Using Nonces in Content Security Policy (CSP)

Introduction

Content Security Policy (CSP) is a security feature that helps prevent a variety of attacks such as Cross-Site Scripting (XSS) and data injection attacks. One of the key features of CSP is the use of nonces, which are unique tokens that allow you to control which scripts are executed on your webpage.

What is CSP?

CSP is a declarative policy that defines what sources of content are considered safe for a web application. It can specify allowed sources for scripts, styles, images, and other resources.

What is a Nonce?

A nonce (number used once) is a randomly generated value that is included in your CSP header and in your script tags. It allows inline scripts to be executed only if they match the nonce specified in the CSP policy.

How to Use Nonces

1. Generate a nonce value on the server-side for each request.
2. Add the nonce to your CSP header. For example:

   
   Content-Security-Policy: script-src 'self' 'nonce-';
                       

3. Add the same nonce to your inline script tags:

   
   <script nonce="">
       // Your inline script here
   </script>
                       

4. Ensure to test your CSP policy to verify that it allows the intended resources while blocking others.

Best Practices

• Always use a unique nonce for each request.
• Keep your nonce secret and do not expose it in client-side code.
• Regularly review and update your CSP policies as your application evolves.
• Consider using a reporting mechanism to monitor CSP violations.

FAQ