Abstract: Usage Control Enforcement with Data Flow Tracking for X11 Alexander Pretschner and Matthias Büchler and Matus Harvan and Christian Schaefer and Thomas Walter Usage control requirements specify restrictions and compulsory actions that relate to the future handling of data. The enforcement of such requirements can and must happen at different levels of abstraction. This is because propositions in policies such as ``delete,'' ``copy,'' ``play,'' etc. can be assigned different semantics, depending on the level of abstraction that is chosen. We provide a formal model for data flow tracking at one such level, namely that of the X11 system, implement monitors that check different copy&paste policies, and use these monitors to ensure that, for instance, screenshots of a window cannot be taken if it contains sensitive data.