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Prevent XSS Attacks Using JavaScript Trusted Types

Last updated: December 13, 2024

Cross-site scripting (XSS) is a prevalent security vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users. The consequences of such attacks can range from stealing sensitive information to completely taking over user accounts. JavaScript Trusted Types is a modern security feature that can help developers prevent XSS by enforcing the use of security-sensitive APIs in a safer way.

In this article, we will explore what Trusted Types are and how you can use them to protect your web applications from XSS attacks.

What are Trusted Types?

Trusted Types is a feature available in most modern browsers that control the creation of values that can be dangerous such as strings used in the DOM APIs, thereby reducing the risk of XSS. The concept is to forbid the direct use of strings in security-sensitive contexts and instead require the use of 'trusted types,' which are safely created objects representing these strings.

Here’s a simple analogy: instead of allowing any random package to enter a secured area, you only allow those with proper identification. Trusted Types work similarly by ensuring that only values that arise from trusted sources (validated code paths) are used in DOM operations, decreasing the likelihood of unintentionally introducing vulnerabilities.

How to Implement Trusted Types in Your Application

Begin by implementing a policy enforcing Trusted Types for your application. A policy can define several functions that transform a string into a trusted type, effectively acting as gates through which validated data can pass.


// Step 1: Create a policy
let myPolicy = window.trustedTypes.createPolicy('my-policy', {
  createHTML: (input) => input, // Return validated input
  createScript: (input) => input,
  createScriptURL: (input) => input
});

In this JavaScript code snippet, we create a simple Trusted Types policy named my-policy, which defines methods to createHTML, createScript, and createScriptURL. These methods receive an input and, after proper validation, use it safely.

Note: It's crucial to ensure the input is sanitized before passing it to these methods; otherwise, Trusted Types will not be effective in preventing XSS when your code directly returns unchecked data.

Integrating Trusted Types with JavaScript

Once the policy is set up, you enforce its usage by modifying your DOM-manipulating code to use Trusted Types safely:


// Safe DOM assignment using Trusted Types
let safeHTML = myPolicy.createHTML('Safe content');

// This assignment will only work if Trusted Types are available,
// otherwise fall back to not assigning the element.
try {
  document.getElementById('content').innerHTML = safeHTML;
} catch (e) {
  console.error('Failed to set innerHTML using trusted types.', e);
}

This example demonstrates a basic use case where you generate a trusted HTML string using a policy and safely use it to set the innerHTML of a DOM element. Should the construction using Trusted Types fail, the catch block will handle the situation gracefully by logging an error rather than introducing potential XSS.

Setting Trusted Types as Mandatory in Your Application

To get the full benefits of Trusted Types, they need to be enforced application-wide. This can often be achieved by adding a specific HTTP header to your server responses:


Content-Security-Policy: trusted-types my-policy;

By using this Content-Security-Policy header, you instruct browsers to comply with Trusted Types guidelines, limiting exposure to XSS through JavaScript.

Conclusion

Implementing Trusted Types in your applications can significantly reduce the risk of XSS attacks by ensuring strings are safely created and utilized. As you integrate Trusted Types into your web development practices, it complements other security measures like regular content sanitation, resulting in a more robust security posture for your applications.

Next Article: Define and Validate Safe HTML Sources in JavaScript

Previous Article: Enforce Safe DOM Interactions with the Trusted Types API in JavaScript

Series: Web APIs – JavaScript Tutorials

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