JavaScript injection is a type of security vulnerability that occurs when an attacker is able to insert malicious JavaScript code into a web application. This can happen when user input is not properly validated or sanitized, allowing the attacker to manipulate the website's behavior or steal sensitive information. When a user visits a compromised page, the injected JavaScript can execute in their browser, potentially leading to unauthorized actions, data theft, or even spreading malware. To prevent JavaScript injection, developers should implement proper input validation, use security libraries, and follow best practices for web application security.