Enable Cross-Origin Requests (CORS) in ASP.NET Core

By Rick Anderson and Kirk Larkin

This article shows how to enable CORS in an ASP.NET Core app.

Browser security prevents a web page from making requests to a different domain than the one that served the web page. This restriction is called the same-origin policy. The same-origin policy prevents a malicious site from reading sensitive data from another site. Sometimes, you might want to allow other sites to make cross-origin requests to your app. For more information, see the Mozilla CORS article.

Cross Origin Resource Sharing (CORS):

  • Is a W3C standard that allows a server to relax the same-origin policy.
  • Is not a security feature, CORS relaxes security. An API is not safer by allowing CORS. For more information, see How CORS works.
  • Allows a server to explicitly allow some cross-origin requests while rejecting others.
  • Is safer and more flexible than earlier techniques, such as JSONP.

View or download sample code (how to download)

Same origin

Two URLs have the same origin if they have identical schemes, hosts, and ports (RFC 6454).

These two URLs have the same origin:

  • https://example.com/foo.html
  • https://example.com/bar.html

These URLs have different origins than the previous two URLs:

  • https://example.net: Different domain
  • https://www.example.com/foo.html: Different subdomain
  • http://example.com/foo.html: Different scheme
  • https://example.com:9000/foo.html: Different port

Enable CORS

There are three ways to enable CORS:

Using the [EnableCors] attribute with a named policy provides the finest control in limiting endpoints that support CORS.

Warning

UseCors must be called in the correct order. For more information, see Middleware order. For example, UseCors must be called before UseResponseCaching when using UseResponseCaching.

Each approach is detailed in the following sections.

CORS with named policy and middleware

CORS Middleware handles cross-origin requests. The following code applies a CORS policy to all the app's endpoints with the specified origins:

var  MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: MyAllowSpecificOrigins,
                      builder =>
                      {
                          builder.WithOrigins("http://example.com",
                                              "http://www.contoso.com");
                      });
});

// services.AddResponseCaching();

builder.Services.AddControllers();

var app = builder.Build();
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseCors(MyAllowSpecificOrigins);

app.UseAuthorization();

app.MapControllers();

app.Run();

The preceding code:

With endpoint routing, the CORS middleware must be configured to execute between the calls to UseRouting and UseEndpoints.

See Test CORS for instructions on testing code similar to the preceding code.

The AddCors method call adds CORS services to the app's service container:

var  MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: MyAllowSpecificOrigins,
                      builder =>
                      {
                          builder.WithOrigins("http://example.com",
                                              "http://www.contoso.com");
                      });
});

// services.AddResponseCaching();

builder.Services.AddControllers();

var app = builder.Build();
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseCors(MyAllowSpecificOrigins);

app.UseAuthorization();

app.MapControllers();

app.Run();

For more information, see CORS policy options in this document.

The CorsPolicyBuilder methods can be chained, as shown in the following code:

var MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(MyAllowSpecificOrigins,
                          builder =>
                          {
                              builder.WithOrigins("http://example.com",
                                                  "http://www.contoso.com")
                                                  .AllowAnyHeader()
                                                  .AllowAnyMethod();
                          });
});

builder.Services.AddControllers();

var app = builder.Build();
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseCors(MyAllowSpecificOrigins);

app.UseAuthorization();

app.MapControllers();

app.Run();

Note: The specified URL must not contain a trailing slash (/). If the URL terminates with /, the comparison returns false and no header is returned.

CORS with default policy and middleware

The following highlighted code enables the default CORS policy:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddDefaultPolicy(
        builder =>
        {
            builder.WithOrigins("http://example.com",
                                "http://www.contoso.com");
        });
});

builder.Services.AddControllers();

var app = builder.Build();

app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseCors();

app.UseAuthorization();

app.MapControllers();

app.Run();

The preceding code applies the default CORS policy to all controller endpoints.

Enable Cors with endpoint routing

Enabling CORS on a per-endpoint basis using RequireCors does not support automatic preflight requests. For more information, see this GitHub issue and Test CORS with endpoint routing and [HttpOptions].

With endpoint routing, CORS can be enabled on a per-endpoint basis using the RequireCors set of extension methods:

var MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: MyAllowSpecificOrigins,
                      builder =>
                      {
                          builder.WithOrigins("http://example.com",
                                              "http://www.contoso.com");
                      });
});

builder.Services.AddControllers();
builder.Services.AddRazorPages();

var app = builder.Build();

app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseCors();

app.UseAuthorization();

app.UseEndpoints(endpoints =>
{
    endpoints.MapGet("/echo",
        context => context.Response.WriteAsync("echo"))
        .RequireCors(MyAllowSpecificOrigins);

    endpoints.MapControllers()
             .RequireCors(MyAllowSpecificOrigins);

    endpoints.MapGet("/echo2",
        context => context.Response.WriteAsync("echo2"));

    endpoints.MapRazorPages();
});

app.Run();

In the preceding code:

  • app.UseCors enables the CORS middleware. Because a default policy hasn't been configured, app.UseCors() alone doesn't enable CORS.
  • The /echo and controller endpoints allow cross-origin requests using the specified policy.
  • The /echo2 and Razor Pages endpoints do not allow cross-origin requests because no default policy was specified.

The [DisableCors] attribute does not disable CORS that has been enabled by endpoint routing with RequireCors.

See Test CORS with endpoint routing and [HttpOptions] for instructions on testing code similar to the preceding.

Enable CORS with attributes

Enabling CORS with the [EnableCors] attribute and applying a named policy to only those endpoints that require CORS provides the finest control.

The [EnableCors] attribute provides an alternative to applying CORS globally. The [EnableCors] attribute enables CORS for selected endpoints, rather than all endpoints:

  • [EnableCors] specifies the default policy.
  • [EnableCors("{Policy String}")] specifies a named policy.

The [EnableCors] attribute can be applied to:

  • Razor Page PageModel
  • Controller
  • Controller action method

Different policies can be applied to controllers, page models, or action methods with the [EnableCors] attribute. When the [EnableCors] attribute is applied to a controller, page model, or action method, and CORS is enabled in middleware, both policies are applied. We recommend against combining policies. Use the [EnableCors] attribute or middleware, not both in the same app.

The following code applies a different policy to each method:

[Route("api/[controller]")]
[ApiController]
public class WidgetController : ControllerBase
{
    // GET api/values
    [EnableCors("AnotherPolicy")]
    [HttpGet]
    public ActionResult<IEnumerable<string>> Get()
    {
        return new string[] { "green widget", "red widget" };
    }

    // GET api/values/5
    [EnableCors("Policy1")]
    [HttpGet("{id}")]
    public ActionResult<string> Get(int id)
    {
        return id switch
        {
            1 => "green widget",
            2 => "red widget",
            _ => NotFound(),
        };
    }
}

The following code creates two CORS policies:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy("Policy1",
        builder =>
        {
            builder.WithOrigins("http://example.com",
                                "http://www.contoso.com");
        });

    options.AddPolicy("AnotherPolicy",
        builder =>
        {
            builder.WithOrigins("http://www.contoso.com")
                                .AllowAnyHeader()
                                .AllowAnyMethod();
        });
});

builder.Services.AddControllers();

var app = builder.Build();

app.UseHttpsRedirection();

app.UseRouting();

app.UseCors();

app.UseAuthorization();

app.MapControllers();

app.Run();

For the finest control of limiting CORS requests:

  • Use [EnableCors("MyPolicy")] with a named policy.
  • Don't define a default policy.
  • Don't use endpoint routing.

The code in the next section meets the preceding list.

See Test CORS for instructions on testing code similar to the preceding code.

Disable CORS

The [DisableCors] attribute does not disable CORS that has been enabled by endpoint routing.

The following code defines the CORS policy "MyPolicy":

var MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: "MyPolicy",
        builder =>
        {
            builder.WithOrigins("http://example.com",
                                "http://www.contoso.com")
                    .WithMethods("PUT", "DELETE", "GET");
        });
});

builder.Services.AddControllers();
builder.Services.AddRazorPages();

var app = builder.Build();

app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseCors();

app.UseAuthorization();

app.MapControllers();
app.MapRazorPages();

app.Run();

The following code disables CORS for the GetValues2 action:

[EnableCors("MyPolicy")]
[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    // GET api/values
    [HttpGet]
    public IActionResult Get() =>
        ControllerContext.MyDisplayRouteInfo();

    // GET api/values/5
    [HttpGet("{id}")]
    public IActionResult Get(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);

    // PUT api/values/5
    [HttpPut("{id}")]
    public IActionResult Put(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);


    // GET: api/values/GetValues2
    [DisableCors]
    [HttpGet("{action}")]
    public IActionResult GetValues2() =>
        ControllerContext.MyDisplayRouteInfo();

}

The preceding code:

See Test CORS for instructions on testing the preceding code.

CORS policy options

This section describes the various options that can be set in a CORS policy:

AddPolicy is called in Program.cs. For some options, it may be helpful to read the How CORS works section first.

Set the allowed origins

AllowAnyOrigin: Allows CORS requests from all origins with any scheme (http or https). AllowAnyOrigin is insecure because any website can make cross-origin requests to the app.

Note

Specifying AllowAnyOrigin and AllowCredentials is an insecure configuration and can result in cross-site request forgery. The CORS service returns an invalid CORS response when an app is configured with both methods.

AllowAnyOrigin affects preflight requests and the Access-Control-Allow-Origin header. For more information, see the Preflight requests section.

SetIsOriginAllowedToAllowWildcardSubdomains: Sets the IsOriginAllowed property of the policy to be a function that allows origins to match a configured wildcard domain when evaluating if the origin is allowed.

var MyAllowSpecificOrigins = "_MyAllowSubdomainPolicy";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: MyAllowSpecificOrigins,
        builder =>
        {
            builder.WithOrigins("https://*.example.com")
                .SetIsOriginAllowedToAllowWildcardSubdomains();
        });
});

builder.Services.AddControllers();

var app = builder.Build();

Set the allowed HTTP methods

AllowAnyMethod:

  • Allows any HTTP method:
  • Affects preflight requests and the Access-Control-Allow-Methods header. For more information, see the Preflight requests section.

Set the allowed request headers

To allow specific headers to be sent in a CORS request, called author request headers, call WithHeaders and specify the allowed headers:

using Microsoft.Net.Http.Headers;

var MyAllowSpecificOrigins = "_MyAllowSubdomainPolicy";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: MyAllowSpecificOrigins,
       builder =>
       {
           builder.WithOrigins("http://example.com")
                  .WithHeaders(HeaderNames.ContentType, "x-custom-header");
       });
});

builder.Services.AddControllers();

var app = builder.Build();

To allow all author request headers, call AllowAnyHeader:

var MyAllowSpecificOrigins = "_MyAllowSubdomainPolicy";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: MyAllowSpecificOrigins,
        builder =>
        {
            builder.WithOrigins("https://*.example.com")
                   .AllowAnyHeader();
        });
});

builder.Services.AddControllers();

var app = builder.Build();

AllowAnyHeader affects preflight requests and the Access-Control-Request-Headers header. For more information, see the Preflight requests section.

A CORS Middleware policy match to specific headers specified by WithHeaders is only possible when the headers sent in Access-Control-Request-Headers exactly match the headers stated in WithHeaders.

For instance, consider an app configured as follows:

app.UseCors(policy => policy.WithHeaders(HeaderNames.CacheControl));

CORS Middleware declines a preflight request with the following request header because Content-Language (HeaderNames.ContentLanguage) isn't listed in WithHeaders:

Access-Control-Request-Headers: Cache-Control, Content-Language

The app returns a 200 OK response but doesn't send the CORS headers back. Therefore, the browser doesn't attempt the cross-origin request.

Set the exposed response headers

By default, the browser doesn't expose all of the response headers to the app. For more information, see W3C Cross-Origin Resource Sharing (Terminology): Simple Response Header.

The response headers that are available by default are:

  • Cache-Control
  • Content-Language
  • Content-Type
  • Expires
  • Last-Modified
  • Pragma

The CORS specification calls these headers simple response headers. To make other headers available to the app, call WithExposedHeaders:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy("MyExposeResponseHeadersPolicy",
        builder =>
        {
            builder.WithOrigins("https://*.example.com")
                   .WithExposedHeaders("x-custom-header");
        });
});

builder.Services.AddControllers();

var app = builder.Build();

Credentials in cross-origin requests

Credentials require special handling in a CORS request. By default, the browser doesn't send credentials with a cross-origin request. Credentials include cookies and HTTP authentication schemes. To send credentials with a cross-origin request, the client must set XMLHttpRequest.withCredentials to true.

Using XMLHttpRequest directly:

var xhr = new XMLHttpRequest();
xhr.open('get', 'https://www.example.com/api/test');
xhr.withCredentials = true;

Using jQuery:

$.ajax({
  type: 'get',
  url: 'https://www.example.com/api/test',
  xhrFields: {
    withCredentials: true
  }
});

Using the Fetch API:

fetch('https://www.example.com/api/test', {
    credentials: 'include'
});

The server must allow the credentials. To allow cross-origin credentials, call AllowCredentials:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy("MyMyAllowCredentialsPolicy",
        builder =>
        {
            builder.WithOrigins("http://example.com")
                   .AllowCredentials();
        });
});

builder.Services.AddControllers();

var app = builder.Build();

The HTTP response includes an Access-Control-Allow-Credentials header, which tells the browser that the server allows credentials for a cross-origin request.

If the browser sends credentials but the response doesn't include a valid Access-Control-Allow-Credentials header, the browser doesn't expose the response to the app, and the cross-origin request fails.

Allowing cross-origin credentials is a security risk. A website at another domain can send a signed-in user's credentials to the app on the user's behalf without the user's knowledge.

The CORS specification also states that setting origins to "*" (all origins) is invalid if the Access-Control-Allow-Credentials header is present.

Preflight requests

For some CORS requests, the browser sends an additional OPTIONS request before making the actual request. This request is called a preflight request. The browser can skip the preflight request if all the following conditions are true:

  • The request method is GET, HEAD, or POST.
  • The app doesn't set request headers other than Accept, Accept-Language, Content-Language, Content-Type, or Last-Event-ID.
  • The Content-Type header, if set, has one of the following values:
    • application/x-www-form-urlencoded
    • multipart/form-data
    • text/plain

The rule on request headers set for the client request applies to headers that the app sets by calling setRequestHeader on the XMLHttpRequest object. The CORS specification calls these headers author request headers. The rule doesn't apply to headers the browser can set, such as User-Agent, Host, or Content-Length.

The following is an example response similar to the preflight request made from the [Put test] button in the Test CORS section of this document.

General:
Request URL: https://cors3.azurewebsites.net/api/values/5
Request Method: OPTIONS
Status Code: 204 No Content

Response Headers:
Access-Control-Allow-Methods: PUT,DELETE,GET
Access-Control-Allow-Origin: https://cors1.azurewebsites.net
Server: Microsoft-IIS/10.0
Set-Cookie: ARRAffinity=8f8...8;Path=/;HttpOnly;Domain=cors1.azurewebsites.net
Vary: Origin

Request Headers:
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Access-Control-Request-Method: PUT
Connection: keep-alive
Host: cors3.azurewebsites.net
Origin: https://cors1.azurewebsites.net
Referer: https://cors1.azurewebsites.net/
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
User-Agent: Mozilla/5.0

The preflight request uses the HTTP OPTIONS method. It may include the following headers:

If the preflight request is denied, the app returns a 200 OK response but doesn't set the CORS headers. Therefore, the browser doesn't attempt the cross-origin request. For an example of a denied preflight request, see the Test CORS section of this document.

Using the F12 tools, the console app shows an error similar to one of the following, depending on the browser:

  • Firefox: Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://cors1.azurewebsites.net/api/TodoItems1/MyDelete2/5. (Reason: CORS request did not succeed). Learn More
  • Chromium based: Access to fetch at 'https://cors1.azurewebsites.net/api/TodoItems1/MyDelete2/5' from origin 'https://cors3.azurewebsites.net' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

To allow specific headers, call WithHeaders:

using Microsoft.Net.Http.Headers;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy("MyAllowHeadersPolicy",
        builder =>
        {
        builder.WithOrigins("http://example.com")
                   .WithHeaders(HeaderNames.ContentType, "x-custom-header");
        });
});

builder.Services.AddControllers();

var app = builder.Build();

To allow all author request headers, call AllowAnyHeader:

using Microsoft.Net.Http.Headers;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy("MyAllowAllHeadersPolicy",
        builder =>
        {
            builder.WithOrigins("https://*.example.com")
                   .AllowAnyHeader();
        });
});

builder.Services.AddControllers();

var app = builder.Build();

Browsers aren't consistent in how they set Access-Control-Request-Headers. If either:

  • Headers are set to anything other than "*"
  • AllowAnyHeader is called: Include at least Accept, Content-Type, and Origin, plus any custom headers that you want to support.

Automatic preflight request code

When the CORS policy is applied either:

  • Globally by calling app.UseCors in Program.cs.
  • Using the [EnableCors] attribute.

ASP.NET Core responds to the preflight OPTIONS request.

Enabling CORS on a per-endpoint basis using RequireCors currently does not support automatic preflight requests.

The Test CORS section of this document demonstrates this behavior.

[HttpOptions] attribute for preflight requests

When CORS is enabled with the appropriate policy, ASP.NET Core generally responds to CORS preflight requests automatically. In some scenarios, this may not be the case. For example, using CORS with endpoint routing.

The following code uses the [HttpOptions] attribute to create endpoints for OPTIONS requests:

[Route("api/[controller]")]
[ApiController]
public class TodoItems2Controller : ControllerBase
{
    // OPTIONS: api/TodoItems2/5
    [HttpOptions("{id}")]
    public IActionResult PreflightRoute(int id)
    {
        return NoContent();
    }

    // OPTIONS: api/TodoItems2 
    [HttpOptions]
    public IActionResult PreflightRoute()
    {
        return NoContent();
    }

    [HttpPut("{id}")]
    public IActionResult PutTodoItem(int id)
    {
        if (id < 1)
        {
            return BadRequest();
        }

        return ControllerContext.MyDisplayRouteInfo(id);
    }

See Test CORS with endpoint routing and [HttpOptions] for instructions on testing the preceding code.

Set the preflight expiration time

The Access-Control-Max-Age header specifies how long the response to the preflight request can be cached. To set this header, call SetPreflightMaxAge:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy("MySetPreflightExpirationPolicy",
        builder =>
        {
            builder.WithOrigins("http://example.com")
                   .SetPreflightMaxAge(TimeSpan.FromSeconds(2520));
        });
});

builder.Services.AddControllers();

var app = builder.Build();

How CORS works

This section describes what happens in a CORS request at the level of the HTTP messages.

  • CORS is not a security feature. CORS is a W3C standard that allows a server to relax the same-origin policy.
    • For example, a malicious actor could use Cross-Site Scripting (XSS) against your site and execute a cross-site request to their CORS enabled site to steal information.
  • An API isn't safer by allowing CORS.
    • It's up to the client (browser) to enforce CORS. The server executes the request and returns the response, it's the client that returns an error and blocks the response. For example, any of the following tools will display the server response:
  • It's a way for a server to allow browsers to execute a cross-origin XHR or Fetch API request that otherwise would be forbidden.
    • Browsers without CORS can't do cross-origin requests. Before CORS, JSONP was used to circumvent this restriction. JSONP doesn't use XHR, it uses the <script> tag to receive the response. Scripts are allowed to be loaded cross-origin.

The CORS specification introduced several new HTTP headers that enable cross-origin requests. If a browser supports CORS, it sets these headers automatically for cross-origin requests. Custom JavaScript code isn't required to enable CORS.

The PUT test button on the deployed sample

The following is an example of a cross-origin request from the Values test button to https://cors1.azurewebsites.net/api/values. The Origin header:

  • Provides the domain of the site that's making the request.
  • Is required and must be different from the host.

General headers

Request URL: https://cors1.azurewebsites.net/api/values
Request Method: GET
Status Code: 200 OK

Response headers

Content-Encoding: gzip
Content-Type: text/plain; charset=utf-8
Server: Microsoft-IIS/10.0
Set-Cookie: ARRAffinity=8f...;Path=/;HttpOnly;Domain=cors1.azurewebsites.net
Transfer-Encoding: chunked
Vary: Accept-Encoding
X-Powered-By: ASP.NET

Request headers

Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Connection: keep-alive
Host: cors1.azurewebsites.net
Origin: https://cors3.azurewebsites.net
Referer: https://cors3.azurewebsites.net/
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
User-Agent: Mozilla/5.0 ...

In OPTIONS requests, the server sets the Response headers Access-Control-Allow-Origin: {allowed origin} header in the response. For example, the deployed sample, Delete [EnableCors] button OPTIONS request contains the following headers:

General headers

Request URL: https://cors3.azurewebsites.net/api/TodoItems2/MyDelete2/5
Request Method: OPTIONS
Status Code: 204 No Content

Response headers

Access-Control-Allow-Headers: Content-Type,x-custom-header
Access-Control-Allow-Methods: PUT,DELETE,GET,OPTIONS
Access-Control-Allow-Origin: https://cors1.azurewebsites.net
Server: Microsoft-IIS/10.0
Set-Cookie: ARRAffinity=8f...;Path=/;HttpOnly;Domain=cors3.azurewebsites.net
Vary: Origin
X-Powered-By: ASP.NET

Request headers

Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Access-Control-Request-Headers: content-type
Access-Control-Request-Method: DELETE
Connection: keep-alive
Host: cors3.azurewebsites.net
Origin: https://cors1.azurewebsites.net
Referer: https://cors1.azurewebsites.net/test?number=2
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
User-Agent: Mozilla/5.0

In the preceding Response headers, the server sets the Access-Control-Allow-Origin header in the response. The https://cors1.azurewebsites.net value of this header matches the Origin header from the request.

If AllowAnyOrigin is called, the Access-Control-Allow-Origin: *, the wildcard value, is returned. AllowAnyOrigin allows any origin.

If the response doesn't include the Access-Control-Allow-Origin header, the cross-origin request fails. Specifically, the browser disallows the request. Even if the server returns a successful response, the browser doesn't make the response available to the client app.

Display OPTIONS requests

By default, the Chrome and Edge browsers don't show OPTIONS requests on the network tab of the F12 tools. To display OPTIONS requests in these browsers:

  • chrome://flags/#out-of-blink-cors or edge://flags/#out-of-blink-cors
  • disable the flag.
  • restart.

Firefox shows OPTIONS requests by default.

CORS in IIS

When deploying to IIS, CORS has to run before Windows Authentication if the server isn't configured to allow anonymous access. To support this scenario, the IIS CORS module needs to be installed and configured for the app.

Test CORS

The sample download has code to test CORS. See how to download. The sample is an API project with Razor Pages added:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: "MyPolicy",
                builder =>
                {
                    builder.WithOrigins("http://example.com",
                        "http://www.contoso.com",
                        "https://cors1.azurewebsites.net",
                        "https://cors3.azurewebsites.net",
                        "https://localhost:44398",
                        "https://localhost:5001")
                            .WithMethods("PUT", "DELETE", "GET");
                });
});

builder.Services.AddControllers();
builder.Services.AddRazorPages();

var app = builder.Build();

app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseCors();

app.UseAuthorization();

app.MapControllers();
app.MapRazorPages();

app.Run();

Warning

WithOrigins("https://localhost:<port>"); should only be used for testing a sample app similar to the download sample code.

The following ValuesController provides the endpoints for testing:

[EnableCors("MyPolicy")]
[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    // GET api/values
    [HttpGet]
    public IActionResult Get() =>
        ControllerContext.MyDisplayRouteInfo();

    // GET api/values/5
    [HttpGet("{id}")]
    public IActionResult Get(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);

    // PUT api/values/5
    [HttpPut("{id}")]
    public IActionResult Put(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);


    // GET: api/values/GetValues2
    [DisableCors]
    [HttpGet("{action}")]
    public IActionResult GetValues2() =>
        ControllerContext.MyDisplayRouteInfo();

}

MyDisplayRouteInfo is provided by the Rick.Docs.Samples.RouteInfo NuGet package and displays route information.

Test the preceding sample code by using one of the following approaches:

  • Use the deployed sample app at https://cors3.azurewebsites.net/. There is no need to download the sample.
  • Run the sample with dotnet run using the default URL of https://localhost:5001.
  • Run the sample from Visual Studio with the port set to 44398 for a URL of https://localhost:44398.

Using a browser with the F12 tools:

  • Select the Values button and review the headers in the Network tab.

  • Select the PUT test button. See Display OPTIONS requests for instructions on displaying the OPTIONS request. The PUT test creates two requests, an OPTIONS preflight request and the PUT request.

  • Select the GetValues2 [DisableCors] button to trigger a failed CORS request. As mentioned in the document, the response returns 200 success, but the CORS request is not made. Select the Console tab to see the CORS error. Depending on the browser, an error similar to the following is displayed:

    Access to fetch at 'https://cors1.azurewebsites.net/api/values/GetValues2' from origin 'https://cors3.azurewebsites.net' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

CORS-enabled endpoints can be tested with a tool, such as curl, Fiddler, or Postman. When using a tool, the origin of the request specified by the Origin header must differ from the host receiving the request. If the request isn't cross-origin based on the value of the Origin header:

  • There's no need for CORS Middleware to process the request.
  • CORS headers aren't returned in the response.

The following command uses curl to issue an OPTIONS request with information:

curl -X OPTIONS https://cors3.azurewebsites.net/api/TodoItems2/5 -i

Test CORS with endpoint routing and [HttpOptions]

Enabling CORS on a per-endpoint basis using RequireCors currently does not support automatic preflight requests. Consider the following code which uses endpoint routing to enable CORS:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: "MyPolicy",
                builder =>
                {
                    builder.WithOrigins("http://example.com",
                        "http://www.contoso.com",
                        "https://cors1.azurewebsites.net",
                        "https://cors3.azurewebsites.net",
                        "https://localhost:44398",
                        "https://localhost:5001")
                            .WithMethods("PUT", "DELETE", "GET");
                });
});

builder.Services.AddControllers();
builder.Services.AddRazorPages();

var app = builder.Build();

app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseCors();

app.UseAuthorization();

app.MapControllers();
app.MapRazorPages();

app.Run();

The following TodoItems1Controller provides endpoints for testing:

[Route("api/[controller]")]
[ApiController]
public class TodoItems1Controller : ControllerBase
{
    // PUT: api/TodoItems1/5
    [HttpPut("{id}")]
    public IActionResult PutTodoItem(int id)
    {
        if (id < 1)
        {
            return Content($"ID = {id}");
        }

        return ControllerContext.MyDisplayRouteInfo(id);
    }

    // Delete: api/TodoItems1/5
    [HttpDelete("{id}")]
    public IActionResult MyDelete(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);

    // GET: api/TodoItems1
    [HttpGet]
    public IActionResult GetTodoItems() =>
        ControllerContext.MyDisplayRouteInfo();

    [EnableCors]
    [HttpGet("{action}")]
    public IActionResult GetTodoItems2() =>
        ControllerContext.MyDisplayRouteInfo();

    // Delete: api/TodoItems1/MyDelete2/5
    [EnableCors]
    [HttpDelete("{action}/{id}")]
    public IActionResult MyDelete2(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);
}

Test the preceding code from the test page of the deployed sample.

The Delete [EnableCors] and GET [EnableCors] buttons succeed, because the endpoints have [EnableCors] and respond to preflight requests. The other endpoints fails. The GET button fails, because the JavaScript sends:

 headers: {
      "Content-Type": "x-custom-header"
 },

The following TodoItems2Controller provides similar endpoints, but includes explicit code to respond to OPTIONS requests:

[Route("api/[controller]")]
[ApiController]
public class TodoItems2Controller : ControllerBase
{
    // OPTIONS: api/TodoItems2/5
    [HttpOptions("{id}")]
    public IActionResult PreflightRoute(int id)
    {
        return NoContent();
    }

    // OPTIONS: api/TodoItems2 
    [HttpOptions]
    public IActionResult PreflightRoute()
    {
        return NoContent();
    }

    [HttpPut("{id}")]
    public IActionResult PutTodoItem(int id)
    {
        if (id < 1)
        {
            return BadRequest();
        }

        return ControllerContext.MyDisplayRouteInfo(id);
    }

    // [EnableCors] // Not needed as OPTIONS path provided
    [HttpDelete("{id}")]
    public IActionResult MyDelete(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);

    [EnableCors]  // Rquired for this path
    [HttpGet]
    public IActionResult GetTodoItems() =>
        ControllerContext.MyDisplayRouteInfo();

    [HttpGet("{action}")]
    public IActionResult GetTodoItems2() =>
        ControllerContext.MyDisplayRouteInfo();

    [EnableCors]  // Rquired for this path
    [HttpDelete("{action}/{id}")]
    public IActionResult MyDelete2(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);
}

Test the preceding code from the test page of the deployed sample. In the Controller drop down list, select Preflight and then Set Controller. All the CORS calls to the TodoItems2Controller endpoints succeed.

Additional resources

By Rick Anderson and Kirk Larkin

This article shows how to enable CORS in an ASP.NET Core app.

Browser security prevents a web page from making requests to a different domain than the one that served the web page. This restriction is called the same-origin policy. The same-origin policy prevents a malicious site from reading sensitive data from another site. Sometimes, you might want to allow other sites to make cross-origin requests to your app. For more information, see the Mozilla CORS article.

Cross Origin Resource Sharing (CORS):

  • Is a W3C standard that allows a server to relax the same-origin policy.
  • Is not a security feature, CORS relaxes security. An API is not safer by allowing CORS. For more information, see How CORS works.
  • Allows a server to explicitly allow some cross-origin requests while rejecting others.
  • Is safer and more flexible than earlier techniques, such as JSONP.

View or download sample code (how to download)

Same origin

Two URLs have the same origin if they have identical schemes, hosts, and ports (RFC 6454).

These two URLs have the same origin:

  • https://example.com/foo.html
  • https://example.com/bar.html

These URLs have different origins than the previous two URLs:

  • https://example.net: Different domain
  • https://www.example.com/foo.html: Different subdomain
  • http://example.com/foo.html: Different scheme
  • https://example.com:9000/foo.html: Different port

Enable CORS

There are three ways to enable CORS:

Using the [EnableCors] attribute with a named policy provides the finest control in limiting endpoints that support CORS.

Warning

UseCors must be called in the correct order. For more information, see Middleware order. For example, UseCors must be called before UseResponseCaching when using UseResponseCaching.

Each approach is detailed in the following sections.

CORS with named policy and middleware

CORS Middleware handles cross-origin requests. The following code applies a CORS policy to all the app's endpoints with the specified origins:

public class Startup
{
    readonly string MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddPolicy(name: MyAllowSpecificOrigins,
                              builder =>
                              {
                                  builder.WithOrigins("http://example.com",
                                                      "http://www.contoso.com");
                              });
        });

        // services.AddResponseCaching();
        services.AddControllers();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseHttpsRedirection();
        app.UseStaticFiles();
        app.UseRouting();

        app.UseCors(MyAllowSpecificOrigins);

        // app.UseResponseCaching();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

The preceding code:

With endpoint routing, the CORS middleware must be configured to execute between the calls to UseRouting and UseEndpoints.

See Test CORS for instructions on testing code similar to the preceding code.

The AddCors method call adds CORS services to the app's service container:

public class Startup
{
    readonly string MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddPolicy(name: MyAllowSpecificOrigins,
                              builder =>
                              {
                                  builder.WithOrigins("http://example.com",
                                                      "http://www.contoso.com");
                              });
        });

        // services.AddResponseCaching();
        services.AddControllers();
    }

For more information, see CORS policy options in this document.

The CorsPolicyBuilder methods can be chained, as shown in the following code:

public void ConfigureServices(IServiceCollection services)
{
    services.AddCors(options =>
    {
        options.AddPolicy(MyAllowSpecificOrigins,
                          builder =>
                          {
                              builder.WithOrigins("http://example.com",
                                                  "http://www.contoso.com")
                                                  .AllowAnyHeader()
                                                  .AllowAnyMethod();
                          });
    });

    services.AddControllers();
}

Note: The specified URL must not contain a trailing slash (/). If the URL terminates with /, the comparison returns false and no header is returned.

CORS with default policy and middleware

The following highlighted code enables the default CORS policy:

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddDefaultPolicy(
                builder =>
                {
                    builder.WithOrigins("http://example.com",
                                        "http://www.contoso.com");
                });
        });

        services.AddControllers();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseHttpsRedirection();
        app.UseStaticFiles();
        app.UseRouting();

        app.UseCors();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

The preceding code applies the default CORS policy to all controller endpoints.

Enable Cors with endpoint routing

Enabling CORS on a per-endpoint basis using RequireCors does not support automatic preflight requests. For more information, see this GitHub issue and Test CORS with endpoint routing and [HttpOptions].

With endpoint routing, CORS can be enabled on a per-endpoint basis using the RequireCors set of extension methods:

public class Startup
{
    readonly string MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddPolicy(name: MyAllowSpecificOrigins,
                              builder =>
                              {
                                  builder.WithOrigins("http://example.com",
                                                      "http://www.contoso.com");
                              });
        });

        services.AddControllers();
        services.AddRazorPages();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseHttpsRedirection();
        app.UseStaticFiles();
        app.UseRouting();

        app.UseCors();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapGet("/echo",
                context => context.Response.WriteAsync("echo"))
                .RequireCors(MyAllowSpecificOrigins);

            endpoints.MapControllers()
                     .RequireCors(MyAllowSpecificOrigins);

            endpoints.MapGet("/echo2",
                context => context.Response.WriteAsync("echo2"));

            endpoints.MapRazorPages();
        });
    }
}

In the preceding code:

  • app.UseCors enables the CORS middleware. Because a default policy hasn't been configured, app.UseCors() alone doesn't enable CORS.
  • The /echo and controller endpoints allow cross-origin requests using the specified policy.
  • The /echo2 and Razor Pages endpoints do not allow cross-origin requests because no default policy was specified.

The [DisableCors] attribute does not disable CORS that has been enabled by endpoint routing with RequireCors.

See Test CORS with endpoint routing and [HttpOptions] for instructions on testing code similar to the preceding.

Enable CORS with attributes

Enabling CORS with the [EnableCors] attribute and applying a named policy to only those endpoints that require CORS provides the finest control.

The [EnableCors] attribute provides an alternative to applying CORS globally. The [EnableCors] attribute enables CORS for selected endpoints, rather than all endpoints:

  • [EnableCors] specifies the default policy.
  • [EnableCors("{Policy String}")] specifies a named policy.

The [EnableCors] attribute can be applied to:

  • Razor Page PageModel
  • Controller
  • Controller action method

Different policies can be applied to controllers, page models, or action methods with the [EnableCors] attribute. When the [EnableCors] attribute is applied to a controller, page model, or action method, and CORS is enabled in middleware, both policies are applied. We recommend against combining policies. Use the [EnableCors] attribute or middleware, not both in the same app.

The following code applies a different policy to each method:

[Route("api/[controller]")]
[ApiController]
public class WidgetController : ControllerBase
{
    // GET api/values
    [EnableCors("AnotherPolicy")]
    [HttpGet]
    public ActionResult<IEnumerable<string>> Get()
    {
        return new string[] { "green widget", "red widget" };
    }

    // GET api/values/5
    [EnableCors("Policy1")]
    [HttpGet("{id}")]
    public ActionResult<string> Get(int id)
    {
        return id switch
        {
            1 => "green widget",
            2 => "red widget",
            _ => NotFound(),
        };
    }
}

The following code creates two CORS policies:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddPolicy("Policy1",
                builder =>
                {
                    builder.WithOrigins("http://example.com",
                                        "http://www.contoso.com");
                });

            options.AddPolicy("AnotherPolicy",
                builder =>
                {
                    builder.WithOrigins("http://www.contoso.com")
                                        .AllowAnyHeader()
                                        .AllowAnyMethod();
                });
        });

        services.AddControllers();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseHttpsRedirection();

        app.UseRouting();

        app.UseCors();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

For the finest control of limiting CORS requests:

  • Use [EnableCors("MyPolicy")] with a named policy.
  • Don't define a default policy.
  • Don't use endpoint routing.

The code in the next section meets the preceding list.

See Test CORS for instructions on testing code similar to the preceding code.

Disable CORS

The [DisableCors] attribute does not disable CORS that has been enabled by endpoint routing.

The following code defines the CORS policy "MyPolicy":

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddPolicy(name: "MyPolicy",
                builder =>
                {
                    builder.WithOrigins("http://example.com",
                                        "http://www.contoso.com")
                            .WithMethods("PUT", "DELETE", "GET");
                });
        });

        services.AddControllers();
        services.AddRazorPages();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseHttpsRedirection();
        app.UseStaticFiles();
        app.UseRouting();

        app.UseCors();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
            endpoints.MapRazorPages();
        });
    }
}

The following code disables CORS for the GetValues2 action:

[EnableCors("MyPolicy")]
[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    // GET api/values
    [HttpGet]
    public IActionResult Get() =>
        ControllerContext.MyDisplayRouteInfo();

    // GET api/values/5
    [HttpGet("{id}")]
    public IActionResult Get(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);

    // PUT api/values/5
    [HttpPut("{id}")]
    public IActionResult Put(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);


    // GET: api/values/GetValues2
    [DisableCors]
    [HttpGet("{action}")]
    public IActionResult GetValues2() =>
        ControllerContext.MyDisplayRouteInfo();

}

The preceding code:

See Test CORS for instructions on testing the preceding code.

CORS policy options

This section describes the various options that can be set in a CORS policy:

AddPolicy is called in Startup.ConfigureServices. For some options, it may be helpful to read the How CORS works section first.

Set the allowed origins

AllowAnyOrigin: Allows CORS requests from all origins with any scheme (http or https). AllowAnyOrigin is insecure because any website can make cross-origin requests to the app.

Note

Specifying AllowAnyOrigin and AllowCredentials is an insecure configuration and can result in cross-site request forgery. The CORS service returns an invalid CORS response when an app is configured with both methods.

AllowAnyOrigin affects preflight requests and the Access-Control-Allow-Origin header. For more information, see the Preflight requests section.

SetIsOriginAllowedToAllowWildcardSubdomains: Sets the IsOriginAllowed property of the policy to be a function that allows origins to match a configured wildcard domain when evaluating if the origin is allowed.

options.AddPolicy("MyAllowSubdomainPolicy",
    builder =>
    {
        builder.WithOrigins("https://*.example.com")
            .SetIsOriginAllowedToAllowWildcardSubdomains();
    });

Set the allowed HTTP methods

AllowAnyMethod:

  • Allows any HTTP method:
  • Affects preflight requests and the Access-Control-Allow-Methods header. For more information, see the Preflight requests section.

Set the allowed request headers

To allow specific headers to be sent in a CORS request, called author request headers, call WithHeaders and specify the allowed headers:

options.AddPolicy("MyAllowHeadersPolicy",
    builder =>
    {
        // requires using Microsoft.Net.Http.Headers;
        builder.WithOrigins("http://example.com")
               .WithHeaders(HeaderNames.ContentType, "x-custom-header");
    });

To allow all author request headers, call AllowAnyHeader:

options.AddPolicy("MyAllowAllHeadersPolicy",
    builder =>
    {
        builder.WithOrigins("https://*.example.com")
               .AllowAnyHeader();
    });

AllowAnyHeader affects preflight requests and the Access-Control-Request-Headers header. For more information, see the Preflight requests section.

A CORS Middleware policy match to specific headers specified by WithHeaders is only possible when the headers sent in Access-Control-Request-Headers exactly match the headers stated in WithHeaders.

For instance, consider an app configured as follows:

app.UseCors(policy => policy.WithHeaders(HeaderNames.CacheControl));

CORS Middleware declines a preflight request with the following request header because Content-Language (HeaderNames.ContentLanguage) isn't listed in WithHeaders:

Access-Control-Request-Headers: Cache-Control, Content-Language

The app returns a 200 OK response but doesn't send the CORS headers back. Therefore, the browser doesn't attempt the cross-origin request.

Set the exposed response headers

By default, the browser doesn't expose all of the response headers to the app. For more information, see W3C Cross-Origin Resource Sharing (Terminology): Simple Response Header.

The response headers that are available by default are:

  • Cache-Control
  • Content-Language
  • Content-Type
  • Expires
  • Last-Modified
  • Pragma

The CORS specification calls these headers simple response headers. To make other headers available to the app, call WithExposedHeaders:

options.AddPolicy("MyExposeResponseHeadersPolicy",
    builder =>
    {
        builder.WithOrigins("https://*.example.com")
               .WithExposedHeaders("x-custom-header");
    });

Credentials in cross-origin requests

Credentials require special handling in a CORS request. By default, the browser doesn't send credentials with a cross-origin request. Credentials include cookies and HTTP authentication schemes. To send credentials with a cross-origin request, the client must set XMLHttpRequest.withCredentials to true.

Using XMLHttpRequest directly:

var xhr = new XMLHttpRequest();
xhr.open('get', 'https://www.example.com/api/test');
xhr.withCredentials = true;

Using jQuery:

$.ajax({
  type: 'get',
  url: 'https://www.example.com/api/test',
  xhrFields: {
    withCredentials: true
  }
});

Using the Fetch API:

fetch('https://www.example.com/api/test', {
    credentials: 'include'
});

The server must allow the credentials. To allow cross-origin credentials, call AllowCredentials:

options.AddPolicy("MyMyAllowCredentialsPolicy",
    builder =>
    {
        builder.WithOrigins("http://example.com")
               .AllowCredentials();
    });

The HTTP response includes an Access-Control-Allow-Credentials header, which tells the browser that the server allows credentials for a cross-origin request.

If the browser sends credentials but the response doesn't include a valid Access-Control-Allow-Credentials header, the browser doesn't expose the response to the app, and the cross-origin request fails.

Allowing cross-origin credentials is a security risk. A website at another domain can send a signed-in user's credentials to the app on the user's behalf without the user's knowledge.

The CORS specification also states that setting origins to "*" (all origins) is invalid if the Access-Control-Allow-Credentials header is present.

Preflight requests

For some CORS requests, the browser sends an additional OPTIONS request before making the actual request. This request is called a preflight request. The browser can skip the preflight request if all the following conditions are true:

  • The request method is GET, HEAD, or POST.
  • The app doesn't set request headers other than Accept, Accept-Language, Content-Language, Content-Type, or Last-Event-ID.
  • The Content-Type header, if set, has one of the following values:
    • application/x-www-form-urlencoded
    • multipart/form-data
    • text/plain

The rule on request headers set for the client request applies to headers that the app sets by calling setRequestHeader on the XMLHttpRequest object. The CORS specification calls these headers author request headers. The rule doesn't apply to headers the browser can set, such as User-Agent, Host, or Content-Length.

The following is an example response similar to the preflight request made from the [Put test] button in the Test CORS section of this document.

General:
Request URL: https://cors3.azurewebsites.net/api/values/5
Request Method: OPTIONS
Status Code: 204 No Content

Response Headers:
Access-Control-Allow-Methods: PUT,DELETE,GET
Access-Control-Allow-Origin: https://cors1.azurewebsites.net
Server: Microsoft-IIS/10.0
Set-Cookie: ARRAffinity=8f8...8;Path=/;HttpOnly;Domain=cors1.azurewebsites.net
Vary: Origin

Request Headers:
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Access-Control-Request-Method: PUT
Connection: keep-alive
Host: cors3.azurewebsites.net
Origin: https://cors1.azurewebsites.net
Referer: https://cors1.azurewebsites.net/
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
User-Agent: Mozilla/5.0

The preflight request uses the HTTP OPTIONS method. It may include the following headers:

If the preflight request is denied, the app returns a 200 OK response but doesn't set the CORS headers. Therefore, the browser doesn't attempt the cross-origin request. For an example of a denied preflight request, see the Test CORS section of this document.

Using the F12 tools, the console app shows an error similar to one of the following, depending on the browser:

  • Firefox: Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://cors1.azurewebsites.net/api/TodoItems1/MyDelete2/5. (Reason: CORS request did not succeed). Learn More
  • Chromium based: Access to fetch at 'https://cors1.azurewebsites.net/api/TodoItems1/MyDelete2/5' from origin 'https://cors3.azurewebsites.net' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

To allow specific headers, call WithHeaders:

options.AddPolicy("MyAllowHeadersPolicy",
    builder =>
    {
        // requires using Microsoft.Net.Http.Headers;
        builder.WithOrigins("http://example.com")
               .WithHeaders(HeaderNames.ContentType, "x-custom-header");
    });

To allow all author request headers, call AllowAnyHeader:

options.AddPolicy("MyAllowAllHeadersPolicy",
    builder =>
    {
        builder.WithOrigins("https://*.example.com")
               .AllowAnyHeader();
    });

Browsers aren't consistent in how they set Access-Control-Request-Headers. If either:

  • Headers are set to anything other than "*"
  • AllowAnyHeader is called: Include at least Accept, Content-Type, and Origin, plus any custom headers that you want to support.

Automatic preflight request code

When the CORS policy is applied either:

  • Globally by calling app.UseCors in Startup.Configure.
  • Using the [EnableCors] attribute.

ASP.NET Core responds to the preflight OPTIONS request.

Enabling CORS on a per-endpoint basis using RequireCors currently does not support automatic preflight requests.

The Test CORS section of this document demonstrates this behavior.

[HttpOptions] attribute for preflight requests

When CORS is enabled with the appropriate policy, ASP.NET Core generally responds to CORS preflight requests automatically. In some scenarios, this may not be the case. For example, using CORS with endpoint routing.

The following code uses the [HttpOptions] attribute to create endpoints for OPTIONS requests:

[Route("api/[controller]")]
[ApiController]
public class TodoItems2Controller : ControllerBase
{
    // OPTIONS: api/TodoItems2/5
    [HttpOptions("{id}")]
    public IActionResult PreflightRoute(int id)
    {
        return NoContent();
    }

    // OPTIONS: api/TodoItems2 
    [HttpOptions]
    public IActionResult PreflightRoute()
    {
        return NoContent();
    }

    [HttpPut("{id}")]
    public IActionResult PutTodoItem(int id)
    {
        if (id < 1)
        {
            return BadRequest();
        }

        return ControllerContext.MyDisplayRouteInfo(id);
    }

See Test CORS with endpoint routing and [HttpOptions] for instructions on testing the preceding code.

Set the preflight expiration time

The Access-Control-Max-Age header specifies how long the response to the preflight request can be cached. To set this header, call SetPreflightMaxAge:

options.AddPolicy("MySetPreflightExpirationPolicy",
    builder =>
    {
        builder.WithOrigins("http://example.com")
               .SetPreflightMaxAge(TimeSpan.FromSeconds(2520));
    });

How CORS works

This section describes what happens in a CORS request at the level of the HTTP messages.

  • CORS is not a security feature. CORS is a W3C standard that allows a server to relax the same-origin policy.
    • For example, a malicious actor could use Cross-Site Scripting (XSS) against your site and execute a cross-site request to their CORS enabled site to steal information.
  • An API isn't safer by allowing CORS.
    • It's up to the client (browser) to enforce CORS. The server executes the request and returns the response, it's the client that returns an error and blocks the response. For example, any of the following tools will display the server response:
  • It's a way for a server to allow browsers to execute a cross-origin XHR or Fetch API request that otherwise would be forbidden.
    • Browsers without CORS can't do cross-origin requests. Before CORS, JSONP was used to circumvent this restriction. JSONP doesn't use XHR, it uses the <script> tag to receive the response. Scripts are allowed to be loaded cross-origin.

The CORS specification introduced several new HTTP headers that enable cross-origin requests. If a browser supports CORS, it sets these headers automatically for cross-origin requests. Custom JavaScript code isn't required to enable CORS.

The PUT test button on the deployed sample

The following is an example of a cross-origin request from the Values test button to https://cors1.azurewebsites.net/api/values. The Origin header:

  • Provides the domain of the site that's making the request.
  • Is required and must be different from the host.

General headers

Request URL: https://cors1.azurewebsites.net/api/values
Request Method: GET
Status Code: 200 OK

Response headers

Content-Encoding: gzip
Content-Type: text/plain; charset=utf-8
Server: Microsoft-IIS/10.0
Set-Cookie: ARRAffinity=8f...;Path=/;HttpOnly;Domain=cors1.azurewebsites.net
Transfer-Encoding: chunked
Vary: Accept-Encoding
X-Powered-By: ASP.NET

Request headers

Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Connection: keep-alive
Host: cors1.azurewebsites.net
Origin: https://cors3.azurewebsites.net
Referer: https://cors3.azurewebsites.net/
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
User-Agent: Mozilla/5.0 ...

In OPTIONS requests, the server sets the Response headers Access-Control-Allow-Origin: {allowed origin} header in the response. For example, the deployed sample, Delete [EnableCors] button OPTIONS request contains the following headers:

General headers

Request URL: https://cors3.azurewebsites.net/api/TodoItems2/MyDelete2/5
Request Method: OPTIONS
Status Code: 204 No Content

Response headers

Access-Control-Allow-Headers: Content-Type,x-custom-header
Access-Control-Allow-Methods: PUT,DELETE,GET,OPTIONS
Access-Control-Allow-Origin: https://cors1.azurewebsites.net
Server: Microsoft-IIS/10.0
Set-Cookie: ARRAffinity=8f...;Path=/;HttpOnly;Domain=cors3.azurewebsites.net
Vary: Origin
X-Powered-By: ASP.NET

Request headers

Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Access-Control-Request-Headers: content-type
Access-Control-Request-Method: DELETE
Connection: keep-alive
Host: cors3.azurewebsites.net
Origin: https://cors1.azurewebsites.net
Referer: https://cors1.azurewebsites.net/test?number=2
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
User-Agent: Mozilla/5.0

In the preceding Response headers, the server sets the Access-Control-Allow-Origin header in the response. The https://cors1.azurewebsites.net value of this header matches the Origin header from the request.

If AllowAnyOrigin is called, the Access-Control-Allow-Origin: *, the wildcard value, is returned. AllowAnyOrigin allows any origin.

If the response doesn't include the Access-Control-Allow-Origin header, the cross-origin request fails. Specifically, the browser disallows the request. Even if the server returns a successful response, the browser doesn't make the response available to the client app.

Display OPTIONS requests

By default, the Chrome and Edge browsers don't show OPTIONS requests on the network tab of the F12 tools. To display OPTIONS requests in these browsers:

  • chrome://flags/#out-of-blink-cors or edge://flags/#out-of-blink-cors
  • disable the flag.
  • restart.

Firefox shows OPTIONS requests by default.

CORS in IIS

When deploying to IIS, CORS has to run before Windows Authentication if the server isn't configured to allow anonymous access. To support this scenario, the IIS CORS module needs to be installed and configured for the app.

Test CORS

The sample download has code to test CORS. See how to download. The sample is an API project with Razor Pages added:

public class StartupTest2
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddPolicy(name: "MyPolicy",
                builder =>
                {
                    builder.WithOrigins("http://example.com",
                        "http://www.contoso.com",
                        "https://cors1.azurewebsites.net",
                        "https://cors3.azurewebsites.net",
                        "https://localhost:44398",
                        "https://localhost:5001")
                            .WithMethods("PUT", "DELETE", "GET");
                });
        });

        services.AddControllers();
        services.AddRazorPages();
    }

    public void Configure(IApplicationBuilder app)
    {
        app.UseHttpsRedirection();
        app.UseStaticFiles();
        app.UseRouting();

        app.UseCors();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
            endpoints.MapRazorPages();
        });
    }
}

Warning

WithOrigins("https://localhost:<port>"); should only be used for testing a sample app similar to the download sample code.

The following ValuesController provides the endpoints for testing:

[EnableCors("MyPolicy")]
[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    // GET api/values
    [HttpGet]
    public IActionResult Get() =>
        ControllerContext.MyDisplayRouteInfo();

    // GET api/values/5
    [HttpGet("{id}")]
    public IActionResult Get(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);

    // PUT api/values/5
    [HttpPut("{id}")]
    public IActionResult Put(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);


    // GET: api/values/GetValues2
    [DisableCors]
    [HttpGet("{action}")]
    public IActionResult GetValues2() =>
        ControllerContext.MyDisplayRouteInfo();

}

MyDisplayRouteInfo is provided by the Rick.Docs.Samples.RouteInfo NuGet package and displays route information.

Test the preceding sample code by using one of the following approaches:

  • Use the deployed sample app at https://cors3.azurewebsites.net/. There is no need to download the sample.
  • Run the sample with dotnet run using the default URL of https://localhost:5001.
  • Run the sample from Visual Studio with the port set to 44398 for a URL of https://localhost:44398.

Using a browser with the F12 tools:

  • Select the Values button and review the headers in the Network tab.

  • Select the PUT test button. See Display OPTIONS requests for instructions on displaying the OPTIONS request. The PUT test creates two requests, an OPTIONS preflight request and the PUT request.

  • Select the GetValues2 [DisableCors] button to trigger a failed CORS request. As mentioned in the document, the response returns 200 success, but the CORS request is not made. Select the Console tab to see the CORS error. Depending on the browser, an error similar to the following is displayed:

    Access to fetch at 'https://cors1.azurewebsites.net/api/values/GetValues2' from origin 'https://cors3.azurewebsites.net' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

CORS-enabled endpoints can be tested with a tool, such as curl, Fiddler, or Postman. When using a tool, the origin of the request specified by the Origin header must differ from the host receiving the request. If the request isn't cross-origin based on the value of the Origin header:

  • There's no need for CORS Middleware to process the request.
  • CORS headers aren't returned in the response.

The following command uses curl to issue an OPTIONS request with information:

curl -X OPTIONS https://cors3.azurewebsites.net/api/TodoItems2/5 -i

Test CORS with endpoint routing and [HttpOptions]

Enabling CORS on a per-endpoint basis using RequireCors currently does not support automatic preflight requests. Consider the following code which uses endpoint routing to enable CORS:

public class StartupEndPointBugTest
{
    readonly string MyPolicy = "_myPolicy";

    // .WithHeaders(HeaderNames.ContentType, "x-custom-header")
    // forces browsers to require a preflight request with GET

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddPolicy(name: MyPolicy,
                builder =>
                {
                    builder.WithOrigins("http://example.com",
                                        "http://www.contoso.com",
                                        "https://cors1.azurewebsites.net",
                                        "https://cors3.azurewebsites.net",
                                        "https://localhost:44398",
                                        "https://localhost:5001")
                           .WithHeaders(HeaderNames.ContentType, "x-custom-header")
                           .WithMethods("PUT", "DELETE", "GET", "OPTIONS");
                });
        });

        services.AddControllers();
        services.AddRazorPages();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        app.UseHttpsRedirection();
        app.UseStaticFiles();
        app.UseRouting();

        app.UseCors();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers().RequireCors(MyPolicy);
            endpoints.MapRazorPages();
        });
    }
}

The following TodoItems1Controller provides endpoints for testing:

[Route("api/[controller]")]
[ApiController]
public class TodoItems1Controller : ControllerBase
{
    // PUT: api/TodoItems1/5
    [HttpPut("{id}")]
    public IActionResult PutTodoItem(int id)
    {
        if (id < 1)
        {
            return Content($"ID = {id}");
        }

        return ControllerContext.MyDisplayRouteInfo(id);
    }

    // Delete: api/TodoItems1/5
    [HttpDelete("{id}")]
    public IActionResult MyDelete(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);

    // GET: api/TodoItems1
    [HttpGet]
    public IActionResult GetTodoItems() =>
        ControllerContext.MyDisplayRouteInfo();

    [EnableCors]
    [HttpGet("{action}")]
    public IActionResult GetTodoItems2() =>
        ControllerContext.MyDisplayRouteInfo();

    // Delete: api/TodoItems1/MyDelete2/5
    [EnableCors]
    [HttpDelete("{action}/{id}")]
    public IActionResult MyDelete2(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);
}

Test the preceding code from the test page of the deployed sample.

The Delete [EnableCors] and GET [EnableCors] buttons succeed, because the endpoints have [EnableCors] and respond to preflight requests. The other endpoints fails. The GET button fails, because the JavaScript sends:

 headers: {
      "Content-Type": "x-custom-header"
 },

The following TodoItems2Controller provides similar endpoints, but includes explicit code to respond to OPTIONS requests:

[Route("api/[controller]")]
[ApiController]
public class TodoItems2Controller : ControllerBase
{
    // OPTIONS: api/TodoItems2/5
    [HttpOptions("{id}")]
    public IActionResult PreflightRoute(int id)
    {
        return NoContent();
    }

    // OPTIONS: api/TodoItems2 
    [HttpOptions]
    public IActionResult PreflightRoute()
    {
        return NoContent();
    }

    [HttpPut("{id}")]
    public IActionResult PutTodoItem(int id)
    {
        if (id < 1)
        {
            return BadRequest();
        }

        return ControllerContext.MyDisplayRouteInfo(id);
    }

    // [EnableCors] // Not needed as OPTIONS path provided
    [HttpDelete("{id}")]
    public IActionResult MyDelete(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);

    [EnableCors]  // Rquired for this path
    [HttpGet]
    public IActionResult GetTodoItems() =>
        ControllerContext.MyDisplayRouteInfo();

    [HttpGet("{action}")]
    public IActionResult GetTodoItems2() =>
        ControllerContext.MyDisplayRouteInfo();

    [EnableCors]  // Rquired for this path
    [HttpDelete("{action}/{id}")]
    public IActionResult MyDelete2(int id) =>
        ControllerContext.MyDisplayRouteInfo(id);
}

Test the preceding code from the test page of the deployed sample. In the Controller drop down list, select Preflight and then Set Controller. All the CORS calls to the TodoItems2Controller endpoints succeed.

Additional resources