Insufficient input sanitization and output escaping lead to Cross-Site Scripting (XSS), allowing attackers to inject arbitrary web scripts into pages. This includes both Reflected XSS, where scripts are reflected off a web application to the user's browser, and Stored XSS, where scripts are permanently stored on the server and executed when a user accesses the affected page. Unauthenticated attackers can exploit this to execute scripts in the browsers of other users.