JIT, or Just-In-Time compilation, is a runtime technique that generates machine code on demand instead of ahead of time. A program starts in an interpreted or lower-level form, then the JIT compiles hot paths into native instructions so they run faster. This is common in browsers, language runtimes, and some kernel or database components.
JIT matters in cyber security because dynamic code generation creates a moving target: code pages are created, reused, and discarded continuously. Attackers can abuse JIT systems to shape memory layouts, create useful gadgets, or influence speculative-execution behavior when CPU predictors retain stale history. Defenders reduce risk with code-signing and sandboxing, W^X page protections, strict code-cache isolation, and branch-predictor mitigations such as IBPB or retpoline where appropriate. The security challenge is to control both the generated code and the hardware state around it.