A Deep Dive into DOMPurify with TypeScript

In modern web development, security is of utmost importance. One of the critical security risks is cross - site scripting (XSS) attacks, which can occur when untrusted user input is rendered in the DOM. DOMPurify is a powerful library that helps mitigate this risk by sanitizing HTML input, removing malicious code, and ensuring that only safe HTML is inserted into the DOM. TypeScript, on the other hand, is a superset of JavaScript that adds static typing, making it easier to catch errors early in the development process. Combining DOMPurify with TypeScript provides a secure and type - safe way to handle user - generated HTML. In this blog post, we will explore the fundamental concepts of using DOMPurify with TypeScript, its usage methods, common practices, and best practices.

Table of Contents

  1. Fundamental Concepts
  2. Installation
  3. Usage Methods
  4. Common Practices
  5. Best Practices
  6. Conclusion
  7. References

1. Fundamental Concepts

DOMPurify

DOMPurify is a JavaScript library that takes untrusted HTML as input and returns sanitized HTML. It works by parsing the input HTML, analyzing each node, and removing or modifying any elements or attributes that could potentially be used for XSS attacks. For example, it can remove <script> tags, event handlers like onclick that could contain malicious code, and other dangerous constructs.

TypeScript

TypeScript adds static typing to JavaScript. When using DOMPurify with TypeScript, we can ensure that the types of variables and function parameters are correct. This helps in preventing type - related errors during development and makes the code more maintainable.

2. Installation

First, you need to install DOMPurify in your TypeScript project. You can use npm or yarn for this:

# Using npm
npm install dompurify

# Using yarn
yarn add dompurify

You also need to install the TypeScript type definitions for DOMPurify:

# Using npm
npm install --save-dev @types/dompurify

# Using yarn
yarn add --dev @types/dompurify

3. Usage Methods

Basic Usage

Here is a simple example of using DOMPurify with TypeScript to sanitize user input:

import DOMPurify from 'dompurify';

// Untrusted user input
const untrustedInput = '<script>alert("XSS attack")</script><p>Some safe text</p>';

// Sanitize the input
const clean = DOMPurify.sanitize(untrustedInput);

// Output the sanitized HTML
console.log(clean);

In this example, the <script> tag in the untrustedInput will be removed by DOMPurify, and only the safe <p> tag will be retained.

Configuring DOMPurify

You can also configure DOMPurify to suit your specific needs. For example, you can allow certain tags or attributes:

import DOMPurify from 'dompurify';

const untrustedInput = '<a href="javascript:alert(\'XSS\')">Click me</a>';

// Configure DOMPurify to allow <a> tags but remove dangerous href values
const config = {
    ALLOWED_TAGS: ['a'],
    ALLOWED_ATTR: ['href']
};

const clean = DOMPurify.sanitize(untrustedInput, config);

console.log(clean);

In this case, the <a> tag is allowed, but the javascript: href value (which could be used for an XSS attack) will be removed.

4. Common Practices

Server - Side and Client - Side Sanitization

While DOMPurify is a client - side library, it’s important to also perform sanitization on the server - side. Server - side sanitization acts as an additional layer of security in case the client - side code is bypassed.

Using Sanitized HTML

Once you have sanitized the HTML, you can safely insert it into the DOM. For example:

import DOMPurify from 'dompurify';

const untrustedInput = '<p>Some user - generated text</p>';
const clean = DOMPurify.sanitize(untrustedInput);

const targetElement = document.getElementById('target');
if (targetElement) {
    targetElement.innerHTML = clean;
}

5. Best Practices

Keep DOMPurify Updated

Regularly update the DOMPurify library to ensure that you have the latest security patches. New XSS attack vectors may be discovered over time, and the library developers will release updates to handle them.

Validate Input Before Sanitization

It’s a good practice to validate the input before sanitizing it. For example, if you expect a certain format of input (e.g., a number or a specific string pattern), validate it first. This can reduce the processing time of DOMPurify and prevent unnecessary sanitization.

Limit Allowed Tags and Attributes

When configuring DOMPurify, limit the allowed tags and attributes as much as possible. Only allow tags and attributes that are necessary for your application. This reduces the attack surface and makes your application more secure.

6. Conclusion

DOMPurify is a powerful tool for sanitizing HTML input and preventing XSS attacks. When combined with TypeScript, it provides a type - safe and secure way to handle user - generated HTML. By understanding the fundamental concepts, usage methods, common practices, and best practices, you can effectively use DOMPurify with TypeScript in your web development projects.

7. References