Question

I’ve recently started having trouble logging into my site using Facebook. I keep seeing this error message:

Facebook.FacebookOAuthException: (OAuthException) Error validating verification code.

There have been significant changes to the site and software; before I dig in to the changes, I’d like to eliminate some of the simple, common causes.

Could this be a firewall issue (the site is behind a firewall and not accessible from outside our network)? What are the most common causes for this error message?

Answer 1

Make sure your redirectUri parameter is the same as the redirecturi you used on https://www.facebook.com/dialog/oauth call.

For example, given an initial authentication request

using Facebook;
public ActionResult fbLogOn( string token, string returnUrl )
        {
            var oAuthClient = new FacebookOAuthClient( FacebookApplication.Current );

            oAuthClient.RedirectUri = 
                       new Uri( "http://" + "localhost:3434" + "/Home/fbOAuth" );
<... etc ...>

Make sure the handler for the facebook response sets the redirect URI to the exact same URI:

using Facebook;
public ActionResult fbOAuth(string code, string state )
{
  FacebookOAuthResult oauthResult;
  if (FacebookOAuthResult.TryParse( Request.Url, out oauthResult ))
  {
    string token = HttpUtility.UrlDecode( state );
    if (oauthResult.IsSuccess)
    {
      var oAuthClient = new FacebookOAuthClient( FacebookApplication.Current );
      oAuthClient.RedirectUri = 
                 new Uri( "http://" + "localhost:3434" + "/Home/fbOAuth" );
      dynamic tokenResult = oAuthClient.ExchangeCodeForAccessToken( code );
      string accessToken = tokenResult.access_token;
<... etc ...>