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Insecure or unset HTTP headers - CORS

Need​

Enforcement of secure and restricted Cross-Origin Resource Sharing (CORS) policies

Context​

  • Usage of C# for developing robust and scalable applications
  • Usage of Microsoft.AspNetCore.Cors for enabling Cross-Origin Resource Sharing (CORS) in ASP.NET Core applications

Description​

Non compliant code​

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddPolicy("AllowAll",
                builder =>
                {
                    builder
                    .AllowAnyOrigin() 
                    .AllowAnyMethod()
                    .AllowAnyHeader();
                });
        });
        // Other services
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        app.UseCors("AllowAll");
        // Other middleware
    }
}

The code above is an example of a CORS (Cross Origin Resource Sharing) policy in an ASP.NET Core application that is insecure due to the use of AllowAnyOrigin(), AllowAnyMethod() and AllowAnyHeader() methods.

The AllowAnyOrigin() method allows any domain to access the resources of your application. This means that a malicious website can make a request to your application and receive a response.

The AllowAnyMethod() method allows any HTTP method (GET, POST, DELETE, PUT, etc.) to be used in requests from any origin. This means that a malicious website can make a request with any HTTP method to your application.

The AllowAnyHeader() method allows any HTTP header to be included in requests from any origin. This means that a malicious website can include potentially harmful headers in requests to your application.

This configuration is equivalent to setting the CORS header Access-Control-Allow-Origin to *, which means that the server is indicating that it will accept and process requests from any origin. This is a security risk as it means that your server is openly accessible to any client, regardless of its domain, potentially exposing sensitive user data and allowing for possible CSRF attacks.

Steps​

  • Remove the wildcard (*) from the 'AllowAnyOrigin' method in the CORS policy configuration.
  • Specify the allowed origins explicitly by using the 'WithOrigins' method in the CORS policy configuration.
  • Consider restricting the allowed methods and headers to only those that are necessary for your application.
  • Test the application to ensure that the CORS policy is correctly configured and restricts cross-domain access as intended.

Compliant code​

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddCors(options =>
        {
            options.AddPolicy("AllowSpecific",
                builder =>
                {
                    builder
                    .WithOrigins("http://example.com") // replace with your actual domain
                    .WithMethods("GET", "POST") // replace with your actual methods
                    .WithHeaders("content-type"); // replace with your actual headers
                });
        });
        // Other services
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        app.UseCors("AllowSpecific");
        // Other middleware
    }
}

The updated code now restricts the CORS policy to only allow specific origins, methods, and headers. This is done by replacing the AllowAnyOrigin, AllowAnyMethod, and AllowAnyHeader methods with WithOrigins, WithMethods, and WithHeaders respectively.

In the WithOrigins method, replace "http://example.com" with the actual domain that you want to allow. If you want to allow multiple domains, you can add them as additional parameters to the WithOrigins method.

In the WithMethods method, replace "GET", "POST" with the actual HTTP methods that your application needs. If your application needs to allow other methods, you can add them as additional parameters to the WithMethods method.

In the WithHeaders method, replace "content-type" with the actual headers that your application needs. If your application needs to allow other headers, you can add them as additional parameters to the WithHeaders method.

Finally, in the Configure method, replace "AllowAll" with "AllowSpecific" to use the new CORS policy.

This change will make your application more secure by preventing unauthorized cross-domain access. Be sure to test your application after making these changes to ensure that the CORS policy is correctly configured and works as intended.

References​

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