How to prevent Node.js applications from XSS attacks


Cross-Site Scripting (XSS) is a type of security vulnerability that occurs when an attacker is able to inject and execute malicious scripts in a web application. Reflected XSS is a type of XSS attack in which the injected script is reflected back to the user, typically as part of an error message or search results.

In Node.js, there are several ways to prevent reflected XSS attacks. Here are some best practices to follow:

  1. Sanitize user input: One of the most effective ways to prevent XSS attacks is to sanitize user input before using it in an HTTP response. This means removing any HTML or JavaScript code that could potentially execute malicious scripts. You can use libraries like sanitize-html or xss to sanitize user input in Node.js.
const sanitizeHtml = require('sanitize-html');

const dirty = 'Hello <script>alert("World");</script>';
const clean = sanitizeHtml(dirty);
console.log(clean); // "Hello"
  1. Use HTTP-only cookies: HTTP-only cookies are cookies that can only be accessed by the server, not by client-side JavaScript code. This can prevent attackers from stealing session cookies and using them to execute XSS attacks. To set an HTTP-only cookie in Node.js, use the httpOnly option when calling the cookie function in the cookie-parser middleware:
const cookieParser = require('cookie-parser');


app.get('/', (req, res) => {
  res.cookie('sessionId', '1234', { httpOnly: true });
  res.send('Hello World!');
  1. Use Content Security Policy (CSP): Content Security Policy is a mechanism that allows you to restrict the types of content that can be loaded on a web page. By using CSP, you can prevent malicious scripts from being executed, even if they are injected into the page. To set a CSP policy in Node.js, use the helmet middleware:
const helmet = require('helmet');

  directives: {
    defaultSrc: ["'self'"],
    scriptSrc: ["'self'", "'unsafe-inline'"]

This policy allows only scripts from the same origin ('self') and scripts with inline code ('unsafe-inline'), which can be useful for allowing certain libraries or frameworks that use inline scripts.

  1. Use a template engine with automatic escaping: If you’re using a template engine to generate HTML, make sure it has automatic escaping of user input built in. This means that any user input that is used in the template will automatically be escaped, preventing XSS attacks. For example, the ejs template engine has automatic escaping built in:
const ejs = require('ejs');

const data = {
  name: '<script>alert("XSS");</script>'

const html = ejs.render('<h1><%= name %></h1>', data);
console.log(html); // "<h1>&lt;script&gt;alert(&quot;XSS&quot;);&lt;/script&gt;</h1>"

By following these best practices, you can help prevent reflected XSS attacks in your Node.js application. However, it’s important to note that these are just some of the many steps you can take to improve the security of your application. It’s always a good idea to stay up-to-date with the latest security best practices and to perform regular security audits of your application to ensure that you’re not leaving any vulnerabilities open to attack.

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