokens in ASP.NET Core

As far as I know, there's nothing built-in in ASP.NET Core 3.1 to refresh access tokens automatically. But I've found this convenient library from the IdentityServer4 authors which stores access and refresh tokens in memory (this can be overriden) and refreshes access tokens automatically when you request them from the library.

How to use the library: https://identitymodel.readthedocs.io/en/latest/aspnetcore/web.html.

NuGet package: https://www.nuget.org/packages/IdentityModel.AspNetCore/.

Source code: https://github.com/IdentityModel/IdentityModel.AspNetCore.

Here is what I came up with. Since there are not very many examples that I could find on how to do refresh tokens in ASP.NET Core with cookies, I thought I would post this here. (The one I link to in the question has issues.)

This is just my attempt at getting this working. It has not been used in any production setting. This code goes in the ConfigureServices method.

services.AddAuthentication(options =>
{
    options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
    options.DefaultChallengeScheme = OpenIdConnectDefaults.AuthenticationScheme;
})
.AddCookie(CookieAuthenticationDefaults.AuthenticationScheme, options =>
{
    options.Events = new CookieAuthenticationEvents
    {
        // After the auth cookie has been validated, this event is called.
        // In it we see if the access token is close to expiring.  If it is
        // then we use the refresh token to get a new access token and save them.
        // If the refresh token does not work for some reason then we redirect to 
        // the login screen.
        OnValidatePrincipal = async cookieCtx =>
        {
            var now = DateTimeOffset.UtcNow;
            var expiresAt = cookieCtx.Properties.GetTokenValue("expires_at");
            var accessTokenExpiration = DateTimeOffset.Parse(expiresAt);
            var timeRemaining = accessTokenExpiration.Subtract(now);
            // TODO: Get this from configuration with a fall back value.
            var refreshThresholdMinutes = 5;
            var refreshThreshold = TimeSpan.FromMinutes(refreshThresholdMinutes);

            if (timeRemaining < refreshThreshold)
            {
                var refreshToken = cookieCtx.Properties.GetTokenValue("refresh_token");
                // TODO: Get this HttpClient from a factory
                var response = await new HttpClient().RequestRefreshTokenAsync(new RefreshTokenRequest
                {
                    Address = tokenUrl,
                    ClientId = clientId,
                    ClientSecret = clientSecret,
                    RefreshToken = refreshToken
                });

                if (!response.IsError)
                {
                    var expiresInSeconds = response.ExpiresIn;
                    var updatedExpiresAt = DateTimeOffset.UtcNow.AddSeconds(expiresInSeconds);
                    cookieCtx.Properties.UpdateTokenValue("expires_at", updatedExpiresAt.ToString());
                    cookieCtx.Properties.UpdateTokenValue("access_token", response.AccessToken);
                    cookieCtx.Properties.UpdateTokenValue("refresh_token", response.RefreshToken);
                    
                    // Indicate to the cookie middleware that the cookie should be remade (since we have updated it)
                    cookieCtx.ShouldRenew = true;
                }
                else
                {
                    cookieCtx.RejectPrincipal();
                    await cookieCtx.HttpContext.SignOutAsync();
                }
            }
        }
    };
})
.AddOpenIdConnect(options =>
{
    options.SignInScheme = CookieAuthenticationDefaults.AuthenticationScheme;
    
    options.Authority = oidcDiscoveryUrl;
    options.ClientId = clientId;
    options.ClientSecret = clientSecret;

    options.RequireHttpsMetadata = true;
    
    options.ResponseType = OidcConstants.ResponseTypes.Code;
    options.UsePkce = true;
    // This scope allows us to get roles in the service.
    options.Scope.Add("openid");
    options.Scope.Add("profile");
    options.Scope.Add("offline_access");

    // This aligns the life of the cookie with the life of the token.
    // Note this is not the actual expiration of the cookie as seen by the browser.
    // It is an internal value stored in "expires_at".
    options.UseTokenLifetime = false;
    options.SaveTokens = true;
});

This code has two parts:

1. AddOpenIdConnect: This part of the code sets up OIDC for the application. Key settings here are:
   • SignInScheme: This lets ASP.NET Core know you want to use cookies to store your authentication information.
   • *UseTokenLifetime: As I understand it, this sets an internal "expires_at" value in the cookie to be the lifespan of the access token. (Not the actual cookie expiration, which stays at the session level.)
   • *SaveTokens: As I understand it, this is what causes the tokens to be saved in the cookie.
2. OnValidatePrincipal: This section is called when the cookie has been validated. In this section we check to see if the access token is near or past expiration. If it is then it gets refreshed and the updated values are stored in the cookie. If the token cannot be refreshed then the user is redirected to the login screen.

The code uses these values that must come from your configuration file:

• clientId: OAuth2 Client ID. Also called Client Key, Consumer Key, etc.
• clientSecret: OAuth2 Client Secret. Also called Consumer Secret, etc.
• oidcDiscoveryUrl: Base part of the URL to your IDP's Well Known Configuration document. If your Well Known Configuration document is at https://youridp.domain.com/oauth2/oidcdiscovery/.well-known/openid-configuration then this value would be https://youridp.domain.com/oauth2/oidcdiscovery.
• tokenUrl: Url to your IDP's token endpoint. For example: https:/youridp.domain.com/oauth2/token
• refreshThresholdMinutes: If you wait till the access token is very close to expiring, then you run the risk of failing calls that rely on the access token. (If it is 5 miliseconds from expiration then it could expire, and fail a call, before you get a chance to refresh it.) This setting is the number of minutes before expiration you want to consider an access token ready to be refreshed.

* I am new to ASP.NET Core. As such I am not 100% sure that those settings do what I think. This is just a bit of code that is working for me and I thought I would share it. It may or may not work for you.