Understanding XSS Vulnerabilities

Cross‑Site Scripting (XSS) remains one of the most common security issues in web applications. In this post I'll break down the three main types of XSS, how they are exploited, and practical steps you can take to prevent them.

1. Reflected XSS

Reflected XSS occurs when user‑supplied data is returned immediately in the response without proper sanitization. Attackers craft a malicious link that, when clicked, injects a script into the victim's browser.

2. Stored XSS

Stored XSS is more dangerous because the payload is saved on the server (e.g., in a database) and served to any user viewing the affected page.

3. DOM‑Based XSS

DOM‑Based XSS happens entirely on the client side, where JavaScript manipulates the DOM using tainted data.

Mitigation Strategies

• Encode output based on context (HTML, JavaScript, URL).
• Use a robust Content Security Policy (CSP).
• Validate and sanitize all user inputs.
• Leverage security‑focused libraries like DOMPurify.