XPath is a query language used to select data from XML data sources. It is increasingly common for web applications to use XML data files on the back-end, using XPath to perform queries much the same way SQL would be used against a relational database.
XPath injection, much like SQL injection, exists when a malicious user can insert arbitrary XPath code into form fields and URL query parameters in order to inject this code directly into the XPath query evaluation engine. Doing so would allow a malicious user to bypass authentication (if an XML-based authentication system is used) or to access restricted data from the XML data source.
source : http://www.rapid7.com/vulndb/lookup/spider-xpath-injection
this video is simple demo of xpath injection exploitation. a tool xpath blind explorer is used in this video to perform attack.