Missing token 'access-control-allow-headers' in CORS header 'Access-Control-Allow-Headers' from CORS preflight channel

Question

I have two VS projects : one exposing MVC5 controllers, the other being an angular client. I want the angular client to be able to query the controllers. I read numerous threads and tried the following :

• I added this in the server's web config:

  <system.webServer>
      <httpProtocol>
         <customHeaders>
              <clear />
              <add name="Access-Control-Allow-Origin" value="*" />
          </customHeaders>
      </httpProtocol>
  <system.webServer>
  

• I created and used the the following filter on the controller's action:

  public class AllowCrossSiteJsonAttribute : ActionFilterAttribute
  {
      public override void OnActionExecuting(ActionExecutingContext filterContext)
      {
          filterContext.RequestContext.HttpContext.Response.AddHeader("Access-Control-Allow-Origin", "*");
          base.OnActionExecuting(filterContext);
      }
  }
  

• In the angular client, I created the following interceptor :

  app.factory("CORSInterceptor", [
      function()
      {
          return {
              request: function(config)
              {
                   config.headers["Access-Control-Allow-Origin"] = "*";
                   config.headers["Access-Control-Allow-Methods"] = "GET, POST, OPTIONS";
                   config.headers["Access-Control-Allow-Headers"] = "Content-Type";
                   config.headers["Access-Control-Request-Headers"] = "X-Requested-With, accept, content-type";
                   return config;
              }
       };
  }
  ]);
  
  app.config(["$httpProvider", function ($httpProvider) {
      $httpProvider.interceptors.push("CORSInterceptor");
  }]);
  

According to Firebug, this results in the following request :

OPTIONS //Login/Connect HTTP/1.1
Host: localhost:49815
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Origin: http://localhost:50739
Access-Control-Request-Method: POST
Access-Control-Request-Headers: access-control-allow-headers,access-control-allow-origin,content-type
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache

And the following response :

HTTP/1.1 200 OK
Allow: OPTIONS, TRACE, GET, HEAD, POST
Server: Microsoft-IIS/10.0
Public: OPTIONS, TRACE, GET, HEAD, POST
X-SourceFiles: =?UTF-8?B?RDpcVEZTXElVV2ViXEdhcE5ldFNlcnZlclxBU1BTZXJ2aWNlc1xMb2dpblxDb25uZWN0?=
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Headers: *
Access-Control-Request-Headers: X-Requested-With, accept, content-type
Date: Tue, 01 Sep 2015 13:05:23 GMT
Content-Length: 0

And still, Firefox blocks the request with the following message :

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost:49815//Login/Connect. (Reason: missing token 'access-control-allow-headers' in CORS header 'Access-Control-Allow-Headers' from CORS preflight channel).

Answer 1

Oftentimes, the threads that I read were suggesting several unecessary configuration steps, which created confusion. It's actually very simple...

For the simple purpose of sending a cross site request, from an angular client, to an ASP controller :

• No angular interceptors are required.
• No custom filters on the server side are required.

• The only mandatory modification is to add this in the server's web.config

  <system.webServer>
        <httpProtocol>
            <customHeaders>
                <clear />
                <add name="Access-Control-Allow-Origin" value="*" />
                <add name="Access-Control-Allow-Headers" value="Content-Type"/>
            </customHeaders>
       </httpProtocol>
  </system.webServer>
  

Answer 2

The problem is, some browsers don’t yet allow the * wildcard for Access-Control-Allow-Headers. Specifically, Firefox 69 and earlier doesn’t. See https://bugzilla.mozilla.org/show_bug.cgi?id=1309358.

So to ensure you get expected behavior in all browsers, the Access-Control-Allow-Headers value you send back should explicitly list all the header names you actually need to access from your frontend code; e.g., for the case in the question: Access-Control-Allow-Headers: Content-Type.

A way you can make that happen without needing to hardcode all the header names is: Have your server-side code take the value of the Access-Control-Request-Headers request header the browser sends, and just echo that into the value of the Access-Control-Allow-Headers response header your server sends back.

Or else use some existing library to CORS-enable your server. Echoing the Access-Control-Request-Headers request-header value into the Access-Control-Allow-Headers response-header value is something most CORS libraries will typically do for you.