NetEscapades.AspNetCore.SecurityHeaders

A small package to allow adding security headers to ASP.NET Core websites

Installing

Install using the NetEscapades.AspNetCore.SecurityHeaders NuGet package from the Visual Studio Package Manager Console:

PM> Install-Package NetEscapades.AspNetCore.SecurityHeaders

Or using the dotnet CLI

dotnet package add Install-Package NetEscapades.AspNetCore.SecurityHeaders

Usage

When you install the package, it should be added to your .csproj. Alternatively, you can add it directly by adding:

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp2.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="NetEscapades.AspNetCore.SecurityHeaders" Version="0.13.0" />
  </ItemGroup>
  
</Project>

Simply add the middleware to your ASP.NET Core application by configuring it as part of your normal Startup pipeline. Note that the order of middleware matters, so to apply the headers to all requests it should be configured first in your pipeline.

To use the default security headers for your application, add the middleware using:

public void Configure(IApplicationBuilder app)
{
    app.UseSecurityHeaders();

    // other middleware e.g. static files, MVC etc  
}

This adds the following headers to all responses that pass through the middleware:

• X-Content-Type-Options: nosniff
• Strict-Transport-Security: max-age=31536000; includeSubDomains - only applied to HTTPS responses
• X-Frame-Options: Deny - only applied to text/html responses
• X-XSS-Protection: 1; mode=block - only applied to text/html responses
• Referrer-Policy: strict-origin-when-cross-origin - only applied to text/html responses
• Content-Security-Policy: object-src 'none'; form-action 'self'; frame-ancestors 'none' - only applied to text/html responses

Customising the security headers added to responses

To customise the headers returned, you should create an instance of a HeaderPolicyCollection and add the required policies to it. There are helper methods for adding a number of security-focused header values to the collection, or you can alternatively add any header by using the CustomHeader type. For example, the following would set a number of security headers, and a custom header X-My-Test-Header.

public void Configure(IApplicationBuilder app)
{
    var policyCollection = new HeaderPolicyCollection()
        .AddFrameOptionsDeny()
        .AddXssProtectionBlock()
        .AddContentTypeOptionsNoSniff()
        .AddStrictTransportSecurityMaxAgeIncludeSubDomains(maxAgeInSeconds: 60 * 60 * 24 * 365) // maxage = one year in seconds
        .AddReferrerPolicyStrictOriginWhenCrossOrigin()
        .RemoveServerHeader()
        .AddContentSecurityPolicy(builder =>
        {
            builder.AddObjectSrc().None();
            builder.AddFormAction().Self();
            builder.AddFrameAncestors().None();
        })
        .AddCustomHeader("X-My-Test-Header", "Header value");

    app.UseSecurityHeaders(policyCollection);

    // other middleware e.g. static files, MVC etc  
}

The security headers above are also encapsulated in another extension method, so you could rewrite it more tersely using

public void Configure(IApplicationBuilder app)
{
    var policyCollection = new HeaderPolicyCollection()
        .AddDefaultSecurityHeaders()
        .AddCustomHeader("X-My-Test-Header", "Header value");

    app.UseSecurityHeaders(policyCollection);

    // other middleware e.g. static files, MVC etc  
}

If you want to use the default security headers, but change one specific header, you can simply add another header to the default collection. For example, the following uses the default headers, but changes the max-age on the Strict-Transport-Security header:

public void Configure(IApplicationBuilder app)
{
    var policyCollection = new HeaderPolicyCollection()
        .AddDefaultSecurityHeaders()
        .AddStrictTransportSecurityMaxAgeIncludeSubDomains(maxAgeInSeconds: 63072000);

    app.UseSecurityHeaders(policyCollection);

    // other middleware e.g. static files, MVC etc  
}

There is also a convenience overload for UseSecurityHeaders that takes an Action<HeaderPolicyCollection>, instead of requiring you to instantiate a HeaderPolicyCollection yourself:

public void Configure(IApplicationBuilder app)
{
    app.UseSecurityHeaders(policies =>
        policies
            .AddDefaultSecurityHeaders()
            .AddStrictTransportSecurityMaxAgeIncludeSubDomains(maxAgeInSeconds: 63072000)
    );

    // other middleware e.g. static files, MVC etc  
}

RemoveServerHeader

One point to be aware of is that the RemoveServerHeader method will rarely (ever?) be sufficient to remove the Server header from your output. If any subsequent middleware in your application pipeline add the header, then this will be able to remove it. However Kestrel will generally add the Server header too late in the pipeline to be able to modify it.

Luckily, Kestrel exposes it's own mechanism to allow you to prevent it being added:

var host = new WebHostBuilder()
    .UseKestrel(options => options.AddServerHeader = false)
    //...

In Program.cs, when constructing your app's WebHostBuilder, configure the KestrelServerOptions to prevent the Server tag being added.

AddContentSecurityPolicy

The Content-Security-Policy (CSP) header is a very powerful header that can protect your website from a wide range of attacks. However, it's also totally possible to create a CSP header that completely breaks your app.

The CSP has a dizzying array of options, only some of which are implemented in this project. Consequently, I highly recommend reading this post by Scott Helme, in which he discusses the impact of each "directive". I also highly recommend using the "report only" version of the header when you start. This won't break your site, but will report instances that it would be broken, by providing reports to a service such as report-uri.com.

Set the header to report-only by using the AddContentSecurityPolicyReportOnly() extension. For example:

public void Configure(IApplicationBuilder app)
{
    var policyCollection = new HeaderPolicyCollection()
        .AddContentSecurityPolicyReportOnly(builder => // report-only
        {
            // configure policies
        });
}

or by by passing true to the AddContentSecurityPolicy command

public void Configure(IApplicationBuilder app)
{
    var policyCollection = new HeaderPolicyCollection()
        .AddContentSecurityPolicy(builder =>
        {
            // configure policies
        },
        asReportOnly: true); // report-only
}

You configure your CSP policy when you configure your HeaderPolicyCollection in Startup.Configure. For example:

public void Configure(IApplicationBuilder app)
{
    var policyCollection = new HeaderPolicyCollection()
        .AddContentSecurityPolicy(builder =>
        {
            builder.AddUpgradeInsecureRequests(); // upgrade-insecure-requests
            builder.AddBlockAllMixedContent(); // block-all-mixed-content

            builder.AddReportUri() // report-uri: https://report-uri.com
                .To("https://report-uri.com");

            builder.AddDefaultSrc() // default-src 'self' http://testUrl.com
                .Self()
                .From("http://testUrl.com");

            builder.AddConnectSrc() // connect-src 'self' http://testUrl.com
                .Self()
                .From("http://testUrl.com");

            builder.AddFontSrc() // font-src 'self'
                .Self();

            builder.AddObjectSrc() // object-src 'none'
                .None();

            builder.AddFormAction() // form-action 'self'
                .Self();

            builder.AddImgSrc() // img-src https:
                .OverHttps();

            builder.AddScriptSrc() // script-src 'self' 'unsafe-inline' 'unsafe-eval' 'report-sample'
                .Self()
                .UnsafeInline()
                .UnsafeEval()
                .ReportSample();

            builder.AddStyleSrc() // style-src 'self' 'strict-dynamic'
                .Self()
                .StrictDynamic();

            builder.AddMediaSrc() // media-src https:
                .OverHttps();

            builder.AddFrameAncestors() // frame-ancestors 'none'
                .None();

            builder.AddBaseUri() // base-ri 'self'
                .Self();

            builder.AddFrameSource() // frame-src http://testUrl.com
                .From("http://testUrl.com");

            // You can also add arbitrary extra directives: plugin-types application/x-shockwave-flash"
            builder.AddCustomDirective("plugin-types", "application/x-shockwave-flash");

        })
        .AddCustomHeader("X-My-Test-Header", "Header value");

    app.UseSecurityHeaders(policyCollection);

    // other middleware e.g. static files, MVC etc  
}

AddFeaturePolicy

The Feature-Policy is an experimental header that allows web developers to selectively enable, disable, and modify the behaviour of certain features and APIs in the browser. It is similar to CSP but controls features instead of security behaviour.

With Feature Policy, you opt-in to a set of "policies" for the browser to enforce on specific features used throughout a website. These policies restrict what APIs the site can access or modify the browser's default behaviour for certain features.

By adding Feature-Policy to headers to your website, you can ensure that sensitive APIs like geolocation or the camera cannot be used, even if your site is otherwise compromised, for example by malicious third-party attacks.

For more information about the feature, I recommend the following resources:

• MDN documentation: https://developer.mozilla.org/en-US/docs/Web/HTTP/Feature_Policy
• Scott Helme's introduction to Feature-Policy: https://scotthelme.co.uk/tag/feature-policy/
• The list of policy-controlled features: https://github.com/WICG/feature-policy/blob/master/features.md
• Google's introduction to Feature-Policy: https://developers.google.com/web/updates/2018/06/feature-policy

Note that Feature-Policy is still an experimental header. The features supported by each browser varies, and is rapidly changing. This library does its best to support common features, but is not exhaustive. If there's a feature you wish to see supported, please raise an issue, or use the AddCustomFeature method, shown below:

You configure your CSP policy when you configure your HeaderPolicyCollection in Startup.Configure. For example:

public void Configure(IApplicationBuilder app)
{
    var policyCollection = new HeaderPolicyCollection()
        .AddFeaturePolicy(builder =>
        {
            builder.AddAccelerometer() // accelerometer 'self' http://testUrl.com
                .Self()
                .For("http://testUrl.com");

            builder.AddAmbientLightSensor() // ambient-light-sensor 'self' http://testUrl.com
                .Self()
                .For("http://testUrl.com");

            builder.AddAutoplay() // autoplay 'self'
                .Self();

            builder.AddCamera() // camera 'none'
                .None();

            builder.AddEncryptedMedia() // encrypted-media 'self'
                .Self();

            builder.AddFullscreen() // fullscreen *:
                .All();

            builder.AddGeolocation() // geolocation 'none'
                .None();

            builder.AddGyroscope() // gyroscope 'none'
                .None();

            builder.AddMagnetometer() // magnetometer 'none'
                .None();

            builder.AddMicrophone() // microphone 'none'
                .None();

            builder.AddMidi() // midi 'none'
                .None();

            builder.AddPayment() // payment 'none'
                .None();

            builder.AddPictureInPicture() // picture-in-picture 'none'
                .None();

            builder.AddSpeaker() // speaker 'none'
                .None();

            builder.AddSyncXHR() // sync-xhr 'none'
                .None();

            builder.AddUsb() // usb 'none'
                .None();

            builder.AddVR() // vr 'none'
                .None();

            // You can also add arbitrary extra directives: plugin-types application/x-shockwave-flash"
            builder.AddCustomDirective("plugin-types", "application/x-shockwave-flash");

            // If a new feature policy is added that follows the standard conventions, you can use this overload
            // iframe 'self' http://testUrl.com
            builder.AddCustomDirective("iframe") // 
                .Self()
                .For("http://testUrl.com");
        });

    app.UseSecurityHeaders(policyCollection);

    // other middleware e.g. static files, MVC etc  
}

Using Nonces and generated-hashes with Content-Security-Policy

The use of a secure Content-Security-Policy can sometimes be problematic when you need to include inline-scripts, styles, or other objects that haven't been whitelisted. You can achieve this in two ways - using a "nonce" (or "number-used-once"), or specifying the hash of the content to include.

To help with this you can install the NetEscapades.AspNetCore.SecurityHeaders.TagHelpers package, which provides helpers for generating a nonce per request, which is attached to the HTML element, and included in the CSP header. A similar method helper exists for <style> and <script> tags, which will take a SHA256 hash of the contents of the HTML element and add it to the CSP whitelist.

To use a nonce or an auto-generated hash with your ASP.NET Core application, use the following steps.

1. Install the NetEscapades.AspNetCore.SecurityHeaders.TagHelpers NuGet package, e.g.

dotnet package add Install-Package NetEscapades.AspNetCore.SecurityHeaders.TagHelpers

This adds the package to your .csproj file

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp2.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="NetEscapades.AspNetCore.SecurityHeaders" Version="0.13.0" />
    <PackageReference Include="NetEscapades.AspNetCore.SecurityHeaders.TagHelpers" Version="0.13.0" />
  </ItemGroup>
  
</Project>

2. Configure your CSP to use nonces and/or hashes

Configure your security headers in the usual way. Use the WithNonce() extension method when configuring ContentSecurityPolicy directives to allow whitelisting with a nonce. Use the WithHashTagHelper() extension methods on script-src and style-src directives to allow automatic generation of whitelisted inline-scripts

public void Configure(IApplicationBuilder app)
{
    var policyCollection = new HeaderPolicyCollection()
        .AddContentSecurityPolicy(builder =>
        {
            builder.AddUpgradeInsecureRequests(); 
            builder.AddDefaultSrc() // default-src 'self' http://testUrl.com
                .Self()
                .From("http://testUrl.com");

            builder.AddScriptSrc() // script-src 'self' 'unsafe-inline' 'nonce-<base64-value>'
                .Self()
                .UnsafeInline()
                .WithNonce(); // Allow elements marked with a nonce attribute

            builder.AddStyleSrc() // style-src 'self' 'strict-dynamic' 'sha256-<base64-value>'
                .Self()
                .StrictDynamic()
                .WithHashTagHelper(); // Allow whitelsited elements based on their SHA256 hash value
        })
        .AddCustomHeader("X-My-Test-Header", "Header value");

    app.UseSecurityHeaders(policyCollection);

    // other middleware e.g. static files, MVC etc  
}

3. Whitelist elements using the TagHelpers

Add the NonceTagHelper to an element by adding the asp-add-nonce attribute.

<script asp-add-nonce>
    var body = document.getElementsByTagName('body')[0];
    var div = document.createElement('div');
    div.innerText = "I was added using the NonceHelper";
    body.appendChild(div);
</script>

This will use a unique value per-request and attach the required attribute at runtime, to generate markup similar to the following:

<script nonce="ryPzmoZScSR2xOwV0qTU9mFdFwGPN&#x2B;gy3S2E1/VK1vg=">
    var body = document.getElementsByTagName('body')[0];
    var blink = document.createElement('div');
    blink.innerText = "And I was added using the NonceHelper";
    body.appendChild(blink);
</script>

While the CSP policy would look something like the following:

Content-Security-Policy: script-src 'self' 'unsafe-inline' 'nonce-ryPzmoZScSR2xOwV0qTU9mFdFwGPN+gy3S2E1/VK1vg='; style-src 'self' 'strict-dynamic'; default-src 'self' http://testUrl.com

To use a whitelisted hash instead, use the HashTagHelper, by adding the asp-add-content-to-csp attribute to <script> or <style> tags. You can optionally add the csp-hash-type attribute to choose between SHA256, SHA384, and SHA512:

<script asp-add-content-to-csp>
    var msg = document.getElementById('message');
    msg.innerText = "I'm allowed";
</script>

<style asp-add-content-to-csp csp-hash-type="SHA384">
#message {
    color: @color;
}  
</style>

At runtime, these attributes are removed, but the hash values of the contents are added to the Content-Security-Policy header.

Using the generated nonce without a TagHelpers

If you aren't using Razor, or don't want to use the TagHelpers library, you can access the Nonce for a request using an extension method on HttpContext:

var nonce = HttpContext.GetNonce();

Note that you must have enabled nonce generation by using the WithNonce() method. HttpContext.GetNonce() will return an string.Empty if nonce generation has not been added to the middleware.

Additional Resources

• ASP.NET Core Middleware Docs
• How to add default security headers in ASP.NET Core using custom middleware
• Content Security Policy - An Introduction by Scott Helme
• Content Security Policy Reference
• Content Security Policy (CSP) by Mozilla Developer Network

Note, Building on Travis is currently disabled, due to issues with the mono framework. For details, see

• http://stackoverflow.com/questions/42747722/building-vs-2017-msbuild-csproj-projects-with-mono-on-linux/42861338
• https://github.com/dotnet/sdk/issues/335