Write Secure JavaScript Applications

📆 · ⏳ 6 min read · ·


In the digital age, security isn’t just a buzzword – it’s a necessity. As software engineers, crafting JavaScript applications that are both functional and secure is paramount. Let’s understand more about the pitfalls of common vulnerabilities like Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF).

We’ll also explore practical strategies to shield our web apps from these threats, ensuring robust and safe software in an interconnected world.

Understanding the Threats

Cross-Site Scripting (XSS)

Imagine a scenario where an attacker injects malicious scripts into your application, causing it to execute unintended actions within the user’s browser. These scripts can steal sensitive data or compromise user accounts.

To prevent XSS attacks, rigorously validate and sanitize user input, use context-aware escaping mechanisms, and implement Content Security Policy ↗️ (CSP).

Cross-Site Request Forgery (CSRF)

Picture an attacker tricking your application into performing unauthorized actions on behalf of a user who’s logged in. This can lead to unintended consequences, from unauthorized fund transfers to modifying personal data.

To thwart CSRF attacks, implement random tokens for every user session, validate requests for sensitive operations, and apply the SameSite attribute to cookies.

Read more about app security in another blog that I shared sometime back: 7 Crucial Risks Every Developer Must Mitigate.

Building a Fortified Codebase

Input Validation and Sanitization

User inputs can be a Trojan horse for attackers if not handled carefully. Always validate input on both the client and server sides. On the client side, use HTML5 input types and attributes to enforce data format (like email or number).

However, client-side validation is easily bypassed, so server-side validation is a must. Sanitize user inputs by stripping out potentially harmful characters and data. Libraries like DOMPurify ↗️ for JavaScript can be your allies in this battle.

Add Client-side validation with HTML5 input element.

<input type="email" id="emailInput" required />

Add Server-side validation

const email = req.body.email;
if (!isValidEmail(email)) {
res.status(400).send('Invalid email format');
// Sanitization
const userInput = '<script>alert("XSS Attack!");</script>';
const sanitizedInput = sanitizeHtml(userInput);

The isValidEmail is a function which will validate the email format. The sanitizeHtml function will sanitize the user input.

Context-Aware Escaping

Escaping is like providing armor for your data when it’s being displayed. Depending on where data is rendered, whether in HTML, JavaScript, URLs, or CSS—different escaping techniques are necessary.

For HTML, use libraries like DOMPurify ↗️ or the built-in encodeURIComponent ↗️ function for URLs. Ensuring you use the right method at the right place can greatly reduce the risk of XSS attacks.

// Escaping for HTML content
const userComment = '<script>alert("XSS Attack!");</script>';
const escapedComment = DOMPurify.sanitize(userComment);
// Escaping for JavaScript
const userText = 'console.log("Script injection!");';
const escapedText = JSON.stringify(userText);
// Escaping for URLs
const userURL = 'https://example.com?param=<script>';
const escapedURL = encodeURIComponent(userURL);

Content Security Policy (CSP)

Think of CSP as a security guard for your website. It restricts the sources from which your application can load content like scripts, styles, images, and more.

By defining a strong CSP policy, you prevent the browser from executing any scripts from untrusted sources. This acts as a powerful barrier against many types of attacks, including XSS.

Would highly recommended reading thoroughly about CSP from MDN ↗️.

// Express.js middleware to set CSP header
app.use((req, res, next) => {
// This will allow scripts only from the same origin and cdn.example.com
// and styles only from the same origin
"default-src 'self'; script-src 'self' cdn.example.com; style-src 'self'",

Anti-CSRF Tokens for CSRF Prevention

Expanding on CSRF protection, the usage of anti-CSRF tokens enhances the security of your applications. Anti-CSRF tokens are unique tokens generated on the server and embedded in the client-side forms. When a form is submitted, the server validates the token to ensure that the request is legitimate and not a result of a CSRF attack.

To implement anti-CSRF tokens, generate a random token on the server for each user session. Include this token as a hidden field in your HTML forms. When the form is submitted, validate the token on the server before processing the request. This practice prevents unauthorized actions triggered by malicious sites.

On server side

// Server-side code to generate and attach anti-CSRF token
const antiCsrfToken = generateToken();
res.cookie('csrfToken', antiCsrfToken);

And on the client side

<!-- Hidden field for anti-CSRF token in a form -->
<input type="hidden" name="csrfToken" value="..." />

Secure Data Storage with Encryption

Protecting sensitive data, such as user credentials or payment information, requires secure data storage. Utilize encryption mechanisms like bcrypt ↗️ for hashing passwords and HTTPS for encrypting data transmission.

Storing passwords as hashes ensures that even if the data is compromised, attackers won’t have direct access to user passwords.

// Example of hashing passwords using bcrypt
const bcrypt = require('bcrypt');
const saltRounds = 10;
const hashedPassword = bcrypt.hashSync(password, saltRounds);
// Or if you prefer async
const hashedPassword = await bcrypt.hash(password, saltRounds);

SameSite Cookies

Cookies can be exploited by attackers to perform actions on behalf of users. The SameSite attribute is your defense mechanism here.

By setting cookies to "SameSite=Strict" or "SameSite=Lax", you control when cookies are sent along with cross-site requests.

This drastically reduces the chances of CSRF attacks. Remember, though, that it’s not a silver bullet; it works well when combined with other security measures.

Again MDN’s guide on this is fantastic read: SameSite Cookies explained ↗️.

// Setting a SameSite cookie
res.cookie('sessionId', 'xyz123', { sameSite: 'strict', httpOnly: true });
// Accessing the SameSite cookie
const sessionId = req.cookies.sessionId;

Formulating a Holistic Defense

Security is an ongoing commitment. Stay informed about the latest security threats, trends, and best practices in the realm of web development. Join security forums, attend conferences, and read reputable resources to keep your knowledge current.

Employ security tools like security scanners and static code analyzers to detect vulnerabilities in your codebase. Regular security audits and penetration testing are vital components of maintaining a secure application.

These tests simulate real-world attack scenarios, uncovering potential vulnerabilities before malicious actors can exploit them. Additionally, ensure that your team is well-versed in secure coding practices through regular training and workshops.


Writing secure JavaScript applications requires a combination of robust coding practices, vigilance, and continuous learning. By understanding the importance of input validation, context-aware escaping, using CSP, and implementing SameSite cookies, you can fortify your codebase against XSS and CSRF vulnerabilities.

Remember, security is a shared responsibility across your team and your organization. Through collaborative efforts and an unwavering commitment to best practices, you can build applications that stand as bastions of digital trust and resilience.

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