Cross-Site Scripting (XSS) remains one of the most prevalent and high-impact vulnerabilities affecting web applications today. Even with secure coding standards, input encoding, and output sanitization, attackers can find ways to inject malicious scripts into your site. To establish a reliable line of defense, deploying a robust content security policy csp header is essential.

In this content security policy csp cheat sheet, we will walk through the core directives, detail a five-step implementation strategy, show web server configurations, and provide a reference table for security teams.

What is Content Security Policy (CSP)?

A Content Security Policy (CSP) is an HTTP response header that modern browsers use to restrict the resources (such as JavaScript, CSS, images, and fonts) that a page is allowed to load and execute. By explicitly declaring trusted source domains, CSP helps developers prevent XSS, data injection, clickjacking, and packet sniffing attacks.

5 Steps to Implement a Robust CSP

Deploying a policy can break existing site elements if done incorrectly. Follow these five steps to ensure a safe, incremental implementation:

Step 1: Define a Strict Default Fallback (default-src)

The default-src directive acts as a fallback for other resource types that are not explicitly defined. Setting it to 'self' ensures that the browser will only load resources originating from your website’s exact domain:

Content-Security-Policy: default-src 'self';

Step 2: Restrict Script Execution (script-src)

The majority of XSS attacks exploit JavaScript injection. To mitigate this risk, define where scripts can load from and prohibit unsafe inline scripts (<script>alert(1)</script>) or string evaluation functions (eval()):

Content-Security-Policy: default-src 'self'; script-src 'self' https://trustedscripts.com;

Step 3: Control Styling and Font Sources (style-src & font-src)

Attackers can use malicious CSS stylesheet injections to extract data (such as passwords or tokens) from the page. Secure these assets by declaring style rules:

Content-Security-Policy: default-src 'self'; style-src 'self' https://fonts.googleapis.com; font-src 'self' https://fonts.gstatic.com;

Step 4: Block Framing to Mitigate Clickjacking (frame-ancestors)

To prevent malicious third-party websites from embedding your pages inside transparent iframes, configure the frame-ancestors directive. This serves as a modern replacement for legacy X-Frame-Options settings:

Content-Security-Policy: frame-ancestors 'self';

Step 5: Test Safely with Report-Only Mode

Instead of deploying a policy immediately and risking site breakage, run CSP in testing mode first. The browser will report policy violations to your specified reporting server but will not actually block the resources:

Content-Security-Policy-Report-Only: default-src 'self'; report-uri /csp-reporting-endpoint;

Ecosystem Cheat Sheet: Core CSP Directives

Refer to this cheat sheet table for common directives and recommended values:

Web Server Implementation Examples

To deploy CSP, configure your web server to return the header. For **Nginx**, add this line to your configuration block:

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

For **Apache**, modify your .htaccess file:

Header always set Content-Security-Policy "default-src 'self'; script-src 'self';"

Summary

Deploying a secure **content security policy csp** response header is one of the most effective ways to defend your application from script injection vulnerabilities. By combining CSP with a secure authentication cheat sheet, you protect both session validation keys and overall page integrity. To explore the absolute browser capabilities and standard specifications, read the comprehensive MDN Web Content Security Policy Documentation.