A template runtime is the part of a web application that combines templates with data to generate pages. It resolves variables, applies filters, runs template logic, and outputs the final HTML that users see.
It matters in cyber security because this layer sits close to user input and often has powerful rendering features. If untrusted data reaches the runtime without proper escaping or sandboxing, attackers may trigger server-side template injection, leak secrets, or sometimes reach code execution. In real attacks, the danger appears when a template engine accepts expressions, helper calls, or unsafe object access. In defenses, teams harden the runtime by using strict escaping, limiting template features, reviewing custom helpers, and keeping template code separate from raw user-controlled content.