Using Google’s CSP Evaluator to Identify and Bypass Content Security Policy Weaknesses

Content Security Policy (CSP) is a critical web security mechanism designed to prevent malicious scripts from executing, especially in the context of Cross-Site Scripting (XSS) attacks. However, even with an active CSP in place, attackers and security researchers may find ways to bypass these safeguards on various websites. This article explores how Google’s CSP Evaluator can be used both to assess the effectiveness of a site’s CSP and to identify potential bypasses, particularly in environments leveraging AngularJS libraries or JSONP endpoints.

Introduction to Content Security Policy

The Content Security Policy is a widely adopted security standard implemented via HTTP headers that restrict what dynamic resources can be loaded and executed by a web page. CSP helps mitigate XSS attacks by specifying domains trusted for loading scripts, styles, and other assets.

Despite its importance, CSP implementation can be complex and error-prone. Misconfigurations or overly permissive policies can inadvertently leave websites vulnerable to script injection or other attacks.

What Is Google’s CSP Evaluator?

Google’s CSP Evaluator is a free, open-source tool designed to analyze Content Security Policies and highlight potential security gaps or risky directives. It allows users to input either the URL of a website or its exact CSP header value to detect unsafe configurations.

• Helps developers and security professionals validate their CSP implementations.
• Identifies known bypass techniques linked to JSONP endpoints, AngularJS script loading, and other vectors.
• Available both as a web tool and a Chrome extension for convenient integration in development workflows.

How to Use CSP Evaluator Effectively

Using the CSP Evaluator involves a straightforward process:

1. Paste the URL of the target website or directly input the Content Security Policy header’s value.
2. The tool analyzes various CSP directives and flags potential security issues or bypasses.
3. Review detailed findings to understand which parts of the policy may be too permissive or insecure.

For example, analyzing https://codejam.withgoogle.com/2018/ reveals multiple CSP vulnerabilities, as CSP Evaluator surfaces high-severity risks related to JSONP endpoints and AngularJS hosting domains.

Common Bypass Vectors Identified by CSP Evaluator

• JSONP Endpoints: Some hosts, like *.google-analytics.com, serve JSONP APIs that can be exploited to execute arbitrary scripts.
• Trusted Script Hosts: Domains such as *.gstatic.com may host libraries like angular.js, which attackers can leverage to bypass the CSP protections.

Demo: Bypassing CSP Using AngularJS

Consider a CSP that allows script loading from gstatic.com, which hosts AngularJS libraries. An attacker can load AngularJS from this trusted source and exploit AngularJS features to execute arbitrary JavaScript, effectively bypassing the CSP restrictions.

<script src="https://www.gstatic.com/fsn/angular_js-bundle1.js"></script>
<input ng-app autofocus ng-focus="$event.path|orderBy:'[].constructor.from([1],alert)'">

This code snippet creates an AngularJS application that triggers an alert on focus, demonstrating a practical CSP bypass.

Real-World Impact and Research Insights

Several studies and vulnerability reports have demonstrated that CSP implementations often contain gaps or allow specific bypasses. According to research published in the IEEE Security & Privacy Journal, misconfigured CSP policies remain one of the leading causes of persistent XSS vulnerabilities on popular websites.

Additionally, Google maintains an active repository of known CSP bypasses involving JSONP, Flash, and AngularJS. The continuous update of these bypasses reflects the evolving nature of web security threats and the importance of proper CSP configuration.

Practical Tips for Enhancing CSP Security

• Minimize trusted domains: Restrict the policy to only essential domains to reduce the attack surface.
• Avoid JSONP: JSONP endpoints are considered risky; prefer CORS and other safer data retrieval mechanisms.
• Regularly audit CSP: Use tools like CSP Evaluator to continuously test and improve your policies.
• Monitor CSP violation reports: Implement CSP reporting mechanisms to detect real-time policy violations in production.
• Test in all major browsers: CSP support varies; ensure broad compatibility and consistent enforcement.

Additional Resources

• Google’s CSP Evaluator GitHub Repository – Access source code and contribute to the tool.
• Google Developers Guide on Content Security Policy – Comprehensive documentation on CSP best practices.
• UselessCSP.com – Archived listings of CSP flaws found across popular websites.

Conclusion

While Content Security Policy provides a strong defense against many common web attacks, using tools like Google’s CSP Evaluator reveals that even well-intentioned policies may have exploitable weaknesses. Continuous evaluation, combined with up-to-date knowledge of known bypass techniques, is essential to maintaining robust web security.