Cross-site scripting (XSS) attacks occur when an attacker injects malicious scripts into a web page that is then executed in the user’s browser. To prevent XSS, developers should escape user inputs before rendering them in HTML. Libraries like Jinja2, used in Flask, automatically escape inputs unless explicitly told not to. In Django, the template system also escapes inputs by default. Understanding contexts (HTML, JavaScript, CSS) and escaping appropriately is crucial. Additionally, using Content Security Policy (CSP) headers can further mitigate XSS risks.