Insecure or unset HTTP headers - Content-Type

Need

Ensuring secure and properly set HTTP headers

Context

• Usage of C# for developing robust and efficient applications
• Usage of Microsoft.AspNetCore.Builder for configuring and building ASP.NET Core applications
• Usage of Microsoft.AspNetCore.Hosting for hosting ASP.NET Core applications
• Usage of Microsoft.AspNetCore.HttpsPolicy for enforcing HTTPS security in ASP.NET Core applications
• Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core
• Usage of Microsoft.Extensions.Configuration for managing application configuration settings
• Usage of Microsoft.Extensions.DependencyInjection for dependency injection in .NET applications
• Usage of Microsoft.Extensions.Hosting for building and hosting .NET Core applications

Description

Non compliant code

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

        app.UseHttpsRedirection();

        app.UseRouting();

        app.UseAuthorization();

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

In the above code snippet, the Startup class configures the application's request pipeline. However, there's a significant security issue here: the application does not define the Content-Type header in its server responses.

The Content-Type HTTP header is important as it defines the media type of the resource or the data. Without it, the client (browser) may misinterpret the content of the response, leading to security vulnerabilities. For example, if the server returns HTML content without specifying Content-Type: text/html, the browser might interpret it as a different content type, such as JavaScript. This can lead to security issues like Cross-Site Scripting (XSS).

In the given code, the Content-Type header is not explicitly set, making the application vulnerable. The app.UseEndpoints method maps the controllers but does not enforce a specific Content-Type for the responses.

Steps

• Add the necessary code to set the Content-Type header in the server responses.
• Determine the appropriate Content-Type for each response and set it accordingly.
• Use the app.UseMiddleware method to add a custom middleware that sets the Content-Type header.
• Ensure that the Content-Type header is set correctly for all types of responses, including HTML, JSON, XML, etc.
• Test the application to verify that the Content-Type header is now being set correctly in the server responses.

Compliant code

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

        app.UseHttpsRedirection();

        app.UseRouting();

        app.UseAuthorization();

        // Add a custom middleware to set the Content-Type header
        app.Use(async (context, next) =>
        {
            context.Response.OnStarting(() =>
            {
                if (!context.Response.Headers.ContainsKey("Content-Type"))
                {
                    context.Response.Headers.Add("Content-Type", "application/json; charset=utf-8");
                }

                return Task.CompletedTask;
            });

            await next.Invoke();
        });

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

The above code fixes the vulnerability by adding a custom middleware to the application pipeline that sets the Content-Type header for each server response. This middleware is added using the app.Use method, which is a part of the ASP.NET Core middleware pipeline.

The middleware uses the context.Response.OnStarting method to add a callback that will be invoked just before the response headers are sent to the client. This callback checks if the Content-Type header is already set in the response headers. If it is not, it adds the Content-Type header with a value of "application/json; charset=utf-8". This value is appropriate for JSON responses, which is a common response type for many web APIs.

Please note that the Content-Type should be set according to the type of the response. In this case, we have set it to "application/json; charset=utf-8" as an example. If your application returns different types of responses (like HTML, XML, etc.), you should set the Content-Type accordingly.

After adding this middleware, all server responses will have the Content-Type header set correctly, which fixes the vulnerability. You should test your application to verify that the Content-Type header is now being set correctly in all server responses.

References

• 329.Insecure or unset HTTP headers - Content-Type