Today, XML is the most used data interchange format for business-to-business applications. Indeed, an increasing amount of data in XML format is created and published over the Internet every day. Moreover, organizations need more and more to share sets of XML documents usually managed via a common XML repository. XML integrity and authenticity have become strong requirements for applications like web services that exchange messages in such format. XML signature aims to guarantee these properties but it cannot avoid attackers to intercept and change the structure of the XML message. A very common attack to XML Signature called XML Signature Wrapping(XSW) attack represents a big issue in web services security as SOAP messages –which are XML signed files- could be corrupted. In this paper, we propose a countermeasure to the XML Signature wrapping attack that makes use of XML watermarking techniques. In our proposal we express constraints on the schema of the XML document and fix its structure using an absolute coordinate system whose values are embedded within the file as a watermark.