Enable OPTIONS header for CORS on .NET Core Web API

Question

I solved this problem after not finding the solution on Stackoverflow, so I am sharing my problem here and the solution in an answer.

After enabling a cross domain policy in my .NET Core Web Api application with AddCors, it still does not work from browsers. This is because browsers, including Chrome and Firefox, will first send an OPTIONS request and my application just responds with 204 No Content.

Answer 1

Add a middleware class to your project to handle the OPTIONS verb.

using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Hosting;

namespace Web.Middlewares
{
    public class OptionsMiddleware
    {
        private readonly RequestDelegate _next;

        public OptionsMiddleware(RequestDelegate next)
        {
            _next = next;
        }

        public Task Invoke(HttpContext context)
        {
            return BeginInvoke(context);
        }

        private Task BeginInvoke(HttpContext context)
        {
            if (context.Request.Method == "OPTIONS")
            {
                context.Response.Headers.Add("Access-Control-Allow-Origin", new[] { (string)context.Request.Headers["Origin"] });
                context.Response.Headers.Add("Access-Control-Allow-Headers", new[] { "Origin, X-Requested-With, Content-Type, Accept" });
                context.Response.Headers.Add("Access-Control-Allow-Methods", new[] { "GET, POST, PUT, DELETE, OPTIONS" });
                context.Response.Headers.Add("Access-Control-Allow-Credentials", new[] { "true" });
                context.Response.StatusCode = 200;
                return context.Response.WriteAsync("OK");
            }

            return _next.Invoke(context);
        }
    }

    public static class OptionsMiddlewareExtensions
    {
        public static IApplicationBuilder UseOptions(this IApplicationBuilder builder)
        {
            return builder.UseMiddleware<OptionsMiddleware>();
        }
    }
}

Then add app.UseOptions(); this as the first line in Startup.cs in the Configure method.

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    app.UseOptions();
}

Answer 2

I know it has been answered. Just answering with the updated information. So it would help others.

Its now its inbuilt into asp.net core framework.

Just follow https://docs.microsoft.com/en-us/aspnet/core/security/cors

and replace

    app.UseCors(builder =>
   builder.WithOrigins("http://example.com"));

with

        app.UseCors(builder =>
       builder.WithOrigins("http://example.com")
              .AllowAnyHeader()
              .AllowAnyMethod()
              .AllowCredentials());

Answer 3

This worked for me:

Make sure that this:

app.UseCors(builder => {
    AllowAnyOrigin()
    AllowAnyMethod()
    AllowAnyHeader()
});

Occurs before any of these:

app.UseHttpsRedirection();
app.UseDefaultFiles();
app.UseStaticFiles();
app.UseCookiePolicy();

Remember, we are dealing with a "pipeline". The cors stuff has to be first.

-gimzani

Answer 4

I wanted to allow this for a single method, not using a middleware to allow this on any method. This is what I ended doing:

Manual handling of the 'OPTIONS' method

[HttpOptions("/find")]
public IActionResult FindOptions()
{
    Response.Headers.Add("Access-Control-Allow-Origin", new[] { (string)Request.Headers["Origin"] });
    Response.Headers.Add("Access-Control-Allow-Headers", new[] { "Origin, X-Requested-With, Content-Type, Accept" });
    Response.Headers.Add("Access-Control-Allow-Methods", new[] { "POST, OPTIONS" }); // new[] { "GET, POST, PUT, DELETE, OPTIONS" }
    Response.Headers.Add("Access-Control-Allow-Credentials", new[] { "true" });
    return NoContent();
}

[HttpPost("/find")]
public async Task<IActionResult> FindOptions([FromForm]Find_POSTModel model)
{
    AllowCrossOrigin();

    // your code...
}

private void AllowCrossOrigin()
{
    Uri origin = null;
    Uri.TryCreate(Request.Headers["Origin"].FirstOrDefault(), UriKind.Absolute, out origin);

    if (origin != null && IsOriginAllowed(origin))
        Response.Headers.Add("Access-Control-Allow-Origin", $"{origin.Scheme}://{origin.Host}");
}

And of course, you can implement IsOriginAllowed as you wish

private bool IsOriginAllowed(Uri origin)
{
    const string myDomain = "mydomain.com";
    const string[] allowedDomains = new []{ "example.com", "sub.example.com" };

    return 
           allowedDomains.Contains(origin.Host) 
           || origin.Host.EndsWith($".{myDomain}");
}

You can find more details on how to enable CORS for POST requests on a single endpoint

Answer 5

There is no need in an additional middleware. As already mentioned above the only thing needed is the OPTIONS method allowed in Cors configuration. You may AllowAnyMethod as suggested here: https://stackoverflow.com/a/55764660/11921910

But it's safer to just allow the specific stuff like this:

app.UseCors(builder => builder
.WithOrigins("https://localhost", "https://production.company.com") /* list of environments that will access this api */
.WithMethods("GET", "OPTIONS") /* assuming your endpoint only supports GET */
.WithHeaders("Origin", "Authorization") /* headers apart of safe-list ones that you use */
);

Some headers are always allowed: https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header