Cross-Site Scripting

Cross-Site Scripting (XSS) refers to a security flaw where an attacker can embed harmful client-side scripts into a trusted web application or site. When a user unknowingly accesses the affected page, their browser runs the script as it seems to originate from a reliable source, allowing the attacker to circumvent security measures and potentially steal sensitive information such as session cookies or take control of user sessions.

Attackers typically introduce malicious scripts into web applications via input fields that accept user data. The application then transmits this script to the user's browser, where it is executed. The primary methods of attack are categorized based on how the code is delivered and stored.

To mitigate XSS, a comprehensive strategy is necessary, treating all user inputs as untrustworthy. The fundamental guideline is to ensure that any data received by an application is not executed as code in the user's browser. A combination of server-side and client-side defenses is the most effective approach.

Although both XSS and CSRF are vulnerabilities in web applications, they exploit trust in distinct manners.

One notable incident was the MySpace 'Samy' worm, a stored XSS attack that rapidly spread, adding over a million friends to the creator's profile within 24 hours. This case highlighted how a single vulnerability could be leveraged for widespread, automated consequences.