Invalid state from server. Possible forgery! error in Xamarin.Auth - xamarin

Question

Why I get this error message when trying to use the Xamarin.Auth Api?
I am running on Android Plataform and using Xamarin.Forms
OAuth2Authenticator auth = new OAuth2Authenticator
(
clientId: AppKeyDropboxtoken,
scope: "",
authorizeUrl: new Uri("https://www.dropbox.com/oauth2/authorize"),
redirectUrl: new Uri(RedirectUri),
isUsingNativeUI: false
);
auth.Completed += (sender, eventArgs) =>
{
if (eventArgs.IsAuthenticated)
{
// Use eventArgs.Account to do wonderful things
this.AccessToken = eventArgs.Account.Properties["access_token"].ToString();
Debug.WriteLine("AccessToken: " + this.AccessToken);
openDropboxFileList();
}
};
var presenter = new Xamarin.Auth.Presenters.OAuthLoginPresenter();
presenter.Login(auth);

Answer 1

Create a class and add this code below:
public class AuthenticatorExtensions : OAuth2Authenticator
{
public AuthenticatorExtensions(string clientId, string clientSecret, string scope, Uri authorizeUrl, Uri redirectUrl, Uri accessTokenUrl, GetUsernameAsyncFunc getUsernameAsync = null, bool isUsingNativeUI = false) : base(clientId, clientSecret, scope, authorizeUrl, redirectUrl, accessTokenUrl, getUsernameAsync, isUsingNativeUI)
{
}
protected override void OnPageEncountered(Uri url, System.Collections.Generic.IDictionary<string, string> query, System.Collections.Generic.IDictionary<string, string> fragment)
{
// Remove state from dictionaries.
// We are ignoring request state forgery status
// as we're hitting an ASP.NET service which forwards
// to a third-party OAuth service itself
if (query.ContainsKey("state"))
{
query.Remove("state");
}
if (fragment.ContainsKey("state"))
{
fragment.Remove("state");
}
base.OnPageEncountered(url, query, fragment);
}
}
Then use it as below:
[Obsolete]
private void SignInGoogleAuth()
{
try
{
string clientId = null;
string redirectUri = null;
//Xamarin.Auth.CustomTabsConfiguration.CustomTabsClosingMessage = null;
clientId = Constants.GoogleAndroidClientId;
redirectUri = Constants.GoogleAndroidRedirectUrl;
account = store.FindAccountsForService(Constants.AppName).FirstOrDefault();
var authenticator = new AuthenticatorExtensions(
clientId,
null,
Constants.GoogleScope,
new Uri(Constants.GoogleAuthorizeUrl),
new Uri(redirectUri),
new Uri(Constants.GoogleAccessTokenUrl),
null,
true);
authenticator.Completed += OnAuthCompleted;
authenticator.Error += OnAuthError;
AuthenticationState.Authenticator = authenticator;
var presenter = new Xamarin.Auth.Presenters.OAuthLoginPresenter();
presenter.Login(authenticator);
}
catch (Exception ex)
{
ShowAlert("Alert", ex.Message);
}
}
[Obsolete]
async void OnAuthCompleted(object sender, AuthenticatorCompletedEventArgs e)
{
var authenticator = sender as OAuth2Authenticator;
if (authenticator != null)
{
authenticator.Completed -= OnAuthCompleted;
authenticator.Error -= OnAuthError;
}
if (e.IsAuthenticated)
{
// If the user is authenticated, request their basic user data from Google
// UserInfoUrl = https://www.googleapis.com/oauth2/v2/userinfo
var request = new OAuth2Request("GET", new Uri(Constants.GoogleUserInfoUrl), null, e.Account);
var response = await request.GetResponseAsync();
if (response != null)
{
// Deserialize the data and store it in the account store
// The users email address will be used to identify data in SimpleDB
string userJson = await response.GetResponseTextAsync();
StaticVariables.googleProfile = JsonConvert.DeserializeObject<GoogleProfile>(userJson);
}
if (account != null)
{
store.Delete(account, Constants.AppName);
}
await store.SaveAsync(account = e.Account, Constants.AppName);
Application.Current.Properties.Remove("Id");
Application.Current.Properties.Remove("FirstName");
Application.Current.Properties.Remove("LastName");
Application.Current.Properties.Remove("DisplayName");
Application.Current.Properties.Remove("EmailAddress");
Application.Current.Properties.Remove("ProfilePicture");
Application.Current.Properties.Add("Id", StaticVariables.googleProfile.Id);
Application.Current.Properties.Add("FirstName", StaticVariables.googleProfile.GivenName);
Application.Current.Properties.Add("LastName", StaticVariables.googleProfile.FamilyName);
Application.Current.Properties.Add("DisplayName", StaticVariables.googleProfile.Name);
Application.Current.Properties.Add("EmailAddress", StaticVariables.googleProfile.Email);
Application.Current.Properties.Add("ProfilePicture", StaticVariables.googleProfile.Picture);
await Navigation.PushAsync(new GoogleProfilePage());
}
}
[Obsolete]
void OnAuthError(object sender, AuthenticatorErrorEventArgs e)
{
var authenticator = sender as OAuth2Authenticator;
if (authenticator != null)
{
authenticator.Completed -= OnAuthCompleted;
authenticator.Error -= OnAuthError;
}
Debug.WriteLine("Authentication error: " + e.Message);
}

Answer 2

I was getting the infamous "Possible Forgery!" error and overrode OnPageEncountered() to work around it as many have done. This turns out to be unnecessary as well as insecure.
Oauth2Authenticator is stateful so you will get this problem if you don't use the same instance of OAuth2Authenticator to invoke OnPageLoading() as was used to initiate the authentication.
To resolve, just save the instance of OAuth2Authenticator used for initiating authentication and then reuse it when calling OnPageLoading() in your OauthInterceptor.