Need

Enforcement of secure content type options through proper HTTP headers

Context

• Usage of C# 7.0 for modern language features and enhancements
• Usage of Microsoft.AspNetCore for building web applications with .NET Core
• Usage of Microsoft.AspNetCore.Hosting for hosting ASP.NET Core applications
• Usage of Microsoft.AspNetCore.Builder for configuring the application's request pipeline in ASP.NET Core
• Usage of Microsoft.AspNetCore.Http for handling HTTP requests and responses in ASP.NET Core applications
• Usage of Microsoft.Extensions.Hosting for hosting and managing .NET Core applications
• Usage of Microsoft.Extensions.DependencyInjection for dependency injection in .NET applications

Description

Non compliant code

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();

    app.UseRouting();

    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller=Home}/{action=Index}/{id?}");
    });
}
        
        

The above code is a typical configuration for an ASP.NET Core application. It sets up exception handling, HTTPS redirection, static file serving, routing, authorization, and endpoint mapping.

The vulnerability in this code lies in the lack of the X-Content-Type-Options HTTP header. This header is used to protect against MIME type confusion attacks. Without this header, the browser could interpret the MIME type of a response differently than what the server intended. For example, a malicious user could upload a file with a .jpg extension that is actually a script, and the browser could execute it as such if the server doesn't explicitly specify that it should be treated as an image.

The X-Content-Type-Options header should be set to "nosniff" to instruct the browser to not try to interpret the content type of the response. This is a simple but effective measure to mitigate this type of attack.

This vulnerability can be fixed by adding the middleware to set this header in the Configure method.

Steps

• Add the X-Content-Type-Options HTTP header to the server's response.
• Set the value of the X-Content-Type-Options header to 'nosniff'.
• Ensure that the X-Content-Type-Options header is set for all responses, including error responses.

Compliant code

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();

    app.UseRouting();

    app.UseAuthorization();

    app.Use((context, next) =>
    {
        context.Response.Headers.Add("X-Content-Type-Options", "nosniff");
        return next();
    });

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller=Home}/{action=Index}/{id?}");
    });
}
        
        

The above code fixes the vulnerability by adding the X-Content-Type-Options HTTP header to the server's response. This is done by using the Use method on the IApplicationBuilder instance, which allows us to add middleware to the application's request pipeline.

In the Use method, we add a delegate that takes the HTTP context and a function representing the next middleware in the pipeline. In this delegate, we add the X-Content-Type-Options header to the response headers of the context with a value of 'nosniff'. This tells the browser to not try to interpret content differently from the declared content type.

After adding the header, we call the next middleware in the pipeline. This ensures that the X-Content-Type-Options header is set for all responses, including error responses.

This fix helps to prevent attacks based on MIME-type confusion. Without the 'nosniff' option, the browser might try to interpret data based on its content, ignoring the declared MIME type. This can lead to security vulnerabilities.

References

• 132. Insecure or unset HTTP headers - X-Content-Type-Options