Web Security Headers: CSP, CORS & SRI

What Are Web Security Headers?

Web security headers are a set of HTTP response headers that help protect web applications against common vulnerabilities. These headers provide an additional layer of defense by instructing browsers on how to handle certain types of content and requests.

The three key headers we will explore—Content-Security-Policy (CSP), Cross-Origin Resource Sharing (CORS), and Subresource Integrity (SRI)—are designed to mitigate risks such as cross-site scripting, unauthorized data access, and malicious code injection.

How Do They Work?

Content-Security-Policy is a header that allows web developers to define rules for which resources can be loaded by the browser. These rules are enforced by the browser, preventing potentially harmful content from executing.

Cross-Origin Resource Sharing and Subresource Integrity headers work together to ensure that only trusted sources of data and scripts are loaded into your application.

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Why Are They Important?

Implementing these security headers is essential for maintaining the integrity, confidentiality, and availability of web applications. They help prevent common attacks like cross-site scripting (XSS), clickjacking, and data leakage.

By enforcing strict policies on what resources can be loaded from which origins, these headers significantly reduce the attack surface of a website.

Real-World Examples

For instance, Content-Security-Policy can prevent inline scripts or external scripts from being executed if they are not explicitly allowed. This is particularly useful in preventing XSS attacks.

Subresource Integrity ensures that any third-party script loaded from a CDN has not been tampered with by verifying its integrity hash before execution.

Frequently asked questions

What does Content-Security-Policy do?

Content-Security-Policy allows developers to specify which sources of content are trusted and should be loaded. It helps prevent attacks like XSS by blocking scripts from untrusted origins.

How can CORS headers protect a web application?

CORS headers control how a browser handles requests for resources from different domains, ensuring that only requests from specific sources are allowed, thus preventing unauthorized access to sensitive data.

What is Subresource Integrity and why is it important?

Subresource Integrity ensures that any third-party script loaded from a CDN has not been tampered with by verifying its integrity hash before execution. This helps prevent the loading of malicious code even if the URL remains unchanged.

Can these headers be implemented on all websites?

Yes, Content-Security-Policy, CORS, and Subresource Integrity can be implemented on any website to enhance security. However, their effectiveness depends on proper configuration and adherence to best practices.

Try it live

Everything above runs in your browser — open Web Security Headers: CSP, CORS & SRI and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.

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