What is Content Security Policy (CSP)? Learn how to implement CSP headers, directives, examples & best practices to prevent XSS attacks securely.

What is Content Security Policy and Why Should You Care?

Ever wondered what is Content Security Policy (CSP) and why it's crucial for web security? This browser-side firewall blocks malicious scripts, preventing XSS attacks Content Security Policy (CSP) is a browser-side firewall. Without proper safeguards, attackers can inject scripts that steal data, redirect users, or deface websites—a risk that impacts businesses and end-users alike.

⚠️ Warning: Vulnerabilities without CSP
Websites lacking CSP are exposed to cross-site scripting (XSS) attacks, which can lead to severe consequences including data breaches, reputational damage, and legal liabilities without a CSP, websites are vulnerable to XSS attacks that can lead to public relations and legal issues for businesses.

In this guide, you’ll learn:

• What CSP is and how it acts as a protective layer for your web applications Content Security Policy (CSP) is a browser-side firewall
• Key directives that control resource loading and block threats Content Security Policy directives include default-src, script-src, object-src, style-src, img-src, and many others
• Implementation best practices to harden your security posture The recommended approach to implement CSP is via HTTP response headers sent by the server

Content Security Policy Explained: Your Browser's Built-in Security Guard

Content Security Policy (CSP) is more than just another security header—it’s a proactive defense mechanism built into modern browsers. CSP functions as a browser-side firewall that sets strict rules about which resources a webpage is allowed to load. When a browser receives a CSP header, it enforces these rules for every request the page makes, blocking or reporting any resource that doesn’t meet the policy criteria When a browser receives a CSP header, it enforces the rules for every request the page makes, blocking or reporting any resource that does not meet the policy criteria.

How Does Content Security Policy Actually Keep Your Site Safe?

The browser parses CSP directives during page load and applies them to all subsequent requests. This initialization involves the browser parsing the policy directives and enforcing them on the document and its resources. If a resource violates the policy—such as an unauthorized script attempting to execute—the browser blocks it and optionally reports the violation depending on the policy’s disposition If a resource violates the CSP, the browser blocks it and optionally reports the violation depending on the policy disposition.

flowchart TD
    A[Browser Receives CSP Header] --> B[Parse Policy Directives]
    B --> C{Check Resource Request}
    C -->|Allowed| D[Load Resource]
    C -->|Blocked| E[Block & Report Violation]

This enforcement occurs at runtime, meaning even dynamically injected content is evaluated against the policy, providing robust protection against stored, reflected, and DOM-based XSS attacks CSP helps mitigate cross-site scripting (XSS) attacks by restricting the sources from which scripts can be loaded and executed.

Breaking Down the Rules: Understanding CSP Directives

CSP’s power lies in its directives—instructions that define allowed sources for specific resource types. These directives are combined into a single header value, separated by semicolons for clarity CSP directives are separated by semicolons in the Content-Security-Policy header value, allowing multiple resource control rules in one policy.

The Most Common CSP Rules and What They Do

Example CSP Header

Content-Security-Policy: default-src 'self'; script-src 'nonce-{random}'; object-src 'none'; style-src 'self' https:; img-src data:

How Key CSP Rules Behave Behind the Scenes

• default-src acts as a catch-all. If a specific directive like script-src isn’t defined, default-src values apply The default-src directive serves as a fallback for other resource types if their specific directives are not defined in the CSP.
• object-src 'none' is a best practice to disable outdated plugins like Flash, reducing attack surface The object-src directive in CSP is often set to 'none' to prevent loading of plugins like Flash, reducing attack surface.
• Nonces (e.g., 'nonce-{random}') ensure only scripts with a predefined, random value are executed, balancing security with functionality.

By strategically configuring these directives, you can create a defense-in-depth strategy that minimizes the risk of malicious script execution and resource injection.

How to Set Up Content Security Policy (With Real Examples)

Implementing Content Security Policy (CSP) effectively starts with understanding where and how to deploy it. The most secure approach is to use HTTP response headers, which are harder for attackers to manipulate compared to <meta> tags CSP is most often implemented using HTTP response headers, which is preferred over using a <meta> tag because headers are harder for hackers to tamper with. This method ensures the policy applies to every resource loaded by the browser, including subresources like images, scripts, and stylesheets The recommended approach to implement CSP is via HTTP response headers sent by the server, rather than using <meta> tags inside HTML documents.

Why You Should Test CSP in Report-Only Mode First

Before enforcing restrictions, use the Content-Security-Policy-Report-Only header to collect violation reports without blocking legitimate content It is advised to start CSP implementation with the Content-Security-Policy-Report-Only header to collect violation reports without blocking content. This allows you to refine your policy based on real-world usage patterns while avoiding broken functionality during deployment.

Basic vs. Strict CSP: Which Security Level is Right for You?

A basic CSP header might look like this, allowing resources only from your own origin:

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

An example of a basic CSP header is: Content-Security-Policy: default-src 'self', which instructs the browser to only load resources from the website's own origin

For a strict policy, combine directives like nonce for trusted scripts and https; to enforce HTTPS:

Content-Security-Policy: default-src https:; script-src 'nonce-{random}'; object-src 'none'; report-uri /csp-report-endpoint  

Sequence Diagram: CSP Header Delivery

flowchart LR  
    A[Server] -->|Sends HTTP Response| B[Browser]  
    B -->|Parses HTML| C{CSP Header Detected}  
    C -->|Enforces Policy| D[Blocks Unauthorized Resources]  
    D -->|Sends Violation Reports| E[Report Endpoint]  

Stop XSS Attacks in Their Tracks with Content Security Policy

Content Security Policy (CSP) is a powerful defense against cross-site scripting (XSS) attacks by controlling which scripts can execute in the browser CSP helps mitigate cross-site scripting (XSS) attacks by restricting the sources from which scripts can be loaded and executed. It specifically protects against:

• Stored XSS: Malicious scripts embedded in databases (e.g., user comments)
• Reflected XSS: Attackers injecting scripts via URL parameters
• DOM-based XSS: Exploits manipulating the DOM without server-side injection

A strict CSP protects against classical stored, reflected, and some DOM-based XSS attacks and is the optimal goal for CSP implementation.

Top Ways CSP Shields Your Website

• Script Source Restrictions: Only allow scripts from trusted origins or those with a cryptographic nonce
• Inline Script Prevention: Block inline event handlers (onclick) unless explicitly allowed via nonce or hash
• Plugin Disabling: Set object-src 'none' to disable Flash and other outdated plugins

CSP also guards against credit card skimming and ad injection by preventing unauthorized resource loads CSP can prevent credit card skimming and ad injection attacks in addition to cross-site scripting.

Tip: Integrate with Comprehensive Security
For deeper XSS mitigation, combine CSP with How to Prevent Cross-Site Scripting (XSS) Attacks and A Guide to Security Headers for Web Developers.

Common Misconception
Allowing inline scripts without nonce or hash values bypasses CSP protections CSP can be bypassed if inline scripts are allowed without nonces or hashes, which is a common misconception about CSP's effectiveness.

CSP Best Practices: Tips and Browser Compatibility

To maximize security and compatibility, follow these CSP best practices:

1. Use Nonces for Trusted Scripts
   Generate a unique, unpredictable nonce for each page load to allow specific scripts:

   Content-Security-Policy: script-src 'nonce-{random}'  

   A strict CSP policy can require all scripts to have a nonce attribute with a random value, ensuring only trusted scripts are executed

2. Leverage strict-dynamic
   This directive simplifies policies by allowing scripts added dynamically by trusted scripts:

   script-src 'strict-dynamic'  

   The strict-dynamic directive in CSP makes it easier to implement a strict policy by allowing scripts dynamically added by trusted scripts

3. Avoid Wildcards
   While '*' seems convenient, it weakens your policy by allowing content from any source Using wildcards (*) in CSP directives allows certain types of content from any source, but it is generally discouraged as it weakens the policy.

4. Enable Violation Reporting
   Use report-uri or report-to to log violations:

   Content-Security-Policy: report-uri /csp-violation-endpoint  

   CSP violation reports can be collected using the report-uri or report-to directives to monitor and improve policy enforcement

Which Browsers Support Content Security Policy?

CSP is widely supported, but ensure coverage for legacy environments:

Modern browsers except Internet Explorer support the Content-Security-Policy HTTP header. For broader compatibility, pair CSP with A Guide to Security for Frontend Developers to address edge cases.

Strict CSP Example

Content-Security-Policy: default-src https:; script-src 'nonce-{random}'; object-src 'none'; report-to default  

A sample strict CSP header is: Content-Security-Policy: default-src https:; script-src 'nonce-{random}'; object-src 'none', which enforces HTTPS and trusted scripts only

Always send the Content-Security-Policy header in every HTTP response, not just the initial HTML page The Content-Security-Policy header must be sent in all HTTP responses, not just the main HTML page, to ensure comprehensive protection.

Ready to Secure Your Site? Get Started with CSP Now!

Content Security Policy (CSP) is more than just another security header—it's a critical defense layer that transforms how your application handles resources, dramatically reducing the risk of attacks like cross-site scripting (XSS) and data injection CSP helps mitigate cross-site scripting (XSS) attacks. By enforcing strict rules for resource loading, CSP shields your users from malicious scripts and unauthorized content, even in complex, dynamic environments CSP helps protect web applications in complex environments. As web applications grow in complexity, adopting CSP isn't optional—it's a necessary step to protect both your brand reputation and your users' data.

Why Content Security Policy is Essential Today

Modern web ecosystems demand robust security measures. Without CSP, websites remain vulnerable to XSS attacks that can lead to public relations crises and legal liabilities [fact-17]. These attacks aren’t just theoretical: they enable credit card skimming, ad injection, and data theft, directly impacting user trust and compliance [fact-16]. CSP mitigates these threats by restricting script sources, blocking unsafe plugins, and requiring HTTPS for all resources [fact-5][fact-19][fact-12]. For example, a strict policy can require all scripts to use a nonce attribute, ensuring only trusted code executes [fact-6].

Implementing CSP via HTTP response headers—not <meta> tags—ensures policies apply universally across every resource, from APIs to images [fact-2][fact-9][fact-22]. This comprehensive enforcement blocks violators at the browser level, with optional reporting to help refine your policy over time [fact-4][fact-18][fact-24].

5 Simple Steps to Roll Out CSP Right Now

1. Start with Content-Security-Policy-Report-Only
   Deploy your policy in report-only mode first to collect violation data without breaking functionality [fact-10]. This lets you identify and fix issues safely before enforcing blocks.

2. Define a Strong Baseline Policy
   Begin with default-src 'self' to restrict resources to your own origin, then expand with specific directives like script-src, style-src, and img-src [fact-3][fact-7]. Avoid wildcards (*) to maintain policy strength [fact-8].

3. Leverage strict-dynamic for Scripts
   Use script-src 'strict-dynamic' to allow dynamically injected scripts from trusted sources while blocking external scripts [fact-13]. Pair this with nonces for inline scripts.

4. Enforce HTTPS and Block Unsafe Content
   Set default-src https: to require encrypted connections and object-src 'none' to disable plugins like Flash [fact-19][fact-20]. This reduces attack surfaces significantly.

5. Enable Violation Reporting
   Configure report-uri or report-to to log violations to a monitoring endpoint, enabling continuous policy refinement [fact-18].

Pro Tips for Making CSP Work for You

📢 Key Reminders

• Test thoroughly: Use tools like CSP Eval to validate your policy.
• Update regularly: As your application evolves, revisit and tighten your CSP.
• Pair with frontend security: Combine CSP with A Guide to Security for Frontend Developers for end-to-end protection [fact-11][fact-25].

CSP adoption doesn’t have to be overwhelming. By following these steps, you’ll transform your security posture, safeguard user data, and future-proof your application against emerging threats. Start small, monitor closely, and scale your policy to become a true guardian of your digital assets.