SharePoint OPTIONS preflight request - sharepoint

Question

I have a Service Stack service hosted within a SharePoint 2013 site. When attempting to make a cross domain request to one of the services, a preflight OPTIONS request is made, as expected.
The problem is that the response always comes back as 401 Unauthorized, due to the fact that authentication info is not sent across with the request. I have tried putting some request filters via servicestack to try and bypass the authentication, but these filters are not firing - it seems like something prior to service stack is sending the response.
Is there any way of specifying that OPTIONS requests to the sharepoint site do not need to be authenticated? If not, does anyone have a workaround for this scenario?
I tried 'fooling' the browser in to not sending a preflight request by changing the data type from application/json to text/plain in my ajax request, but then the data I send is not being deserialised in to the correct RequestDTO for the service calls on the server side.
Any help would be appreciated.

Answer 1

We ended up having to write our own HTTP module in order to support the options request. We basically add a key specifying which domains to allow the CORS requests from (can support more than one) and then have this HTTP module registered:
public class ECSPreFlightModule : IHttpModule
{
/// <summary>
/// You will need to configure this module in the Web.config file of your
/// web and register it with IIS before being able to use it. For more information
/// see the following link: http://go.microsoft.com/?linkid=8101007
/// </summary>
public void Dispose()
{
//clean-up code here.
}
private const string OptionsHeader = "OPTIONS";
private const string OriginHeader = "ORIGIN";
private const string AccessAllowOrigin = "Access-Control-Allow-Origin";
private string AllowedOriginUrlsArray
{
get
{
return GetWebConfigValue("CORSAllowedOriginUrls");
}
}
private string GetWebConfigValue(string key)
{
var configuration = WebConfigurationManager.OpenWebConfiguration("~");
object o = configuration.GetSection("system.web/httpModules");
var section = o as HttpModulesSection;
return section.CurrentConfiguration.AppSettings.Settings[key].Value;
}
public void Init(HttpApplication context)
{
context.PreSendRequestHeaders += (sender, e) =>
{
var splitUrls = AllowedOriginUrlsArray.Split('|');
var response = context.Response;
var originHeader = context.Request.Headers.Get(OriginHeader);
if (!String.IsNullOrEmpty(originHeader) && splitUrls.Length > 0)
{
foreach (var url in splitUrls)
{
var urlLower = url.ToLower();
var originHeaderLower = originHeader.ToLower();
// if the method being requested is an OPTIONS request and the url is the url specified in the web.config then return an OK response.
if (context.Request.HttpMethod.ToLowerInvariant() == OptionsHeader.ToLowerInvariant() &&
(urlLower == originHeaderLower))
{
response.StatusCode = (int)HttpStatusCode.OK;
}
// If the originating header url is equal to the url specified in the web.config then grant the access control
if (originHeaderLower == urlLower)
{
response.AddHeader(AccessAllowOrigin, originHeader);
break;
}
}
}
};
}
}
}
The above module was wrapped in a sharepoint feature that, when activated, made the appropriate changes to the web.config, namely registering the module and adding the following keys:
<add name='Access-Control-Allow-Credentials' value='true' />
<add name='Access-Control-Allow-Headers' value='Authorization, X-Requested-With, Content-Type, Origin, Accept, X-RequestDigest' />
<add name='Access-Control-Allow-Methods' value='GET,POST,OPTIONS,PUT, DELETE' />