Using CSP Header In ASP.NET Core 2.0

Tahir Naushad
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Article

Problem

How to use Content-Security-Policy header in ASP.NET Core to prevent XSS attacks.

Solution

Create an empty project and update Startup to include service and middleware for MVC.

public void ConfigureServices(
IServiceCollection services)
{
services.AddMvc();
}
public void Configure(
IApplicationBuilder app,
IHostingEnvironment env)
{
app.UseDeveloperExceptionPage();
app.Use(async (context, next) =>
{
context.Response.Headers.Add(
"Content-Security-Policy",
"script-src 'self'; " +
"style-src 'self'; " +
"img-src 'self'");
await next();
});
app.UseStaticFiles();
app.UseMvcWithDefaultRoute();
}
}

Add a _Layout page and include scripts, styles and images from local (wwwroot) and remote domains.

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width" />
<title>@ViewBag.Title</title>
<link rel="stylesheet" href="~/css/site.css" />
<link rel="stylesheet"
href="https://ajax.aspnetcdn.com/ajax/bootstrap/3.3.5/css/bootstrap.min.css" />
</head>
<body>
<img src="~/img/site-banner.jpg" />
<img src="https://media-www-asp.azureedge.net/media/5245130/home-hero-2.png" />
<div>
@RenderBody()
</div>
<script src="~/js/site.js"></script>
<script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.2.1.min.js"></script>
</body>
</html>

Run and observe the errors. When loading the page, the browser has rejected loading from remote sources.

Discussion

Content Security Policy (CSP) is an additional level of security that could help prevent Cross-Site Scripting (XSS) attacks. In these attacks, malicious scripts are executed on user’s browser since the browser doesn’t know whether the source of the script is trustworthy or not.

CSP allows developers to specify the sources (domains) that trustworthy and can serve executable scripts. This whitelisting of domains is achieved by using Content-Security-Type HTTP header, like -

Content-Security-Policy: [policy]

Here, the [policy] is made up of directives describing the type of restrictions and domains to the whitelist. It is a string defining a series of directives and sources separated by a semicolon -

[directive] <source> <…source>; [directive] <source> <…source>; …

Sources

Sources are the trusted domains; the commonly used ones are -

*: allow any URL.
‘self’: allow the origin of the served page. Note that single quotes are required.
‘none’: no source should be allowed. Note that single quotes are required.
Host: allow a specified internet host (by name or IP address). A wildcard (asterisk character) can be used to include all subdomains e.g. http://*.foo.com
‘unsafe-line’: allow inline scripts
‘nonce-[base64-value]’: allow inline scripts with a specific nonce (number used once). The nonce should be encrypted and unique for every HTTP request/response.

Note
Other sources could be found at Mozilla documents for developers; see Other Resources section at the end of this post.

Directives

Directives specify the resource types to put restrictions on. Commonly used ones are,

script-src: defines valid sources of JavaScript
style-src: defines valid sources of stylesheets
img-src: defines valid sources of images
connect-src: defines valid sources to which AJAX calls can be made
font-src: defines valid sources of fonts
object-src: defines valid sources for <object>, <embed> and <applet> elements
media-src: defines valid sources of audio and video
form-action: defines valid sources that can be used as a HTML <form> action.
default-src: specifies the default policy for loading content

Note
Other directives could be found at Mozilla documents for developers, see Other Resources section at the end of this post.

Middleware

We could encapsulate the building and adding of CSP header in a reusable middleware. Below is an example to get you started. You could extend it as necessary. First, create a class to hold sources.

public sealed class CspOptions
{
public List<string> Defaults { get; set; } = new List<string>();
public List<string> Scripts { get; set; } = new List<string>();
public List<string> Styles { get; set; } = new List<string>();
public List<string> Images { get; set; } = new List<string>();
public List<string> Fonts { get; set; } = new List<string>();
public List<string> Media { get; set; } = new List<string>();
}

Create a builder. We’ll use it when setting up the middleware.

public sealed class CspOptionsBuilder
{
private readonly CspOptions options = new CspOptions();
internal CspOptionsBuilder() { }
public CspDirectiveBuilder Defaults { get; set; } = new CspDirectiveBuilder();
public CspDirectiveBuilder Scripts { get; set; } = new CspDirectiveBuilder();
public CspDirectiveBuilder Styles { get; set; } = new CspDirectiveBuilder();
public CspDirectiveBuilder Images { get; set; } = new CspDirectiveBuilder();
public CspDirectiveBuilder Fonts { get; set; } = new CspDirectiveBuilder();
public CspDirectiveBuilder Media { get; set; } = new CspDirectiveBuilder();
internal CspOptions Build()
{
this.options.Defaults = this.Defaults.Sources;
this.options.Scripts = this.Scripts.Sources;
this.options.Styles = this.Styles.Sources;
this.options.Images = this.Images.Sources;
this.options.Fonts = this.Fonts.Sources;
this.options.Media = this.Media.Sources;
return this.options;
}
}
public sealed class CspDirectiveBuilder
{
internal CspDirectiveBuilder() { }
internal List<string> Sources { get; set; } = new List<string>();
public CspDirectiveBuilder AllowSelf() => Allow("'self'");
public CspDirectiveBuilder AllowNone() => Allow("none");
public CspDirectiveBuilder AllowAny() => Allow("*");
public CspDirectiveBuilder Allow(string source)
{
this.Sources.Add(source);
return this;
}
}

Now, we can create a middleware.

public sealed class CspMiddleware
{
private const string HEADER = "Content-Security-Policy";
private readonly RequestDelegate next;
private readonly CspOptions options;
public CspMiddleware(
RequestDelegate next, CspOptions options)
{
this.next = next;
this.options = options;
}
public async Task Invoke(HttpContext context)
{
context.Response.Headers.Add(HEADER, GetHeaderValue());
await this.next(context);
}
private string GetHeaderValue()
{
var value = "";
value += GetDirective("default-src", this.options.Defaults);
value += GetDirective("script-src", this.options.Scripts);
value += GetDirective("style-src", this.options.Styles);
value += GetDirective("img-src", this.options.Images);
value += GetDirective("font-src", this.options.Fonts);
value += GetDirective("media-src", this.options.Media);
return value;
}
private string GetDirective(string directive, List<string> sources)
=> sources.Count > 0 ? $"{directive} {string.Join(" ", sources)}; " : "";
}

And an extension method to set it up.

public static class CspMiddlewareExtensions
{
public static IApplicationBuilder UseCsp(
this IApplicationBuilder app, Action<CspOptionsBuilder> builder)
{
var newBuilder = new CspOptionsBuilder();
builder(newBuilder);
var options = newBuilder.Build();
return app.UseMiddleware<CspMiddleware>(options);
}
}

We can now configure the middleware in the Startup class.

app.UseCsp(builder =>
{
builder.Defaults
.AllowSelf();
builder.Scripts
.AllowSelf()
.Allow("https://ajax.aspnetcdn.com");
builder.Styles
.AllowSelf()
.Allow("https://ajax.aspnetcdn.com");
builder.Fonts
.AllowSelf()
.Allow("https://ajax.aspnetcdn.com");
builder.Images
.AllowSelf()
.Allow("https://media-www-asp.azureedge.net/");
});

Run and observe the network traffic. The browser is allowing local and remote resources.

Other Resources

I’ve only scratched the surface of security headers and CSP. These are some of the other useful resources you could look at -

Pluralsight course by Troy Hunt discussing security headers
Mozilla developer documentation on HTTP headers.
Joonas blog and sample for various security headers.

Source Code

GitHub

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