Cross-site scripting (XSS) is a security vulnerability typically found in web applications. This vulnerability allows attackers to inject malicious scripts into content from otherwise trusted websites. XSS attacks enable attackers to bypass access controls such as the same-origin policy, which is designed to prevent malicious scripts on one page from obtaining access to sensitive data on another page without the user’s consent.
There are three main types of XSS vulnerabilities: stored (or persistent), reflected, and DOM-based. Stored XSS occurs when malicious script is directly injected into vulnerable web applications, such as in a database, message forum, visitor log, or comment field. Reflected XSS happens when an attacker sends a malicious script to a user through emails, messages, or some other means that reflects off of a web application back to the user’s browser. DOM-based XSS involves the manipulation of the Document Object Model (DOM) in the client-side script and does not involve sending the payload to the server.
Preventing XSS vulnerabilities requires secure coding practices, which include validating and sanitizing all user inputs to ensure they do not contain executable code. Additionally, Content Security Policy (CSP) headers can help mitigate the impact of XSS attacks by restricting the sources from which scripts can be executed.