One of the first vulnerability classes one learns in web security are Cross Site Scripting (XSS) and SQL Injection (SQLI). Together with other similar classes like Shell Injection, Server-side Template Injection (STTI), HTTP Header Injection, etc., they form the "Injection" family. In this article, we revisit fundamentals of information security and answer the question why do these kinds of attacks really happen, from a technology design perspective.