What is a cross-site scripting attack, and how do you defend against it?

This question is designed to evaluate your technical knowledge and understanding of web security, specifically regarding cross-site scripting (XSS) attacks. Interviewers aim to assess your ability to identify vulnerabilities and implement effective defenses, as these skills are critical for safeguarding applications from harmful threats. A lack of knowledge in this area may raise red flags about your overall preparedness for a technical role.

To effectively respond, start by defining what a cross-site scripting attack is: a security vulnerability that allows an attacker to inject malicious scripts into webpages viewed by other users. Then, outline defense strategies such as input validation, output encoding, and implementing Content Security Policy (CSP). For example, you could say, "A cross-site scripting attack occurs when an attacker injects malicious scripts into a trusted site, potentially compromising user data. To defend against it, I implement input validation to ensure that user data is safe, output encoding to prevent script execution, and utilize Content Security Policy to control which scripts are allowed to run on the site."