DOM-Based XSS (Cross-Site Scripting) is a type of security vulnerability that occurs when a web application modifies the Document Object Model (DOM) in a way that allows an attacker to inject malicious scripts. This happens when user input is not properly sanitized, enabling the attacker to manipulate the page's content or behavior. In DOM-Based XSS, the attack is executed entirely on the client side, meaning the malicious code runs in the user's browser rather than on the server. This can lead to unauthorized actions, data theft, or session hijacking, making it crucial for developers to validate and sanitize all user inputs.