Content Security Policy (CSP) Header Not Set

Content Security Policy (CSP) is a security feature that helps prevent cross-site scripting (XSS), clickjacking, and other code injection attacks by defining and enforcing a whitelist of approved content sources. It does this by defining a policy, which is a set of rules that specify the types of content that can be loaded and executed on a web page.

CSP can be implemented by setting a special HTTP header, called the Content-Security-Policy header, on the server's HTTP responses. However, if the CSP header is not set correctly, attackers can exploit this vulnerability to inject malicious code into your website.

In this guide, we will discuss how to fix the 'Content Security Policy (CSP) Header Not Set' vulnerability step-by-step. We will also provide examples of how to implement CSP in different web frameworks and programming languages.

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Step 1: Define your CSP policy

The first step in implementing CSP is to define your policy. This policy should specify which sources are allowed to load content on your web page, and which types of content are allowed to be loaded.

A CSP policy is made up of one or more directives, each of which defines a specific aspect of the policy. For example, the default-src directive specifies the default source of content that can be loaded on the page. Other directives include script-src, style-src, font-src, connect-src, and frame-src, among others.

To create your CSP policy, you should first identify all the sources of content that your web page needs to function properly. This may include your own domain, as well as third-party domains for resources such as scripts, stylesheets, fonts, and images.

Here's an example CSP policy that allows content to be loaded from the same domain, as well as from a trusted third-party domain:

Content-Security-Policy: default-src 'self' https://trusted-domain.com;

This policy allows content to be loaded from the same domain as the page, as well as from the domain 'https://trusted-domain.com'.

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Step 2: Set the CSP header on your server

Once you have defined your CSP policy, the next step is to set the CSP header on your server's HTTP responses.

The exact method for setting the CSP header will depend on the web framework or programming language you are using. Here are examples for some popular web frameworks:

In Node.js, you can set the CSP header using the helmet-csp module. Here's an example:

const helmet = require('helmet'); const csp = { directives: { defaultSrc: ["'self'"], styleSrc: ["'self'", 'maxcdn.bootstrapcdn.com'], scriptSrc: ["'self'", 'cdnjs.cloudflare.com'] } }; app.use(helmet({ contentSecurityPolicy: csp }));

This code sets the default source for content to be the same domain as the page, and allows styles to be loaded from 'maxcdn.bootstrapcdn.com' and scripts from 'cdnjs.cloudflare.com'.

In PHP, you can set the CSP header using the header() function. Here's an example:

$csp = "default-src 'self'; script-src 'self' https://trusted-domain.com"; header("Content-Security-Policy: $csp");

This code sets the default source for content to be the same domain as the page, and allows scripts to be loaded from the same domain as the page and from 'https://trusted-domain.com'.

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Step 3: Test your CSP policy

Once you have set your CSP policy, you should test it to make sure that it is working correctly. You can use the Content-Security-Policy-Report-Only header to test your policy without blocking any content.

The Content-Security-Policy-Report-Only header is similar to the Content-Security-Policy header, but it does not actually enforce the policy. Instead, it sends violation reports to a specified endpoint whenever a violation occurs.

Here's an example of how to use the Content-Security-Policy-Report-Only header:

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

This policy allows content to be loaded from the same domain as the page, and sends violation reports to the endpoint '/csp-violation-report-endpoint' whenever a violation occurs.

Once you have set the Content-Security-Policy-Report-Only header, you should monitor the violation reports to make sure that your policy is not blocking any legitimate content. You can use tools like CSP Evaluator or CSP Analyzer to analyze your policy and generate test cases to ensure that your policy is working correctly.

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Step 4: Enforce your CSP policy

After you have tested your CSP policy using the Content-Security-Policy-Report-Only header, you should switch to using the Content-Security-Policy header to enforce your policy.

Here's an example of how to use the Content-Security-Policy header:

Content-Security-Policy: default-src 'self'; report-uri /csp-violation-report-endpoint

This policy allows content to be loaded from the same domain as the page, and sends violation reports to the endpoint '/csp-violation-report-endpoint' whenever a violation occurs.

Once you have switched to using the Content-Security-Policy header, you should monitor your web application to make sure that your policy is not blocking any legitimate content.

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Conclusion

In this guide, we discussed how to fix the 'Content Security Policy (CSP) Header Not Set' vulnerability by implementing CSP on your web application. We provided a step-by-step guide with examples for different web frameworks and programming languages.

By implementing CSP, you can protect your web application from cross-site scripting (XSS), clickjacking, and other code injection attacks. However, it is important to test and monitor your policy to make sure that it is working correctly and not blocking any legitimate content.

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