Is the X-Content-Security-Policy Deprecated? Time to Upgrade Your Headers

In the world of web security, few tools are as powerful as the Content Security Policy (CSP). It is your primary line of defense against Cross-Site Scripting (XSS) and data injection attacks.

However, if you are auditing an older application or configuring a legacy server, you might stumble upon a header that looks slightly different: X-Content-Security-Policy or X-WebKit-CSP.

This raises a critical question for developers: Are these “X” headers still valid? Should you keep them for backward compatibility, or are they just digital clutter?

The short answer is: Yes, they are deprecated. If you are using them today, you are relying on obsolete standards that modern browsers ignore.

Here is the deep dive into what happened to these headers and how you should configure your security policy today.

The Experimental Era: A Brief History

To understand why these headers exist, we have to rewind to roughly 2010–2011.

When the concept of Content Security Policy was first introduced, it wasn’t yet a finalized W3C standard. However, browser vendors (like Mozilla and Google) wanted to start testing this powerful new feature immediately.

Following the conventions of the time, they implemented the feature using experimental prefixes:

1. Firefox (Gecko): Introduced X-Content-Security-Policy in Firefox 4.
2. Chrome/Safari (WebKit): Introduced X-WebKit-CSP in Chrome 14 and Safari 6.
3. Internet Explorer: Partially implemented sandbox restrictions using X-Content-Security-Policy in IE 10/11.

For a few years, developers had to include all three versions in their server configurations to ensure their policy worked across different browsers. It was a messy, verbose time for server admins.

The Shift to Standardization

As the CSP specification matured, the W3C (World Wide Web Consortium) finalized the standard. With the release of CSP Level 1.0 (and subsequently Level 2 and 3), the “X” prefix was officially dropped.

The standard header is now simply:

Content-Security-Policy

Why You Should Stop Using the “X” Headers

You might be tempted to keep the old headers “just in case” to support older browsers. Here is why that is a bad idea:

1. Inconsistent Implementations: The policies supported by X-Content-Security-Policy (Firefox 4 era) are significantly different from the modern CSP standard. Directives have changed names, and behaviors are different. A policy written for the modern header might cause errors or unexpected blocking when read by an ancient browser using the “X” header.
2. Double the Payload: Sending three headers instead of one increases the size of your HTTP response headers unnecessarily.
3. Modern Browsers Ignore Them: Chrome, Firefox, Safari, and Edge have all supported the standard Content-Security-Policy header for over a decade. They generally ignore the “X” versions if the standard one is present, rendering the “X” headers useless code bloat.

Browser Compatibility Breakdown

To visualize just how obsolete these headers are, look at which browser versions actually needed them:

Header Name	Required By	Status
Content-Security-Policy	Chrome 25+, Firefox 23+, Safari 7+, Edge 12+	Standard (Use This)
X-Content-Security-Policy	Firefox 4-22, IE 10-11	Deprecated
X-WebKit-CSP	Chrome 14-24, Safari 6	Deprecated

Unless your user base is strictly using Firefox version 4 (released in 2011) or Chrome version 14, you do not need the “X” headers.

How to Check and Fix Your Site

If you want to ensure your site is using the correct standards, follow these steps:

1. Audit Your Headers

Open your website, inspect the Network tab in your developer tools, and look at the “Response Headers” for your main document request.

If you see this:

X-Content-Security-Policy: default-src 'self';
X-WebKit-CSP: default-src 'self';
Content-Security-Policy: default-src 'self';

Fix: Delete the first two lines.

2. Configure the Standard Header

Ensure your server (Nginx, Apache, or backend code) is sending only the standard header.

For Nginx:

add_header Content-Security-Policy "default-src 'self'; script-src 'self' https://trusted.cdn.com;" always;

For Apache:

Header set Content-Security-Policy "default-src 'self'; script-src 'self' https://trusted.cdn.com;"

Summary

The X-Content-Security-Policy and X-WebKit-CSP headers are relics of the past. They served their purpose during the experimental phase of the web, but that phase ended years ago.

Using them today offers no security benefit and only adds confusion to your codebase. Clean up your server config, stick to the standard Content-Security-Policy, and rest easy knowing you are following modern security best practices.