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ASP.NET Core Web API’s handling of Cross-Origin Resource Sharing, or CORS

Posted on by Pablo Francesco

Cross-Origin Resource Sharing (CORS) is a security feature used by web browsers to prevent web pages from making requests to a different domain than the one that supplied the web page. You may run across CORS-related issues when designing a web application, particularly if it involves sending API queries to a separate domain. We’ll look at fixing CORS problems in an ASP.NET Core Web API in this article.
Comprehending CORS

Prior to delving into remedies, let us comprehend the causes of CORS problems. The browser by default restricts requests made by web pages to domains other than the one from which they originated. This is a security precaution to guard against illegal access to resources located on another domain.

ASP.NET Core’s CORS

Support for managing CORS is integrated into ASP.NET Core. The Startup.cs file contains configuration options for the CORS middleware that let you choose which headers, HTTP methods, and origins your API can accept.
Install Microsoft.AspNetCore in step one.Package Cors

Make sure your project has the Microsoft.AspNetCore.Cors package installed. The following command in the Package Manager Console can be used to install it:

dotnet add package Microsoft.AspNetCore.Cors

Step 2. Configure CORS in Startup.cs

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddCors(options =>
{
    options.AddPolicy("AllowSpecificOrigin",
        builder =>
        {
            builder.WithOrigins("https://hostforlife.eu")
                   .AllowAnyMethod()
                   .AllowAnyHeader();
        });
});

builder.Services.AddControllers();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if(app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

//Use CROS Orgion Policy in Your Application
app.UseCors("AllowSpecificOrigin");

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

In the code above, AllowSpecificOrigin is the name of the CORS policy. You can change it according to your needs. WithOrigins specifies the allowed origins (replace “https://hostforlife.eu” with your actual frontend domain). AllowAnyMethod and AllowAnyHeader allow any HTTP method and headers, but you can customize these based on your requirements.

Now, in the Configure method, use the CORS policy:

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddCors(options =>
{
    options.AddPolicy("AllowSpecificOrigin",
        builder =>
        {
            builder.WithOrigins("https://www.hostforlife.eu")
                   .AllowAnyMethod()
                   .AllowAnyHeader();
        });
});

builder.Services.AddControllers();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if(app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

//Use CROS Origin Policy in Your Application
app.UseCors("AllowSpecificOrigin");

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

Step 3. Test Your API

With CORS configured, you should be able to make requests from your front end to the API without encountering CORS issues. Test your API with your front-end application and make sure that the requests are allowed.

Additional Considerations

  • Wildcard Origins: Instead of specifying a single origin, you can use Builder.AllowAnyOrigin() to allow requests from any origin. Be cautious with this approach, as it may pose security risks.
  • Credentials: If your API and frontend are on different domains and you need to send credentials (e.g., cookies), consider adding.AllowCredentials() to your CORS policy.
  • Fine-grained Control: You can customize the CORS policy further to allow specific methods, headers, and more. Refer to the official documentation for detailed options.

By following these steps, you can resolve CORS issues in your ASP.NET Core Web API and allow secure communication between your API and front-end applications.

Best ASP.NET Core Hosting

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