Shield Your ASP.NET MVC Web Applications with Content Security Policy (CSP)

“One single vulnerability is all an attacker needs.”

–  Window Snyder

Hackers are everywhere today. The world wide web is also a place for worldwide vulnerabilities. In order to safeguard your application, you need a powerful mechanism. In that case, Content Security Policy (CSP)  is at your service with some excellent features.

In this blog post, we will see how to implement CSP in ASP.NET MVC web applications!

Overview

CSP is used to protect your web application. It safeguards it by identifying some types of attacks like cross-site scripting (XSS) and SQL or data injection attacks.

Note: In CSP, some browser features are disabled by default.

If we want to apply CSP to our application, we have to define some CSP content security directives in the desired Content-Security-Policy headers or in the <meta> tags.

When a resource doesn’t match with the defined policy directives, then it won’t be loaded by the browser (scripts and styles from a third-party).

So if a policy restricts images means, then the browser will prevent images from loading when a page contains an <img> tag with a third-party origin in the img src attribute.

All latest versions of browsers Chrome, Edge, Firefox, Opera, and Safari provide support for CSP.

Topics to be covered

• Policy directives.
• Source List Reference.
• Apply the policy.
• Test your policy.
• Browser supports.
• Threats
• Troubleshoot

Policy directives

The CSP is used to restrict unauthorized third-party content resources. There are many directives available for a source (application). Once Content-Security-Policy headers are included in your application, the browser will reject any other content from sources that are not explicitly included or pre-approved using any of the directives.

You can add the directives in your ASP.NET web application’s HTTP response header GUI in the IIS manager or add the following to your Web.config file.

<httpProtocol>
    <customHeaders>
       <add name="Content-Security-Policy" value="default-src 'self'" />
     </customHeaders>
</httpProtocol>

The following are some of the policy directives:

default-src

default-src is used as a fallback if the directives (object-src, img-src, etc.) are not specified. Then, the default-src content policy will be applied for source directives.

• Use default-src ‘self’ to allow content from the current origin.
• Use default-src ‘none’ to block everything that’s not added (pre- approved).

Example:

<add name="Content-Security-Policy" value="default-src 'self'" />

script-src

script-src is used to pre-approve script sources.

1. Use script-src ‘self’ to allow scripts from the current origin.
2. You can pre-approve your scripts using script-src ‘https://www.example.com/scripts/*’. It will allow domain scripts in this URL.
3. script-src ‘*.googleapis.com www.example.*’ is used to allow all domain scripts.
4. Specify unsafe-eval to use eval () methods for creating code from strings.

   <add name="Content-Security-Policy" value="script-src 'self' 'unsafe-inline' 'unsafe-eval' 'https://www.example.com/scripts/*' '*.googleapis.com https://www.example.*’ " / >
   

5. script-src ‘unsafe-inline’ is used to allow inline scripts. In this, you can write <script> </script> directly in the view. Don’t use inline script in your application directly. If you want to use inline script, you should use nonce to avoid security vulnerabilities.

   <script nonce="r@nd0m">
   	doWhatever();
   </script>
   <add name="Content-Security-Policy" value=" script-src ‘r@nd0m’" />

   Nonce browser support
   The nonce directive is supported from CSP Level 2. It is supported by Chrome and Firefox after the version published in 2015, Safari 10+ or Edge 15+. It’s not supported in all Internet Explorer versions; you need to use the Edge browser for nonce support instead of Internet Explorer.

style-src

style-src is used to pre-approve the CSS stylesheet sources.

1. Use style-src ‘self’ to allow stylesheets from the current origin.
2. You can pre-approve your styles using style-src ‘https://www.example.com/styles/*’. It will allow domain styles in this URL.

   <add name="Content-Security-Policy" value=" style-src 'self' 'https://www.example.com/styles/*’ " / >
   

object-src

object-src allows sources for the <object>, <embed>, and <applet> tags. You can specify object-src ‘none’ to prevent loading all URL sources.

<add name="Content-Security-Policy" value="object-src 'none'" / >

img-src

img-src is used to restrict image sources. You can pre-approve third-party images in CSP by specifying the domain.

<add name="Content-Security-Policy" value="img-src 'none'" / >

font-src

font-src is used to mention sources for loading fonts.

<add name="Content-Security-Policy" value="font-src 'none'" / >

media-src

media-src is used to restrict sources from loading sound and video resources.

<add name="Content-Security-Policy" value="media-src 'none'" / >

frame-ancestors

frame-ancestors is used to restrict URLs that can embed the current source in <iframe>, <object>.

<add name="Content-Security-Policy" value="frame-ancestors 'self'"/ >

upgrade-insecure-requests

upgrade-insecure-requests indicates that the content URL from insecure (HTTP) sources should be acquired securely over HTTPS.

<add name="Content-Security-Policy" value="upgrade-insecure-requests">

Source List Reference

Apply the policy

Use <system.webServer> tag to apply the CSP.

Place the directives in the name attribute value. Separate directives with a semicolon (;). Refer to the following code example.

<system.webServer>
    <httpProtocol>
        <customHeaders>
            <add name="Content-Security-Policy" value="upgrade-insecure-requests; default-src 'self'; style-src 'self'; script-src 'self'; img-src 'none'; frame-ancestors 'self'" />
        </customHeaders>
    </httpProtocol>
</system.webServer>

Test your policy

After implementing the CSP in your application, you can validate your CSP directives. It helps to confirm that third-party scripts are not inadvertently blocked. If any of the CSP have failed in your application, you can generate a report to rectify them.

For more information, see MDN web docs: Content-Security-Policy-Report-Only and Google CSP evaluator.

Browser supports

CSP supports all latest versions of modern browsers but not Internet Explorer at all. Refer to the following links.

• Browser support list
• CSP level

Threats

There are many open tools available to scan your application. OWASP is one of the finest applications for finding security vulnerabilities. Once you implement the CSP, you can scan your complete application. This will not affect your application’s performance. You can also scan your production site for:

• Cross-site scripting
• SQL/data injection

See more about security vulnerabilities here.

Troubleshoot

If CSP is not implemented properly in your application, the errors will appear in your browser console. The browser will provide the details about the scripts that are blocked by your webpage. It will also provide the details like:

• • How to change the policy to allow a blocked item.
  • Elements that don’t accept the policy.

Note: CSP is only effective when the client’s browser supports all the included directives. For the latest browser support matrix, check Can I use: Content-Security-Policy.

Conclusion

In this blog, we have seen the steps to implement Content Security Policy (CSP) in your ASP.NET MVC web applications. I hope this blog post was helpful to you.

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