An Introduction to Cross-Site Scripting (XSS)

Cross-Site Scripting, often abbreviated as XSS, is a common type of security vulnerability typically found in web applications. At its core, XSS allows an attacker to inject malicious scripts (most commonly client-side scripts like JavaScript) into web pages viewed by other users. This occurs when an application takes untrusted data and sends it to a web browser without proper validation or escaping. Essentially, the attacker tricks a trusted website into delivering their harmful code to an unsuspecting user's browser.

Imagine a community notice board where anyone can post messages. If the board's administrator doesn't check the messages before pinning them up, a prankster could post a misleading or even harmful notice that looks like it came from the administrator. In the digital world, XSS is similar: a vulnerable website inadvertently delivers a malicious script, which the user's browser then executes because it trusts the source (the website). This can lead to various unwanted actions, from stealing sensitive information like login cookies to defacing websites or redirecting users to malicious sites.