Cross-site scripting (XSS) is a web security vulnerability that allows an attacker to inject malicious code into a web page that is viewed by other users. XSS can compromise the user’s interaction with the web application, steal sensitive data, perform unauthorized actions, and more. To prevent XSS, web developers need to apply various defensive techniques to ensure that user-supplied data is not interpreted as code by the browser. Two of these techniques are:

Incorporate contextual output encoding/escaping: This means that any user-supplied data that is displayed on the web page should be properly encoded or escaped according to the context where it appears. For example, if the data is inserted into an HTML attribute, it should be HTML attribute encoded; if the data is inserted into a JavaScript string, it should be JavaScript string encoded; and so on. This prevents the data from breaking out of its intended context and being executed as code by the browser. Output encoding should be done by using a reliable library or framework that supports different contexts and encodings.

Run untrusted HTML input through an HTML sanitization engine: This means that any user-supplied data that is intended to contain HTML markup should be filtered and validated by a sanitization engine that removes or escapes any potentially dangerous elements, attributes, or scripts. This prevents the attacker from injecting malicious HTML code that can execute scripts, load external resources, redirect the user, or perform other malicious actions. HTML sanitization should be done by using a well-tested and maintained library or framework that follows the best practices and standards for HTML filtering.

References :=

[Implementing and Operating Cisco Security Core Technologies (SCOR) v1.0], Module 5: Securing the Cloud, Lesson 5.2: Web Application Security, Topic 5.2.2: Cross-Site Scripting (XSS)

Cross Site Scripting Prevention Cheat Sheet - OWASP

What is cross-site scripting (XSS) and how to prevent it? - Web Security Academy