Ever activated a security header only to find your WordPress site suddenly breaking? That is often a misconfigured Content Security Policy (CSP), a powerful but tricky web security tool. It is a browser security feature that helps protect websites from common web attacks like cross-site scripting (XSS), clickjacking, and data injection.

Example of a CSP violation message in the browser console

 

In this example, the browser refuses to load an image from unsplash.it because it is not listed in the site’s allowed sources under the img-src directive. This is how CSP prevents untrusted or unexpected content from loading.

CSP works by defining which content sources, such as scripts, styles, and images are allowed to load and execute on your site. Anything not included in your policy is automatically blocked. This significantly lowers the chances of harmful code running on your WordPress site.

 

How CSP Works in WordPress

Developers can implement a CSP in WordPress by adding a Content-Security-Policy header or meta tag. This header tells browsers where it is safe to load scripts, CSS, fonts, and embedded media from.
 For example:

Content-Security-Policy: default-src ‘self’;

This directive ensures that all content is loaded only from your own domain. You can then expand permissions for trusted sources, like:

Content-Security-Policy: default-src ‘self’; script-src ‘self’ coolscripts.example.com;

To support inline scripts securely, WordPress offers the wp_create_nonce() function, which generates unique nonces that can be tied to inline code. The browser only executes these if the nonce matches the one defined in the CSP header.

 

Why CSP is Crucial for Site Security (And The Threats it Protects Against)

CSP is designed to prevent a variety of attacks that exploit untrusted or injected content. The most common ones include:

  • Cross-Site Scripting (XSS): Attackers inject malicious JavaScript into trusted web pages to steal session cookies or perform unauthorized actions.
  • Clickjacking: A user is tricked into clicking hidden or disguised buttons or links, potentially granting access or performing unintended actions.
  • Data Injection: Attackers manipulate system input (like SQL or HTTP headers) to insert harmful code or commands.

While CSP can’t block every form of attack, it greatly reduces the risk of browser-based threats and provides a vital extra layer of protection alongside input sanitization and HTTPS enforcement.

 

Common CSP Errors That Break WordPress and How to Solve Them

While a well-defined CSP improves WordPress security, a misconfigured policy can easily disrupt your site. Below are the most common CSP errors and how to fix them.

a. Inline Scripts and Styles

Many WordPress themes and plugins use inline JavaScript or CSS. Since CSP blocks inline code by default, this often causes functionality to break.
Fix: Move inline code into external files or use nonces/hashes to securely allow specific inline snippets. Avoid using ‘unsafe-inline’ unless absolutely necessary.

b. Conflicts with Other Plugins

Plugins frequently load external scripts or fonts. Installing a new one may cause CSP violations if its sources aren’t included in your policy.
Fix: Audit your plugins and update the script-src, style-src, or related directives whenever you add or update plugins.

c. Broken Website Functionality

Overly strict policies can block legitimate resources, making features stop working.
Fix: Use browser Developer Tools to identify blocked resources and whitelist only the necessary ones.

d. Incorrect Header Syntax

Even small syntax mistakes can cause your entire CSP to fail.
Fix: Validate your header using online CSP checkers before deploying.

e. Browser Compatibility Issues

Older browsers may interpret or ignore certain CSP directives.
Fix: Implement fallback headers like X-Frame-Options and X-XSS-Protection to maintain baseline security.

f. Performance Problems

Large CSPs with many source definitions can slow down page loads.
Fix: Use wildcards judiciously (for trusted subdomains) and resolve repeated violation reports quickly to prevent overhead.

g. HTTPS Mixed Content Issues

If your CSP enforces HTTPS but your site loads assets over HTTP, browsers will block them.
Fix: Serve all assets over HTTPS or use the upgrade-insecure-requests directive to automatically rewrite them.

 

Conclusion

A Content Security Policy is a strong layer of defense, but it is not a complete solution. It should work alongside a broader WordPress security strategy that includes regular plugin audits, secure authentication, and updated coding practices.

If you want to learn more about defining or testing your CSP, check out WordPress’s official Content Security Policy Guide

To explore additional security best practices, visit the official WordPress Security Hardening Guide