Everyone knows about securing apps from SQL injection attacks, but how many of you have given consideration to XPath injection protection?  

This (PDF:"Blind Xpath Injection") is a good introduction. A short excerpt shows why XPath injection can be more dangerous than it's SQL cousin:

This approach is even more powerful with XPath than it is with SQL, due to the following characteristics of XPath:
  • XPath 1.0 is a standard language. SQL has many dialects all based on a common, relatively weak syntax.
  • XPath 1.0 allows one to query all items of the database (XML objects). In some SQL dialects, it is impossible to query for some objects of the database
    using an SQL SELECT query (e.g. MySQL does not provide a table of tables ).
  • XPath 1.0 has no access control for the database , while in SQL, some parts of the database may be inaccessible due to lack of privileges to the application.

Take the general reasoning in the doc, add the lack of a native XPath equivalent to ADO.NET parameterized queries, and this has the potential to cause some major headaches in the future.