When dealing with dynamic, untrusted content, such as on the Web, software behavior must be sandboxed, typically through use of a language like JavaScript. However, even for such specially-designed languages, it is difficult to ensure the safety of highly-optimized, dynamic language runtimes which, for efficiency, rely on advanced techniques such as Just-In-Time (JIT) compilation, large libraries of native-code support routines, and intricate mechanisms for multi-threading and garbage collection. Each new runtime provides a new potential attack surface and this security risk raises a barrier to the adoption of new languages for creating untrusted content.