There are many aspects to web security, but in this article we’ll focus on one in particular. Attackers can use any available information about a target web application to their advantage. Therefore, if your web application is sending out headers revealing the underlying infrastructure of your web application, attackers can use those details to narrow down their attack and attempt to exploit vulnerabilities in that particular software.

Let’s create a new ASP .NET Core web application to see what is returned in the headers by default:

mkdir dotnet-server-header
dotnet new web
dotnet run

This creates a “Hello world” ASP .NET Core application using the “ASP .NET Core Empty” template, and runs it. By default it runs on Kestrel on port 5000. If we access it in a browser and check the response headers (e.g. using the Network tab of the Chrome Developer Tools), we see that there’s this Server header with a value of Kestrel. If it were running under IIS, this value might have been MicrosoftIIS/10.0 instead.

Honestly, this could be worse. Older versions of ASP .NET running on the old .NET Framework used to add X-Powered-By, X-AspNet-Version and X-AspNet-MvcVersion headers with very specific information about the underlying software. While this information can be really useful for statistical purposes (e.g. to identify the most popular web servers, or to identify how prevalent different versions of ASP .NET are), they are also very useful for malicious purposes (e.g. to look for known vulnerabilities in a specific ASP .NET version).

ASP .NET Core, on the other hand, only adds the Server header, which is quite broad. However, the less information we give a potential attacker, the better for us.

There is no harm in removing the Server header, and to do this in ASP .NET Core, we can take a tip from this Stack Overflow answer:

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup<Startup>()
                              .UseKestrel(options => options.AddServerHeader = false);
                });

The highlighted line above, added to Program.cs, has the effect of getting rid of that Server header. In fact, if we dotnet run again now, we find that it is gone:

Update 27th July 2020: Just to clarify, this removes the Server header coming from Kestrel. However, if you use other software (e.g. IIS) to host your web application, you will need to take additional steps to remove it from there as well.

It is always a good idea to do a vulnerability assessment of your web application, and in doing so, remove any excess information that complete strangers do not need to know. What we have seen here is a very small change that can reduce the security risk at least by a little.