Abstract: A controlled system performs internal taint tracking of data items. When a data item is created, the controlled system computes a name and a taint for the data item and performs an initialization function, thus informing a tracking entity that of the name and data of the data item. The taint is propagated to further data items, while the name may change, and when a data item is exported to or imported from a further device, the controlled system informs the tracking entity of the name and taint of the exported or imported data item as well as its source and destination. A controlled system may request a propagation history from the tracking entity. As the tracking entity is shared by more than one controlled system, it is possible to perform taint tracking across controlled systems even if these do not use the same taint tracking framework.