Setting Access-Control-Allow-Origin in ASP Net MVC - simplest possible method

Question

I have a simple actionmethod  that returns some json  It runs on ajax example com  I need to access this from another site someothersite com   If I try to call it  I get the expected      Origin http   someothersite com is not allowed by Access-Control-Allow-Origin    I know of two ways to get around this  JSONP and creating a custom HttpHandler to  set the header   Is there no simpler way   Is it not possible for a simple action to either define a list of allowed origins - or simple allow everyone  Maybe an action filter   Optimal would be      return json mydata  JsonBehaviour IDontCareWhoAccessesMe

User · Answer

If you use IIS  I d suggest trying IIS CORS module  It s easy to configure and works for all types of controllers   Here is an example of configuration          lt system webServer gt           lt cors enabled  true  failUnlistedOrigins  true  gt               lt add origin       gt               lt add origin  https     microsoft com                   allowCredentials  true                   maxAge  120  gt                    lt allowHeaders allowAllRequestedHeaders  true  gt                       lt add header  header1    gt                       lt add header  header2    gt                   lt  allowHeaders gt                   lt allowMethods gt                        lt add method  DELETE    gt                   lt  allowMethods gt                   lt exposeHeaders gt                       lt add header  header1    gt                       lt add header  header2    gt                   lt  exposeHeaders gt               lt  add gt               lt add origin  http      allowed  false    gt           lt  cors gt       lt  system webServer gt

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In Web config enter the following   lt system webServer gt   lt httpProtocol gt     lt customHeaders gt       lt clear   gt            lt add name  Access-Control-Allow-Credentials  value  true    gt       lt add name  Access-Control-Allow-Origin  value  http   localhost 123456 etc     gt     lt  customHeaders gt   lt  httpProtocol gt

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I m using DotNet Core MVC and after fighting for a few hours with nuget packages  Startup cs  attributes  and this place  I simply added this to the MVC action  Response Headers Add  quot Access-Control-Allow-Origin quot    quot   quot     I realise this is pretty clunky  but it s all I needed  and nothing else wanted to add those headers  I hope this helps someone else

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There are different ways we can pass the   Access-Control-Expose-Headers     As jgauffin has explained we can create a new attribute  As LaundroMatt has explained we can add in the web config file  Another way is we can add code as below in the webApiconfig cs file   config EnableCors new EnableCorsAttribute     headers      methods      exposedHeaders   TestHeaderToExpose     SupportsCredentials   true        Or we can add below code in the Global Asax file   protected void Application BeginRequest                         if  HttpContext Current Request HttpMethod     OPTIONS                                   These headers are handling the  pre-flight  OPTIONS call sent by the browser                 HttpContext Current Response AddHeader  Access-Control-Allow-Methods    GET  POST  OPTIONS                    HttpContext Current Response AddHeader  Access-Control-Allow-Headers                         HttpContext Current Response AddHeader  Access-Control-Allow-Credentials    true                    HttpContext Current Response AddHeader  Access-Control-Allow-Origin    http   localhost 4200                    HttpContext Current Response AddHeader  Access-Control-Expose-Headers    TestHeaderToExpose                    HttpContext Current Response End                              I have written it for the options  Please modify the same as per your need   Happy Coding

User · Answer

Sometimes OPTIONS verb as well causes problems   Simply  Update your web config with the following    lt system webServer gt       lt httpProtocol gt           lt customHeaders gt             lt add name  Access-Control-Allow-Origin  value       gt             lt add name  Access-Control-Allow-Headers  value  Origin  X-Requested-With  Content-Type  Accept    gt           lt  customHeaders gt       lt  httpProtocol gt   lt  system webServer gt    And update the webservice controller headers with httpGet and httpOptions      GET api Master Sync  version 12121          HttpGet  HttpOptions          public dynamic Sync string version

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WebAPI 2 now has a package for CORS which can be installed using   Install-Package Microsoft AspNet WebApi Cors -pre -project WebServic   Once this is installed  follow this for the code  http   www asp net web-api overview security enabling-cross-origin-requests-in-web-api

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I ran into a problem where the browser refused to serve up content that it had retrieved when the request passed in cookies  e g   the xhr had its withCredentials true   and the site had Access-Control-Allow-Origin set to     The error in Chrome was   Cannot use wildcard in Access-Control-Allow-Origin when credentials flag is true     Building on the answer from  jgauffin  I created this  which is basically a way of working around that particular browser security check  so caveat emptor   public class AllowCrossSiteJsonAttribute   ActionFilterAttribute       public override void OnActionExecuting ActionExecutingContext filterContext                   We d normally just use     for the allow-origin header              but Chrome  and perhaps others  won t allow you to use authentication if            the header is set to                 TODO  Check elsewhere to see if the origin is actually on the list of trusted domains          var ctx   filterContext RequestContext HttpContext          var origin   ctx Request Headers  Origin            var allowOrigin    string IsNullOrWhiteSpace origin    origin                ctx Response AddHeader  Access-Control-Allow-Origin   allowOrigin           ctx Response AddHeader  Access-Control-Allow-Headers                 ctx Response AddHeader  Access-Control-Allow-Credentials    true            base OnActionExecuting filterContext

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Add this line to your method  If you are using a API   HttpContext Current Response AddHeader  Access-Control-Allow-Origin

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This is really simple   just add this in web config   lt system webServer gt     lt httpProtocol gt       lt customHeaders gt         lt add name  Access-Control-Allow-Origin  value  http   localhost    gt         lt add name  Access-Control-Allow-Headers  value  X-AspNet-Version X-Powered-By Date Server Accept Accept-Encoding Accept-Language Cache-Control Connection Content-Length Content-Type Host Origin Pragma Referer User-Agent    gt         lt add name  Access-Control-Allow-Methods  value  GET  PUT  POST  DELETE  OPTIONS    gt         lt add name  Access-Control-Max-Age  value  1000    gt       lt  customHeaders gt     lt  httpProtocol gt   lt  system webServer gt    In Origin put all domains that have access to your web server  in headers put all possible headers that any ajax http request can use  in methods put all methods that you allow on your server  regards

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After struggling for a whole evening I finally got this to work  After some debugging I found the problem I was walking into was that my client was sending a so called preflight Options request to check if the application was allowed to send a post request with the origin  methods and headers provided  I didn t want to use Owin or an APIController  so I started digging and came up with the following solution with just an ActionFilterAttribute  Especially the  Access-Control-Allow-Headers  part is very important  as the headers mentioned there do have to match the headers your request will send   using System Linq  using System Web  using System Web Mvc   namespace MyNamespace       public class AllowCrossSiteJsonAttribute   ActionFilterAttribute               public override void OnActionExecuting ActionExecutingContext filterContext                        HttpRequest request   HttpContext Current Request              HttpResponse response   HttpContext Current Response                  check for preflight request             if  request Headers AllKeys Contains  Origin    amp  amp  request HttpMethod     OPTIONS                                 response AppendHeader  Access-Control-Allow-Origin                         response AppendHeader  Access-Control-Allow-Credentials    true                    response AppendHeader  Access-Control-Allow-Methods    GET  PUT  POST  DELETE                    response AppendHeader  Access-Control-Allow-Headers    Origin  X-Requested-With  X-RequestDigest  Cache-Control  Content-Type  Accept  Access-Control-Allow-Origin  Session  odata-version                    response End                              else                               HttpContext Current Response Cache SetCacheability HttpCacheability NoCache                   HttpContext Current Response Cache SetNoStore                     response AppendHeader  Access-Control-Allow-Origin                         response AppendHeader  Access-Control-Allow-Credentials    true                    if  request HttpMethod     POST                                         response AppendHeader  Access-Control-Allow-Methods    GET  PUT  POST  DELETE                        response AppendHeader  Access-Control-Allow-Headers    Origin  X-Requested-With  X-RequestDigest  Cache-Control  Content-Type  Accept  Access-Control-Allow-Origin  Session  odata-version                                       base OnActionExecuting filterContext                                     Finally  my MVC action method looks like this  Important here is to also mention the Options HttpVerbs  because otherwise the preflight request will fail    AcceptVerbs HttpVerbs Post   HttpVerbs Options    AllowCrossSiteJson  public async Task lt ActionResult gt  Create MyModel model        return Json await DoSomething model

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public ActionResult ActionName string ReqParam1  string ReqParam2  string ReqParam3  string ReqParam4                this ControllerContext HttpContext Response Headers Add  Access-Control-Allow-Origin                                    --Your code goes here --                     return Json new   ReturnData   Data to be returned   Success true    JsonRequestBehavior AllowGet

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For plain ASP NET MVC Controllers  Create a new attribute  public class AllowCrossSiteJsonAttribute   ActionFilterAttribute       public override void OnActionExecuting ActionExecutingContext filterContext                filterContext RequestContext HttpContext Response AddHeader  Access-Control-Allow-Origin                 base OnActionExecuting filterContext             Tag your action    AllowCrossSiteJson  public ActionResult YourMethod         return Json  Works better         For ASP NET Web API  using System  using System Web Http Filters   public class AllowCrossSiteJsonAttribute   ActionFilterAttribute       public override void OnActionExecuted HttpActionExecutedContext actionExecutedContext                if  actionExecutedContext Response    null              actionExecutedContext Response Headers Add  Access-Control-Allow-Origin                  base OnActionExecuted actionExecutedContext             Tag a whole API controller    AllowCrossSiteJson  public class ValuesController   ApiController     Or individual API calls    AllowCrossSiteJson  public IEnumerable lt PartViewModel gt  Get                 For Internet Explorer  lt   v9  IE  lt   9 doesn t support CORS  I ve written a javascript that will automatically route those requests through a proxy  It s all 100  transparent  you just have to include my proxy and the script    Download it using nuget corsproxy and follow the included instructions   Blog post   Source code

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This tutorial is very useful  To give a quick summary    Use the CORS package available on Nuget  Install-Package Microsoft AspNet WebApi Cors In your WebApiConfig cs file  add config EnableCors   to the Register   method  Add an attribute to the controllers you need to handle cors     EnableCors origins    lt origin address in here gt    headers       methods

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If you are using IIS 7   you can place a web config file into the root of the folder with this  in the system webServer section    lt httpProtocol gt      lt customHeaders gt         lt clear   gt         lt add name  Access-Control-Allow-Origin  value       gt      lt  customHeaders gt   lt  httpProtocol gt    See  http   msdn microsoft com en-us library ms178685 aspx And  http   enable-cors org  how-iis7

[json] Setting Access-Control-Allow-Origin in ASP.Net MVC - simplest possible method

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