Published on

November 27, 2023

10 tips to build a Content Security Policy (CSP) without breaking your site!

There are many benefits to adopting CSP – from blocking attacks (such as XSS), to improving security posture and compliance.

However, many developers that try to use CSP – lack the experience and tooling to do so effectively – create policies that may offer lower protection, and worse – block legitimate parts of their web applications – breaking functionality in production!

So many developers get reports of their site getting broken – only to check and see this dreaded error:

Refused to load the script '...' because it violates the following
Content Security Policy directive: "script-src 'self' 'report-sample'".
Note that 'script-src-elem' was not explicitly set,
so 'script-src' is used as a fallback.

The below 10 tips, learned from years of deploying CSP at scale —  will take you from A to Z with regards to proper deployment of Content-Security-Policy without breaking your website / web application.

1. Always Start Out with a Strict Report-Only Policy to Map and Learn Your Site

   Embarking on the journey of establishing a Content Security Policy (CSP) can be likened to navigating uncharted waters. It's prudent to begin with a strict Report-Only CSP mode. This strategy serves as a reconnaissance mission, allowing you to meticulously map out and understand the intricate workings of your site. By monitoring and logging potential violations without enforcing restrictions, you can gain invaluable insights into how your site operates under various security conditions without causing any disruption to its functionality.

2. Don’t Use the Head <meta> Tag to Set the Policy

   A common misconception in CSP implementation is the utilization of the head <meta> tag. While it seems convenient, this method significantly limits the policy’s scope and flexibility. It's imperative to understand that CSPs set through <meta> tags don't support the reporting directive, a critical component for monitoring potential security breaches. Instead, opt for setting your CSP through HTTP headers, which offers a broader range of directives and enhances overall security efficacy.

3. Maintain Versions of Your Content Security Policies

   Just like any robust software, your CSP should be subject to version control. This practice is not just about preserving previous versions; it's about maintaining a history of changes, understanding the evolution of your security needs, and ensuring rollback capabilities in case of policy misconfigurations. By keeping track of CSP iterations, you can better analyze the impact of each change and make informed decisions moving forward.

4. Integrate CSP into Your Environment or CI/CD (Rather Than Hard-Code It)

   In the realm of modern web development, adaptability is key. Hard-coding your CSP is akin to setting sail with a rigid, unyielding mast. Instead, integrate CSP into your development environment or Continuous Integration/Continuous Deployment (CI/CD) pipeline. This approach ensures that your CSP evolves in tandem with your site, allowing for dynamic updates and seamless security enhancements.

5. Allow CSP Packages for Known Services

   Navigating the complex web of third-party services is a daunting task. To simplify this, consider adopting CSP packages for known services. These pre-configured settings are tailored for popular services like Google Analytics or social media platforms, ensuring compatibility and reducing the risk of inadvertently blocking essential functionalities.

6. Avoid an Overkill with the Allow-List Specifics

   While specificity in CSP is crucial, overly granular allow-lists can lead to an administrative nightmare and potential operational bottlenecks. Strike a balance between security and manageability. It's more pragmatic to focus on broader policy strokes that address major security concerns without getting entangled in the minutiae.

7. Custom Fit Your CSP Directive Choice to the Web Asset

   Each web asset has its unique security needs. A one-size-fits-all approach to CSP directives can be ineffective. Tailor your directives to fit the specific security requirements of different assets. This bespoke approach ensures that each part of your site is optimally protected without unnecessary restrictions on its functionality.

8. Enforce Less Strict Versions of Your Reporting Policy

   As your site evolves, so should your CSP. Gradually transitioning from a strict Report-Only policy to a more permissive but still secure policy can be beneficial. This phased approach allows you to incrementally enforce your policies, minimizing the risk of breaking site functionalities while still maintaining a robust security posture.

9. Keep Calm and Report-Only

   In the world of web security, panic is a luxury you can't afford. When facing potential security breaches, it's crucial to remain composed and rely on your Report-Only policy. This policy acts as your first line of defense, providing valuable insights into potential threats without precipitating hasty decisions that could disrupt site functionality.

10. Ask for Advice

    The complexity of CSPs can sometimes be overwhelming. In such instances, seeking advice from experts can be invaluable. Whether it's consulting with cybersecurity professionals or engaging with developer communities, external insights can provide new perspectives and solutions to enhance your CSP strategy.

Bonus Tip: Add Nonces Separately (Later)

Nonces, or number-used-once tokens, add an extra layer of security to your CSP. However, incorporating them into your policy should be a calculated decision made later in the process. Initially focus on establishing a robust CSP framework. Once this foundation is solid, you can introduce nonces to further fortify your site against script-based attacks.