Trouble with CORS Policy and .NET Core 3.1

Question

I'm not sure what I'm missing, but can't seem to get my CORS Policy working with .NET Core 3.1 and Angular 8 client-side.

Startup.cs:

        public void ConfigureServices(IServiceCollection services)
        {
            // ...

            // Add CORS policy
            services.AddCors(options =>
            {
                options.AddPolicy("foo",
                builder =>
                {
                    // Not a permanent solution, but just trying to isolate the problem
                    builder.AllowAnyOrigin().AllowAnyMethod().AllowAnyHeader();
                });
            });

            services.AddControllers();
        }

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

            app.UseHttpsRedirection();

            // Use the CORS policy
            app.UseCors("foo");

            app.UseRouting();

            app.UseAuthorization();

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

Error Message Client-side:

Access to XMLHttpRequest at 'https://localhost:8082/api/auth/' from origin 'http://localhost:4200' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

UPDATE:

Although I was configuring CORS incorrectly (and the accepted answer below did in fact help with that) the root of issue was unrelated. For additional context, the app was working completely fine when running the API and Angular app using the CLI - I was only having this issue after deploying them both to a web server.

The "actual" issue ended up being related to the SQL connection, which I only discovered after adding flat-file error logging to the API and running a SQL Server trace to find that the app wasn't able to connect to SQL at all.

I would normally expect this to just return a 500 and I would have realized the issue in a matter of 10 seconds - however the CORS mis-configuration meant a 500 was never actually being returned because the CORS middleware failed first. This was immensely frustrating to say the absolute least! . However I want to add that here in case others find themselves in this situation, as I was "chasing the wrong rabbit," if you will. After fixing the CORS configuration, I realized the actual issue was entirely unrelated to CORS.

TL;DR; - Sometimes "Non-CORS" .NET Server-side Errors Can Be Returned as CORS Errors If CORS policies Aren't Set Correctly

References:

https://docs.microsoft.com/en-us/aspnet/core/security/cors?view=aspnetcore-3.1#cors-with-named-policy-and-middleware

https://medium.com/swlh/cors-headers-with-dot-net-core-3-5c9dfc664785

Answer 1

first app.UseRouting(); then app.UseCors("foo");

Change your Configure method like the following :

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

    app.UseHttpsRedirection();



    app.UseRouting();  // first
    // Use the CORS policy
    app.UseCors("foo"); // second

    app.UseAuthorization();

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

It worked for me !

Answer 2

Web API is using app.UseHttpsRedirection(); which cause CORS issue if requesting client is not https based. So in order to use it with http client we need to comment or remove that line.

This issue is not with CORS, the https is causing this issue but thrown error is saying its with CORS.

Answer 3

When adding the Cors Service make sure to include .SetIsOriginAllowed((host) => true) after .WithOrigins("http://localhost:4200")

services.AddCors(options => {
            options.AddPolicy("mypolicy",builder => builder
            .WithOrigins("http://localhost:4200/")
            .SetIsOriginAllowed((host) => true)
            .AllowAnyMethod()
            .AllowAnyHeader());
  });

Answer 4

There might be several issues causing CORS error. Make sure to configure CORS properly first. You can configure it in the below way:

        services.AddCors(options =>
        {
            options.AddPolicy("CorsPolicy",
                builder => builder.AllowAnyOrigin()
                .AllowAnyMethod()
                .AllowAnyHeader());
        });

Then in Configure() of Startup.cs file, the following should be the order of middlewares:

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {

        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }


        // Enable middleware to serve generated Swagger as a JSON endpoint.
        app.UseSwagger();

        // Enable middleware to serve swagger-ui (HTML, JS, CSS, etc.),
        // specifying the Swagger JSON endpoint.
        app.UseSwaggerUI(c =>
        {
            c.SwaggerEndpoint("../swagger/v1/swagger.json", "API");
        });

        app.UseHttpsRedirection();

        app.UseRouting();

        app.UseCors("CorsPolicy");

        app.UseAuthentication();

        app.UseAuthorization();

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

If you have configured the .NET core application to run on HTTPs URL, then make sure to invoke the https URL of the application for API request from Angular as well.

If you are invoking HTTP URL from the Angular and running .NET core application on HTTPs, then remove the app.UseHttpsRedirection() middleware.

To use app.UseHttpsRedirection(), either run .NET core on HTTPs URL and make HTTPs requests from the angular or run .NET core on HTTP URL and make HTTP requests from the angular (in this case, the .NET application must only run on HTTP. It shouldn't be configured to run on both HTTP and HTTPs).

The above things will most probably solve your problem.

Answer 5

I had this error, I used to run my project with the CLI .net core and it did not worked but when I ran the project with IIS Express Visual studio it worked with no CORS problems, that happens for the redirection to https middleware in Configure method of Startup.css class app.UseHttpsRedirection();, like Waseem Ahmad Naeem mentioned.

Either its good to always visit the .Net Core Middleware order, putting them in bad order may bring performance problems to the applications or even make that things does not work as expected. Middleware docs below.

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware/?view=aspnetcore-5.0

Answer 6

As you are using localhost as http://localhost:4200, then try to set it in your configuration:

public void ConfigureServices(IServiceCollection services)
{
    services.AddCors(options => options.AddPolicy("ApiCorsPolicy", build =>
    {                
        build.WithOrigins("http://localhost:4200")
             .AllowAnyMethod()
             .AllowAnyHeader();
        }));
        // ... other code is omitted for the brevity
     }
}

And Configure method:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, IServiceProvider provider)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    app.UseCors("ApiCorsPolicy");
    app.UseHttpsRedirection();
    app.UseAuthentication();
    app.UseMvc();
}

Answer 7

The call to UseCors must be placed after UseRouting, but before UseAuthorization. For more information, see https://docs.microsoft.com/en-us/aspnet/core/security/cors?view=aspnetcore-3.1

Answer 8

Try this util

https://www.nuget.org/packages/Microsoft.AspNetCore.Cors/ Using this you can enable/disable the cors in your .net core application

Answer 9

I resolved my issue by adding EnableCors to my controller

[EnableCors("MyPolicy")]
[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase

Answer 10

In my situation it was related to the Content-Type of the header being application/json.

Only those Content-Type headers are allowed with CORS:

application/x-www-form-urlencoded

multipart/form-data

text/plain

Answer 11

I faced the same issue and I just found out that custom middlewares must go after the useCors()

        app.UseHttpsRedirection();

        app.UseAuthentication();

        app.UsePathBase("/api");

        app.UseRouting();

        app.UseCors("AllowSpecificOrigins");

        app.UseMiddleware<CountryMiddleware>();

        app.UseAuthorization();

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