A JIT compiler is a browser component that turns JavaScript into native machine code while a page is running. By compiling “hot” code on the fly, the browser can execute scripts much faster than interpreting them every time. This speed boost is important for modern web apps, but it also increases complexity inside the browser engine.
In security terms, JIT code is a sensitive attack surface because a bug can cause the engine to generate incorrect machine code from attacker-controlled JavaScript. A malformed page may then steer execution into unintended instructions, sometimes leading to memory corruption or code execution inside the renderer or content process. Defenders reduce this risk by patching browser engines quickly, limiting what sandboxed processes can access, and treating JIT-related flaws as high-priority because they affect the part of the browser that handles untrusted web content.