XML is today the most used data interchange format for business-to-business applications. Indeed, an increasing amount of data is created and published over the Internet every day in XML format. Moreover, organizations need more and more to share sets of XML documents usually managed via a common XML repository. XML copyright protection and source tracking have become strong requirements for collaborative environments. In the literature, XML-specific fingerprinting mechanisms have been proposed, inspired by similar work on relational data. However their robustness is impaired by the fact that an XML file can undergo a set of updates that change both the file structure and content. In this paper, we propose a novel watermarking schema for XML files. This solution is based first on an adequate selection of XML locators, i.e. document fragments targeted to embed the watermark. Watermark retrieval is achieved thank to a set of personalized fuzzy queries that reconstruct the locators that contain the watermark. We show theoretically the watermark robustness against possible XML file transformations; then, we present some initial experiments that validate the approach.