Web applications are vulnerable to cross-site scripting attacks that enable data thefts. Information flow tracking in web browsers can prevent communication of sensitive data to unintended recipients and thereby stop such data thefts. Unfortunately, existing solutions have focused on incorporating information flow into browsers’ JavaScript interpreters, rather than just-in-time compilers, rendering the resulting performance noncompetitive. Few users will switch to a safer browser if it comes at the cost of significantly degrading web application performance. We present the first information flow tracking JavaScript engine that is based on a true just-in-time compiler, and that thereby outperforms all previous interpreter-based information flow tracking JavaScript engines by more than a factor of two. Our JIT-based engine (i) has the same coverage as previous interpreter- based solutions, (ii) requires reasonable implementation effort, and (iii) introduces new optimizations to achieve acceptable performance. When evaluated against three industry-standard JavaScript benchmark suites, there is still an average slowdown of 73% over engines that do not support information flow, but this is now well within the range that many users will find an acceptable price for obtaining substantially increased security.