Hardening JavaScript JITs: Practical Mitigations for Modern Engines

[Why JavaScript JITs Are High-Value Targets] [Common JIT Vulnerability Classes and How Exploits Chain] [Applying CFI, PAC, and Memory Tagging Without Killing Performance] [Process-Level JIT Sandboxing and Isolation Patterns] [Fuzzing JS Engines: Targeted Strategies and Metrics] [Practical Hardening Checklist and Rollout Plan] The web’s fastest code is also its most dangerous: Just‑In‑Time compilers convert untrusted JavaScript into optimized native code with assumptions that are fragile under adversarial inputs, and those optimizations give attackers powerful primitives when they break. Treating JITs as the high-risk surface—not an afterthought—changes the defensive design choices you make in the renderer and the JS engine. The browser stack shows the symptoms you already see in incident queues: repeated high-severity V8 crashes tied to type confusion and use‑after‑free , chains that start in JS types and escalate to native code execution and sandbox escapes. Those crash trends are exa