Blind XPath Injection

About XPath
XPath is a sort of query language that describes how to locate specific elements (including attributes, processing instructions, etc.) in an XML document. Since it is a query language, XPath is somewhat similar to Structured Query Language (SQL). However, XPath can be used to reference almost any part of any XML document without access control restrictions, whereas with SQL, a "user" (which is a term undefined in the XPath/XML context) may be restricted to certain tables, columns or queries. 

Blind XPath Injection
Using Blind XPath Injection, an attacker can extract a complete XML document for XPath querying without prior knowledge of the query. The attacker can access the entire XML "database" used in the XPath query which can be powerful against sites that use XPath queries (and XML "databases") for authentication, searching and other uses.