JIT, or Just-In-Time compilation, is a browser optimization that converts frequently used JavaScript into native machine code while a page is running. This makes scripts execute faster than interpreting them line by line, which improves responsiveness for modern web apps.
In cyber security, JIT matters because it creates a complex performance layer inside the browser’s attack surface. If the compiler or its generated code handles values incorrectly, a malicious page may trigger memory corruption, type confusion, or sandbox escapes. Attackers often target JIT paths because they run automatically when a page loads and because optimized code can behave differently from the original JavaScript, making bugs harder to reason about. Defenders care about JIT flaws because patching the browser engine closes a direct route from web content to process compromise. Hardening settings that reduce or disable JavaScript can lower exposure, but the main defense is keeping the engine fully updated.