Securing Front-End Code from Common Vulnerabilities

A common software security fallacy is assuming that backend firewalls and API authentication are sufficient to protect users. Frontend applications execute directly inside user browsers—an environment where attackers can inject malicious scripts, manipulate client state, or exfiltrate session data.

📘 Table of Contents

1. DOM-Based Cross-Site Scripting (XSS)
2. Dangerous Sinks: innerHTML, document.write, and eval()
3. Preventing Client-Side API Secret Leaks
4. Subresource Integrity (SRI) for Third-Party CDNs
5. Frontend Security Checklist
6. FAQ

⚠️ Avoiding Dangerous DOM Sinks

Never inject unsanitized user input into innerHTML or outerHTML. Always use textContent or trusted DOM sanitization libraries (such as DOMPurify):

// Insecure:
targetElement.innerHTML = userProvidedString;

// Secure:
targetElement.textContent = userProvidedString;

Protecting Client-Side JavaScript Applications

Modern web applications must protect themselves from malicious script injections and credential extraction.

1. Never Expose Secrets in Client Bundles

Environment variables embedded during build time (process.env) are bundled into client JavaScript and readable by anyone viewing source code. Never store database credentials, private API keys, or JWT secret signing keys in frontend code.

2. Defending Against Prototype Pollution

Freeze sensitive shared utility prototypes (Object.freeze) and use Object.create(null) for lookup maps to prevent attackers from altering native JavaScript prototype methods.