enabling cross-origin resource sharing on IIS7


I recently ran into with posting Javascript requests to another domain. By default XHR posting to other domains is not allowed.

Following the instructions from http://enable-cors.org/, I enabled this on the other domain.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
 <system.webServer>
  <httpProtocol>
    <customHeaders>
      <add name="Access-Control-Allow-Origin" value="*" />
      <add name="Access-Control-Allow-Methods" value="GET,PUT,POST,DELETE,OPTIONS" />
      <add name="Access-Control-Allow-Headers" value="Content-Type" />
    </customHeaders>
  </httpProtocol>
 </system.webServer>
</configuration>

enter image description here

Everything works fine now, however it is still return a 405 response before sending back the working 200 response.

Request URL:http://testapi.nottherealsite.com/api/Reporting/RunReport
Request Method:OPTIONS
Status Code:405 Method Not Allowed
Request Headersview source
Accept:*/*
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-GB,en-US;q=0.8,en;q=0.6
Access-Control-Request-Headers:origin, content-type, accept
Access-Control-Request-Method:POST
Connection:keep-alive
Host:testapi.nottherealsite.com
Origin:http://test.nottherealsite.com
Referer:http://test.nottherealsite.com/Reporting
User-Agent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/21.0.1180.89 Safari/537.1
Response Headersview source
Access-Control-Allow-Headers:Content-Type
Access-Control-Allow-Methods:GET,PUT,POST,DELETE,OPTIONS
Access-Control-Allow-Origin:*
Allow:POST
Cache-Control:private
Content-Length:1565
Content-Type:text/html; charset=utf-8
Date:Tue, 18 Sep 2012 14:26:06 GMT
Server:Microsoft-IIS/7.5
X-AspNet-Version:4.0.30319
X-Powered-By:ASP.NET

Update: 3/02/2014

There is a recently updated article in MSDN magazine. Detailing CORS Support in ASP.NET Web API 2.

http://msdn.microsoft.com/en-us/magazine/dn532203.aspx

It is likely a case of IIS 7 'handling' the HTTP OPTIONS response instead of your application specifying it. To determine this, in IIS7,

  1. Go to your site's Handler Mappings.

  2. Scroll down to 'OPTIONSVerbHandler'.

  3. Change the 'ProtocolSupportModule' to 'IsapiHandler'

  4. Set the executable: %windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll

Now, your config entries above should kick in when an HTTP OPTIONS verb is sent.

Alternatively you can respond to the HTTP OPTIONS verb in your BeginRequest method.

    protected void Application_BeginRequest(object sender,EventArgs e)
    {
        HttpContext.Current.Response.AddHeader("Access-Control-Allow-Origin", "*");

        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, PUT, DELETE");
            HttpContext.Current.Response.AddHeader("Access-Control-Allow-Headers", "Content-Type, Accept");
            HttpContext.Current.Response.AddHeader("Access-Control-Max-Age", "1728000" );
            HttpContext.Current.Response.End();
        }

    }

I can't post comments so I have to put this in a separate answer, but it's related to the accepted answer by Shah.

I initially followed Shahs answer (thank you!) by re configuring the OPTIONSVerbHandler in IIS, but my settings were restored when I redeployed my application.

I ended up removing the OPTIONSVerbHandler in my Web.config instead.

<handlers>
    <remove name="OPTIONSVerbHandler"/>
</handlers>

I found the information found at http://help.infragistics.com/Help/NetAdvantage/jQuery/2013.1/CLR4.0/html/igOlapXmlaDataSource_Configuring_IIS_for_Cross_Domain_OLAP_Data.html to be very helpful in setting up HTTP OPTIONS for a WCF service in IIS 7.

I added the following to my web.config and then moved the OPTIONSVerbHandler in the IIS 7 'hander mappings' list to the top of the list. I also gave the OPTIONSVerbHander read access by double clicking the hander in the handler mappings section then on 'Request Restrictions' and then clicking on the access tab.

Unfortunately I quickly found that IE doesn't seem to support adding headers to their XDomainRequest object (setting the Content-Type to text/xml and adding a SOAPAction header).

Just wanted to share this as I spent the better part of a day looking for how to handle it.

<system.webServer>
    <httpProtocol>
        <customHeaders>
            <add name="Access-Control-Allow-Origin" value="*" />
            <add name="Access-Control-Allow-Methods" value="GET,POST,OPTIONS" />
            <add name="Access-Control-Allow-Headers" value="Content-Type, soapaction" />
        </customHeaders>
    </httpProtocol>
</system.webServer>

The 405 response is a "Method not allowed" response. It sounds like your server isn't properly configured to handle CORS preflight requests. You need to do two things:

1) Enable IIS7 to respond to HTTP OPTIONS requests. You are getting the 405 because IIS7 is rejecting the OPTIONS request. I don't know how to do this as I'm not familiar with IIS7, but there are probably others on Stack Overflow who do.

2) Configure your application to respond to CORS preflight requests. You can do this by adding the following two lines underneath the Access-Control-Allow-Origin line in the <customHeaders> section:

<add name="Access-Control-Allow-Methods" value="GET,PUT,POST,DELETE" />
<add name="Access-Control-Allow-Headers" value="Content-Type" />

You may have to add other values to the Access-Control-Allow-Headers section based on what headers your request is asking for. Do you have the sample code for making a request?

You can learn more about CORS and CORS preflight here: http://www.html5rocks.com/en/tutorials/cors/


Elaborating from DavidG answer which is really near of what is required for a basic solution:

  • First, configure the OPTIONSVerbHandler to execute before .Net handlers.

    1. In IIS console, select "Handler Mappings" (either on server level or site level; beware that on site level it will redefine all the handlers for your site and ignore any change done on server level after that; and of course on server level, this could break other sites if they need their own handling of options verb).
    2. In Action pane, select "View ordered list..." Seek OPTIONSVerbHandler, and move it up (lots of clicks...).

    You can also do this in web.config by redefining all handlers under <system.webServer><handlers> (<clear> then <add ...> them back, this is what does the IIS console for you) (By the way, there is no need to ask for "read" permission on this handler.)

  • Second, configure custom http headers for your cors needs, such as:

    <system.webServer>
      <httpProtocol>
        <customHeaders>
          <add name="Access-Control-Allow-Origin" value="*"/>
          <add name="Access-Control-Allow-Headers" value="Content-Type"/>
          <add name="Access-Control-Allow-Methods" value="POST,GET,OPTIONS"/>
        </customHeaders>
      </httpProtocol>
    </system.webServer>
    

    You can also do this in IIS console.

This is a basic solution since it will send cors headers even on request which does not requires it. But with WCF, it looks like being the simpliest one.

With MVC or webapi, we could instead handle OPTIONS verb and cors headers by code (either "manually" or with built-in support available in latest version of webapi).


With ASP.net Web API 2 install Microsoft ASP.NET Cross Origin support via nuget.

http://enable-cors.org/server_aspnet.html

public static void Register(HttpConfiguration config)
{
 var enableCorsAttribute = new EnableCorsAttribute("http://mydomain.com",
                                                   "Origin, Content-Type, Accept",
                                                   "GET, PUT, POST, DELETE, OPTIONS");
        config.EnableCors(enableCorsAttribute);
}

The solution for me was to add :

<system.webServer>
    <modules runAllManagedModulesForAllRequests="true">
      <remove name="WebDAVModule"/>
    </modules>
</system.webServer>

To my web.config


Alsalaam Aleykum.

The first way is to follow the instructions in this link:

http://help.infragistics.com/Help/NetAdvantage/jQuery/2013.1/CLR4.0/html/igOlapXmlaDataSource_Configuring_IIS_for_Cross_Domain_OLAP_Data.html

Which corresponds to these configuration:

<handlers>
  <clear />
  <add name="OPTIONSVerbHandler" path="*" verb="OPTIONS" type="" modules="ProtocolSupportModule" scriptProcessor="" resourceType="Unspecified" requireAccess="Read" allowPathInfo="false" preCondition="" responseBufferLimit="4194304" />
  <add name="xamlx-ISAPI-4.0_64bit" path="*.xamlx" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="4194304" />
  <add name="xamlx-ISAPI-4.0_32bit" path="*.xamlx" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="4194304" />
  <add name="xamlx-Integrated-4.0" path="*.xamlx" verb="GET,HEAD,POST,DEBUG" type="System.Xaml.Hosting.XamlHttpHandlerFactory, System.Xaml.Hosting, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" modules="ManagedPipelineHandler" scriptProcessor=""
  resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="rules-ISAPI-4.0_64bit" path="*.rules" verb="*" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness64"
  responseBufferLimit="4194304" />
  <add name="rules-ISAPI-4.0_32bit" path="*.rules" verb="*" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness32"
  responseBufferLimit="4194304" />
  <add name="rules-Integrated-4.0" path="*.rules" verb="*" type="System.ServiceModel.Activation.ServiceHttpHandlerFactory, System.ServiceModel.Activation, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" modules="ManagedPipelineHandler"
  scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="xoml-ISAPI-4.0_64bit" path="*.xoml" verb="*" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness64"
  responseBufferLimit="4194304" />
  <add name="xoml-ISAPI-4.0_32bit" path="*.xoml" verb="*" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness32"
  responseBufferLimit="4194304" />
  <add name="xoml-Integrated-4.0" path="*.xoml" verb="*" type="System.ServiceModel.Activation.ServiceHttpHandlerFactory, System.ServiceModel.Activation, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" modules="ManagedPipelineHandler"
  scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="svc-ISAPI-4.0_64bit" path="*.svc" verb="*" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness64"
  responseBufferLimit="4194304" />
  <add name="svc-ISAPI-4.0_32bit" path="*.svc" verb="*" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness32"
  responseBufferLimit="4194304" />
  <add name="svc-Integrated-4.0" path="*.svc" verb="*" type="System.ServiceModel.Activation.ServiceHttpHandlerFactory, System.ServiceModel.Activation, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" modules="ManagedPipelineHandler" scriptProcessor=""
  resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="ISAPI-dll" path="*.dll" verb="*" type="" modules="IsapiModule" scriptProcessor="" resourceType="File" requireAccess="Execute" allowPathInfo="true" preCondition="" responseBufferLimit="4194304" />
  <add name="AXD-ISAPI-4.0_64bit" path="*.axd" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="PageHandlerFactory-ISAPI-4.0_64bit" path="*.aspx" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="SimpleHandlerFactory-ISAPI-4.0_64bit" path="*.ashx" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script"
  allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="WebServiceHandlerFactory-ISAPI-4.0_64bit" path="*.asmx" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script"
  allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="HttpRemotingHandlerFactory-rem-ISAPI-4.0_64bit" path="*.rem" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script"
  allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="HttpRemotingHandlerFactory-soap-ISAPI-4.0_64bit" path="*.soap" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script"
  allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="aspq-ISAPI-4.0_64bit" path="*.aspq" verb="*" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness64"
  responseBufferLimit="0" />
  <add name="cshtm-ISAPI-4.0_64bit" path="*.cshtm" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="cshtml-ISAPI-4.0_64bit" path="*.cshtml" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="vbhtm-ISAPI-4.0_64bit" path="*.vbhtm" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="vbhtml-ISAPI-4.0_64bit" path="*.vbhtml" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="TraceHandler-Integrated-4.0" path="trace.axd" verb="GET,HEAD,POST,DEBUG" type="System.Web.Handlers.TraceHandler" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0"
  responseBufferLimit="4194304" />
  <add name="WebAdminHandler-Integrated-4.0" path="WebAdmin.axd" verb="GET,DEBUG" type="System.Web.Handlers.WebAdminHandler" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0"
  responseBufferLimit="4194304" />
  <add name="AssemblyResourceLoader-Integrated-4.0" path="WebResource.axd" verb="GET,DEBUG" type="System.Web.Handlers.AssemblyResourceLoader" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="PageHandlerFactory-Integrated-4.0" path="*.aspx" verb="GET,HEAD,POST,DEBUG" type="System.Web.UI.PageHandlerFactory" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="SimpleHandlerFactory-Integrated-4.0" path="*.ashx" verb="GET,HEAD,POST,DEBUG" type="System.Web.UI.SimpleHandlerFactory" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="WebServiceHandlerFactory-Integrated-4.0" path="*.asmx" verb="GET,HEAD,POST,DEBUG" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" modules="ManagedPipelineHandler"
  scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="HttpRemotingHandlerFactory-rem-Integrated-4.0" path="*.rem" verb="GET,HEAD,POST,DEBUG" type="System.Runtime.Remoting.Channels.Http.HttpRemotingHandlerFactory, System.Runtime.Remoting, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
  modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="HttpRemotingHandlerFactory-soap-Integrated-4.0" path="*.soap" verb="GET,HEAD,POST,DEBUG" type="System.Runtime.Remoting.Channels.Http.HttpRemotingHandlerFactory, System.Runtime.Remoting, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
  modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="aspq-Integrated-4.0" path="*.aspq" verb="GET,HEAD,POST,DEBUG" type="System.Web.HttpForbiddenHandler" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0"
  responseBufferLimit="4194304" />
  <add name="cshtm-Integrated-4.0" path="*.cshtm" verb="GET,HEAD,POST,DEBUG" type="System.Web.HttpForbiddenHandler" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0"
  responseBufferLimit="4194304" />
  <add name="cshtml-Integrated-4.0" path="*.cshtml" verb="GET,HEAD,POST,DEBUG" type="System.Web.HttpForbiddenHandler" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0"
  responseBufferLimit="4194304" />
  <add name="vbhtm-Integrated-4.0" path="*.vbhtm" verb="GET,HEAD,POST,DEBUG" type="System.Web.HttpForbiddenHandler" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0"
  responseBufferLimit="4194304" />
  <add name="vbhtml-Integrated-4.0" path="*.vbhtml" verb="GET,HEAD,POST,DEBUG" type="System.Web.HttpForbiddenHandler" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0"
  responseBufferLimit="4194304" />
  <add name="ScriptHandlerFactoryAppServices-Integrated-4.0" path="*_AppService.axd" verb="*" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" modules="ManagedPipelineHandler"
  scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="ScriptResourceIntegrated-4.0" path="*ScriptResource.axd" verb="GET,HEAD" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" modules="ManagedPipelineHandler"
  scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="4194304" />
  <add name="AXD-ISAPI-4.0_32bit" path="*.axd" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="PageHandlerFactory-ISAPI-4.0_32bit" path="*.aspx" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="SimpleHandlerFactory-ISAPI-4.0_32bit" path="*.ashx" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="WebServiceHandlerFactory-ISAPI-4.0_32bit" path="*.asmx" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script"
  allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="HttpRemotingHandlerFactory-rem-ISAPI-4.0_32bit" path="*.rem" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script"
  allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="HttpRemotingHandlerFactory-soap-ISAPI-4.0_32bit" path="*.soap" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script"
  allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="aspq-ISAPI-4.0_32bit" path="*.aspq" verb="*" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false" preCondition="classicMode,runtimeVersionv4.0,bitness32"
  responseBufferLimit="0" />
  <add name="cshtm-ISAPI-4.0_32bit" path="*.cshtm" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="cshtml-ISAPI-4.0_32bit" path="*.cshtml" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="vbhtm-ISAPI-4.0_32bit" path="*.vbhtm" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="vbhtml-ISAPI-4.0_32bit" path="*.vbhtml" verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="TRACEVerbHandler" path="*" verb="TRACE" type="" modules="ProtocolSupportModule" scriptProcessor="" resourceType="Unspecified" requireAccess="None" allowPathInfo="false" preCondition="" responseBufferLimit="4194304" />
  <add name="ExtensionlessUrlHandler-ISAPI-4.0_32bit" path="*." verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
  <add name="ExtensionlessUrlHandler-ISAPI-4.0_64bit" path="*." verb="GET,HEAD,POST,DEBUG" type="" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
  <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="GET,HEAD,POST,DEBUG" type="System.Web.Handlers.TransferRequestHandler" modules="ManagedPipelineHandler" scriptProcessor="" resourceType="Unspecified" requireAccess="Script" allowPathInfo="false"
  preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="0" />
  <add name="StaticFile" path="*" verb="*" type="" modules="StaticFileModule,DefaultDocumentModule,DirectoryListingModule" scriptProcessor="" resourceType="Either" requireAccess="Read" allowPathInfo="false" preCondition="" responseBufferLimit="4194304"
  />
</handlers>

The second way is as to respond to the HTTP OPTIONS verb in your BeginRequest method.

  protected void Application_BeginRequest(object sender, EventArgs e)
{
    HttpContext.Current.Response.AddHeader("Access-Control-Allow-Origin", "*");
    if (HttpContext.Current.Request.HttpMethod == "OPTIONS")
    {
        HttpContext.Current.Response.AddHeader("Access-Control-Request-Method", "GET ,POST, PUT, DELETE");

        HttpContext.Current.Response.AddHeader("Access-Control-Allow-Headers", "Origin,Content-Type, Accept");
        HttpContext.Current.Response.AddHeader("Access-Control-Max-Age", "86400"); // 24 hours
        HttpContext.Current.Response.End();
    }
}

One thing that has not been mentioned in these answers is that if you are using IIS and have sub applications with their own separate web.config, you may need to have a web.config in the parent directory containing the following code.

<httpProtocol>
  <customHeaders>
    <add name="Access-Control-Allow-Origin" value="*"/>
    <add name="Access-Control-Allow-Headers" value="Content-Type"/>
    <add name="Access-Control-Allow-Methods" value="POST,GET,OPTIONS"/>
  </customHeaders>
</httpProtocol>

It took Microsoft years to identify the gaps and ship an out-of-band CORS module to solve this problem.

  1. Install the module from Microsoft
  2. Configure it with snippets

as below

<configuration>
    <system.webServer>
        <cors enabled="true" failUnlistedOrigins="true">            
            <add origin="http://*" allowed="true" />
        </cors>
    </system.webServer>
</configuration>

In general, it is much easier than your custom headers and also offers better handling of preflight requests.

In case you need the same for IIS Express, use some PowerShell scripts I wrote.