Protecting data from accidental loss or theft is crucial in today's world of mobile computing. Data Tethers provides flexible environmental policies, which can be attached to data, specifying security requirements that must be met before accessing that data. Data Tethers uses fine-grain data flow tracking to maintain these policies on derivative data. This is implemented by dynamic recompilation of legacy applications without the need to recompile from source. We demonstrate the system's feasibility with microbenchmarks that show individual component performance and benchmarks of real user applications like word processors and spreadsheets.