HTML/JavaScript injection is a cybersecurity vulnerability where attackers insert malicious HTML or JavaScript code into web pages viewed by other users. This typically occurs when user input is not properly sanitized, allowing harmful scripts to execute in the context of a trusted website. Such attacks can lead to data theft, session hijacking, website defacement, or redirection to malicious sites. Preventing this vulnerability involves validating and escaping user input, implementing Content Security Policy (CSP), and keeping web applications updated. Both website owners and users must be aware of these risks to ensure web safety and protect sensitive information.