/ˌɛks-ɛs-ˈɛs/
n. “Sneaky scripts slipping where they shouldn’t.”
XSS, short for Cross-Site Scripting, is a class of web security vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users. Unlike server-side attacks, XSS exploits the trust a user has in a website, executing code in their browser without their consent or knowledge.
There are three main types of XSS: Reflected, Stored, and DOM-based. Reflected XSS occurs when malicious input is immediately echoed by a web page, such as through a search query or URL parameter. Stored XSS involves the attacker saving the payload in a database or message forum so it executes for anyone viewing that content. DOM-based XSS happens when client-side JavaScript processes untrusted data without proper validation.
A classic example: a user clicks on a seemingly normal link that contains JavaScript in the query string. If the website fails to sanitize or escape the input, the script runs in the victim’s browser, potentially stealing cookies, session tokens, or manipulating the page content. XSS attacks can escalate into full account takeover, phishing, or delivering malware.
Preventing XSS relies on a combination of techniques: input validation, output encoding, and content security policies. Frameworks often include built-in escaping functions to ensure that user input does not become executable code. For example, in HTML, characters like < and > are encoded to prevent interpretation as tags. In modern web development, using libraries that automatically sanitize data, alongside Content Security Policy, greatly reduces risk.
XSS remains one of the most common vulnerabilities in web applications, making awareness critical. Even large, popular sites can fall victim if validation and sanitization practices are inconsistent. Testing tools, such as automated scanners, penetration tests, and bug bounty programs, often prioritize XSS detection due to its prevalence and impact.
In essence, XSS is about trust and control. Users trust a website to deliver content safely; attackers exploit that trust to execute unauthorized scripts. Proper sanitization, rigorous coding practices, and security policies are the antidotes, turning a website from a potential playground for malicious scripts into a secure, trustworthy environment.