Protect your dotnet core website with Content Security Policy

The new Content-Security-Policy HTTP response header helps you reduce XSS risks on modern browsers by declaring what dynamic resources are allowed to load via a HTTP Header.

For example, with the CSP header you can block inline scripts from executing, effectively stopping simple XSS attacks.


What is the Content Security Policy

HTTP headers are used by Servers and Browsers to talk to one another. For example the server can tell the browser what kind of content it is sending using the Content-Type and Content-Length header. Every web page is built-up from different content sources, for example the html comes from the server, but your style might come from a CDN server such as bootstrap. The new Content-Security-Policy is used by the server to tell the browser which content-sources it can use, for example:

Content-Security-Policy:default-src 'self'; style-src 'self' https://ajax.aspnetcdn.com

This header tells the browser to only use html from the server itself, and only to use styles from the server and the aspnetcdn server. Browsers that support CSP will not use any other content sources.

But wait! There is more. You can also tell the browser not to load your content into another page, protecting against Clickjacking. You can use the frame-ancestors directive for that:

Content-Security-Policy:default-src 'self'; script-src 'self' https://ajax.aspnetcdn.com ; style-src 'self' https://ajax.aspnetcdn.com; frame-ancestors 'none';

All the different kinds of content sources and directives can be found here


Using inline scripts with CSP

Maybe you are using some external inline script, such as Google Analytics or Microsoft Application Insights. Normally CSP will block any inline script or style. So how can I use my inline script?

One option would be to use the 'unsafe-inline' content source, but this allows any inline script to execute! Luckily there is another way.

Using hashes to allow inline scripts and styles

When CSP has been enabled, and you have an inline script on your page, your browser will not execute it. You will find in your browser's Console window some remark about it. For example in Chrome:

Refused to execute inline script because it violates the following Content Security Policy directive

Chrome also suggest a way to get around this, and even displays a hash for the inline script:

Either the 'unsafe-inline' keyword, a hash ('sha256-e3wuJEA9ZnrbftKXWc68bpGC5pLCehsGKmy02Qh9h74='), or a nonce ('nonce-...') is required to enable inline execution.

So the solution is to include this hash value in your content sources:

default-src 'self'; script-src 'self' https://ajax.aspnetcdn.com 'sha256-gKHd+pSZOJ3MwBsFalomyNobAcinjJ44ArqbIKlcniQ='; style-src 'self' https://ajax.aspnetcdn.com 'sha256-pTnn8NGuYdfLn7/v3BQ2pYxjz73VjHU2Wkr6HjgUgVU='; frame-ancestors 'none';

As you can see in the example above, you can also use this for inline styles.

Using nonces to allow inline scripts and styles

The mayor disadvantage of the hash is that you need to recalculate and update the hash value whenever you update the script/style. So if you update the script (or dynamically generate it) you will want to use a nonce.

A nonce is a 'number used only once'

You need to generate for each request a unique nonce for each inline script and inline style and include it in the CSP header:

Content-Security-Policy:default-src 'self'; script-src 'self' https://ajax.aspnetcdn.com 'sha256-gKHd+pSZOJ3MwBsFalomyNobAcinjJ44ArqbIKlcniQ=' 'nonce-1LCV8O37L47QVufyugd6rqoebY+OAQGq8iajMbdy3B8='; style-src 'self' https://ajax.aspnetcdn.com 'sha256-pTnn8NGuYdfLn7/v3BQ2pYxjz73VjHU2Wkr6HjgUgVU=' 'nonce-ZUqNLKpiwM9Hru6BjlIx6DtREfGXO2c38CCzMAW6TQ0='; frame-ancestors 'none';

You also need to attribute your scripts and styles with a nonce attribute, matching the nonce from the header.

<script nonce="1LCV8O37L47QVufyugd6rqoebY&#x2B;OAQGq8iajMbdy3B8=">

Adding the CSP header in .NET Core

To make adding the CSP header easy in .NET Core I have built two NuGet packages: U2U.AspNetCore.Security.Headers and U2U.AspNetCore.Security.Headers.TagHelpers.

So start by adding them to your project:

<PackageReference Include="U2U.AspNetCore.Security.Headers" Version="1.1.0" />
<PackageReference Include="U2U.AspNetCore.Security.Headers.TagHelpers" Version="1.1.0" /> 

U2U.AspNetCore.Security.Headers

This package allows you to add headers to your response, such as the CSP header. Call UseResponseHeaders in your Startup.Configure method:

app.UseResponseHeaders(builder =>
{
  ...
}

You can set any header like this:

builder.SetHeader("SomeHeader", "SomeValue")

You can set the CSP header using the SetContentSecurityPolicy method:

builder.SetContentSecurityPolicy(new ContentSecurityPolicy()
  {
    ...
  }

Now select the directives you need with their content sources:

DefaultSrc = new List<string> {
  ContentSecurityPolicy.Source.Self
},
ScriptSrc = new List<string> {
  ContentSecurityPolicy.Source.Self,
  "https://ajax.aspnetcdn.com",
  "'sha256-gKHd+pSZOJ3MwBsFalomyNobAcinjJ44ArqbIKlcniQ='"
},
StyleSrc = new List<string> {
  ContentSecurityPolicy.Source.Self,
  "https://ajax.aspnetcdn.com",
  "'sha256-pTnn8NGuYdfLn7/v3BQ2pYxjz73VjHU2Wkr6HjgUgVU='"
}

And what about nonces?

U2U.AspNetCore.Security.Headers.TagHelpers

Using nonces means that you need to generate a cryptographically randon nonce, and attach it to the header and the script of style tag. This package makes that easy through a nonce taghelper.

If you're not familiar with taghelpers, click this link

First of all enable nonces in Startup.Configure:

builder.SetContentSecurityPolicy(new ContentSecurityPolicy()
{
  SupportNonces = true,

Next add the nonce taghelper to your views. The easiest way is to add following to _ViewImports.cshtml:

@addTagHelper *, U2U.AspNetCore.Security.Headers.TagHelpers

Now look for your inline script and style tag(s) and add the nonce attribute:

<script nonce="true">alert('Use the Nonce!');</script>

Start your website and use the browser debugger to look at the CSP header:

Content-Security-Policy:default-src 'self'; 
script-src 'self' https://ajax.aspnetcdn.com 
'nonce-Gl9JnGKKw9+0+fThsPtVdYtraPLwxWDtB4Qq7qMKH0w=';
style-src 'self' https://ajax.aspnetcdn.com  'nonce-KX0fql/urMHxnZGnDqNoyOljycR/e8nNv2bsjk//sS8='; 

Your script tags should also include the nonce value:

<script nonce="Gl9JnGKKw9&#x2B;0&#x2B;fThsPtVdYtraPLwxWDtB4Qq7qMKH0w=">
  alert('Use the Nonce!');
</script>
<script nonce="stjl3RNNOKDytWwDlWb8Rr2FGmNAmEdykWaCCPc10TQ=">
  alert('Also ok!');
</script>

Does your browser support CSP?

The easiest way to find out is to visit CSP-Browser-Test

But generally, if you are using the latest version of a modern browser, it should support CSP.

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