Cross-Site Scripting (XSS)
Cross-Site Scripting (XSS) is a web security vulnerability that lets an attacker inject malicious JavaScript into a page viewed by other users, allowing them to steal data, hijack sessions, or impersonate victims in their browser.
Definition
Cross-Site Scripting (XSS) is a web security vulnerability that lets an attacker inject malicious JavaScript into a page viewed by other users, allowing them to steal data, hijack sessions, or impersonate victims in their browser.
Overview
XSS occurs when an application includes untrusted input in its output without proper sanitization or encoding, allowing an attacker's script to execute in another user's browser under the trust of the vulnerable site. There are three main forms: stored XSS, where malicious input is saved on the server (e.g., in a comment field) and served to every visitor; reflected XSS, where the payload is part of a request (such as a URL parameter) and immediately echoed back in the response; and DOM-based XSS, where the vulnerability lives entirely in client-side JavaScript that unsafely manipulates the page. XSS has consistently ranked among the OWASP Top 10 web application risks. Because the injected script runs with the same privileges as the legitimate page, it can read cookies, capture keystrokes, redirect users to phishing pages, or perform actions on the victim's behalf — often combined with Cross-Site Request Forgery (CSRF) for greater impact. Prevention relies on output encoding, input validation, a strict Content Security Policy, and frameworks like React that escape output by default. Developers building anything user-facing should treat XSS defense as a baseline requirement; the Web App Security (OWASP) course covers detection and remediation patterns in depth, and the blog post Cybersecurity for Developers: The OWASP Top 10 Explained walks through real examples.
Key Concepts
- Three variants: stored, reflected, and DOM-based XSS
- Exploits untrusted input rendered without proper output encoding
- Can steal session cookies, keystrokes, and credentials
- Frequently chained with CSRF or phishing for greater impact
- Mitigated with output encoding, input validation, and Content Security Policy
- Modern frameworks like React and Vue auto-escape output by default
- Consistently listed in the OWASP Top 10 web application risks
Use Cases
Frequently Asked Questions
From the Blog
Cross-Site Scripting (XSS) Explained
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